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Abstract

Until relatively recently, the extent of microbiota presence in the human breast was under-appreciated. A high-throughput sequencing study and culture-based studies have demonstrated the extensive presence of microbes in human milk, with their origin believed to be from the skin, oral cavity and via gut translocation. Since formula milk substitutes do not contain these bacteria, what benefits are denied to these infants? The addition of probiotic bacteria to some infant formula is meant to provide some benefits, but these only contain one species and the dose is relatively high compared with breast milk. Many questions of importance to women's health arise from these findings. When, how and what types of microbes colonize the breast at different stages of a woman's life, including postlactation, and what effect do they have on the host in the short and long term? This article discusses some aspects of these questions.

Keywords

bacteria breast breast milk banking human milk oligosaccharide microbiota probiotic translocation

The Human Microbiome Project has been a catalyst for studies designed to log the microbes that inhabit the body and determine what role they play in health and disease. Some of these studies have examined the unique anatomical sites of women, namely in the reproductive tract, revealing a plethora of microbes and a dynamic milieu that can change in composition within days, or show signs of stability over months [1 –5]. The high-throughput sequencing tools that have emerged in recent times have allowed in-depth coverage of the organisms that can inhabit different sites. These tools have complemented culture-based methods used to isolate microorganisms. The application of these systems to study the female breast has begun to reveal the breadth of microbes present in this gland, at least from the perspective of organisms in human milk.

Breast milk is the first food given to newborn humans and it contains all that the child requires with regards to nutritional requirements. The average amount of milk consumed by infants varies with age. A study by Kent et al. showed that breast-fed infants (15 weeks old) fed 11-times a day ingested approximately 788 ml of milk each day [6]. The microbiological content of milk has been routinely recorded to be as high as 1 × 10⁵ colony-forming units (CFU) per milliliter. Probiotic formula designed to mimic human milk delivers around 1 × 10⁶ CFU/ml [7].

Milk is easily digested by the immature intestinal tract, and the initial colostrum, which delivers protective antibodies, immune competent cell types (e.g., macrophages, granulocytes and T and B lymphocytes) [8] and a complex network of chemoattractants, activators and anti-inflammatory cytokines that help prevent disease while the neonatal immune system is still developing. Milk also provides iron-binding protein, lactoferrin, lysozyme, lactoperoxidase and hydrogen peroxide and oligosaccharides, which inhibit bacterial invasion of the infant gut. Infant formula milk does not deliver the full immunological, host-derived products of breast milk.

The main focus of this article is to discuss the current understanding of bacteria found in the female breast, particularly in milk, and how these might influence the microbial composition and health of the newborn. We will also discuss how probiotics may modulate the composition of human milk and the well-being of the infant, and how breast milk banking and infant formula substitutes may eventually better mimic human milk. In addition, we discuss a relatively new area of breast microbiota influencing health beyond lactation, such as cancer.

What bacteria are in breast milk?

As long ago as 1924, streptococci were isolated from human milk [9] and since then numerous studies have detected Streptococcus salivarius, Streptococcus mitis and other oral species, likely transferred from the infant's mouth [10 –15]. Wysham et al. observed an interchanging bacterial biota between the breast surface and infant's mouth during suckling [16], while McCarthy et al. demonstrated that streptococci, especially the nonhemolytic S. salivarius, were common mouth isolates in infants under the age of 1 year old [17,18].

In the 1970s there was a rapid increase in the number of breast milk banks for neonates in intensive care and also due to the controversy of using infant milk formulas. Microbiological and biochemical analyses of donor milk again revealed the presence of Gram-positive cocci [2,19 –23]. Williamson et al. reported that healthy, full-term infants ingested milk from their mothers with bacterial counts exceeding 1 × 10² CFU/ml and these originated from the alveolar skin and the oral cavity of the infant [23]. Carroll et al. performed aerobic cultures of 207 samples of drip breast milk from 70 mothers and showed that six (3%) were sterile, 82% contained only coagulase-negative staphylococci and Streptococcus viridans (a pseudo taxonomic term for α-hemolytic streptococci), 15% grew potential pathogens such as Staphylococcus aureus and enterobacteria or group B streptococci in three samples (2%) [20]. Similarly, Eidelman and Szilagyi collected samples from 44 postpartum mothers and recovered bacteria in each, with collection counts averaging 2 × 10⁴ CFU/ml [10]. Of these samples, 84% had organisms identical to those cultured from the mother's nipple, while in 30%, potential pathogens were detected [10].

It is intriguing that none of the reports suggested that this range of organisms, many with the capability of inducing infection at other body sites, were causing pathogenesis in the breast. Why, we might ask? If the bacteria are multiplying, presumably they could be producing toxins, hemolysins and some of the known range of virulence factors found in streptococci, staphylococci and oral cavity inhabitants [24 –26]. The breast is known to have a range of antimicrobial defenses from first-line lactoferrin, complement, lysozyme, cytokines and immunoglobulins to pattern recognition receptors and Toll-like receptors that activate defenses [27]. With macrophages, lymphocytes and neutrophils there is no shortage of factors that under other circumstances would induce an inflammatory response and signs and symptoms of the host fighting an infection. In human milk, secretory IgA is in abundance and yet none of these factors eradicate the microbiota. Bacterial biofilm formation or immune tolerance may explain this failure, but at the very least these are worthy of further investigation. Biofilms are aggregates of bacteria that adhere to each other and to other surfaces. Quiescent biofilms have been described in many human tissues, for example the bladder [28].

The first noncultural analysis of human milk started occurring in the year 2000 by the use of denaturing gradient gel electrophoresis and 16S rRNA clone library analysis, which confirmed the general composition described by culture studies but showed that a wider array of microorganism diversity is present [11,14]. It also emerged that individuals had somewhat distinct microbiota profiles [14]. The first deep sequencing analysis was performed by Hunt et al. on breast milk collected from 16 healthy women (20–40 years of age) nursing infants [29]. While more bacterial types were detected, including Serratia, Ralstonia, Prevotella and Corynebacterim, once again Streptococcus and Staphylococcus were the major phylotypes present. Previous studies have shown that Lactobacillus and Bifidobacterium species are common but are minority members of human milk (2–3% abundance). The list of species identified in human milk is summarized in Table 1 and includes organisms considered to be pathogenic.

Table 1.

Bacterial types identified in human breast milk by culture and culture-independent techniques.

Genera	Species
Acinetobacter	A. calcoaceticus
Bifidobacterium	B. breve, B. adolescentis, B. bifidum, B. longum, B. animalis, B. catenulatum
Enterococcus	E. faecal is, E. faecium, E. durans, E. hirae, E. mundtii
Escherichia	E. coli
Gemella	G. haemolysans
Klebsiella	K. oxytoca
Kocuria	K. kristinae
Lactobacillus	L. rhamnosus, L. crispatus, L. gasseri, L. fermentum, L plantarum, L. brevis, L. oris, L. animails
Lactococcus	L. lactis ssp. lactis
Leuconostoc	L. mesenteroides, L. citreum, L. fallax
Propionibacterium	P. acnes
Pseudomonas	P. synxantha, P. fluorescens
Rothia	R. mucilaginosa
Serratia	S. proteomaculans
Staphylococcus	S. aureus, S. epidermidis, S. haemolyticus, S. hominis, S. pasteuri, S. warneri
Streptococcus	S. salivarius, S. mitis, S. gallolyticus, S. australis, S. vestibularis, S. parasanguis, S. pneumoniae
Weisella	W. cibaria, W. confusa
Other bacterial genera detected but not assigned to species
Corynebacterium, Sphingomonas, Granulicatella, Bradyrhizobium, Prevotella, Ralstonia, Actinomyces, Clostridium, Veillonella

Data taken from [12 –14,29,30,114].

How do the bacteria get to the breast?

Until recently, it was generally assumed that the newborn gut becomes colonized by microbes through the birthing process, handling and the environment. But a 2011 DNA-based study of 700 bacterial isolates showed that the same strains of bacteria could be recovered from human milk, the mothers feces and the new-born's feces, supporting the hypothesis that vertical transfer of intestinal bacteria from the mother's gut to her milk occurs [30]. Three potential mechanisms now exist on how bacteria reach the breast in addition to via the skin and nipple: translocation across the mucosal membranes of the mouth, intestine, urogenital tract and skin, entry through the bloodstream via oral hygiene care, and dendritic cell (DC) sampling of the intestine and transfer through the bloodstream and mesenteric lymph nodes ( Figure 1 ) [31]. Clearly, studies are needed to verify the third mechanism and to determine why some classic intestinal species have not yet been recovered from the breast. It seems hard to imagine that the DC sampling is so specific to exclude some strains, or that the defenses in the breast eradicate only certain bacterial types. Then, from the bloodstream, what is the process by which the bacteria transfer from DCs to reside in the mammary lobes and ducts? The isolation of strains of Enterococcus, Streptococcus, Staphylococcus and Propionibacterium in neonatal umbilical cord blood of cesarean-born babies further supports the idea that bacteria can reach the mammary ducts via the bloodstream [32]. An interesting finding in a cesarean-born baby in Korea, was that on the first day of life, Enterobacter, Lactococcus lactis, Leuconostoc citreum and S. mitis were present in the newborn's feces [33]. Either these organisms reached the baby prior to birth, which in itself is fascinating, or they were passed onto the baby from the mother and handlers, and rapidly colonized the gut. L. citreum is often used in Kimchi, a popular Korean food, but as shown in Table 1 , it was also detected in four mothers in Spain, thus it could have colonized the mother then the baby via any one of the three mechanisms herein proposed.

Figure 1.

Possible mechanism of bacterial translocation from the gut to the breast. The lumen of the gut is inhabited by trillions of bacteria and is separated from the interior by a thin layer of cells called the epithelium. DCs can sample bacteria directly from the lumen by the extension of dendrites or can capture bacteria that have translocated across specialized epithelial cells called ‘M cells.’ DCs laden with bacteria can either prime T cells in the Peyer's patches or migrate to the MLN to prime T-cell reactions there. Unlike primed T cells, DCs do not normally move past the MLN. However, during pregnancy and lactation, it is believed that bacteria-carrying DCs migrate out of the MLN and into the mammary glands. These glands are composed of approximately 15–20 lobes that circle around the nipple, albeit not as definitively visible during surgery as depicted in this diagram. Inside these lobes are lobules and at the end of each lobule are tiny alveoli that produce milk. The milk is carried via the ducts into the nipple. DC: Dendritic cell; MLN: Mesenteric lymph node.

One study of breast milk and peripheral blood collected aseptically from healthy donors at various times after delivery examined bacterial ribosomal DNA content in milk cells, maternal peripheral blood mononuclear cells and feces, and corresponding infant feces [31]. Breast milk contained alow total concentration of microbes (1 × 10³ CFU/ml) and maternal blood and milk cells contained the genetic material of a large biodiversity of enteric bacteria. Some bacterial signatures were common to infant feces and to samples of maternal origin, for example S. salivarius appeared to be prevalent.

Mucosal translocation can occur with bacteria crossing over the gut epithelium into the bloodstream either through gap junctions between epithelial cells or by invading cells or through uptake in DCs. Rather than always being detrimental, this is a protective mechanism that provides the host's defenses with constant external antigen contact, similar to a monitoring system. The process was shown in healthy animals and low numbers of bacteria penetrated systemically into the tissues of the spleen and liver [34]. An elegant study in mice showed increased bacterial translocation from the gut during pregnancy and lactation, and the presence of DCs loaded with bacteria in lactating breast tissue [35]. In addition, they showed that human peripheral blood mononuclear cells and breast milk cells contained bacteria during lactation.

Bacterial interactions in the breast

Apart from studies showing that some pathogens cause infection in the breast, it is not at all clear what the bulk of the organisms are doing to the breast. As depicted in Figure 2 , the bacteria isolated to date reside in the ducts and lobules, but preliminary findings from our laboratory show that bacteria also reside in tissues. The breast is primarily composed of adipose and glandular tissue, so theoretically these would provide different nutrients for the organisms. There is nothing obvious to suggest that any nutrient differences are sufficient to restrict bacterial growth, given that Gram positive, Gram negative, aerobes and anaerobes have been detected. Nevertheless, much remains to be determined about how the organisms proliferate, avoid eradication from the immune system and each other and how hormonal changes and other factors affect their persistence. For now, the structures of the microbial communities and how, if at all, metabolic by-products influence the dynamics are not known.

Figure 2.

Localization of bacteria within the breast. Bacteria have been found in (1) the ducts and (2) the lobules. (3) The presence of bacteria in the fatty tissue is yet to be determined. The nature of the bacterial interactions in these communities remains to be elucidated as portrayed by the ‘?’ symbol.

From the nature of the species identified to date, and the fact that many different types appear to coexist [29], one may predict that several bacteria–bacteria interactions occur. This may include bacteriocin production, cell–cell signaling and symbiotic relationships based upon each species' nutritional requirements. Studies in the oral cavity reveal how these types of bacterial communities are established and provide examples of these interactions taking place. The breast microbiome may or may not be as well organized as the oral cavity in terms of succession of strains colonizing and specific ways they coaggregate with one another [36,37], but as with any community, there is likely to be competition. Compounds produced by one bacterium to inhibit another are common, and Heikkila et al. showed that approximately 20% of Staphylococcus epidermidis and 50% of S. salivarius isolated from human milk produced bacteriocin lantibiotics that suppressed growth of S. aureus [12]. In addition, Enterococcus faecalls isolated from 7.5% of samples and lactic acid bacteria (e.g., Lactobacillus rhamnosus, Lactobacillus crispatus, Lactococcus lactis and Leuconoctoc mesenteroides) isolated from 12.5% of samples also had some antimicrobial activities against S. aureus. This indicates that not only are bacteriocin-producing bacteria present, but there are a number of different bacterial types that have this ability. This does not mean that S. aureus would then be wiped out, as that is not the case, but rather at the local bacterial community level, there are ways for other strains to secure a niche.

In such ecosystems, bacteria communicate using signaling molecules, some of which can also be bacteriocins. A recent review covers this area in depth and not only discusses the bacteria–bacteria communication systems, but also how some bacteria can hijack host hormones and how interkingdom (bacteria–host) communication, such as epinephrine/norepinephrine/auto-inducer-3, can benefit the organisms [38]. The same research group has also shown that in the gut, bacteria utilize chemical-sensing systems to determine which niche will be good for colonization [39]. This may explain why some species have not yet been detected in the breast.

Are the breast bacteria important for the infant?

The effect that the transfer of nonpathogens and potentially deleterious bacteria have on infants has not been well studied, but these species can influence the risk of allergy, diabetes, inflammatory bowel disease (IBD), celiac disease, obesity and diseases such as multiple sclerosis [40 –44]. This could be due to several mechanisms, the most plausible being immune modulation, and through metabolites from bacterial growth in breast or formula milk.

Studies indicate that early bacterial colonizers in the newborn play a role in defense against pathogens [45], maturation of both the systemic and mucosal immune systems and micro vasculature of the gut [46 –48], establishment of the microbiome across body sites [49,50], directing the immune system and in some cases errantly, to increase the risk of allergic responses, synthesis of vitamins and digestion of insoluble compounds, and increased integrity of the gut epithelium (i.e., to prevent noxious substances and pathogens from gaining access to the periphery). Mounting evidence in recent years has shown that the gut microbiota plays an important role in modulating the risk of IBD, Type II diabetes and allergy [48]. In an animal model studying necrotizing enterocolitis (NEC) it was shown that the transfer of an adult commensal flora to neonatal mice lead to severe NEC, however, pups that were dam fed were protected against the disease [51]. In studies that have sought to identify which microbes colonize the host's gut in the days following birth, prior to predominant bifidobacteria, some degree of random succession occurs. In some vaginally born infants, Escherichia coli and similar fecal organisms are inherited from the mother [49]. In others, Bacteroides thetaiotaomicron have been identified, which then play an important role in gut microvasculature by inducing angiogenesis via Paneth cells [46,52], while a bacterial polysaccharide (PSA) from Bacteroides fragilis directs the cellular and physical maturation of the developing immune system [47]. Yet, in a study of women who gave birth vaginally, none had B. thetaiotaomicron or B. fragilis in their vaginal samples suggesting that these species were not transferred to the newborns at delivery, nor did they colonize through environmental contamination [50]. This raises an important question: is lack of these bacterial species detrimental to the host or do other species substitute the functionalities they confer? Studies are needed to examine this. This is especially the case for formula-fed babies, where their intestinal microbiota is already less diverse than breast-fed babies. Interestingly, in one study it was shown that babies at genetic risk of celiac disease had a high prevalence of certain Bacteroides species such as Bacteroides uniformis [44], meaning there could be major differences within a genera and between strains on how the host is affected by the early colonization process.

Another example of possible aberrations from the mother–child bacterial transfer and programming scenario is that some diseases of the gut have been rising quite significantly in recent years. Some researchers have suggested that the increase in childhood IBD is associated with elevated numbers of Gram-negative bacteria [53,54]. Thus, conceptually, the abundance profile of the premature infant may suggest a predisposition to IBD, especially if Serratia, Citrobacter and organisms such as E. coli dominate for several months or years [55]. In perhaps the most in-depth study of the gut microbiota of a premature infant using metagenomic profiling, Serratia and Citrobacter strains emerged after day 10 to dominate the gut microbiota abundance by day 16 [56]. Although only one child was assessed, the study illustrates the fluidity of the infant gut and potentially how alterations can then impact health.

Breast milk banking as a substitute

‘Wet nursing’ is not new, nor is the collection and donation of human milk, which is often conducted on an informal basis between mothers where required. However there are now companies that sell pasteurized fortified human milk to hospitals for use in neonatal intensive care [201]. There have been breast milk banks in North America and other parts of the world for approximately 100 years but they largely fell out of favor in the 1950s with the availability of powdered infant formula. Now, with additional health benefits of human milk being discovered, there is a renewed interest in its use.

The delivery of human milk goes against the convention of hygiene requirements for food, that is, while it contains numerous and diverse bacterial types, it is not fermented. Bjorksten noted that although breast milk is “contaminated”, it produces no ill effects, perhaps because it also contains antimicrobial properties that protect against infection [57]. He therefore concluded that pasteurization of donated human milk is not necessary (or recommended). Others, such as El-Mohandes [58] and Wright [59], decided that milk containing more than 10⁵ organisms per ml should be discarded before pooling with other samples. However, as with other transference of entire microbiota communities between individuals, such as to treat Clostridum difficile infections [60], there is now more concern over transmission of diseases such as HIV and hepatitis C [61], thus pasteurization of human milk is required. In 1985, the Human Milk Banking Association of North America was established with one of the main goals being to create standards for all North American milk banks [202]. Human milk from these facilities is pasteurized. Thus, at most, the infants consuming these products acquire some bacterial components left over from pasteurization, which may or may not provide benefit, for example by stimulating the immune system [62].

Infant formula as a human milk substitute & its pitfalls

The advocates for breastfeeding have been strong since the late 1960s, with universal agreement that human milk is the best source of nourishment for newborns and infants. The WHO recommends that breastfeeding starts within 1 h of birth and that infants should be exclusively fed for 6 months. Globally, less than 40% of infants under 6 months are actually exclusively breast fed [203]. In a survey conducted by Statistics Canada in 2009, it was shown that medical factors were the main reasons cited for mothers not breastfeeding (e.g., C-section, premature birth, multiple births or medical condition of mother or baby) [204].

Unfortunately, in less developed countries, especially where unhygienic conditions exist, the incidence of diarrheal diseases in formula-fed babies is significantly elevated compared with those who are breastfed [63]. This is largely attributable to the contaminated water used to mix the infant formula, but also because the babies do not receive all of the nutritional and protective benefits of the mothers milk, such as the passive protection by immune molecules. Cow's milk also contains immunoglobulin, but in different quantities and proportions of IgG, IgA (lower levels) and IgM classes [64], and unless specifically hyper-immunized against human pathogens to make ‘immune milk’, this will not provide protection for infants in these regions [65].

Possibilities of improving infant formula with probiotics & prebiotics

Probiotics are defined as “live microorganisms which, when administered in adequate amounts, confer a health benefit on the host” [205]. Since 2001, probiotic bacteria have been added to some infant formulas in the USA, with the intent of helping to prevent gastrointestinal infections and antibiotic-associated diarrhea, reduce colic-associated crying, reduce or prevent allergy and improve stool consistency and frequency [66].

A review by the Supplementation of Infant Formula with Probiotics and/or Prebiotics Committee on Nutrition (ESPGHAN) reiterated that the long-term effects of probiotics in infant formula still require monitoring for negative outcomes [66]. A criticism by ESPGHAN was the inadequacy of studies to look for and record adverse events. Potentially, another way to deliver probiotics to the infant would be via ingestion by the mother and then translocation from her gut to the mammary gland. However, this has not been tested so far. In terms of dosage, as noted earlier, the total CFU/ml is higher than that of human milk, and significantly higher in bifidobacterial numbers. Since these products have obtained regulatory approval, they should be safe, but it remains to be seen whether this disproportionate ingestion of bifidobacteria is truly beneficial in all infants, and more so than human milk. One study suggests it is beneficial in vulnerable populations, specifically in infants born to mothers with HIV [67].

Prebiotics are defined as “the selective stimulation of growth and/or activity(ies) of one or a limited number of microbial genus (era)/species in the gut microbiota that confer(s) health benefits to the host” [68]. The use of prebiotics, specifically human milk oligosaccharides (HMOs), are reviewed elsewhere [69]. Suffice to say, breast milk contains 5–10 g/l [56] of HMOs with the highest amounts found at the early stages of lactation [70 –74]. Preterm breast milk has higher levels of oligosaccharides than term milk [73,74]. Some infant formulas attempt to match the HMOs. Analysis of the human milk glycobiome (genes that bacteria use to break down complex carbohydrates) have been published relating to intestinal bacteria [75]. Human milk contains hundreds of diverse and complex oligosaccharides, many indigestible by the developing infant, therefore gastrointestinal microbiota are needed, particularly bifidobacteria, to break them down [75]. Interestingly, glycobiome analysis showed that certain bifidobacterial probiotic strains added to infant formula could not utilize HMOs as widely as indigenous species. Also, Bifidobacterium species more commonly found in adults, such as Bifidobacterium adolescentis, and nonhumans, such as Bifidobacterium lactis/animalis, were not able to metabolize as wide an array of breast milk HMOs. This makes it all the more important that the probiotic strains added to infant formula need to be aligned with the prebiotics and the gut organisms that are being targeted.

HMOs also help prevent infection in breastfed infants by interfering with pathogen adhesion to the host [76]. More widely metabolized prebiotic oligosaccharides have already been added to infant milk formula with positive benefits, such as an increase in lactic acid bacteria and bifidobacteria, decreased diarrhea and decreased rates of infection and allergies [77 –81].

Beyond infant feeding: could bacteria have a role in inflammation & breast cancer?

Breast carcinomas occur mostly (75%) in the ducts, followed by the lobes, areola and nipple (Paget's disease). There is also a rare but highly malignant form called inflammatory carcinoma, which appears to occur in younger women [82]. It has been suggested that intestinal bacteria affect the circulation and absorption of female hormones such as estrogen, and this may hold clues as to why certain people develop breast cancer and others do not. Furthermore, high amounts of fat in the diet may increase the risk of breast cancer, and this too may relate to the types of bacteria in the intestine and how they process the fat.

Many bacteria detected in human milk are able to produce carcinogens or other toxic compounds [83 –89]. No studies have yet determined if these compounds are produced in breast milk or lactating women, but the possibility is worth pursuing. Secondary amines and high levels of nitrites have been detected and could potentially be used by bacteria, such as E. coli, to form nitrosamines, which are carcinogenic, mutagenic and occasionally teratogenic [90,91].

The ability of a number of bacterial species found in breast milk to induce inflammation has been shown, and inflammatory processes have been linked to a number of cancers. Using a systems-biology approach, a recent mouse study showed that during breast-tumor induction and progression, a series of inflammatory processes occurred [92]. Inflammation induced by strains of E. coli has been detected in the bovine mammary gland with upregulation of caseins, whey protein serum albumin, β-lactoglobulin, α-lactalbumin, as well as lactoferrin, transferrin, apolipoprotein AI, fibrinogen, glycosylation-dependent cell adhesion molecule-1, peptidoglycan recognition receptor protein and cyclic dodecapeptide-1 [93].

Other reviews more adequately cover the topic of the role of inflammation as a risk factor in breast cancer and the development of more aggressive, therapy-resistant estrogen receptor-positive breast cancers [94]. But, clearly an inflammatory microenvironment within the breast, tumor-associated macrophages and proinflammatory cytokines that can act on nearby breast cancer cells and modulate tumor phenotype, and activation of the NF-κB pathway and its cross talk with estrogen receptors, are all important in the cancer process. Bacteria can also induce these responses, but studies are needed to determine if they do this in women who go on to develop breast cancer.

How might lactic acid bacteria reduce the risk of breast cancer?

A case–control study in The Netherlands found a higher incidence of breast cancer among women who consumed significantly lower amounts of fermented milk [95]. From this, the authors concluded that fermented foods could protect against breast cancer, and suggested it was either due to lactic bacteria (especially Streptococcus spp. and Leuconostoc spp. used in buttermilk) interfering with enterohepatic circulation or stimulating some sort of immunological activity.

Another study that followed women between 1989 and 2006 reported a median reduction in breast cancer mortality (as distinct from incidence of disease), with decreases of 12% in Finland and 16% in Sweden, two of the highest per capita consumers of fermented milks [96]. Yet, in these and the other Scandinavian countries, breast cancer rates have risen steadily since the 1970s and 1980s when more fermented and probiotic milk products became available [97]. So, the issue remains unresolved and with many confounding factors that contribute to health and disease, it will not be an easy one to decipher.

Lactic acid bacteria have been shown to confer anti-inflammatory and anticarcinogenic effects. In a murine model, milk fermented with Lactobacillus helveticus R389 delayed breast tumor growth by decreasing IL-6 and increasing anti-inflammatory IL-10 in the serum and mammary glands and tumor-infiltrating immune cells [98]. Likewise, administration of kefir and a kefir cell-free fraction to mice injected with breast tumor cells resulted in anti-inflammatory and delayed carcinogenesis in the mammary gland [99]. This ex novo effect is supported by an earlier study on milk fermented by bifidobacteria and lactobacilli [100]. The implication is that microbial by-products from fermented milk are involved in signaling processes that may help to protect the host from cancer. Of note, anti-inflammatory effects cannot be generalized for all lactobacilli. Two human milk isolates of lactobacilli showed opposite effects on rodent bone marrow-derived macrophages with L. salivarius CECT5713, being anti-inflammatory and L. fermentum CECT5716 being immunostimulatory [101].

A study of 352 women with infectious mastitis showed that daily ingestion of L. fermentum CECT5716 or L. salivarius CECT5713 for 3 weeks resulted in an improved cure and a lower recurrence over antibiotic therapy [102]. Interestingly, the probiotic strains were recovered from the breast milk in some cases, indicating the likelihood of gut translocation to the breast. This also shows that probiotic therapy to the mother may not only help to treat or prevent disease in the breast, but may also be a way to colonize the neonatal gut and impart other health benefits.

A huge study of over 140,000 women in 30 countries concluded that the relative risk of breast cancer decreased by 4.3% for every 12 months of breastfeeding in addition to a decrease of 7.0% for each birth, and this was independent of her age when the first child was born [103]. This is contradicted by a more recent study that found that young women (<25 years of age) can have an increased breast cancer risk associated with a completed pregnancy [104], and this is more often fatal two years after delivery, irrespective of breastfeeding [105]. The trend of increased risk of disease persists for up to 15 years. If pathogenic bacteria play a role in breast cancer, the vital step could be the arrival of the organisms due to the translocation process described above [34], not necessarily the persistent production and release of milk. For some younger women, perhaps specific aberrations in their mucosal microbiota play a role.

If we hypothesize that lobes and ducts abundant with lactic acid bacteria lower the risk of cancer, what mechanisms might this involve? One possibility is that these organisms metabolize or secrete compounds that break down carcinogens. For example, lactosylceramide is an effector molecule with an important role in stimulus/agonist-mediated signaling and regulation of cellular processes. It is proinflammatory and potentially protumorigenic, and it may play a role in multidrug resistance in tumor cells [106]. Some organisms found in the gut and human milk can degrade lactosylceramide and thereby increase ceramide proapoptotic signaling molecules, so this is one means for possibly lowering carcinogenesis [107]. In addition, members of the intestinal microbiota, Clostridium spp., Eubacterium ramulus, Bifidobacterium and Eggerthella spp. [108] have been shown to metabolize the phytoestrogen, daidzein, a soybean isoflavonoid, into the more bioactive components, equol and O-desmethylangolensin, a reaction which may be protective against breast cancer development. Phytoestrogens are believed to compete with human estrogen for receptors, and induce less breast cell division. The drug tamoxifen is administered in patients with hormone-dependent breast cancer to block the estrogen receptor. Countering this protective notion of phytoestrogens, in vitro studies have shown that phytoestrogens and their metabolites can bind to estrogen receptor cells, actually stimulating cell growth [109,110]. As phytoestrogens have been detected in breast tissue, it raises the possibility that bacteria in the gland could metabolize these precursors and alter cancer risk [111]. Although the following study did not test this concept, it acquired some interesting data. It comprised a randomized, crossover design of 20 postmenopausal breast cancer survivors and 20 healthy postmenopausal women who took four 42-day diets of isolated soy protein, isolated milk protein, soy plus probiotic capsules or milk plus probiotic capsules. The probiotic supplements contained Lactobacillus acidophilus DDS-1, Bifidobacterium longum and 15–20 mg fructo-oligosacchatide prebiotic. The results showed that plasma levels of estrogens, follicle-stimulating hormone, androgens or sex hormone-binding globulin were not affected by soy, probiotic supplements or equol producer status [112]. In addition, neither cancer status nor equol producer status altered the effects of soy or probiotics. It would have been interesting to identify which bacteria were present in the breast of these women and whether or not they have the capacity to affect phytoestrogens.

Conclusion

The detection of multiple bacterial types in human milk indicates that an association between milk, microbes and humans has evolved to ensure that newborns become colonized upon entry into the microbial-dominated world. Hopefully, this transfers an optimal set of bacterial partners that stimulate the immune system provide sources of nutrition and prevent various acute diseases or ones that emerge later in life (e.g., allergies, diabetes and IBD). But, with obstetrical practices resulting in more cesarean births, widespread changes in the foods we eat, and more and more drugs and pollutants in our water supply and food chain, the microbiome that is transferred may not be optimal.

Breast milk ‘banking’ of donated human milk appears to be having a resurgence and while products from these are now typically pasteurized and may not offer a ‘microbiota transplant’, they may still contain bacterial and HMO components, which could influence the indigenous microbiota and the immune system. Some infant formulas now contain pro- and pre-biotics, but they remain substantially different from human milk naturally in many ways, including not having the streptococci and staphylococci that dominate the latter. While some women do not have a choice but to use infant formula, and the babies do indeed receive a number of nutritional benefits from infant formula, there is a lot more work required to make it more closely mimic the natural product.

The ability of bacteria to reach the mammary ducts, potentially through several mechanisms, raises the possibility that such processes could be detrimental to the host. Of the organisms isolated from mammary ducts and lobes to date, a number have the capacity to produce carcinogens or to induce inflammatory processes that may increase the risk of cancer. On the other hand, bacteria long associated with milk through fermentation and production of yogurt, kefir, cheese and other products may lower the risk of some women developing cancer. These studies are certainly worthy of being undertaken.

Future perspective

The tools are now available to decipher the inter-relationship between microbes at various body sites and the evolution of human life from fetus to infant, child and adult. In the majority of cases, the first 1000 days of life are greatly influenced by the microbiome of the mother and her breast milk. We need to devise studies that interrogate this time period to understand which factors (i.e., bacteria and their metabolites, nutrients and host genetics) have the optimal outcome for the health of our progeny. Further disease associations between nonbreastfeeders, bacteria and disease are likely to emerge and we may even see direct links between different microbiota types in the gut and breast linked as cofactors to more serious ailments, especially since bacteria have been detected in the breast irrespective of lactation [113]. For the infants fed with formula, we need to determine, more rigorously, the benefits and deficiencies, preferably into adulthood. If we are to make a prediction of exciting steps that will occur in the future, it is that various probiotic and prebiotic formulations will be developed to try and program human life for improved health and longevity.

Executive summary

Background

The human breast, nipple, ducts and lobules are colonized by a wide range of bacteria, not just during periods of lactation.

Bacteria in human milk

Breast milk is important for nutrition and as a source of bacteria for maturation of the infant's immune system and gut. While the composition of human milk is diverse and unique to each individual, commensal staphylococci and streptococci usually predominate and many of these produce antimicrobial molecules.

How bacteria reach the breast

Bacteria appear to reach the breast mostly through the nipple, but some translocate from the gut and oral microbiotas, likely via dendritic cells.

Breast milk ‘banking’

The banking of human milk may offer a ‘microbiota transplant’ when milk is not available. Although most donated milk is now pasteurized, it may still contain certain bacterial components beneficial for immune and gut development, as well as oligosaccharides that support the growth of indigenous bacteria in the infant gut.

Beneficial microbes

The use of probiotic bacteria, fermented food products and prebiotics hold great potential to influence maternal and newborn health, and possibly even reduce the risk of more serious illnesses.

Footnotes

This work is supported by a grant from Natural Sciences and Engineering Research Council and Dairy Farmers of Canada. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.

References

Papers of special note have been highlighted as:

of interest.

of considerable interest.

Hummelen

Fernandes

Macklaim

Deep sequencing of the vaginal microbiota of women with HIV. PLoS one 5(8), E12078 (2010).

Ravel

Gajer

Abdo

Vaginal microbiome of reproductive-age women. Proc. Natl Acad. Sci. USA 108(Suppl. 1), 4680–4687 (2011).

Oakley

Fiedler

Marrazzo

Fredricks

. Diversity of human vaginal bacterial communities and associations with clinically defined bacterial vaginosis. Appl. Environ. Microbiol. 74(15), 4898–4909 (2008).

Hill

Goh

Money

Characterization of vaginal microflora of healthy, nonpregnant women by chaperon in-60 sequence-based methods. Am. J. Obstet. Gynecol. 193(3 Pt 1), 682–692 (2005).

Kim

Thomas

Heterogeneity of vaginal microbial communities within individuals. J. Clin. Microbiol. 47(4), 1181–1189 (2009).

Kent

Mitoulas

Cregan

Ramsay

Doherty

Hartmann

. Volume and frequency of breastfeedings and fat content of breast milk throughout the day. Pediatrics 117(3), E387–E395 (2006).

10.

Holscher

Czerkies

Cekola

Bifidobacterium lactis Bb12 enhances intestinal antibody response in formula-fed infants: a randomized, double-blind, controlled trial. J. Parenter. Enteral. Nutr. 36(Suppl. 1), S106–S117 (2012).

11.

Clare

Catignani

Swaisgood

. Bio defense properties of milk: the role of antimicrobial proteins and peptides. Curr. Pharm. Des. 9, 1239–1255 (2003).

12.

West

Hewitt

Murphy

. Influence of methods of collection and storage on the bacteriology of human milk. J. Appl. Bacteriol. 46(2), 269–277 (1979).

13.

Eidelman

Szilagyi

. Patterns of bacterial colonization of human milk. Obstet. Gynecol. 53(5), 550–552 (1979).

14.

Delgado

Arroyo

Martin

Rodriguez

. PCR-DGGE assessment of the bacterial diversity of breast milk in women with lactational infectious mastitis. BMC Infect. Dis. 8, 51 (2008).

15.

Heikkila

Saris

. Inhibition of Staphylococcus aureus by the commensal bacteria of human milk. J. Appl. Microbiol. 95(3), 471–478 (2003).

16.

Martin

Langa

Reviriego

The commensal microflora of human milk: new perspectives for food bacteriotherapy and probiotics. Trends Food Sci. Technol. 15, 121–127 (2004).

17.

Martin

Heilig

Zoetendal

Cultivation-independent assessment of the bacterial diversity of breast milk among healthy women. Res. Microbiol. 158(1), 31–37 (2007).

18.

Kononen

Jousimies-Somer

Bryk

Kilp

Kilian

. Establishment of streptococci in the upper respiratory tract: longitudinal changes in the mouth and nasopharynx up to 2 years of age. J. Med. Microbiol. 51(9), 723–730 (2002).

19.

Wysham

Mulhern

Navarre

La Veck

Kennan

Giedt

. Staphylococcal infections in an obstetric unit. II. Epidemiologic studies of puerperal mastitis. N. Engl. J. Med. 257(7), 304–306 (1957).

20.

McCarthy

Snyder

Parker

. The indigenous oral flora of man. I. The newborn to the 1-year-old infant. Arch. Oral Biol. 10, 61–70 (1965).

21.

Plueckhahn

Banks

. Breast abscess and staphylococcal disease in a maternity hospital. Br. Med. J. 2, 414–418 (1964).

22.

Carrol

Osman

Davies

Broderick

. Raw donor breast milk for newborn babies. Br. Med. J. 2, 1711–1711 (1978).

23.

Carroll

Osman

Davies

McNeish

. Bacteriological criteria for feeding raw breast-milk to babies on neonatal units. Lancet 2, 732–733 (1979).

24.

Carroll

Osman

Davies

McNeish

. Bacteriology of raw breast milk. Lancet 2, 1186–1186 (1979).

25.

West

Hewitt

Murphy

. Influence of methods of collection and storage on the bacteriology of human milk. J. Appl. Bacteriol. 46, 269–277 (1979).

26.

Williamson

Hewitt

Finucane

Gamsu

. Organisation of bank of raw and pasteurized human milk for neonatal intensive care. Br. Med. J. 1, 393–396 (1978).

27.

Kuramitsu

. Virulence properties of oral bacteria: impact of molecular biology. Curr. Issues Mol. Biol. 3(2), 35–36 (2001).

28.

Krause

. A half-century of streptococcal research: then & now. Ind. J. Med. Res. 115, 215–241 (2002).

29.

Klingenberg

Rønnestad

Anderson

Persistent strains of coagulase-negatfve staphylococci in a neonatal intensive care unit: virulence factors and invasiveness. Clin. Microbiol. Infect. 13(11), 1100–1111 (2007).

30.

Aitken

Corl

Sordillo

. Immunopathology of mastitis: insights into disease recognition and resolution. J. Mammary Gland Biol. Neoplasia 16(4), 291–304 (2011).

31.

Blango

Mulvey

. Persistence of uropathogenic Escherichia coli in the face of multiple antibiotics. Antimicrob. Agents Chemother. 54, 1855–1863 (2010).

32.

Hunt

Foster

Forney

Characterization of the diversity and temporal stability of bacterial communities in human milk. PLoS one 6(6), E21313 (2011).

33.

Elegant and insightful paper showing the wide range of bacterial types that are present in human milk, and how the abundance profiles can change from day to day .

34.

Albesharat

Ehrmann

Korakli

Yazaji

Vogel

. Phenotypic and genotypic analyses of lactic acid bacteria in local fermented food, breast milk and faeces of mothers and their babies. Syst. Appl. Microbiol. 34, 148–155 (2011).

35.

Perez

Dore

Leclerc

Bacterial imprinting of the neonatal immune system: lessons from maternal cells?

Pediatrics 119(3), E724–E732 (2007).

36.

Jimenez

Fernandez

Marin

Isolation of commensal bacteria from umbilical cord blood of healthy neonates born by cesarean section. Curr. Microbiol. 51(4), 270–274 (2005).

37.

Park

Shim

Kim

Molecular analysis of colonized bacteria in a human newborn infant gut. J. Microbiol. 43, 345–353 (2005).

38.

Supports the hypothesis that bacteria can be administered to the mother as a means of colonizing the baby .

39.

Nikitenko VI Stadnikov

Kopylov

. Bacterial translocation from the gastrointestinal tract in healthy and injured rats. J. Wound Care 20, 114–122 (2011).

40.

Supports the process of gut translocation as a means for bacteria to be transferred through milk to the newborn .

41.

Donnet-Hughes

Duc

Serrant

Vidal

Schiffrin

. Bioactive molecules in milk and their role in health and disease: the role of transforming growth factor-β. Immunol. Cell. Biol. 78(1), 74–79 (2000).

42.

Kolenbrander

. Multispecies communities: interspecies interactions influence growth on saliva as sole nutritional source. Int. J. Oral Sci. 3(2), 49–54 (2011).

43.

Kolenbrander

Palmer

Jr Periasamy

Jakubovics

. Oral multispecies biofilm development and the key role of cell-cell distance. Nat. Rev. Microbiol. 8(7), 471–480 (2010).

44.

Curtis

Sperandio

. A complex relationship: the interaction among symbiotic microbes, invading pathogens, and their mammalian host. Mucosal. Immunol. 4(2), 133–138 (2011).

45.

Hughes

Terekhova

Liou

Chemical sensing in mammalian host-bacterial commensal associations. Proc. Natl Acad. Sci. USA 107(21), 9831–9836 (2010).

46.

Kim

Kwon

Ahn

Effect of probiotic mix (Bifidobacterium bifidum, Bifidobacterium lactis, Lactobacillus acidophilus) in the primary prevention of eczema: a double-blind, randomized, placebo-controlled trial. Pediatr. Allergy Immunol. 21(2 Pt 2), E386–E393 (2010).

47.

Calcinaro

Dionisi

Marinaro

Oral probiotic administration induces interleukin-10 production and prevents spontaneous autoimmune diabetes in the non-obese diabetic mouse. Diabetologia 48, 1565–1575 (2005).

48.

Lavasani

Dzhambazov

Nouri

A novel probiotic mixture exerts a therapeutic effect on experimental autoimmune encephalomyelitis mediated by IL-10 producing regulatory T cells. PLoS one 5 (2), E9009 (2010).

49.

Ilmonen

Isolauri

Poussa

Laitinen

. Impact of dietary counselling and probiotic intervention on maternal anthropometric measurements during and after pregnancy: a randomized placebo-controlled trial. Clin. Nutr. 30, 156–164 (2011).

50.

Sanchez

De Palma

Capilla

Influence of environmental and genetic factors linked to celiac disease risk on infant gut colonization by Bacteroides species. Appl. Environ. Microbiol. 77(15), 5316–5323 (2011).

51.

Jara

Sanchez

Vera

Cofre

Castro

. The inhibitory activity of Lactobacillus spp. isolated from breast milk on gastrointestinal pathogenic bacteria of nosocomial origin. Anaerobe 17(6), 474–477 (2011).

52.

Stappenbeck

Hooper

Gordon

. Developmental regulation of intestinal angiogenesis by indigenous microbes via Paneth cells. Proc. Natl Acad. Sci. USA 99, 15451–15455 (2002).

53.

Mazmanian

Liu

Tzianabos

Kasper

. An immunomodulatory molecule of symbiotic bacteria directs maturation of the host immune system. Cell 122, 107–118 (2005).

54.

Macpherson

Harris

. Interactions between commensal intestinal bacteria and the immune system. Nat. Rev. Immunol. 4(6), 478–485 (2004).

55.

Reid

Younes

Van der Mei

Gloor

Knight

Busscher

. Microbiota restoration: natural and supplemented recovery of human microbial communities. Nat. Rev. Microbiol. 9, 27–38 (2011).

56.

Illustrates how indigenous and probiotic bacteria can be involved in the natural resolution of diseases in humans .

57.

Dominguez-Bello

Costello

Delivery mode shapes the acquisition and structure of the initial microbiota across multiple body habitats in newborns. Proc. Natl Acad. Sci. USA 107(26), 11971–11975 (2010).

58.

Tian

Liu

Williams

Characterization of a necrotizing enterocolitis model in newborn mice. Int. J. Clin. Exp. Med. 3(4), 293–302 (2010).

59.

Favier

de Vos

Akkermans

. Development of bacterial and bifidobacterial communities in feces of newborn babies. Anaerobe 9(5), 219–229 (2003).

60.

Chouraki

Savoye

Dauchet

The changing pattern of Crohn's disease incidence in northern France: a continuing increase in the 10- to 19-year-old age bracket (1988–2007). Aliment Pharmacol. Ther. 33, 1133–1142 (2011).

61.

Vejborg

Hancock

Petersen

Krogfelt

Klemm

. Comparative genomics of Escherichia coli isolated from patients with inflammatory bowel disease. BMC Genomics 12, 316 (2011).

62.

Higgins

Frankel

Douce

Dougan

MacDonald

. Citrobacter rodentium infection in mice elicits a mucosal Th1 cytokine response and lesions similar to those in murine inflammatory bowel disease. Infect. Immun. 67(6), 3031–3039 (1999).

63.

Morowitz

Denef

Costello

Strain-resolved community genomic analysis of gut microbial colonization in a premature infant. Proc. Natl Acad. Sci. USA 108, 1128–1133 (2011).

64.

Excellent in-depth study of the gut microbiota of a premature infant .

65.

Bjorksten

Burman

Collecting and banking human milk: to heat or not to heat?

Br. Med. J. 281, 765–769 (1980).

66.

el-Mohandes

Keiser

Johnson

Refat

Jackson

. Aerobes isolated in fecal microflora of infants in the intensive care nursery: relationship to human milk use and systemic sepsis. Am. J. Infect. Control. 21, 231–234 (1993).

67.

Wright

Feeney

. The bacteriological screening of donated human milk: laboratory experience of British Paediatric Association's published guidelines. J. Infect. 36, 23–27 (1998).

68.

Allen-Vercoe

Reid

Viner

A Canadian Working Group report on fecal microbial therapy: microbial ecosystems therapeutics. Can. J. Gastroenterol. (2012) (In Press).

69.

Bakken

Borody

Brandt

Treating Clostridium difficile infection with fecal microbiota transplantation. Clin. Gastroenterol. Hepatol. 9(12), 1044–1049 (2011).

70.

Zhang

Caicedo

Neu

. Alive and dead Lactobacillus rhamnosus GG decrease tumor necrosis factor-α-induced interleukin-8 production in Caco-2 cells. J. Nutr. 135(7), 1752–1756 (2005).

71.

Riordan

. The cost of not breastfeeding: a commentary. J. Hum. Lact. 13(2), 93–97 (1997).

72.

Hurley

Theil

. Perspectives on immunoglobulins in colostrum and milk. Nutrients 3 (4), 442–474 (2011).

73.

Hodgkinson

Cannon

Holmes

Fischer

Willix–Payne

. Production from dairy cows of semi-industrial quantities of milk-protein concentrate (MPC) containing efficacious anti-Candida albicans IgA antibodies. J. Dairy Res. 74(3), 269–275 (2007).

74.

Braegger

Chmielewska

Decsi

Supplementation of infant formula with probiotics and/or prebiotics: a systematic review and comment by the ESPGHAN committee on nutrition. J. Pediatr. Gastroenterol. Nutr. 52(2), 238–250 (2011).

75.

Steenhout

Rochat

Hager

. The effect of Bifidobacterium lactis on the growth of infants: a pooled analysis of randomized controlled studies. Ann. Nutr. Metab. 55(4), 334–340 (2009).

76.

Roberfroid

Gibson

Hoyles

Prebiotic effects: metabolic and health benefits. Br. J. Nutr. 104(Suppl. 2), S1–S63 (2010).

77.

Chichlowski

German

Lebrilla

Mills

. The influence of milk oligosaccharides on microbiota of infants: opportunities for formulas. Annu. Rev. Food Sci. Technol. 2, 331–351 (2011).

78.

Bode

. Human milk oligosaccharides: prebiotics and beyond. Nutr. Rev. 67(Suppl. 2), S183–S191 (2009).

79.

Kunz

Rudloff

Baier

Klein

Strobel

. Oligosaccharides in human milk: structural, functional, and metabolic aspects. Annu. Rev. Nutr. 20, 699–722 (2000).

80.

Coppa

Gabrielli

Pierani

Catassi

Carlucci

Giorgi

. Changes in carbohydrate composition in human milk over 4 months of lactation. Pediatrics 91(3), 637–641 (1993).

81.

Coppa

Pierani

Zampini

Lactose, oligosaccharide and monosaccharide content of milk from mothers delivering preterm newborns over the first month of lactation. Minerva Pediatr. 49, 471–475 (1997).

82.

Gabrielli

Zampini

Galeazzi

Preterm milk oligosaccharides during the first month of lactation. Pediatrics 128, 1520–1531 (2011).

83.

Zivkovic

German

Lebrilla

Mills

. Human milk glycobiome and its impact on the infant gastrointestinal microbiota. Proc. Natl Acad. Sci. USA 108(Suppl. 1), 4653–4658 (2011).

84.

Kunz

Rudloff

. Potential anti-inflammatory and anti-infectious effects of human milk oligosaccharides. Adv. Exp. Med. Biol. 606, 455–465 (2008).

85.

Veereman–Wauters

Staelens

Van de Broek

Physiological and bifidogenic effects of prebiotic supplements in infant formulae. J. Pediatr. Gastroenterol. Nutr. 52, 763–771 (2011).

86.

Haarman

Knol

. Quantitative real-time PCR assays to identify and quantify fecal Bifidobacterium species in infants receiving a prebiotic infant formula. Appl. Environ. Microbiol. 71, 2318–2324 (2005).

87.

Haarman

Knol

. Quantitative real-time PCR analysis of fecal Lactobacillus species in infants receiving a prebiotic infant formula. Appl. Environ. Microbiol. 72, 2359–2365 (2006).

88.

Ziegler

Vanderhoof

Petschow

Term infants fed formula supplemented with selected blends of prebiotics grow normally and have soft stools similar to those reported for breast-fed infants. J. Pediatr. Gastroenterol. Nutr. 44, 359–364 (2007).

89.

Bruzzese

Volpicelli

Squeglia

A formula containing galacto- and fructo-oligosaccharides prevents intestinal and extra-intestinal infections: an observational study. Clin. Nutr. 28, 156–161 (2009).

90.

Goldfarb

Pippen

. Inflammatory breast cancer: the experience of Baylor University Medical Center at Dallas. Proc. (Bayl. Univ. Med. Cent.) 24, 86–88 (2011).

91.

Kwon

Weiss

. Production of 3-nitrosoindole derivatives by Escherichia coli during anaerobic growth. J. Bacteriol. 191, 5369–5376 (2009).

92.

Guo

McCay

Brown

Glycidol modulation of the immune responses in female B6C3F1 mice. Drug Chem. Toxicol. 23, 433–457 (2000).

93.

Blitman

Steiner

Bell

Wilks-Gallo

. Pulmonary nodules associated with Gemella bacteremia: CT findings in two children with osteosarcoma. J. Thorac. Imaging 22, 182–184 (2007).

94.

Koren

Spor

Felin

Human oral, gut, and plaque microbiota in patients with atherosclerosis. Proc. Natl Acad. Sci. USA 108(Suppl. 1), 4592–4598 (2011).

95.

Alexeyev

Bergh

Marklund

Association between the presence of bacterial 16S RNA in prostate specimens taken during transurethral resection of prostate and subsequent risk of prostate cancer (Sweden). Cancer Causes Control 17, 1127–1133 (2006).

96.

Nagy

Sonkodi

Szöke

Nagy

Newman

. The microflora associated with human oral carcinomas. Oral Oncol. 34, 304–308 (1998).

97.

Adris

Chung

. Metabolic activation of bladder procarcinogens, 2-aminofluorene, 4-aminobiphenyl, and benzidine by Pseudomonas aeruginosa and other human endogenous bacteria. Toxicol. In Vitro 20, 367–374 (2006).

98.

Ayanaba

Alexander

. Microbial formation of nitrosamines in vitro. Appl. Microbiol. 25, 862–868 (1973).

99.

Ohta

Tsukahara

Ohshima

Nitric oxide metabolites and adrenomedullin in human breast milk. Early Hum. Dev. 78, 61–65 (2004).

100.

Pitteri

Kelly-Spratt

Gurley

Tumor microenvironment%-derived proteins dominate the plasma proteome response during breast cancer induction and progression. Cancer Res. 71(15), 5090–5100 (2011).

101.

Boehmer

Ward

Peters

Shefcheck

McFarland

Bannerman

. Proteomic analysis of the temporal expression of bovine milk proteins during coliform mastitis and label-free relative quantification. J. Dairy Sci. 93, 593–603 (2010).

102.

Baumgarten

Frasor

. Minireview: inflammation: an instigator of more aggressive estrogen receptor (ER) positive breast cancers. Mol. Endocrinol. 26(3), 360–371 (2012).

103.

van't Veer

Dekker

Lamers

J W

Consumption of fermented milk products and breast cancer: a case–control study in The Netherlands. Cancer Res. 49(14), 4020–4023 (1989).

104.

Very interesting study, difficult to coordinate and with an outcome that could have practical implications for women's risk of cancer .

105.

Autier

Boniol

La Vecchia

Disparities in breast cancer mortality trends between 30 European countries: retrospective trend analysis of WHO mortality database. BMJ 341, C3620 (2010).

106.

Pukkala

Martinsen

Lynge

Occupation and cancer – follow-up of 15 million people in five Nordic countries. Acta Oncol. 48, 646–790 (2009).

107.

de Moreno de Leblanc

Perdigon

. The application of probiotic fermented milks in cancer and intestinal inflammation. Proc. Nutr. Soc. 69(3), 421–428 (2010).

108.

Supportive of the idea that probiotics could play a role in preventing breast cancer .

109.

de Moreno de Leblanc

Matar

Farnworth

Perdigon

. Study of immune cells involved in the antitumor effect of kefir in a murine breast cancer model. J. Dairy Sci. 90(4), 1920–1928 (2007).

110.

Biffi

Coradini

Larsen

Riva

Di Fronzo

. Antiproliferative effect of fermented milk on the growth of a human breast cancer cell line. Nutr. Cancer 28(1), 93–99 (1997).

111.

Diaz-Ropero

Martin

Sierra

Two Lactobacillus strains, isolated from breast milk, differently modulate the immune response. J. Appl. Microbiol. 102(2), 337–343 (2007).

112.

Arroyo

Martin

Maldonado

Jimenez

Fernandez

Rodriguez

. Treatment of infectious mastitis during lactation: antibiotics versus oral administration of lactobacilli isolated from breast milk. Clin. Infect. Dis. 50, 1551–1558 (2010).

113.

Collaborative Group on Hormonal Factors in Breast Cancer. Breast cancer and breastfeeding: collaborative reanalysis of individual data from 47 epidemiological studies in 30 countries, including 50302 women with breast cancer and 96973 women without the disease. Lancet 360, 187–195 (2002).

114.

Lyons

Schedin

Borges

. Pregnancy and breast cancer: when they collide. J. Mammary Gland Biol. Neoplasia 14(2), 87–98 (2009).

115.

Dodds

Fell

Joseph

Relationship of time since childbirth and other pregnancy factors to premenopausal breast cancer prognosis. Obstet. Gynecol. 111, 1167–1173 (2008).

116.

Ogretmen

Hannun

. Biologically active sphingolipids in cancer pathogenesis and treatment. Nat. Rev. Cancer 4, 604–616 (2004).

117.

Falk

Hoskins

Larson

. Bacteria of the human intestinal microbiota produce glycosidases specific for lacto-series glycosphingolipids. J. Biochem. 108(3), 466–474 (1990).

118.

Yokoyama

Oshima

Nomura

Hattori

Suzuki

. Complete genomic sequence of the equol-producing bacterium Eggerthella sp. strain YY7918, isolated from adult human intestine. J. Bacteriol. 193(19), 5570–5571 (2011).

119.

Duncan

Phipps

Kurzer

. Phyto-oestrogens. Best Pract. Res. Clin. Endocrinol. Metab. 17(2), 253–271 (2003).

120.

Kinjo

Tsuchihashi

Morito

Interactions of phytoestrogens with estrogen receptors α and β (III). Estrogenic activities of soy isoflavone aglycones and their metabolites isolated from human urine. Biol. Pharm. Bull. 27(2), 185–188 (2004).

121.

Bolca

Urpi-Sarda

Blondeel

Disposition of soy isoflavones in normal human breast tissue. Am. J. Clin. Nutr. 91(4), 976–984 (2010).

122.

Nettleton

Greany

Thomas

Wangen

Adlercreutz

Kurzer

. Short-term soy and probiotic supplementation does not markedly affect concentrations of reproductive hormones in postmenopausal women with and without histories of breast cancer. J. Altern Complement Med. 11(6), 1067–1074 (2005).

123.

Rahal

Júnior

Reis

Pimenta

Netto

Paulinelli

. Prevalence of bacteria in the nipple discharge of patients with duct ectasia. Int. J. Clin. Pract. 59(9), 1045–1050 (2005).

124.

Martin

Langa

Reviriego

Human milk is a source of lactic acid bacteria for the infant gut. J. Pediatr. 143(6), 754–758 (2003).

125.

Important paper as it led to more studies on lactic acid bacteria in the breast, and on the potential for dendritic cells transferring them from the maternal gut .

126.

Prolacta Bioscience. www.prolacta.com.

127.

Human Milk Banking Association of North America. www.hmbana.org.

128.

WHO. 10 facts on breastfeeding. www.who.int/features/factfiles/breastfeed.ing/en/index.html.

129.

Statistics Canada: Breastfeeding (2009). www.statcan.gc.ca/pub/82–625-x/2010002/article/11269-eng.htm.

130.

Food and Agriculture Organization of the UN and WHO. Health and nutritional properties of probiotics in food including powder milk with live lactic acid bacteria. (2001). www.who.int/foodsafety/publications/fs_management/en/probiotics.pdf.

Breast,Milk and Microbes: A Complex Relationship that Does Not End with Lactation

Abstract

Keywords

What bacteria are in breast milk?

How do the bacteria get to the breast?

Bacterial interactions in the breast

Are the breast bacteria important for the infant?

Breast milk banking as a substitute

Infant formula as a human milk substitute & its pitfalls

Possibilities of improving infant formula with probiotics & prebiotics

Beyond infant feeding: could bacteria have a role in inflammation & breast cancer?

How might lactic acid bacteria reduce the risk of breast cancer?

Conclusion

Future perspective

Footnotes

References