Taurolidine susceptibility of methicillin-resistant Staphylococcus pseudintermedius isolates

Abstract

Infections caused by methicillin-resistant Staphylococcus pseudintermedius (MRSP) can be challenging to treat because they are often resistant to many other antimicrobial agents. We assessed the susceptibility of 29 MRSP isolates from dogs to taurolidine in vitro. There was no growth at 0.12% taurolidine and light growth at 0.06% for all isolates. Taurolidine was reliable at inhibiting growth of MRSP at a concentration of 1,200 µg/mL.

Methicillin-resistant Staphylococcus pseudintermedius (MRSP) is a facultative pathogen primarily of dogs, and, since 2004, the isolation of this organism has increased in frequency. ¹⁴ MRSP is one of the most common bacteria causing surgical site infection in dogs.^3,15,17,20 Surgical site infections with MRSP are a potentially serious problem in that they increase postoperative morbidity, overall cost of treatment, duration of hospitalization, and can, in some cases, result in catastrophic outcomes. ¹ Additionally, MRSP strains often are resistant to various other antimicrobial classes. ⁹ A multicenter study from Europe and North America showed that MRSP isolates commonly are resistant to virtually all antimicrobial drug classes approved for use in dogs, with 90% of MRSP isolates resistant to ciprofloxacin, clindamycin, erythromycin, kanamycin, streptomycin, and trimethoprim, and 57% resistant to chloramphenicol. ¹⁴ This can make treatment challenging and sometimes unsolvable.

Taurolidine is a synthetic derivative of the amino acid taurine. It has been studied as a broad-spectrum antimicrobial agent and has demonstrated significant antibacterial effect on established dental plaque biofilm. ² Taurolidine has an antimicrobial effect on gram-positive and -negative bacteria as well as fungi. ¹⁸ The mechanism of action of taurolidine involves chemical reactions with the microbial cell wall, endotoxins, and exotoxins, ⁶ and it also inhibits microbial adhesion ¹⁰ to inert and living surfaces. There are no reports of bacterial resistance toward taurolidine, ¹¹ and evidence of the development of microbial adaptation to taurolidine after prolonged use of this agent has not been found. ¹³ Taurolidine remains active in a serum-rich environment. ⁸ Taurolidine has been reported to prevent formation of biofilms, which is probably based on the irreversible reaction of methylol taurinamid with microbial cell wall constituents resulting in the prevention of microbial adherence. ²¹ Taurolidine as a 2% solution can be given safely to dogs. ¹² Given the properties of taurolidine and the need to find novel therapies against MRSP, we assessed the susceptibility of MRSP to taurolidine in vitro.

Twenty-nine epidemiologically unrelated MRSP isolates collected from dogs from around the United States were studied. Bacterial species identification and methicillin resistance status of the isolates were confirmed by species-specific thermonuclease gene (nuc) and methicillin resistance gene (mecA) PCRs, respectively, as described previously.^4,16 Taurolidine susceptibility was determined by agar dilution using Clinical and Laboratory Standards Institute (CLSI) protocols. ⁷ As well, CLSI-recognized, quality-control organisms Escherichia coli ATCC 25922, Staphylococcus aureus ATCC 29213, Pseudomonas aeruginosa ATCC 27853, and Enterococcus faecalis ATCC 29212 were used for quality control of media production. Isolates were grown overnight in pure culture on Columbia agar with 5% sheep blood. After overnight incubation, individual isolates were suspended in phosphate-buffered saline to achieve a 0.5 McFarland standard (~10⁸ CFU/mL). Using a Steer replicator, these suspensions were inoculated onto Mueller–Hinton agar supplemented with taurolidine (Austin Chemical, Buffalo Grove, IL). The Mueller–Hinton-supplemented plates ranged in taurolidine concentration from 2% to 0.03%. Results were interpreted as growth (resistant) or no-growth (susceptible) per EUCAST guidelines (http://www.eucast.org/).

Proper base media production was assured by the growth of the 4 quality-control organisms. All 29 MRSP isolates investigated were inhibited at 0.12% (1,200 µg/mL) of taurolidine, and all had light growth at 0.06% (600 µg/mL). Therefore, both the MIC₅₀ and MIC₉₀ were between 1,200 µg/mL and 600 µg/mL. Our results compare to results of previous studies in which the MIC₅₀ and MIC₉₀ for taurolidine were 500 µg/mL and 1,000 µg/mL, respectively, for both oxacillin-resistant Staphylococcus aureus and oxacillin-resistant, coagulase-negative staphylococci. ¹⁹

The maximum concentration of taurolidine achieved in the plasma of dogs has been reported to be 140.5 ± 40.3 µg/mL. ¹² Consequently, it appears unlikely that taurolidine would be useful for systemic therapy against MRSP, but it may have an important role when used topically or infused locally.

The ability of taurolidine to maintain antimicrobial activity in the presence of a biofilm ² and to prevent biofilm formation ²¹ may prove very important when treating infection in which biofilm formation plays an important role in the pathogenesis. Microbial colonization of biomaterials and tissues is dramatically affected by the ability of the bacteria to produce a biofilm. Biofilm resident bacteria are highly resistant to antimicrobial agents. ⁵ Our study indicates that MRSP isolates are susceptible to taurolidine in vitro. Further studies, including in vivo, are warranted to better define the role of taurolidine against MRSP, particularly in the clinical setting.