Antimicrobial in vitro susceptibility of actinomyces israelii and arachnia propionica.

46 reference strains and clinical isolated of Actinomyces israelli and 8 strains of Arachnia propionica--the causative microorganisms of actinomycosis in man--were tested for their in vitro susceptibility to penicillin, sulfaisodimidine, erythromycin, tetracycline, lincomycin, clindamycin, metronidazole and tinidazole by the agar dilution method. All strains were susceptible to benzylpenicillin (minimum inhibitory concentrations, MICs of less than or equal to 0.064 mug/ml). Many strains were resistant in vitro to sulfaisodimidine (MICs 4.0-32.0 mug/ml). Erythromycin, tetracycline, clindiamycin and lincomycin possessed in vitro activity at concentrations readily attainable in serum. Due to adverse side-effects associated with clindamycin and lincomycin therapy, erythromycin and tetracycline may be the best alternative drugs to penicillin in the treatment of actinomycosis. In contrast, in vitro resistance (MICs greater than or equal to 4.0 mug/ml) of the pathogenic actinomycetes to metronidazole and tinidazole implies that these drugs may not be valuable in the therapy of human actinomycosis.