Antimicrobial in Vitro Susceptibility of Actinomyces Israelii and Arachnia Propionica

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Abstract
46 reference strains and clinical isolates of Actinomyces israelii 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 ≤0.064 μg/ml). Many strains were resistant in vitro to sulfaisodimidine (MICs 4.0–32.0 μg/ml). Erythromycin, tetracycline, clindamycin 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 ≥4.0 μg/ml) of the pathogenic actinomycetes to metronidazole and tinidazole implies that these drugs may not be valuable in the therapy of human actinomycosis.