Rapid method for detection of minimal bactericidal concentration of antibiotics

We developed a test for rapid determination of the minimal bactericidal concentration (MBC) of antibiotics. MBC is determined by calculating the relative proportion of live and dead bacteria by means of two fluorescence dyes: Syto 9 (which detects only live bacteria) and Propidium iodide (which labels live and dead bacteria). Testing was performed in micro-titer plates. Determination of bacterial viability was performed after exposure to antibiotics for 4 h. Reference strains and clinical isolates of Escherichia coli and Pseudomonas aeruginosa were tested. Fluorescence-based determination (MBC/F) and reference methods (MBC/L) were comparable (± 1 Dilution step), except for E. coli with piperacillin and cefotaxime, which differed up to two steps. Bactericidal activity against E. coli/P. aeruginosa was observed: Meropenem (100%/100%), gentamicin (100%/9.3%), ciprofloxacin (93.3%/86.7%), cefotaxime (E. coli only: 86.7%) ceftazidime (P. aeruginosa only: 80%), piperacillin (56.6%/76.7%). The calculation of MBC has been achieved by graphical extrapolation and mathematical approximation method.