Comparison of kill-kinetic studies with agar and broth microdilution methods for determination of antimicrobial activity of selected agents against members of the Bacteroides fragilis group.

Kill-kinetic studies often are used to determine the rate and degree of killing of aerobic bacteria by antimicrobial agents. Few studies, however, make use of this method for determining antimicrobial activity against anaerobic bacteria. To evaluate kill-kinetic studies for anaerobes, kill-kinetic studies were performed for selected antimicrobial agents against members of the Bacteroides fragilis group and compared with MICs obtained by using a reference agar dilution method and a broth microdilution method. Results of the kill-kinetic studies showed that the degree of killing over a 24-h test period was related to the MIC for the test organism. In general, the higher the MIC of an antimicrobial agent for a test organism, the less the killing observed. In addition, these studies demonstrate subtle differences in bactericidal activity at various concentrations of the antimicrobial agents, which cannot be determined by agar or broth dilution methods. Kill-kinetic studies are a useful addition to dilution methods for the evaluation of antimicrobial agents against anaerobes.