ANTIBIOTIC SUSCEPTIBILITY TESTING FOR CHLAMYDIA-TRACHOMATIS USING FLOW-CYTOMETRY

The aim of this study was to compare the effectiveness of two methods of antibiotic susceptibility testing performed on Chlamydia trachomati s-infected cells: a flow cytometric detection method and the standard method, which consists of a microscopic reading of minimal inhibitory concentration (MIG), The L2 reference strain and 13 clinical strains i solated from six patients presenting recurrent infections were tested. McCoy cells infected with an inoculum of 10(5) inclusion forming unit s (IFU)/ml were incubated with serial dilutions of doxycycline, ofloxa cin, and erythromycin. Mean fluorescence intensity (MFI) of cells was determined by flow cytometry after staining of chlamydial inclusions w ith an anti-Chlamydia fluorescent monoclonal antibody, The end-point v alues determined by flow cytometry and microscopic reading were equiva lent but presented the same imprecision, Calculation of the inhibitory concentration 50 (IC50) by now cytometry, defined as the antibiotic c oncentration required to reduce the drug-free control MFI by 50%, allo wed a more objective and precise evaluation of antibiotic activity tha n MIG. Moreover, IC50 values were reproducible, independent of the ant ibiotic dilution series tested, and could be used to compare the in vi tro efficiency of various drugs on C. trachomatis. No resistant strain was found among the 13 clinical isolates of C. trachomatis tested, (C ) 1998 Wiley-Liss, Inc.