Evaluation of broth microdilution susceptibility results for anaerobic organisms by use of a rapid direct colony...

A direct colony inoculum suspension procedure was compared with the overnight suspension procedure recommended for the broth microdilution anaerobic commercial system (Micro-Media Systems, Inc., Potomac, Md.). Six National Committee for Clinical Laboratory Standards-recommended quality control organisms, Bacteroides fragilis ATCC 25285, Clostridium perfringens ATCC 13124, Bacteroides thetaiotaomicron ATCC 29741, Bacteroides vulgatus ATCC 29327, Peptococcus magnus ATCC 29328, Peptococcus asaccharolyticus ATCC 29743, and 50 anaerobic clinical isolates were tested against seven commonly tested antimicrobial agents. The minimum inhibitory concentration results from each suspension method (using the quality control organisms) were identical in 18 (78%) instances, and within +/- 1 log2 dilution in 96% of the comparisons. Results with the fresh clinical isolates also compared satisfactorily with the overnight procedure (97% were identical or within one dilution). The Wilkins-Chalgren test medium failed to support the growth of most anaerobic gram-positive cocci and Bacteroides melaninogenicus strains.