The effects of biofi lm production on the antimicrobial susceptibility of Pseudomonas aeruginosa strains isolated from dogs

SUMMARY The aim of this study was to evaluate the in vitro activity of marbofl oxacin, gentamicin, colistin and their combinations. The authors compared the sensitivity of planktonic cells with bacteria in biofi lms. The susceptibility of the planktonic cells was determined to marbofl oxacin, marbofl oxacin-gentamicin and marbofl oxacin-colistin combination with the microdilution method by determining MIC values according to CLSI (Clinical Laboratory Standards Institute) guidelines. In the second part of the study the bacterial strains were incubated without antimicrobials in microplate wells for 12 hours to form biofi lms. To determine the sensitivity of biofi lm-grown cells a photometric method utilizing MTS-formazan formation was performed. Among the examined planktonic P. aeruginosa strains, 91.7% was sensitive to marbofl oxacin and all of the strains were susceptible to the 1 : 1 combination of the antimicrobials. In case of marbofl oxacin, MICs ranged from 0.25 to 4 μg/ml, in case of the marbofl oxacin-gentamicin and marbofl oxacin-colistin combinations these were between 0.0625 and 1 μg/ml. The MIC50 and MIC90 values of marbofl oxacin were 1 μg/ml and 2 μg/ml, respectively. These values for the marbofl oxacin:gentamicin combination were 0.5 μg/ml and 1 μg/ml, respectively, while for the marbofl oxacin:colistin combination 0.0625 μg/ml and 0.5 μg/ml, respectively. To examine the susceptibility of biofi lm-grown strains EC50 (effective concentration 50%) was used. Concerning marbofl oxacin these values ranged from 64 to 4096 μg/ml. In case of marbofl oxacin-gentamicin combination the EC50 was found between 8-2048 μg/ml, while the 1 : 1 combination of marbofl oxacin-colistin showed EC50 values between 8-128 μg/ml. Based on the results, the susceptibility of the examined strains was signifi cantly decreased in biofi lms compared to planktonic cells, and the antimicrobial combinations increased the sensitivity of these strains