Impact of different statistical methodologies on the evaluation of the in-vitro MICs for Bacteroides fragilis of selected cephalosporins and cephamycins.

The purpose of this study was to assess the effect of different MIC statistics on the in-vitro evaluation of cefoxitin, cefotetan, ceftizoxime, cefotaxime, desacetylcefotaxime, and cefotaxime:desacetylcefotaxime (1:1 ratio) against clinical isolates of Bacteroides fragilis from the Medical University of South Carolina. MICs were determined by the agar dilution method following NCCLS guidelines. Statistical analyses included arithmetic and geometric means, median, mode, MIC50, MIC90, and range. Differences between antimicrobial agents were determined using parametric and nonparametric methods. Consistent with previous in-vitro studies on anaerobes with these agents, a wide range in the MICs was observed. The arithmetic mean MIC was lowest for cefotetan, but the geometric mean MIC was lowest for ceftizoxime. Using the MIC90, cefotetan was at least a two-fold dilution more active than the other agents. After natural logarithmic transformation of the MIC data, analysis of variance with the Scheffé post-hoc test demonstrated that the MICs of ceftizoxime were significantly lower than those of cefoxitin (P < 0.001), cefotetan (P < 0.05), cefotaxime (P < 0.05), and desacetylcefotaxime (P < 0.001). Median and mode MICs were lowest for ceftizoxime and cefotaxime:desacetylcefotaxime. Using published breakpoints, cumulative percent susceptibility was similar for all agents studied. In this analysis of the antimicrobial susceptibility of B. fragilis in our institution, the in-vitro activity of the cephalosporins and cephamycins tested appeared to be very similar when all statistical data were evaluated. Since apparent in-vitro activity may be influenced by the statistic evaluated, we suggest that all MIC data be reported to allow for a more complete analysis of microbiological potency.