Veterinary session

External otitis caused by Pseudomonas aeruginosa is a frequently diagnosed disease in veterinary dermatology. In companion animals these clinical cases require a long duration of antimicrobial therapy, that can promote development of resistance in the pathogens. The incidence of this phenomenon might be decreased by the combined application of certain antimicrobials.

In this study, the susceptibility of P. aeruginosa, a bacterium frequently multiresistant and responsible for therapeutic failure in dermatology, was determined to marbofloxacin, colistin and their combination. In addition, the ability of colistin to reduce resistance development against marbofloxacin was also investigated. The examined strains (n=16) were obtained in Hungary from dogs suffering from otitis externa. The aim of this study was to evaluate sensitivity of P. aeruginosa isolates to the above mentioned antimicrobials with the microdilution method by determining minimal inhibitory concentration (MIC) values according to CLSI (Clinical and Laboratory Standards Institute) guidelines. The interactions between the antibacterials were evaluated with fractional inhibitory concentration (FIC) determination. In the study emphasizing the resistance delaying effect of colistin, bacteria were passaged for 8 days in broths containing subinhibitory concentrations of marbofloxacin and the marbofloxacin:colistin combination. The respective MIC values for each strain were determined each day.

Among the examined isolates, 62.5% was sensitive to marbofloxacin, all of the strains were susceptible to colistin and the marbofloxacin:colistin combination. According to our results, the MIC50 and MIC90 values of marbofloxacin were 2 µg/ml and 8 µg/ml. Concerning colistin the values found were much lower, 0.25 µg/ml (MIC50) and 0.5 µg/ml (MIC90), respectively. The same values applied for the marbofloxacin:colistin combination. According to fractional inhibitory concentration values the combination was considered as indifferent.

The 8 day serial passage of bacteria in broths containing only marbofloxacin showed that the susceptibility of all the strains decreased. At the end of the investigation, all of the isolates became resistant with one exception to the fluoroquinolone according to EUCAST epidemiological cutoff values.

MIC50 and MIC90 values on the last day proved to be 32 µg/ml, and 64 µg/ml, respectively. In contrary, serial passage in broth containing marbofloxacin supplemented with colistin, the susceptibilities of all isolates increased, MICs became much lower.

In conclusion, all of the P. aeruginosa isolates investigated showed high susceptibility to colistin, while several strains proved to be moderately susceptible or resistant to marbofloxacin. Passaging bacteria containing broth with the fluoroquinolone alone resulted in rapid development of resistance, while colistin was able to inhibit and reverse this phenomenon.