Postantibiotic effects with Bacteroides fragilis determined by viable counts and CO2 generation

Objective: To study the postantibiotic effect (PAE) for Bacteroides fragilis after exposure to common anaerobic antimicrobials with two different methods, by viable counting and by measuring CO₂ generation in a BACTEC® blood culture system.
Method: Four strains of B. fragilis were exposed for 1,2 and 4 h to cefoxitin, chloramphenicol, clindamycin, imipenem or metronidazole at concentrations from 1 to 16 X MIC. The drugs were removed by dilution into BACTEC 7A® vials and growth determined with viability counts and CO₂ production.
Results: The durations of the PAEs determined by the two methods correlated well ( r =0.913, p <0.005). PAEs of up to 4–5 h were induced by imipenem and metronidazole with achievable concentrations and exposure durations. Chloramphenicol induced short or no PAEs, but cefoxitin and clindamycin induced PAEs up to 2 h with high AUC values. The imipenem PAEs and the short cefoxitin and clindamycin PAEs were dependent on AUC.
Conclusions: Significant PAEs against B. fragilis were induced by imipenem and metronidazole. Determining PAE by measuring CO₂ production is an accurate and less time-consuming alternative to the conventional method of viable counts.