ANTIBIOTIC SUSCEPTIBILITY TESTING (AGAR DISK DIFFUSION AND AGAR DILUTION) OF CLINICAL ISOLATES OF ENTEROCOCCUS-FAECALIS AND ENTEROCOCCUS-FAECIUM - COMPARISON OF MUELLER-HINTON, ISO-SENSITEST, AND WILKINS-CHALGREN AGAR MEDIA

Forty-two isolates of Enterococcus faecalis and 56 isolates of Enteroc occus faecium, including 8 vancomycin-resistant strains, were examined for comparative susceptibility to 27 antimicrobial drugs with the aga r dilution method, employing Mueller-Hinton (MHA), Iso-Sensitest (ISTA ), and Wilkins-Chalgren (WCA) agar. The Bauer-Kirby agar disk diffusio n method was used to comparatively test 24 of the agents in parallel. The enterococci yielded better growth on ISTA and WCA. However, WCA co mpletely antagonized co-trimoxazole and, though less, fosfomycin. Impo rtantly, WCA slightly reduced the activities of teicoplanin (minimal i nhibitory concentrations, MICs, raised up to twofold) and vancomycin ( MICs raised two- to fourfold) against enterococci and staphylococcal q uality control strains. Therefore, WCA was judged unsuitable for susce ptibility testing of enterococci. For E. faecalis no discrepancies bet ween agar dilution MICs and inhibition zone diameters were encountered with augmentin, ampicillin, ampicillin-sulbactam, chloramphenicol, mu pirocin, oxacillin, teicoplanin, and co-trimoxazole. Overall, MHA yiel ded fewer very major (category I) and major (category II) discrepancie s than ISTA. However, numerous minor (category III), slight (category IV), minimal (category V), and/or negligible (category VI) discrepanic es were encountered with ciprofloxacin, doxycycline, erythromycin, fos fomycin, fusidic acid, meropenem, ofloxacin and rifampin, With respect to E. faecium, only cefotaxime, mupirocin, oxacillin, and teicoplanin yielded nondiscrepant results. Several very major (I) and major (II) discrepancies were observed with augmentin, ampicillin, ampicillin-sul bactam, doxycycline, fusidic acid, imipenem, and penicillin G. Minor d iscrepancies (categories III-VI) were particularly numerous with augme ntin, chloramphenicol, ciprofloxacin, doxycycline, and piperacillin. T he largest numbers of negligible (VI) discrepancies were noted with fo sfomycin, fusidic acid, and ofloxacin. It is recommended to test one c ephalosporin (cefuroxime or the like) in parallel for educational purp oses and to exclude fosfomycin, fusidic acid, and rifampin from test b atteries because of the wide scatter of test results. The large number of minimal (V) discrepancies of ciprofloxacin against E.faecalis, the numerous minor (III) and slight (IV) discrepancies of chloramphenicol against E. faecium, and the not insignificant number of very major (I ) and minor (III) discrepancies observed with meropenem against isolat es of E. faecalis necessitated proposals for new disk intermediate sus ceptibility criteria.