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 pe riod to demonstrate antibiotic action is necessary because such tests rely on growth (or the lack of it), when the microorganisms are incuba ted in the presence of the antibiotic. In an effort to improve antibio tic susceptibility testing, we developed a flow cytometric technique f or Staphylococcus aureus in which antibiotic action is determined by m onitoring drug-induced membrane potential changes, using the potential -sensitive dye 3,3'-dipentyloxacarbocyanine iodide. Three ATCC referen ce strains of S. aureus and 13 unknown strains of the same microorgani sm were tested for susceptibility to penicillin G and oxacillin. Our r esults indicate that susceptibility of S. aureus to these antibiotics can be measured reliably at 90 min after addition of the antibiotic, a nd the results are comparable to those obtained with conventional susc eptibility tests. (C) 1993 Wiley-Liss, Inc.