COMPARISON OF E-TEST WITH BROTH MICRODILUTION AND DISK DIFFUSION FOR SUSCEPTIBILITY TESTING OF CORYNEFORM BACTERIA

The susceptibilities of 135 coryneform bacteria isolated from clinical samples to ampicillin (AMP), cephalothin (CR), cefoxitin (FOX), cefot axime (CTX), erythromycin (E), ciprofloxacin (CIP), tetracycline (TE), amikacin (AK), vancomycin (VA), and rifampin (R) were determined by d isk diffusion, broth microdilution, and the E-test. The following spec ies (number of isolates in parentheses) were included: Corynebacterium urealyticum (30), Corynebacterium minutissimum (20), coryneform CDC g roup ANF-1 (20), Corynebacterium striatum (20), Corynebacterium jeikei um (15), coryneform CDC group I2 (8), Listeria monocytogenes (7), Cory nebacterium xerosis (5), and other coryneform bacteria (10). Agreement within one twofold dilution between the E-test and broth microdilutio n was 31% (VA), 64% (AK), 71% (CTX), 77% (FOX and CIP), 79% (TE), 84% (AMP), 87% (E), and 88% (CR and R). For the 1,350 combinations of micr oorganisms and antimicrobial agents, 85 (6.3%) discrepancies in interp retive category were found (4.2% minor, 1.2% major, and 0.9% very majo r). Seventy (5.1%) disagreements in interpretive category were found b etween disk diffusion and the E-test (3.8% minor, 0.4% major, and 0.9% very major), and 85 (6.3%) disagreements were found between microdilu tion (reference method) and disk diffusion (4.2% minor, 0.5% major, an d 1.5% very major). MICs obtained with the E-test were highly reproduc ible, No category discrepancy was observed for VA, despite quantitativ e results. Considering interpretive categories, there is a good overal l agreement between the three methods studied here, but further evalua tion of current methodologies for susceptibility testing is required w hen considering coryneform bacteria and determination of quantitative activity of antimicrobial agents.