Disk diffusion, agar dilution and the E-test for susceptibility testing of Corynebacterium jeikeium

OBJECTIVE: The susceptibilities to penicillin, tetracycline, erythromycin, gentamicin, vancomycin and teicoplanin of 58 strains of Corynebacterium jeikeium were assessed by disk diffusion and agar dilution reference methods. METHODS: Zone sizes and minimal inhibitory concentrations (MIC) by agar dilution were interpreted using the ranges in the NCCLS tables for organisms other than Haemophilus, Neisseria gonorrhoeae, and Streptococcus pneumoniae. RESULTS: By agar dilution, 14%, 88%, 17% and 26% of the 58 isolates were susceptible to penicillin, tetracycline, erythromycin, and gentamicin, respectively. Using the breakpoints for Listeria monocytogenes, all strains showed concordant results for penicillin by disk diffusion. Discrepancies in the interpretative categories by disk diffusion were found in four cases (two very major and two minor) for tetracycline, in nine (two very major, two major, and five minor) for erythromycin, and in 1 case (very major) for gentamicin. All 58 strains were susceptible to vancomycin and teicoplanin by agar dilution and disk diffusion. The overall agreement of interpretative disk diffusion for all six antibiotics was 95.9%. In addition, all strains were susceptible to both glycopeptides by E-test. However, for vancomycin the MIC results in 58.6% were two log2 dilutions and in 1.7% more than two log2 dilutions higher by E-test than by agar dilution, whereas for teicoplanin agreement within one log2 dilution was 100%. CONCLUSIONS: Further evaluation of methodologies of disk diffusion is required to obtain a better agreement for erythromycin and tetracycline. The criteria of the NCCLS for interpretation of disk diffusion are adequate for susceptibility testing of C. jeikeium to penicillin, gentamicin, vancomycin and teicoplanin.