Rapid flow cytometric antibiotic susceptibility assay for Staphylococcus aureus

Conventional techniques for antibiotic susceptibility testing usually require 24 h or more to produce accurate results. This long waiting period to demonstrate antibiotic action is necessary because such tests rely on growth (or the lack of it), when the microorganisms are incubated in the presence of the antibiotic. In an effort to improve antibiotic susceptibility testing, we developed a flow cytometric technique for Staphylococcus aureus in which antibiotic action is determined by monitoring drug-induced membrane potential changes, using the potential-sensitive dye 3,3'-dipentyloxacarbocyanine iodide. Three ATCC reference strains of S. aureus and 13 unknown strains of the same microorganism were tested for susceptibility to penicillin G and oxacillin. Our results indicate that susceptibility of S. aureus to these antibiotics can be measured reliably at 90 min after addition of the antibiotic, and the results are comparable to those obtained with conventional susceptibility tests.