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The in vitro activity of cefoxitin was compared with those of metronidazole and clindamycin against 322 strains of anaerobic bacteria collected from several hospitals during 1982 and tested by an agar dilution method. Metronidazole and cefoxitin inhibited at least 89% of strains tested, whereas clindamycin was less active.

The "in vitro" susceptibility to roxithromycin and three other macrolides of 236 anaerobes isolated from clinical samples in 1984/1985 was determined by an agar-dilution method on Wilkins Chalgren medium. 90% of Gram positive cocci were susceptible to both roxithromycin and josamycin (MIC less than 1 mg/l, whereas 1 mg/l erythromycin and 2 mg/l spiramycin were able to inhibit respectively 46 and 86% of the same tested strains. No resistance to the four macrolides was observed among Eubacterium, propionibacterium and Bifidobacterium. Two C. perfringens strains and one C. difficile strain were resistant to all four macrolides, while 97% of Clostridium sp. strains were inhibited by 4 mg/l erythromycin, josamycin or roxithromycin. Against Gram positive anaerobes, roxithromycin was equal or superior to erythromycin and spiramycin. At a concentration of 4 mg/l, roxithromycin inhibited 82% of B. fragilis strains. Roxithromycin and josamycin were more active against Gram negative bacilli that erythromycin and spiramycin. Macrolides had no effect on Fusobacterium strains. In this study, 4 mg/l roxithromycin inhibited 217 of the 236 anaerobic strains investigated (92%).

The in vitro activity of four new quinolones against 355 obligate anaerobes was investigated. MICs were determined using the reference method of Sutter et al. [14]. The four fluorinated quinolones tested differ from nalidixic acid which is inactive on most anaerobes except for some Clostridium perfringens strains. All strains tested were inhibited by 64 and 16 mg/l pefloxacin and ofloxacin respectively. Except for Bacteroides, all strains were inhibited by 64 mg/l norfloxacin or enoxacin. Clostridium strains other than C. perfringens exhibited heterogeneity; C. difficile and C. ramosum had the highest MICs. Bacteroides fragilis proved less susceptible to fluorinated quinolones than the other obligate anaerobes. Ofloxacin showed the greatest activity, with 4 mg/l inhibiting 100% of C. perfringens and 83% of all anaerobes investigated.

The activity of metronidazole combined with ofloxacin was investigated by the checkerboard method in liquid medium against 60 obligate anaerobes. The bacteriostatic effect of the combination was assessed by calculating the FIC index. Two metronidazole resistant strains of Propionibacterium acnes (MIC greater than 32 mg/l) were inhibited by 0.125 mg/l of this former antibiotic in presence of an ofloxacin concentration equal to half the value of the MIC. On the 58 other anaerobic strains, the combination of metronidazole plus ofloxacin had an additive bacteriostatic effect (30 strains) or a synergistic effect (26 strains). No antagonism was noted with any strain. For selected anaerobic or mixed infections the combination of metronidazole and ofloxacin may be useful. Killing curves demonstrated that, under good anaerobic conditions, metronidazole acted rapidly against Bacteroides fragilis. At a concentration of 8 mg/l, a decrease of log10 of the population was observed after 2 hours. After a contact ranging from 1 to 8 hours, depending of the investigated strains, a bactericidal effect was observed with metronidazole.

Minimal inhibitory concentrations of 105 anaerobic Gram negative bacilli were determined by two-fold dilution in Wilkins - Chalgren agar following the reference method proposed by Sutter et al. No resistance was observed with thienamycin, which is especially active against the Bacteroides fragilis group (MIC 50 = 0.125 mg/l). Among the beta-lactams investigated, cefoxitin and moxalactam proved more active than ceftizoxime and ceftriaxone, while cefotiam exhibited poor activity. While most Fusobacteria were susceptible to studied beta-lactams, some Fusobacterium varium demonstrated resistance to cephalosporins and cephamycins .

The in vitro activity of imipenem, a new penem antibiotic, was determined against 210 clinical Gram-positive anaerobic isolates and compared with the activities of metronidazole, clindamycin, cefoxitin, moxalactam, ceftizoxime, ceftriaxone and cefotiam. All investigated strains were inhibited by a 4-mg/l concentration of imipenem. Cefoxitin demonstrated good activity against most strains with exception of some Clostridium difficile and Clostridium ramosum strains. Cephalosporins were classed in decreasing activity order as follows: cefoxitin moxalactam ceftriaxone ceftizoxime cefotiam. Metronidazole had better activity against Clostridium spp. than against non-sporulated bacilli. Clindamycin resistance rates were superior to 10% for most groups, with the exception of Clostridium perfringens. Actinomyces and Bifidobacterium. Imipenem was the most potent inhibitor, as 81% and 95% of tested strains were inhibited at concentrations of 0.25 and 0.5 mg/l respectively.