Susceptibility of anaerobic bacteria to metronidazole: relative resistance of non-spore-forming gram-positive baccilli.

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.