Study objectives: To detect biofilm formation in Staphylococcus aureus strains and to determine the minimal biofilm inhibition concentrations (MBIC) and the minimal biofilm eradicating concentrations (MBEC) of vancomycin, gentamicin and rifampin. To compare the MBIC and MBEC with the minimal inhibition concentration (MIC) and minimal bactericidal concentration (MBC) data for planktonic Staphylococcus aureus forms that are commonly used in antimicrobial susceptibility testing for the purposes of individualized therapy. Patients and Methods: Fifteen S. aureus strains isolated from central venous catheters, intratracheal tubes and wound drainage tubes from the patients of the University Hospital, Bratislava-Staré Mesto were included in the study. Selected virulence factors were characterized. The biofilm formation potential was measured by a modified crystal violet micro-assay. The presence of viable cells biofilm in was tested using 3-(4,5-dimethylthiazol- -2-yl)-2,5-diphenyl tetrazolium bromide (MTT). The MIC and MBC of vancomycin, gentamicin and rifampin was tested in planktonic S. aureus forms by the broth microdilution method. The MBIC and MBEC of these antimicrobial drugs for biofilm S. aureus forms were determined by a modified microdilution method. Student’s t-test was used for statistical analysis of the results. Results: All of the study strains formed biofilm, with only two of them having a low biofilm formation potential. MTT revealed moderate to high metabolic activity of bacteria biofilm in Vancomycin MICs and MBICs were identical in 80 % of the study strains. Vancomycin MBECs are higher than MBCs in all the study strains, are interpreted as resistance according to the criteria of the Clinical and Laboratory Standards Institute (CLSI) and make the drug unsuitable for use in the treatment. In vitro gentamicin MBICs indicated susceptibility according to the CLSI criteria but gentamicin MBECs were interpreted as gentamicin resistance. Rifampin MICs and MBICs of the study strains revealed susceptibility. Rifampin MBCs were interpreted as susceptibility, but based on MBECs, 13 % of the study strains were considered as resistant and 13 % of the study strains showed intermediate susceptibility. The differences between gentamicin and rifampin MICs and MBICs and those between MBCs and MBECs of all antimicrobials tested were statistically significant. Conclusion: The tested biofilm S. aureus forms showed high MBECs of vancomycin, gentamicin and rifampin, with rifampin only being suitable for therapeutic use. To provide reliable results for individualized antibiotic therapy, it will be needed to test in vitro biofilm formation, to determine MBIC and MBEC of antimicrobial drugs using a standardized method, to interpret the test results in relation to biofilm S. aureus forms and to establish the interpretation criteria for MBIC and MBEC similarly to MIC and MBC.

Mikrobiologický ústav LFUK a FNsP Bratislava

Susceptibility of Staphylococcus aureus biofilms to vancomycin, gentamicin and rifampin

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