An in vitro pharmacodynamic investigation was conducted to explore whether the area under the concentration time curve from 0 to 24 h (AUC0–24)/MIC ratio could predict fluoroquinolone performance against Bacteroides fragilis. An in vitro model was used to generate kill curves for trovafloxacin (TVA) and levofloxacin (LVX) at AUC0–24/MIC ratios of 1 to 406 against three strains of B. fragilis (ATCC 25285, ATCC 23745, and clinical isolate M97-117). TVA and LVX were bolused prior to the start of experiments to achieve the corresponding AUC0–24/MIC ratio. Experiments were performed in duplicate over 24 h and in an anaerobic environment. Analyses of antimicrobial performance were conducted by comparing the rates of bacterial kill (K) using nonlinear regression analysis with 95% confidence intervals. Statistical significance was defined as a lack of overlap in the 95% confidence limits generated from the slope of each kill curve. For both TVA and LVX, K was maximized once an AUC0–24/MIC ratio of ≥40 was achieved and was not further increased despite a 10-fold increase in AUC0–24/MIC from approximately 40 to 400 against all three strains of B. fragilis. No significant differences were found in K between AUC0–24/MIC ratios of approximately 40 to 200. In experiments where AUC0–24/MIC ratios that were ≥ 5 and ≤ 44 were conducted, 64% demonstrated regrowth at 24 h. Resistant strains were selected in 50% of those experiments, demonstrating regrowth, which resulted in increased MICs of two- to 16-fold for both TVA and LVX. Regrowth did not occur, nor were resistant strains selected in any studies with an AUC/MIC that was > 44. Our findings suggest that fluoroquinolones provide antibacterial effects against B. fragilis in a concentration-independent manner associated with an AUC0–24/MIC ratio of ≥40. Also, the potential for the selection of resistant strains of B. fragilis may increase with an AUC0–24/MIC ratio of ≤44.

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