Determination of MICs for staphylococci using the API ATB quinolone and API ATB macrolide systems

The determination of minimal inhibitory concentrations (MICs) is cumbersome, but remains necessary in certain cases. We tested the two ATB MIC experimental strips (Biomérieux SA), of which each contains 4 antimicrobials of the same class. These strips can be read automatically. The MIC quinolone strip contains nalidixic acid, pefloxacin, ofloxacin and ciprofloxacin, whereas the MIC macrolide (lincosamide-streptogramin) strip contains erythromycin, clindamycin, lincomycin, and pristinamycin. In order to evaluate these strips, 102 S. aureus and 63 coagulase negative staphylococci were used. Correlation coefficients for these MICs (micrograms/ml) and disk diffusion inhibition zone diameters (mm) were nalidixic acid -0.59, pefloxacin -0.95, ofloxacin -0.95, ciprofloxacin -0.91, erythromycin -0.98, clindamycin -0.96, lincomycin -0.96, and pristinamycin -0.64. Using the Biomic system (Giles Scientific USA), the same zone diameters were converted to MICs (micrograms/ml). Rates of agreement (+/- 1 dilution) between ATB MICs and Biomic MICs were nalidixic acid 96 p. cent, ciprofloxacin 98 p. cent, erythromycin 99 p. cent and clindamycin 98 p. cent. Rates of agreement between MICs for the same strains determined using agar dilution and ATB MICs were nalidixic acid 93 p. cent, pefloxacin 100 p. cent, ciprofloxacin 99 p. cent, ofloxacin 94 p. cent and erythromycin 96 p. cent. The ATB MIC strips are an easy-to-use tool for MIC determination and their composition is well-suited to the study of phenotypic resistance and detection of low-level resistance.