EVALUATION OF ALAMAR COLORIMETRIC MIC METHOD FOR ANTIMICROBIAL SUSCEPTIBILITY TESTING OF GRAM-NEGATIVE BACTERIA

The Alamar (Alamar Biosciences, Inc., Sacramento, Calif.) colorimetric antimicrobial susceptibility testing method is a new approach to the determination of broth microdilution MICs. The method uses a color ind icator to detect growth of microorganisms within the wells of a microd ilution tray. The color changes can be read visually or with a fluorom eter. The system contains growth and sterility control wells and 20 an timicrobial agents per MIC tray with eight twofold dilutions for each antimicrobial agent. We tested 186 multiresistant, gram-negative bacte rial isolates against 33 antimicrobial agents and compared the results to those obtained by agar dilution. Categorical agreement for all age nts was 90.9% and ranged from 78.2% for ampicillin-sulbactam to 98.1% for amikacin. Percent agreement for MIC results (within +/-1 log(2) di lution) was 91.0% for all agents and ranged from 69.1% for gentamicin to 97.9% for ciprofloxacin. Most of the disagreements were with the pe nicillins and cephalosporins for beta-lactamase-producing strains. The Alamar MIC system is very easy to read visually and appears to be a s atisfactory addition to currently used MIC determination methods.