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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.
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