Multidrug and Pan-Antibiotic Resistance—The Role of Antimicrobial and Synergistic Essential Oils: A Review

Abstract

Bacterial resistance to antibiotics continues to be a grave threat to human health. Because antibiotics are no longer a lucrative market for pharmaceutical companies, the development of new antibiotics has slowed to a crawl. The World Health Organization reported that the 8 new bacterial agents approved since July 2017 had limited clinical benefits. While a cohort of biopharmaceutical companies recently announced plans to develop 2-4 new antibiotics by 2030, we needn’t wait a decade to find innovative antibiotic candidates. Essential oils (EOs) have long been known as antibacterial agents with wide-ranging arsenals. Many are able to penetrate the bacterial membrane and may also be effective against bacterial defenses such as biofilms, efflux pumps, and quorum sensing. EOs have been documented to fight drug-resistant bacteria alone and/or combined with antibiotics. This review will summarize research showing the significant role of EOs as nonconventional regimens against the worldwide spread of antibiotic-resistant pathogens. The authors conducted a 4-year search of the US National Library of Medicine (PubMed) for relevant EO studies against methicillin-resistant Staphylococcus aureus, multidrug-resistant (MDR) Escherichia coli, EO combinations/synergy with antibiotics, against MDR fungal infections, showing the ability to permeate bacterial membranes, and against the bacterial defenses listed above. EOs are readily available and are a needed addition to the arsenal against resistant pathogens.

The antibiotic era began in 1941 with the welcomed release of penicillin. However, Alexander Fleming, who discovered penicillin, warned that microbes are educated to resist penicillin. The Centers for Disease Control and Prevention (CDC) reported that bacterial resistance to penicillin developed just 1 year later in 1942. Antibacterial resistance has continued to evolve, assisted by the overuse of antibiotics in humans, and antibiotics used to increase food animal growth in factory farming. Ultimately, 2015 brought news that Pan-resistance to antibiotics used for Gram-negative bacteria had arrived with colistin resistance that developed in China.

Researchers from Chinese, British, and US universities published in Lancet Infectious Diseases their discovery of a new form of resistance to the antibiotic of last resort, colistin. The initial colistin resistance gene is called mcr-1. ¹ Since this resistance presumably came from massive agricultural use of colistin in China, National Geographic headlined the news story “Apocalypse Pig.” ² Before ending its use of colistin in animals at the end of April 2017, China had been using more than 8000 metric tons of colistin per year. By 2019, there were 9 mobilized mcr genes identified (mcr-1-mcr-9).

The German Center for Infection Research reported on the plasmid-mediated colistin resistance mechanism mcr-1 in an article entitled “Dangerous and ‘jumping’ mcr-1 resistance gene.” In studying the transferability of the easily transferred gene, they stated that the “mcr-1 gene not only exists on mobile plasmids, but [can] be integrated into chromosomes as well. Consequently, it can be reliably passed on to next generations.” ³

The “jumping” plasmid mcr-1 indeed jumped. Klebsiella pneumoniae, Shigella flexneri, Acinetobacter baumannii, Escherichia coli, and Pseudomonas aeruginosa are also now resistant to all known antibiotics.

Research from the ensuing years has not been encouraging. While human-to-human spread of colistin resistance is to be expected, research has shown that even our waters are now carriers. Plasmid-mediated colistin-resistant ESBL-producing E. coli was retrieved from seawater at a public beach in Norway. According to the study, “This report illustrates that E. coli strains containing plasmid-mediated colistin resistance genes have also reached areas where this drug is hardly used at all. Surveillance of colistin resistance in environmental, veterinary, and human strains is warranted also in countries where colistin resistance is rare in clinical settings.” ⁴ A colistin-resistant ESBL-producing Enterobacteriaceae was isolated from river water and imported vegetable samples in Switzerland. ⁵ Sewage in Barcelona, Spain, carried mcr-1 Enterobacteriaceae. ⁶ Colistin-resistant mcr-1 E. coli was found on 2 Brazilian beaches, including Santos, the major beachfront city of the region with the largest shipping terminal in Latin America. ⁷ The colistin resistance gene was found in 18 different locations on the Haihe River in China. ⁸ ESBL, KPC-type, and mcr-1.2-producing Enterobacteriaceae contaminated wells, river water, and wastewater treatment plants in Northern Italy, ⁹ while a pan-drug-resistant isolate of A. baumannii was recovered in municipal waste water in Zagreb, Croatia. ¹⁰ Finally, we note a September 2019 study published in Infection Control & Hospital Epidemiology. Henig et al report using real-time polymerase chain reaction to identify 4 patients with the mobile colistin resistance mcr-1 gene in a hospital in Michigan. ¹¹

With extensive drug resistance and pan resistance now worldwide, microbial transfer has taken on fearful new capabilities. A February 2017 study in Nature Microbiology linked flies for the first time to the spread of carbapenem resistance. The researchers reported: “Furthermore, local bla_NDM dissemination (flies and dogs) increases… the probability of carriage by migratory birds such as swallows, the migratory winter destinations of which are usually South East Asia (including Cambodia, Vietnam, Laos, Thailand, Malaysia and Indonesia).” ¹² A 2018 Sichuan University concurred stating “the movable colistin-resistance gene mcr-1 was detected among the colistin-resistant E. coli strains isolated from the river water and egret feces, which indicated the possibility of the environmental dissemination of this gene.” ¹³

The Multiple Mechanisms of Essential Oils

Schnaubelt discussed the antibacterial efficacy of essential oils (EOs): “Unlike antibiotics, which are active due to inhibiting an easily identifiable single target, the activity of EOs, impairing a bacterium in multiple physiological systems as well as in membrane functionality, is only now understood.” ¹⁴

A 2019 study lists some of the antibacterial pathways of EOs: “The most frequently reported mechanism of antibacterial action of both isomers [carvacrol and thymol] involves the disruption of bacterial membrane leading to bacterial lysis and leakage of intracellular contents… Other proposed mechanisms of antibacterial action include the inhibition of efflux pumps, prevention in the formation and disruption of preformed biofilms, inhibition of bacterial motility, and inhibition of membrane ATPases.” ¹⁵

A lesser-known aspect of EOs is while powerful against pathogenic bacteria, some studies suggest they have little effect on beneficial bacteria. An abstract presented at the Experimental Biology 2019 meeting reported: “Using cinnamon essential oil as an antibiotic alternative has shown to be a promising option with limited detrimental effects against the commensal bacteria.” Lead author Victoria R. Adams said further evaluation was needed to determine the benefits of utilizing a natural alternative. ¹⁶

A 2009 study showed “Carum carvi [caraway], Lavandula angustifolia Mill. [lavender], Trachysperum copticum [ajowan], and Citrus aurantium var. amara [neroli] essential oils displayed the greatest degree of selectivity, inhibiting the growth of potential pathogens at concentrations that had no effect on the beneficial bacteria examined.” ¹⁷ Si et al reported, “Thymol, cinnamon oil and carvacrol have previously demonstrated a broad spectrum of antimicrobial activities… Our results not only confirmed the activity of these compounds against E. coli and Salmonella, but also demonstrated their selectivity towards the pathogens with little effect on the beneficial gut bacteria.” ¹⁸ Also, a 2015 study in Microbiology reported that “thymol and geraniol at around 100 ppm could be effective in suppressing pathogens in the small intestine, with no concern for beneficial commensal colonic bacteria in the distal gut.” ¹⁹

Abreu et al explain that “plants, rather than relying on single metabolites, use a combination of strategies and a highly efficient defense system that includes very diverse molecules ranging from proteins to H₂O₂ and oxygen radicals, which most likely complement each other, to deal with microbial threats. The defense strategies may thus involve the synergistic activity of 2 or more compounds, which could act via different mechanisms and/or targets… It is important to bear in mind that millions of years of evolution have resulted in plant defense systems that have proved to be not readily susceptible to microbial resistance mechanisms.” ²⁰

EOs Against Methicillin-Resistant Staphylococcus aureus

A 2019 case study and literature review in Therapeutic Advances in Infectious Disease reported that in “2013, the Centers for Disease Control and Prevention (CDC) highlighted methicillin-resistant Staphylococcus aureus (MRSA) as a serious drug-resistant threat citing over 80,000 invasive MRSA infections leading to over 11,000 deaths each year in the United States. MRSA frequently enters the blood stream and metastasizes resulting in endocarditis and osteomyelitis. This represents a major burden on the healthcare system, with 30- and 90-day mortality rates as high as 30% and 50% respectively.” ²¹ Deaths in the United States from antibiotic-resistant infections are more than 35 000 yearly. ²²

In the study mentioned above, Abreu et al identified plant extract compounds that could act as antibiotic adjuvants “showing their great potential for application in the clinical therapy of infections with antibiotic-resistant microorganisms such as MRSA.” ²⁰

A review by Chambers and Deleo noted CA MRSA has characteristics not found in HA MRSA. Tissue-destructive infections that include necrotizing fasciitis and fulminant, necrotizing pneumonia may be a result of CA MRSA genes that encode the virulence factor Panton-Valentine Leukocidin, which induces leukocyte destruction and tissue necrosis. ²³ Please see Table 1 for more substantiated data.

Table 1

EO/Constituent Against MRSA

EO/Constituent	Method	Bioactivity	Critical finding	Ref.
Nigella sativa L./ EO/carvacrol, thymoquinone, p-cymene	Broth microdilution/PCR/biofilm formation assay	Membrane integrity	Results show components p-cymene, thymoquione, and carvacrol disrupted bacterial membrane of the MRSA strain.	24
Origanum vulgare L., carvacrol, thymol	Agar diffusion method	Antimicrobial	The MIC for carvacrol: 0.015%-0.03%, v/v; thymol: 0.03‐0.06, v/v; oregano oil: 0.06%-0.125%, v/v. Oregano oil shows potential as a topical antibacterial agent against MRS strains.	25
CPV orange oil Citrus sinensis, L. Osbeck	Disc diffusion/agar diffusion	Cell lysis	The CPV orange oil showed inhibition and bactericidal effect on MRSA as well as vancomycin intermediate-resistant S. aureus strains in the in vitro model.	26
Eugenol/citral from Syzygium aromaticum (L.) Merr. & L.M. Perry/citrus plants	Agar dilution, microdilution, crystal violet assay	Antimicrobial	Sequential exposure of S. aureus L. monocytogenes strains to eugenol or citral did not result in development of resistance to the oil component itself or to antibiotics.	27
91 EOs/EO blends were tested. Including Cymbopogon flexuosus (Nees ex Steud.) W.Watson/R.C. blend: Eucalypus globulus Labill., Myrtus communis L., Origanum majorana L., Pinus sylvestris L., E. radiata A. Cunn ex DC., E. citriodora Hook., Lavandula angustifolia Mill., Cupressus sempervirens L., Picea mariana Mill./Britton, Sterns & Poggenb., Mentha × piperita L.	Disc diffusion assay	Antimicrobial	The concentration of each oil or blend tested against MRSA was 30 µL. While 78 oils had measurable inhibitory activity, the Cymbopogon flexuosus ZOI was >83 mm as was the R.C. blend’s. The authors state that both Cymbopogon flexuosus and R.C. blend inhibited all MRSA growth on the plate (not noting any role vapor might play).	28
Melaleuca alternifolia (Maiden & Betchel) Cheel	Disc diffusion/modified broth microdilution	Antimicrobial	All 60 MRSA isolates tested against tea tree were susceptible. There was no difference in susceptibility between mupirocin-resistant/sensitive S. aureus. Universal susceptibility to tea tree oil of all MRSA and MSSA isolates tested represents a significant result in the application for MRSA control.	29
Chamaecyparis obtusa (Sielbold & Zucc.) Endl.	MIC: serial dilution method	Antimicrobial	Effective against MDR MRSA/VRE as well as M. luteus ATCC 9341, M. smegmatis ATCC 9341, Pseudomonas aeruginosa KCTC 1637, and E. faecalis ATCC 29212.	30
Melaleuca alternifolia (Maiden & Betchel) Cheel	In vivo	Antimicrobial	While treatment with mupirocin 2% nasal ointment and a tea tree regimen had similar results, tea tree may be useful as an alternative to mupirocin in areas of high mupirocin resistance.	31
Tea tree Melaleuca alternifolia (Maiden & Betchel) Cheel	PCR detection of SA442 and MecA genes/disk diffusion assay	Anti-biofilm	Low concentration of TTO was able to eliminate biofilm formation and subsequently cells communication (quorum sensing).	32
Eucalyptus varieties: E. globulus Labill. (fruit and leaf), Eucalyptus radiata A. Cunn. Ex DC., Eucalyptus citriodora Hook. (leaf)	Broth microdilution	Antimicrobial	All Eucalyptus varieties were active against MRSA NCTC 10422 (and 9 other strains), but it was E. globulus (fruit) oil that showed the best activity (MIC between 250 and 1000 µg/mL). This strong showing could be attributed to aromadendrene’s lipophilic ability to disrupt the MRSA biomembrane.	33
Anethum graveolens L. (Dill EO)	BALB/c mice model/RT-PCR analysis	Antimicrobial	Dill EO shortens the MRSA inflammatory stage and induces apoptosis by promoting p53 and caspase-3 expression.	34
Aromadendrene/1,8-cineole from Eucalyptus globulus Labill. (fruit)	Checkerboard method, time-kill assay	Antimicrobial	Aromadendrene apparently contributes significantly to the antimicrobial activity of EGF. Combinations of aromadendrene and 1,8-cineole showed additive effects in most cases, but also synergistic behavior in the time-kill assay. EGF exhibited a pronounced antimicrobial effect toward MDR bacteria.	35
Piper cubeba L.	Transmission electronic microscope/atomic force microscopy	Antimicrobial	EOs from P. cubeba L. show activity as an antibacterial agent against methicillin- and oxacillin-resistant S. aureus, acting on the bacterial cell wall and plasma membrane.	36
Lavandula coronopifolia Poir.	Broth micro-well dilution	Antimicrobial	L. coronifolia was found to be effective against MRSA with ZOI (mm) of 16.0 ± 1.00.	37
Cymbopogon flexuosus (Nees ex Steud.) W.Watson	Disc diffusion, MBIC, MBEC	Antimicrobial	Lemongrass and citral both completely cleared the plate of bacterial growth w/ZOI of >8.60 cm. Demonstrating lemongrass EO possesses anti-biofilm activity at low concentrations.	38
Tagetes minuta L.	Disc diffusion test	Antimicrobial	EO of Tagetes minuta showed a high activity against MRSA with an inhibition zone of 23 mm.	39
Melaleuca alternifolia (Maiden & Betchel) Cheel	MIC/MBC	Antimicrobial	Results might provide the basis for a possible role of TTO as part of nonconventional regimens against both MSSA/MRSA and Gram-negative MDR/PDR microorganisms.	40
Schinus areira L.	Broth microdilution assay	Antibacterial	Antibacterial effectiveness could be synergistic effects of α-phellandrene and limonene or disruption of bacterial membrane integrity.	41
Pogostemon cablin (Blanco) Benth./pogostone	In vitro/in vivo Kunming mice	Antibacterial	Pogostone showed antibacterial activity against MRSA with an average MIC of 400 µg/mL. The in vivo animal study against MRSA ATCC43300 showed MIC at 400.000 ± 0.000.	42

Abbreviations: AHLs, N-acyl homoserine lactones; CA MRSA, Community Associated Methicillin-resistant Staphylococcus aureus; CDR, Candida auris resistant; CPV, Cold-pressed Valencia; EGF, epidermal growth factor; EO, essential oil; FICI, Fractional Inhibitory Concentration; LVO, Lavender essential oil; MBC, Minimum bacterial concentration; MBEC, Minimum biofilm eradication concentration; MBIC, Minimum biofilm inhibitory concentration; MDR, multidrug-resistant; MFC, Minimum fungicidal concentration; MIC, minimum inhibitory concentration; MRSA, methicillin-resistant Staphylococcus aureus; OSEO, Ocimum sanctum essential oil; PCR, polymerase chain reaction; UTIs, Urinary tract infections ; ZOI, Zone of inhibition.

EO/Antibiotic Additivity & Synergism

Rosato et al reported peppermint EO/gentamicin and ampicillin synergism. The “results against Gram-negative bacteria [such] as K. pneumoniae and Pseudomonas aeruginosa are of particular interest as these bacteria are difficult to treat with commonly employed antibacterial drugs.” ⁴³

The 2016 review by Aelenei et al noted that the intrinsic difference between Gram-positive and the more-difficult-to-kill Gram-negative bacteria is “due to an additional outer membrane acting as an effective barrier for amphipathic agents and over expression of efflux pumps responsible for innate antimicrobial resistance.” The authors concluded that their review “reveals that essential oils and their purified components enhance the efficacy of antibiotics against Gram-negative bacteria, being promising candidates for the development of new effective formulations against Gram-negative bacteria.” ⁴⁴

In their research against MDR Pseudomonas aeruginosa, I Utchariyakiat et al found “cinnamon bark oil showed the strongest antimicrobial activity against all clinical-isolated MDR-PA strains with MIC of 0.0562 to 0.225% v/v and MBC of 0.1125 to 1.8% v/v.” This study also reported that “cinnamon bark oil and cinnamaldehyde combined with colistin demonstrated synergistic rates at 16.7% and 10%, respectively.” ⁴⁵ Please see Table 2 for more substantiated data.

Table 2

EOs Combined With Antimicrobials—Additivity/Synergistic Effects.

EO/Antibiotic/Pathogen	Method	Critical finding	Ref.
Myrtus communis L./polymyxin B, ciprofloxacine/MDR-Acinetobacter baumannii	Checkerboard method, time-kill curve method	Myrtle EO’s action concerns cell wall and membrane structures, changing their permeability and leading to release of intracellular contents outside of cell.	61
Cinnamomum tamala (Buch.-Ham)/DNase1/DNase MBD (Marine bacteria)/Pseudomonas aeruginosa	MIC, crystal violet assay, scanning electron microscopy	This study shows synergism of EO C. tamala with DNases. This use may be an effective treatment of Pseudomonas aeruginosa biofilm infections.	62
Rosmarinus officinalis L. (REO)/neomycin, gentamicin, metronidazole, amikacin, metronidazole, benzoilmetronidazole, nystatin, anphotericin B/MDR S. aureus, Escherichia coli; and Candida strains albicans and krusei	Broth microdilution	Evidence was shown of the modifying-antibiotic resistance activity of REO when tested in combination with aminoglycoside against MDR S. aureus, E. coli. Although REO sensitized the C. krusei strain to benzoilmetronidazole.	63
Lavandula angustifolia Mill./Chloramphenicol, Ciprofloxacin, Fusidic acid, Nystatin/C. albicans, S. aureus, Pseudomonas aeruginosa	Microdilution assay	Lavender EO has potential to improve antibacterial efficacy of some antibiotics. EOs affect many antibacterial pathways and combined with antibiotics offer possible potentiation through synergistic effects.	64
Pituranthos chloranthus (Coss. & Durieu) Schinz, Teucruim ramosissimum Desf. and Pistacia lentiscus L./amoxicillin, tetracycline, piperacillin, ofloxacin, oxacillin, novobiocin/ESBL-E. coli	MTT assay, in vitro cytotoxic assay, time-kill assay, broth microdilution checkerboard method	The data support the potential of natural antimicrobials to enhance antibiotics actions toward clinical MDR bacteria. This suggests the potential use of these oils with pharmaceutical products, diminishing harmful side effects and treatment costs of the synthetic drugs.	65
Lavender EO/Octenidine dihydrochloride	MIC, time-killing curves, FTIR analysis	Based on results of time-killing curve it may be assumed that LEO has a synergistic effect on OCT, thereby enhancing its permeation into bacterial cells.	66
Origanum vulgare L., Thymus vulgaris L., Lavandula angustifolia Mill., Mentha piperita, and Melaleuca alternifolia (Maiden & Betchel) Cheel/erythromycin-resistant Group A Streptococci (GAS)	Live/dead assay, time-kill curves, checkerboard assay	Synergism of carvacrol and erythromycin supports reuse of erythromycin against GAS strains.	67
Carvacrol, thymol, and eugenol/nalidixic acid/S. typhimurium	MIC, EtBr accumulation assays	All tested compounds found to present synergistic effect with NA against S. typhimurium; thymol most effective.	68
Mentha × piperita L./antibacterials: gentamicin, ampicillin, antifungal: amphotericin B, FLC, miconazole/B. cereus, B. subtilis, Staphylococcus spp. A. baumannii, Pseudomonas aeruginosa. Fungals: Candida spp. C. guillier-mondii, C. glabrata, C. krusei, C. kefyr	Checkerboard microdilution method	With M. piperita the MIC for gentamicin is markedly reduced, more than 30-fold lower against 6 out of the 10 bacteria strains considered. Strong synergy between gentamicin and peppermint against Gram-negative Pseudomonas aeruginosa and Klebsiella pneumoniae is much lower than normally required.	43
Nine EOs, including Mentha × piperita L. and Carum carvi L. (caraway) with gentamicin against ESBL-producing and NDM-producing K. pneumoniae isolates	Checkerboard method	Peppermint oil combined with gentamicin showed synergistic activity against both control, ESBL-producing and NDM-1 producing isolates. Caraway EO demonstrated synergy with gentamicin toward ESBL-producing and gentamicin-resistant strains.	69
Mentha × piperita L./FLC/Candida spp.	MIC, Checkerboard assay, scanning electron microscopy	FICI of mint EO in combination with FLC against all FLC-sensitive and FLC-resistant Candida strains studied. ranged from 0.28 to 0.41 and 0.26 to 0.35, respectively, indicating significant synergy.	70
Origanum vulgare L. EO/silver nanoparticles bio-AgNP /S. aureus, S. mutans, K. pneumoniae, ESBL-E. coli	Antibacterial comb. assay, time-kill assay, scanning electron microscopy	The combination of bio-AgNP with OEO resulted in synergistic and additive antimicrobial activities against the MDR bacterial strains of E. coli, A. baumannii and MRSA.	71
Ocimum basilicum L., Salvia sclarea L., Rosmarinus officinalis L. (out of 8 oils tested)/gentamicin, ciprofloxacin, amikacin, netilmicin, tobramycin, chloramphenicol, tetracycline, tigecycline, trimethoprim-sulfamethoxazole, cefoxitin, erythromycin, clindamycin, rifampicin, linezolid, fusidic acid, qunupristin/dalfopristin, kanamycin, mupirocin, vancomycin/Gram-positive and Gram-negative wound infection bacteria	Disc diffusion assay, MIC, MBC	Lipophilic nature of monoterpenes causes expansion of the membrane, increased membrane fluidity and permeability, disturbance of membrane-embedded proteins, inhibition of respiration, and alteration of ion transport processes. Additive and synergistic effects of the 3 oils with antibiotics were noted.	72
Cinnamomum cassia (L.) J Presl./streptomycin, ampicillin, chloramphenicol/MDR E. coli, Staphylococcus aureus, P. aeruginosa	Agar disc diffusion, MIC, checkerboard assay	Antibiotics with cinnamon EO reduces MICs 2-fold for Pseudomonas aeruginosa, 2‐8-fold for S. aureus, and 2‐4-fold for E. coli. There were synergistic effects of cinnamon with ampicillin or chloramphenicol against S. aureus, and by combination EO and chloramphenicol against E. coli.	73
Citrus hystrix DC. EO/chlorhexidine/P. gingivalis, S. mutans, S. sanguinis	Time-kill assay, checkerboard microdilution assay	Bacterial cell membrane disrupted at 4× MIC for 4 hours; at 4× MIC cell membrane completely destroyed at 8 hours.	74
Cinnamomum verum J. Presl/meropenem/P. pneumoniae	Time-kill assay CLSI guideline; checkerboard assay	Study showed that additivity and synergistic interaction between CBO and antibiotic were comparable in ability to cause bacterial membrane disruption.	75
Paired combinations piperacillin and Cinnamomum verum J.Presl; piperacillin & Lavandula angustifolia Mill.; piperacillin & Mentha × piperita L.; meropenem & Mentha × piperita L/beta-lactamase-producing E. coli	MIC, Checkerboard assay	Two- to four-fold reduction in the MIC of the antibiotics was observed in some paired combinations: piperacillin/cinnamon bark and pipercillin and lavender.	76
Nepeta nuda L. ssp. nuda/1,8 cineole, tetracycline, and streptomycin/E. coli, K. pneumoniae, Proteus mirabilis, Pseudomonas aeruginosa, Staphylococcus aureus	Microdilution checkerboard assay, chemoinformatics method	N. nuda oil-antibiotic and 1,8-cineole- antibiotic resulted in antagonistic interactions. Chemoinformatics confirms antagonistic interactions via membrane dissipation.	77
Satureja kitaibelii Wierzb. ex Heuff., geraniol/tetracycline, chloramphenicol/E. coli, K. pneumoniae, Proteus mirabilis, Pseudomonas aeruginosa, Staphylococcus aureus	Microdilution checkerboard assay, chemometric method	Synergistic interactions of EO and antibiotic; constituent and antibiotic, reduced MIC values against Gram-negative bacteria.	78
Libanotis montana Crantz subsp. leiocarpa (Heuff.) Soó/ tetracycline, streptomycin, chloramphenicol/E. coli, K. pneumoniae, Proteus mirabilis, Pseudomonas aeruginosa, Staphylococcus aureus	Microdilution checkerboard assay, chemometric method	Synergistic or additive combinations reduced the minimum effective antibiotic dose, which minimized side effects. Strong synergy, especially against Gram-negative bacteria, was noted.	79

Abbreviations: EO, essential oil; FLC, fluconazole; FTIR, Fourier transform infrared; MDR, multidrug-resistant; MIC, minimum inhibitory concentration; MRSA, methicillin-resistant Staphylococcus aureus; NDM, New Delhi metallo-β-lactamase-1.

Escherichia coli: 2 Devasting Clone-Sequence Types: ST 405 and 1193

Gram-negative E. coli has increased in virulence. In 2016 in New Jersey, 2 mobile genes in E. coli were found to carry the mcr-1 and blaNDM-5 genes conferring resistance to colistin and carbapenems. ⁴⁶ Co-author in this study, Barry Kreiswirth later stated: “The bad news is that… there are clearly other strains out there we haven’t detected yet. Both the carbapenem resistance and the colistin resistance genes are on separate plasmids, which means in principle they could spread to other bacteria.”

Tchesnokova et al discovered in 2019 a new MDR E. coli affecting younger adults (under age 40) E. coli clonal group, sequence type 1193 emerging in multiple US cities. This MDR-E. coli is 100% resistant to fluoroquinolones, 55% resistant to trimethoprim-sulfamethaoxazole, and 53% resistant to tetracycline. ⁴⁷ The multipronged effects of EOs may provide a boost to antibiotics that are now ineffective for MDR E. coli. Please see Table 3 for more substantiated data.

Table 3

EOs Against Escherichia coli and MDR-E. coli.

Essential oil/Constituent/Pathogen	Method	Critical finding	Ref.
Eucalyptus globulus Labill., Cinnamomum verum J.Presl, Rosmarinus officinalis L., Daucus carota L., and Camelina sativa/E. coli	Broth microdilution	Two blends of EOs were effective against antibiotic-resistant ESBL-E. coli and MRSA among others.	48
Origanum compactum Benth./E. coli	MIC, MBC	Irreversible damage on the cell wall and membrane was caused by O. compactum EO leading to the leakage of proteins and genetic materials (DNA and RNA).	49
Origanum majorana L., Thymus zygis L., and Rosmarinus officinalis L./E. coli	Disc diffusion, MIC, MBC	1,8 cineole from Rosmarinus officinale does more to inhibit biofilm formation compared to the linalool and terpinen-4-ol present in T. zygis and O. majorana EOs in E. coli UTI patients.	50
Thymol, vancomycin, EDTA/MDR-E. coli	MIC, MBC, checkerboard dilution, time kill	This synergistic combination showed 16-fold sensitivity against MDR-E. coli, which is normally insensitive to vancomycin.	51
Origanum onites L./ESBL-E. coli	Disc diffusion, agar diffusion	O. onites EO was found effective against nosocomial ESBL-E. coli isolates with 2 methods.	52
Origanum vulgare L. and red thyme Thymus vulgare L. EOs, carvacrol and thymol/uropathogenic E. coli	Crystal-violet static formulation assay, CLSM/COMSTAT analysis	Oregano and thyme red oils/carvacrol and thymol exhibited high biofilm activity; reduced virulence of UPEC by limiting ability to agglutinate erythrocytes and to survive in human whole blood.	53
EO components citronellol, citronellal, carveol, and carvone/E. coli	MIC, MBC	Components compromised E. coli cytoplasmic membrane integrity causing intracellular potassium leakage.	54
Origanum heracleoticum L. EO, carvacrol/E. coli	RT-qPCR	O. heracleoticum showed inhibition of 4 tested genes (ler, stx2B, fliC, and luxS) at both sub-MICs. Carvacrol may be the major active component in oregano EO that is able to influence virulence gene expression and also the expression of the luxS gene involved in QS.	55
Litsea cubeba (Lour.) Pers. testing 1,8-cineole (LC19) and linalool (BV27) types/E. coli	MIC, MBC, time kill	E. coli was selected as the model strain to investigate mechanism of action of the 2 [types]. BV27 (linalool) led to cell permeabilization, which also changes in nucleoid morphology indicating disruption of membrane integrity.	56
Iranian EOs: Lallemantia royleana (Benth.), Pulicaria vulgaris Gaertn, Achillea wilhelmsii K. Koch, Echinophra platyloba D.C., Salvia nemorosa L., Satureja intermedia C.A.Mey./ESBL-producing E. coli	MIC, double-disc synergy test, PCR	L. royleana and P. vulgaris had great antibacterial activity against ESBL-producing E. coli isolates. A. wilhelmsii, E. platyloba, S. nemorosa, as did S. intermedia.	57
Cinnamomum zeylanicum Blume, Syzygium aromaticum L. Merr. & L.M. Perry, Cymbopogon flexuosus (Nees ex Steud.)WWatson, Origanum vulgare L., Rosmarinus officinalis L., Thymus vulgaris L., Melaleuca alternifolia (Maiden & Betchel) Cheel, Leptospermum scoparium J.R.Forst. & G.Forst./Gram-positive, -negative clinical isolates	Disc diffusion protocol	This study tested both Gram-positive and -negative pathogens. When cinnamon bark EO was tested 19 times (n = 19) against ESBL E. coli, the mean kill zone diameter was 29.7 mm with an SD of 0.82 mm. The median and range of the kill zone were 30 and 3 mm, respectively.	58
Allicin from garlic EO, Allium sativum L./uropathogenic E. coli	Sub-MIC	Sub-MICs of allicin can decrease UPEC CFT073 and J96 biofilm, aiding eradication of infection. Effects may be due to influence of allicin on type 1 pili adhesion FimH and BarA/UvrY/Csr pathway gene uvrY and csrA.	59
Cinnamomum verum J. Presl/piperacillin/MDR-E. coli strain	Time-kill assay	These results agree with those of eugenol, one of the major components of cinnamon bark EO, which disrupts bacterial cell membrane. Anti-quorum sensing ability of this oil showed promising inhibitory properties for long- and short-chain AHL quorum sensing systems.	60

Abbreviations: EO, essential oil; MDR, multidrug-resistant; MIC, minimum inhibitory concentration; MRSA, methicillin-resistant Staphylococcus aureus.

Candida auris: The Deadly MDR Fungi Hospitals Would Not Disclose

April 2017 news reports told of an outbreak of pan-resistant Candida auris in New York, New Jersey, and Illinois. One deadly case was at Northwestern Memorial Hospital in Chicago contracted by a patient from a catheter or intravenous line. It has been reported that nearly half the people who contract this die within 90 days. There was no announcement to the community about the outbreak. ⁸⁰

This nonaction followed a model of suppression set by Royal Brompton Hospital outside London. Only after the intensive care unit at Royal Brompton was shut down for 50 Candida auris cases did they acknowledge the outbreak. Patients were moved to another floor for 11 days.

A newspaper article told how London hospital workers used a device to spray aerosolized hydrogen peroxide throughout the room of a patient with Candida auris. ⁸¹ After a week’s treatment the “settle plate” used to confirm bacterial efficacy showed just 1 organism grew back: Candida auris.

The 2016 European C. auris outbreak was discussed by Schelenz et al who reported, “Despite a comprehensive review of modern technologies for environmental decontamination there is currently no published data in the literature on the effectiveness of cleaning agents or decontamination of the environment for C. auris specifically.” ⁸²

In 2018, China had its first cases of pan-echinocandin-resistant Candida tropicalis and pan-echinocandin-resistant Candida glabrata, which suggests monitoring for antifungal susceptibility trends in all Candida species. ⁸³

We note a 2014 study in Acta Biochem Pol. as just one of many showing antifungal effects of EOs. Budzyńska et al state: “This report includes the results proving the influence of clove oil, geranium oil, lemon balm and citronella oil on possible mechanisms reported to be relevant for Candida pathogenesis, namely germ tube and mycelium formation, adhesive and invasive properties and extracellular production of various enzymes.” ⁸⁴ Please see Table 4 for more substantiated data.

Table 4

EOs Against MDR Fungal Infections.

EO	Pathogen	Method	Mode of action	Critical finding	Ref.
Mentha × piperita L.	Candida albicans, C. tropicalis, C. glabrata	Broth dilution method/checkerboard assay/scanning electron microscopy	Disrupts cell membrane/target ergosterol biosynthesis pathway	Mint EO and compounds penetrate the cell membrane and target ergosterol biosynthesis pathway, thus impairing its biosynthesis.	68
Melaleuca alternifolia (Maiden & Betchel) Cheel	Flucon-resistant candidiasis in AIDS patients	In vitro susceptibility studies, MIC	Antifungal	Six of 12 patients improved, while an additional 2 were clinically cured.	85
Melissa officinalis EO with methylcellulose hydrogel	C. albicans	Disc diffusion method, time-kill assay	Anti-Candida action	The antifungal properties of the hydrogels were confirmed by both zone of inhibition method and time-kill assays and the 2% (v/v) hydrogel significantly reduced the retention of C. albicans.	86
Eugenia uniflora L.	C. albicans, C. krusei, C. tropicalis	Broth microdilution assay/MFC	Anti-Candida action	EO inhibited the growth of C. albicans filamentous structures at all concentrations. In C. tropicalis, this effect was indicated only at the highest concentration. The oil was inefficient against C. krusei.	87
Thymus capitatus (L.) Hoffmans & Link, Rosmarinus officinalis L., Myrtus communis L., Salvia officinalis L., Myrtus communis Pistacia lentiscus L., Pelargonium graveolens L’Héŕ, Eucalyptus globulus Labill., Cinnamomum verum J.Presl, Syzygium aromaticum (L.) Merr. & L.M. Perry, and Nigella sativa L.	Variety of Candida strains	MIC/checkerboard technique, time-kill assay	Broad-spectrum activity against pathogenic Candida strains	C. verum and cinnamaldehyde equally inhibited Candida species growth. C. verum, P. graveolens, S. aromaticum, and T. capitatus were potent against Candida while P. graveolens and C. verum showed synergistic effects with fluconazole against fluconazole-resistant strains of C. albicans.	88
Thymol/carvacrol monoterpenes	C. albicans	Checkerboard microdilution assay, time-kill curves	Synergy w/fluconazole	Thymol and carvacrol augment the efficacy of fluconazole by chemo-sensitizing fungal cells to the drug and decreasing its efflux by CDR1 and MDR1 efflux pumps. These monoterpenes could be useful in antifungal chemotherapy.	89
Myrtus communis L., Zingiber officinale Roscoe, Matricaria chamomilla L., Trachyspermum ammi (L.) Sprague, and Origanum vulgare L.	Fluconazole-resistant C. albicans	Disc diffusion method, MIC, MFC	Antifungal	O. vulgare and T. ammi EOs showed high antifungal activity. Interestingly, differences on susceptibility between groups of C. albians were observed, indicating that the FLU-resistant group was more susceptible to these EOs than the FLU-susceptible group. Both oregano and T. ammi oils are high in thymol, which may explain their antifungal activity.	90
Cinnamaldehyde	Fluconazole-resistant clinical isolates	Disc diffusion halo assay, time-kill curves	Antifungal	Candida isolates showed susceptibility to cinnamaldehyde and inhibition of H+-ATPase-mediated proton pumping. The study showed that cinnamaldehyde exhibits fungicidal but not fungistatic activity.	91
Origanum vulgare L., Cinnamomum zeylanicum Blume, Lippia graveolens Kunth, Thymus vulgarus L., Salvia officinalis L., Rosmarinus officinalis L., Ocimum basilicum L., Zingiber officinale Roscoe	Fluconazole-resistant and -sensitive C. glabrara	Broth microdilution, MIC, MFC	Antifungal, anti-efflux pump	C. glabrata resistance to azoles results from efflux of antifungal agents. Oregano EO showed the lowest MIC and MFC values against both the fluconazole-susceptible and -resistant isolates. Mexican oregano (Lippia graveolens) showed MICs (μg/mL) of 400 to >3200 for 2 C. glabrata fluconazole-resistant and -sensitive isolates.	92
EO-ACs thymol, carvacrol, citral	Cryptcoccus neoformans, C. laurentii	Scanning electron microscopy, confocal laser scanning	Antifungal	Thymol best inhibited Cryptcoccus sp.: planktonic cells, biofilm formation, and mature biofilms, followed by carvacrol and citral. The biofilm of C. laurentii was more susceptible to EO-ACs in comparison to C. neoformans showing species-specific and drug-specific differences.	93
Pinus sylvestris L., Origanum vulgare L., Thymus vulgaris L. components/itraconazole	Cryptococccus neoformans strains	Checkerboard assays, time-kill studies	Antifungal	Pine oil’s MIC showed the highest inhibitory activity: 0.07‐0.27 mg/mL. Oregano was the most effective against the azole nonsusceptible isolate. Synergistic and additive effects were shown with combinations of pine, oregano, and thyme red with itraconazle.	94
Mentha × piperita L.	C. albicans	MTT assay/quantitative real-time RT-PCR	Antifungal	The oil and its vapor distorted C. albicans’ cell wall surface. Biofilm formation was decreased while Mentha × piperita disrupted mature biofilm.	95
Hyptis suaveolens (L.) Poit	Aspergillus spp. A. fumigatus ATCC-40640 and A. parasiticus ATCC-15517	Macrodilution broth	Antifungal	At 40 and 80 µL/mL, the EO inhibited mycelial growth of both fungi. At 40 and 80 µL/mL Hyptis suaveolens had a 100% lethal effect during a 14-day exposure. Changes in mycelial structure included lack of sporulation, cytoplasm content/pigmentation loss, with distortion of hyphae development.	96
Cuminum cyminum L., Ziziphora clinopodioides Lam., Nigella sativa L.	Aspergillus fumigatus and A. flavus	Broth macro- and microdilution methods	Antifungal	Fungicidic activity against both Aspergillus sp was shown by C. cyminum at 0.25 mg/mL, Z. clinopodioides at 0.5 and 0.25 mg/mL, N. sativa at 1.5 mg/mL. The oils changed cell uniformity by interacting with either the cell wall or cytoplasmic membranes.	97
Cymbopogon martini (Roxb.) W.Watson, Foeniculum vulgare Mill. Trachyspermum ammi (L.) Sprague	Aspergillus flavus, A. niger	Agar dilution methods	Antifungal	These 3 EOs all showed better fungitoxic activity against aflatoxigenic Aspergillus spp. than the chemical preservative sodium benzoate. T. ammi had the best effect of the 3 oils with MICs of 1 µL/mL against both A. flavus and A. niger.	98
Zataria multiflora Boiss.	Aspergillus parasiticus	MIC/MFC broth dilution/HPLC	Antifungal	Z. multiflora EO reduced fungal growth and mycelial dry weight by the fifth day in cultures of 100 ppm (62.91%). However, on day 9, amounts of aflatoxins AFB1, AFB2, AFg1, AFC2, and AFTotal increased. This is possibly caused by the oils’ volatility.	99

Abbreviations: EO-ACs, essential oil active components; HPLC, high-performance liquid chromatography; MDR, multidrug-resistant; MIC, minimum inhibitory concentration; RT-PCR, reverse transcriptase polymerase chain reaction.

EO Effects on Bacterial Cell Membrane

A 2013 Italian study described EO mechanisms of action against microbes. “Toxic effects on membrane structures and function are generally used to explain the antimicrobial activity of EOs. In fact, the mechanisms of action of the EOs include the degradation of the cell wall, damaging the membrane proteins, increased permeability leading to leakage of the cell contents, reducing the proton motive force, reducing the intracellular ATP pool via decreased ATP synthesis and augmented hydrolysis that is separate from the increased membrane permeability and reducing the membrane potential via increased membrane permeability.” ¹⁰⁰ Please see Table 5 for more substantiated data.

Table 5

EO Effects on Bacterial Cell Membrane.

EO/Bacteria	Method	Critical finding	Ref.
Ocimum sanctum/60 clinical, 5 standard laboratory isolates of Candida	MIC, MFC, confocal scanning laser microscopy, sterol quantitation method	OSEO and the major components, methyl chavicol and linalool, effectively target the cytoplasmic membrane in Candida. This action may originate with inhibition of ergosterol biosynthesis.	101
Thymus maroccanus Ball/Escherichia coli, Enterobacter aerogenes, Salmonella enterica Typhimurium	Measuring optical density, MIC and MIC/2, β-lactamase assay	Thymus maroccanus was shown to permeabilize both the outer and inner membranes of the bacteria tested allowing the release of proteins such as β-lactamase, β-galactosidase, or EfTu.	102
Monarda punctata L. EO/Streptococcus pyogenes, MRSA, Streptococcus pneumoniae, Haemophilus influenzae, E. coli	Broth microdilution method, time-kill dynamic curves, SEM	Monarda punctate EO was the most successful with S. pyogenes, E. coli, and S. pneumonia with the lowest MIC and MBC values. The most resistant bacterial strain was MRSA. The oil’s major compounds are thymol, p-cymenem, and limonene. In testing, the whole EO was found to be more potent than the individual compounds.	103
EO components: citronellol, citronellal, carveol, and carvone/E. coli, Staphylococcus aureus	MIC and MBC/membrane integrity: propidium iodide uptake	While all compounds showed antibacterial activity against E. coli and S. aureus, carvone lacked inhibitory action against S. aureus. Generally, citronellol was the most effective compound. The EO compounds’ antibacterial activity resulted in membrane/cell wall permeabilization.	54
Juniperus rigida Siebold & Zucc./Klebsiella pneumoniae	MIC and MBC, growth curve	The EO caused irreversible damage to the bacterial cell wall and membrane leading to leakage of proteins. With 61 components to the oil there may be more than 1 antibacterial mechanism.	104
Lavandula angustifolia Mill./E. coli	Time-kill assay, SEM, anti-QS assay	Lavender EO is able to disrupt bacterial membranes of Gram-negative bacteria. When combined with antibiotics lavender increases penetration of the antibiotic into the bacteria.	105
Lavandula angustifolia Mill. EO/K. pneumoniae	MIC, checkerboard assay, influx/efflux assay, SEM	Our study showed LVO is a promising antimicrobial extract, which can be used in combinatory therapy and may possibly be effective in reviving the efficacy of carbapenem and other antibiotics against other resistant bacteria. We propose the overall mode of action of LVO: disrupting the bacterial membrane via oxidative stress.	106
Isodon melissoides (Benth.) H. Hara/12 pathogenic bacteria including MRSA, AAA-44, MRSE	MIC, protein leakage assays	Isodon melissoides showed significant antibacterial activity against 11 bacterial strains out of 12 tested by damaging the cell membrane and changing cell membrane permeability.	107
Origanum compactum Benth.	MIC, MBC	The bacterial cell membrane was damaged by OCEO measured by release of cell contents.	49

Abbreviations: EO, essential oil; MIC, minimum inhibitory concentration; MRSA, methicillin-resistant Staphylococcus aureus; SEM, scanning electron microscopy.

EO Effect on Biofilms/Efflux Pumps/Quorum Sensing

In 2013, Bjarnsholt reported on the “category of chronic infections caused by bacteria growing in slime-enclosed aggregates known as biofilms. Biofilm infections, such as pneumonia in cystic fibrosis patients, chronic wounds, chronic otitis media and implant- and catheter-associated infections, affect millions of people in the developed world each year and many deaths as a consequence.” ¹²³ Wang et al noted that “The increasing multidrug resistance has become a major threat to the public health. Overexpression of multidrug efflux pumps is one of the major mechanisms of drug resistance in bacteria.” ¹²⁴ Defoirdt stated: “Many important plant, animal, and human pathogens regulate virulence by quorum sensing, bacterial cell-to-cell communication with small signal molecules.” ¹²⁵ EOs and their components have multifaceted effects against these powerful bacterial defenses. We recommend the 2019 review “Anti-quorum Sensing and Antimicrobial Effect of Mediterranean Plant Essential Oils Against Phytopathogenic Bacteria” by Camele et al. Please see Table 6 for more substantiated data.

Table 6

EOs Against Biofilm, Efflux Pump, Quorum Sensing.

EO/Constituent/Pathogen	Method	Critical finding	Ref.
Biofilm
Black pepper (Piper nigrum L.), cananga (Canaga odorata Lam. Hook.f. & Thomson), and myrrh (Commiphora myrrha [Nees]) Engl. EOs, and the constituent cis-nerolidol	Crystal violet assay, confocal laser microscopy, SEM assay	Black pepper, cananga, and myrrh oils and their common constituent cis-nerolidol at 0.01% markedly inhibited S. aureus biofilm formation. Confocal laser microscopy was used to observe biofilm formation on plate bottoms where this inhibition was observed.	108
Oregano (Origanum vulgare L.?), red thyme (as neither is listed, it is assumed Thymus vulgaris L. as oregano contains little thymol), EO, thymol, carvacrol	SEM	Thymol and oregano EO have a better result on antibiofilm formation of E. coli than carvacrol. Carvacrol and thymol have a better result on anti-biofilm formation of Salmonella than oregano EO.	109
Artemisia vestita Wall. Ex Besser	Micro-well dilution method, SEM	The EO Artemsia vestita and its major component, grandisol, offer potential for development as a novel antibacterial drug for the treatment of respiratory infections, particularly against Streptococcus pyogenes.	110
Of 11 EOs, most effective: Cinnamomum cassia L. J. Presl. and Salvia officinalis L.	MIC, MBC, MBIC, MBEC against S. aureus	Three microemulsions: C. cassia, S. officinalis, or both caused a >3 logarithmic reduction in S. aureus 24-h-old biofilms and desiccated biofilms; and up to 68% of biofilm removal after 90 min of exposure.	111
Lemongrass cymbopogon flexuosus (Nees ex Steud.) W.Watson, grapefruit (Citrus paradisi Macfad), EO, citral	Disc diffusion method, SEM, MIC, MBC, MBEC	Lemongrass and citral prevented biofilm formation at 0.06% (v/v) for hospital MSSA strains/0.125% for other strains tested. It did not remove already formed biofilms. No antibiofilm activity shown for grapefruit EO.	38
Efflux pump
Thymol and carvacrol from thyme (Thymus vulgare L.) EO	Checkerboard microdilution method	Both thymol and carvacrol decreased expression of CDR and MDR genes, which are known to encode for the efflux pumps and contribute to FLC resistance.	112
Rosmarinus officinalis L. (EO) and main constituent 1,8-cineole	Adapted diffusion method, serial binary microdilution, flow cytometry assessment	R. officinalis L. and 1,8-cineole exhibited very good antimicrobial activity against Pseudomonas aeruginosa and Acinetobacter baumannii MDR strains and a synergistic activity with ciprofloxacin by inducing cellular wall permeabilization and efflux pumps inhibitory activity.	113
Thymol and carvacrol	MIC, MBC	Thymol and carvacrol inhibited the ethidium bromide (EtBr) cell efflux in a concentration-dependent manner.	114
Chenopodium ambrosioides L. and α-terpinene	MIC, efflux pump inhibition by MIC reduction	α-terpinene was not relevant in this study despite being the major compound of the oil. The oils’ success is possibly from blocking the protein source for the MFS efflux pump in the SA-1199B strain combined with destabilizing the cell membrane.	115
Chenopodium ambrosioides L. and α-terpinene	Microdilution method	The expected power of α-terpinene was not seen. Components in lower concentrations may be responsible for the oils’ ability to alter cytoplasmic membrane fluidity state resulting in destabilizing efflux pumps. Synergistic effects were seen: the oil decreased the tetracycline MIC by 2× and of EtBr by 6×.	116
Salvia fruticose Mill., Salvia officinalis Wall, Salvia sclarea L.	MIC, broth checkerboard microdilution method	The FICI for the combination of tetracycline and EO Salvia officinalis and S. sclarea indicated synergistic and in some cases additive interactions.	117
Quorum sensing
Micromeria thymifolia (Scop.) Fritsch	MIC	M. thymifolia exhibited anti-quorum sensing activity by decreasing pyocyanin, pyovertine, elastase, and also biofilm production by Pseudomonas aeruginosa PA01.	118
Thymus daenensis Celak, Satureja hortensis L., Origanum vulgare L.	Microtiter-plate test, SEM	The results of qPCR demonstrate that part of anti-pneumococcal biofilm properties of studied EOs, especially Thymus and Satureja may be due to QSI activity.	119
Carum copticum (L.) Benth. & Hook.f.	Disc diffusion, broth microdilution method	The EO showed anti-QS activity while the major component thymol did not show anti-QS in C. violaceum. However, minor compounds such as linalool and cuminaldehyde showed anti-QS activity.	120
Syzygium aromaticum (L.) Merr. & L.M. Perry	Disc diffusion, agar well diffusion, QS: C. violaceum (CV 12472) and Pseudomonas aeruginosa (PAO1)	Inhibition of swarming motility by Syzygium aromaticum by clove oil has further strengthened its anti-QS behavior.	121
Lippia alba (Mill) N.E.Br. ex Britton & P.Wilson, gathered from 5 locations in Colombia.	Broth dilution method, anti-QS activity detected through inhibition of QS-controlled violacein pigment production by the sensor bacteria	Results showed that 2 L. alba EOs, one containing the greatest geranial:neral and the other the highest limonene:carvone concentrations, were the most effective QS inhibitors.	122

Abbreviations: EO, essential oil; EtBr, ethidium bromide; MDR, multidrug-resistant; qPCR, quantitative polymerase chain reaction; SEM, scanning electron microscopy.

MDR and XDR Tuberculosis

It is important to include EO impact on the major health problem, MDR and extensively drug-resistant (XDR) strains of tuberculosis, which affect millions. The US CDC reports that tuberculosis is one of the world’s deadliest diseases with one-fourth of the world’s population infected with it. According to the World Health Organization (WHO): “A total of 1.5 million people died from TB in 2018. Multidrug-resistant TB (MDR-TB) remains a public health crisis and a health security threat. WHO estimates that there were 484,000 new cases with resistance to rifampicin—the most effective first-line drug, of which 785 had MDR-TB.” Of the 10 million individuals who became ill with TB in 2018, approximately three million were “missed” by health systems and do not get the care they need, allowing the disease to continue to be transmitted. ¹²⁶

The 2018 study by Kazemian et al noted the lengthy treatment time for TB, hepatotoxicity of drugs, and the emergence of multidrug-resistance and extreme-drug-resistant strains. Their research on medicinal plants of the Lamiaceae family showed MDR-TB was completely inhibited by Zataria multiflora Boiss. at 78 µg/mL while Satureja rechingeri and Satureja khuzestianica showed MICs of 156 µg/mL against MDR Mycobacterium tuberculosis. ¹²⁷

Other EOs showing efficacy in studies against MDR- and XDR-TB are Myrtus communis L. ¹²⁸ and Nigella damascena. ¹²⁹

Conclusion

On April 21, 2019, a segment on CBS News asked: “Could Antibiotic-Resistant ‘Super Bugs’ Become a Bigger Killer Than Cancer?” Interviewee Ramanan Laxminarayan, senior research scholar at Princeton University, has tracked the rise of superbugs for nearly 20 years. He discussed what would not be possible without effective antibiotics. “Everything that we think of whether it’s cancer chemotherapy, transplants, hip replacements, knee replacements, colorectal surgery, all of these require effective antibiotics to perform.” ¹³⁰

What has been the response to the antibiotic-resistance threat? “By 2006, the European Union had banned all nonmedicinal antibiotics in animals… In 2005, the emergence of fluoroquinolone-resistant Campylobacter jejuni in the clinical setting in conjunction with fluoroquinolone administration in animals prompted the FDA to ban fluoroquinolone use in poultry.” ¹³¹ The authors of the study just quoted reported that following the avoparin ban in Europe, “the prevalence of VRE in farm animals rapidly declined… However, this did not translate directly to a decrease of VRE in humans.”

A systematic review and meta-analysis on reducing antibiotic use in food-producing animals was published in Lancet Planet Health in November 2017. The Lancet study concluded:

Interventions that restrict antibiotic use in food-producing animals are associated with a reduction in the presence of antibiotic-resistant bacteria in these animals. A smaller body of evidence suggests a similar association in the studied human populations, particularly those with direct exposure to food-producing animals. The implications for the general human population are less clear, given the low number of studies. The overall findings have directly informed the development of WHO guidelines on the use of antibiotics in food-producing animals. ¹³²

Acting on causes of antibiotic resistance is certainly a part of solving this bacterial crisis. But foremost is research for deterrents. The synchronicity of EOs and antibiotics has been reported by Owen and Laird “as a potential solution to antibiotic resistance.” They note, however, that “a lack of consistency in methods and interpretation criteria makes drawing conclusions of efficacy of studied combinations difficult.” ¹³³ The multipronged effects of EOs on resistant microorganisms must be added to the antibacterial arsenal. With a paucity of new antibiotics in the pipeline, the call is now urgent for more and better designed EO/antibiotic synergy research.

Lahmar et al documented the often-surprising power of combining an EO with an antibiotic:

In the interactive experiment essential oils were found highly effective in reducing the resistance of Methicillin-resistant Staphylococcus aureus to amoxicillin, tetracycline, piperacillin, ofloxacin and oxacillin and resistance of Acinetobacter baumannii to amoxicillin and of ofloxacin in interactive manner. Furthermore, the results proved synergism among essential oils and both antibiotics ofloxacin and novobiocin against the Extended-Spectrum Beta-Lactamase producing E. coli (ESBL). ¹³⁴

We close by recommending the 2014 review by Langeveld et al that stated: “Several modes of action have been put forward by which antibiotics and the EO components may act synergistically, such as by affecting multiple targets; by physiochemical interactions and inhibiting antibacterial-resistance mechanisms. Many reported assays show additivity or moderate synergism, indicating that EOs may offer possibilities for reducing antibiotic use.” ¹³⁵