Susceptibility of 358 clinical isolates of obligate anaerobes to metronidazole was determined by an agar-dilution technique. Only 66% of all isolates were inhibited by 6.25 mug/ml, whereas 30% required larger than or equal to 50 mug/ml. Considerable variation in susceptibility was observed among different genera and species of bacteria. Fusobacterium was most senstitive, followed by Clostridium, Bacteroides and Peptococcus, Peptostreptococcus, Veillonella and Acidaminococcus, and non-spore-forming gram-positive bacilli. Bacteroides fragilis was more sensitive than other species of Bacteroides. Similarly, Clostridium perfringens was more susceptible than other species of Clostriduim. While metronidazole appears to be a promising antimicrobial agent for infections caused by Fusobacterium, Clostrididium, and B.fragilis, therapy for infections with other anaerobic bacteria should be guided by in vitro tests of sensitiivity.

The comparative susceptibility of 622 recent clinical isolates of anaerobic bacteria to minocycline, doxycycline, and tetracycline was determined by an agar-dilution technique. In addition to Bacteroides fragilis, a variety of other anaerobic bacteria was resistant to achievable blood concentrations of tetracycline (55% inhibited by 6.25 gg/ml) and doxycycline (58% inhibited by 2.5 gg/ml). In contrast, minocycline was significantly more active (P < 0.05) than both doxycycline and tetracycline, and 70% of strains were inhibited by achievable blood concentrations of this antibiotic (2.5 lAg/ml). The enhanced activity of minocycline was particularly striking for Peptococcus asaccharolyticus, P. magnus, P. prevotii, Peptostreptococcus anaerobius, and Bacteroides melaninogenicus. Further evaluation of the clinical efficacy of minocycline against anaerobic infections is indicated.

In vitro susceptibility of 11 recent clinical isolates of Campylobacter fetus to 22 antimicrobial agents was determined by an agar dilution technique. Unlike most obligate anaerobic gram-negative bacilli, C. fetus isolates tested were relatively resistant to penicillin and cephalosporins, but exquisitely susceptible to tetracyclines and aminoglycosides. All strains were also inhibited at concentrations achievable in serum by clindamycin, chloramphenicol, metronidazole, carbenicillin, ticarcillin, and with rare exceptions, ampicillin. They were variably susceptible to lincomycin and erythromycin and highly resistant to vancomycin.

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