A flow cytometric study of antibiotic-induced damage and evaluation as a rapid antibiotic susceptibility test for methicillin-resistant Staphylococcus aureus.

Flow cytometry using the anionic membrane potential-sensitive fluorescent probe, bis-(1,3-dibutylbarbituric acid) trimethine oxonol (DiBAC4(3)), enabled assessment of antibiotic-induced membrane perturbation in five clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA) and two antibiotic-sensitive reference strains, NCTC 6571 and 8325-4, after establishment of steady-state growth in liquid cultures inoculated from single colonies. Flow cytometric indications of the enhanced DiBAC4(3) uptake after treatment with vancomycin at 0.1, 1, 4 and 10 x MIC showed excellent comparison with viability losses quantified as cfu on solid agar in MRSA isolate QC. The antibiotic susceptibility patterns to benzylpenicillin, methicillin and vancomycin for all isolates used in this study could be determined in 2-4 h from an overnight plate culture. This technique thus provides a rapid and reproducible antibiotic sensitivity test which may be applicable in routine clinical practice.