Susceptibility of Anaerobic Bacteria to Metronidazole: Relative Resistance of Non-Spore-Forming Gram-Positive Baccilli

Abstract

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 µg/ml, whereas 30% required ≥ 50 µg/ml. Considerable variation in susceptibility was observed among different genera and species of bacteria. Fusobacterium was most sensitive, followed by Clostridium, Bacteroides and Peptococcus, Peptostreptococcus, Veillonella and Addaminococcus, and non-sporeforming gram-positive bacilli. Bacteroides jragilis was more sensitive than other species of Bacteroides. Similarly, Clostridium perjringens was more susceptible than other species of Clostridium. While metronidazole appears to be a promising antimicrobial agent for infections caused by Fusobacterium, Clostridium, and B. Fragilis, therapy for infections with other anaerobic bacteria should be guided by in vitro tests of sensitivity.