[Determination of sensitivity of biofilm-positive forms of microorganisms to antibiotics]

Nosocomial infections caused by biofilm-positive microorganisms are a serious therapeutic problem. In the biofilm, microorganisms are protected against adverse effects of the external environment, including the action of antibiotics. It is well known that the values of minimum inhibitory concentrations (MIC) determined for planktonic forms do not correspond to the actual concentrations of antibiotics necessary for the eradication of bacteria in a biofilm. The purpose of the study was to propose a method of determining minimum biofilm inhibitory concentrations (MBIC) and minimum biofilm eradication concentrations (MBEC) and to compare these values with MIC values. Biofilm-positive strains of Staphylococcus epidermidis were cultured so as to form a biofilm layer on polystyrene pegs. The biofilm on the pegs was then exposed to the action of antibiotics and after 18 hours we determined the minimum biofilm inhibitory concentration (MBIC). The evaluation of minimum biofilm eradication concentrations was done colorimetrically from the metabolic activity of surviving cells. MBIC and MBEC values were many times higher than MIC values. We selected such a duration of the biofilms cultivation on the pegs of the plate, which ensured that the number of bacterial cells corresponded to standard MIC assessment. The MBEC values established in our study indicate that the currently used concentrations of tested antibiotics cannot be used in monotherapy for an efficacious eradication of a biofilm. The MBEC determination is a far more laborious and time-consuming method than the determination of MIC, but the use of plates with pegs facilitates the handling of biofilms. The advantage of our method is the possibility of standardization of the size of the inoculum and thus of the whole MBEC assessment.