In vitro activity of azlocillin, metronidazole and its hydroxy metabolite against anaerobes. Bacteriostatic synergism studies

The in vitro inhibitory activity of azlocillin (Securopen), metronidazole (Cloni) and its hydroxy metabolite was determined against 27 gram-negative and 13 gram-positive species of anaerobes by means of agar dilution tests. The 63 anaerobic strains were also tested against the pairs azlocillin-metronidazole and azlocillin-hydroxy metabolite by means of agar dilution tests. Gram-negative species (Bacteroides spp., Fusobacterium spp. etc.) were inhibited by 0.125-256 micrograms/ml azlocillin, 0.01-4 micrograms/ml metronidazole and 0.01-4 micrograms/ml hydroxy metabolite. With gram-positive anaerobes (Clostridium spp., Peptococcaceae etc.) the MIC ranges were 0.125-4 micrograms/ml for azlocillin, 0.03-1 micrograms/ml for metronidazole and 0.125-2 micrograms/ml for the hydroxy metabolite. A synergistic effect was observed exclusively with gram-negative anaerobes (Bacteroides fragilis, B. thetaiotaomicron, B. disiens etc.). There were few instances of antagonism, likewise with gram-negative species. The preponderant combination effect against gram-positive anaerobes was addition. In view of the broad antiaerobic spectrum of azlocillin, the present in vitro findings do not preclude combined therapy with metronidazole in cases of anaerobic-aerobic poly-bacterial infections.