The minimal inhibitory concentrations of 45 antimicrobial agents were determined for 14 strains of Bacterium actinomycetem comitans (Actinobacillus actinomycetem-comitans). All the strains showed good susceptibility to tetracyclines and chloramphenicol, but not to lincomycins. Some strains were clearly resistant to penicillins, cephalosporins, and nitroimidazoles. The lowest minimum inhibitory concentrations were observed with tetracycline, rolitetracycline, methacycline, minocycline, chloramphenicol, and cotrimoxazole.

The minimal inhibitory concentrations of 45 antimicrobial agents were determined for 13 strains of Bacteroides melaninogenicus. All the strains showed good susceptibility to those agents known to be active against anaerobic gram-negative rods, such as tetracyclines and lincomycins. The lowest MICs were observed with lincomycin, clindamycin, methacycline, doxycycline, minocycline, and erythromycin. As expected, aminoglycosides were only poorly active, also some strains showed clear resistance to penicillins, cephalosproins (including the newer ones), and nitroimidazoles.

The minimal inhibitory concentrations of 18 newer antibiotics were determined for 33 strains of cutaneous propionibacteria, i.e., P.acnes, P. granulosum and P. avidum. All the strains showed high sensitivity to the penicillin and cephalosporin antibiotics tested. They were mostly resistant to aminoglycosides in therapeutically achievable concentrations, and nitroimidazole compounds were completely ineffective.

Reference data on the overall resistance patterns of anaerobic bacteria are needed since in individual clinical cases susceptibility testing is complicated by the long generation time and difficulties of test conditions. Benzylpenicillin and tetracyclines are the drugs of choice against Clostridium perfringens. Newer cephalosporins are also active, whereas other Clostridium species have been found resistant. Beta-lactam antibiotics show high activity against Peptococcaceae; resistance to clindamycin should be noted, however. The good in vitro activity of tetracyclines, clindamycin and erythromycin against Propionibacterium acnes could be confirmed in the systemic and local therapy of acne vulgaris. In contrast to gram-positive anaerobic bacteria, only some penicillin and cephalosporin derivatives are sufficiently active against gram-negative rods of the Bacteroides group. Sporadic resistance is seen to the newer tetrayclines, nitroimidazole compounds and to lincomycins. Nevertheless, these substances must be considered as standard therapeutics for infections caused by anaerobic gram-negative bacteria. It should be mentioned that cefoxitin may offer new therapeutic possibilities.