The minimal inhibitory concentrations (MIC) of six penicillins (ampicillin, carbenicillin, ticarcillin, piperacillin, mezlocillin, and Bacteroides species, particularly Bacteroides fragilis, are relatively resistant to penicillin G and ampicillin (12). Because ticarcillin, carbenicillin, and cefoxitin are only moderately active against B. fragilis (12), serious infections involving B. fragilis often require the use of either clindamycin or chloramphenicol with their recognized serious toxicity. Therefore, the in vitro activities of some newer semisynthetic penicillins, i.e., mezlocillin, piperacillin, and Bay k 4999, which are substituted in the a-amino position of ampicillin, were compared with those of carbenicillin, ticarcillin, and ampicillin against Bacteroides species. Bay k 4999 [6(D-2-(3-furfurylidenamino-2-oxoimidazolidine-1-carboxamido) -2-(4-hydroxy-phenyl)-acetamido) penicillanic acid (15) In vitro susceptibility tests. The minimal inhibitory concentrations (MICs) of the penicillins (except mecillinam) were determined by an agar dilution method. The antibiotics were diluted in twofold steps in Mueller-Hinton broth. One milliliter of each dilution of antibiotic was added to 9 ml of Mueller-Hinton agar to obtain final antibiotic concentrations of 0.98 to 500 ug/ml. Bacteria, grown for 24 h in brain heart infusion broth, were inoculated onto the surface of these freshly prepared plates, using the replicating device of Steers et al. (11

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