Estimation of Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) of Cell-Free Extracts of Bifidobacterium Species Against Methicillin-Resistant Staphylococcus aureus in vitro

The aim of this study was to examine if extract of bifidobacteria, a major species of the human colonic microflora participates in the barrier effect against enteropathogens by developing antimicrobial activity against virulent bacteria. Six human bifidobacteria strains were isolated from infant stools. They were characterized and identified through physiological, biochemical tests and API 20 A test system. The isolates belonged to the three species: B. breve, B.longum and B.infantis . The cell extracts of the isolates were examined for antimicrobial activity by determination of minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC). For this purpose, the methicillin-resistant S. aureus (MRSA) was chosen as an indicator. MRSA treated with cell-free supernatants (CFS) from bifidobacteria were examined. All the Bifidobacterium isolates used have been identified as novel probiotics with a greater ability to survive at low pH and high concentrations of bile salt in vitro . 0.5 McFarland standard (10 8 CFU/ml) of a confirmed MRSA strain was challenged with the CFS strains by employing the tube dilution method and subculture on MRS agar assays. The cell-free supernatants of the 6 LAB strains exhibited MIC values between 50 µl/ml and 200 µl/ml. Only two CFS of bifidobacteria (b3 and b 4) had no MIC and MBC values with the concentrations under the current study. The b1, BL and BI strains showed highest antibacterial activity by MIC value with 100 conc. and by MBC value with 150 conc. Increased concentration levels of the cell-free extract (CFE) correlated with a decrease in MRSA viability. MRS broth medium (control) showed a high growth rate of MRSA without CFE.These results may provide a basis for alternative therapies for the treatment of MRSA superbug.