Post-antibiotic effect in Bacteroides fragilis group.

Post-antibiotic effect (PAE) is the transient suppression of bacterial growth after brief antimicrobial exposure. While numerous reports have described PAE with aerobic and facultative microorganisms, virtually no studies have been conducted with anaerobic isolates. Intraabdominal isolates of the Bacteroides fragilis group were exposed for one hour to antibiotic (cefoxitin, cefotetan, and imipenem) concentrations two to four times the minimal inhibitory concentration (MIC). Post-antibiotic effect was described as the difference between the time required for microbial growth in the test versus the control to increase one Log10 above the quantitation observed immediately after drug removal. Bacteroides fragilis, B. ovatus, B. thetaiotaomicron and B. vulgatus exhibit PAEs for all test compounds. The time intervals for the PAEs were strain variable and ranged from six to 50 hours. No PAE was demonstrated with B. distasonis strains by the broth dilution technique. The results suggest that brief high dose exposure of some members of the B. fragilis group to anaerobe active beta-lactams produces a prolonged suppression in growth. In theory, a prolonged PAE could influence the dosage regimentation of selective antibiotics.

Comparative in vitro beta-lactam activity against aerobic and anaerobic surgical isolates.

The comparative in vitro activity of cefoxitin, piperacillin, cefotetan, and ceftizoxime was compared against 843 surgical microbial isolates. All compounds exhibited excellent activity against the streptococcal isolates. Antimicrobial activity was poor for Staphylococcus aureus and S. epidermidis. Cefotetan activity against the Enterobacteriaceae was comparable to ceftizoxime (greater than 95% susceptible). Resistance rates of 0, 1, 5, and 2% were observed with cefoxitin, piperacillin, cefotetan, and ceftizoxime against the anaerobic cocci and anaerobic gram-positive non-spore-forming rods. Ninety-six and ninety-nine percent of the clostridial strains were susceptible to cefotetan and piperacillin, respectively. Piperacillin, cefotetan, and ceftizoxime exhibited similar activity against Bacteroides fragilis (resistance less than 10%). Cefotetan and cefoxitin exhibited poor activity against Bacteroides distasonis, B. ovatus, and B. thetaiotaomicron. Antimicrobial activities were comparable for the four drugs against other Bacteroides and Fusobacterium species. The results demonstrate that all four compounds exhibited broad antimicrobial activity against facultative and obligate anaerobic surgical isolates from intraabdominal and soft tissue infections.