SENSITIVITIES OF COLONIES AND SUSPENSIONS OF ACTINOMYCES ISRAELII TO PENICILLINS, TETRACYCLINES, AND ERYTHROMYCIN

Susceptibility of Actinomyces israelii to antibiotics, sodium hypochlorite and calcium hydroxide

Actinomyces israelii has been repeatedly implicated as a cause of failure of endodontic therapy. This study investigated the antimicrobial effect of antibiotics as well as intracanal medicaments, sodium hypochlorite solution and calcium hydroxide, on this important pathogen. Growth of A. israelii was inhibited by low concentrations of antibiotics, yet high concentrations were not bactericidal for A. israelii over 1 week. When A. israelii was exposed for 2-6 weeks at concentrations equivalent to clinical serum levels, the antibiotics were lethal. The results reveal a species-specific antibiotic tolerance for A. israelii. Both sodium hypochlorite solution and calcium hydroxide were found to be highly effective in killing A. israelii.

Special methodological problems in antibiotic susceptibility testing of fermentative actinomycetes

Knowledge on the antibiotic suceptibility of pathogenic fermentative actinomycetes is still fragmentary and contradictory, which is one reason why even today the chemotherapy of human actinomycosis may still present problems. Complete and reliable data on the in vitro sensitivity of these pathogens can be obtained, however, when standardized and specially adapted methods of susceptibility testing are employed. Special adaptation should cover the general technique of in vitro testing, as well as the choice of test media, the preparation of inocula, incubation methods and method of reading results. Provided that all factors are taken into account which might cause difficulties in test reproducibility and therapeutic relevance of the results, in vitro tests appear to contribute to the revision and updating of treatment regimes for actinomycosis. Our findings indicate that, apart from ampicillin the clinical efficacy of which has already often been proved, mezlocillin, cefoxitin, clindamycin, tetracyclines, rifampicin and erythromycin may be recommended as alternative drugs which would seem to offer good chances of success.

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.

Susceptibility of pathogenic actinomycetes to antimicrobial compounds

Current interest in antimicrobial susceptibility testing of anaerobic pathogens and recent recognition that actinomycetes other than Actinomyces israelii may cause actinomycosis in man prompted this in vitro survey of 74 strains of actinomycetes, representing seven species. Minimum inhibitory concentrations (MICs) for 24 antimicrobials were determined by inhibition of gross colonial enlargement in semisolid antibiotic agar after incubation at 37 C for 48 h under anaerobic conditions. Erythromycin and rifampin were the most active drugs in vitro (MICs of 0.008 to 0.25 mug/ml), although a small number of non-israelii strains were conspicuously more resistant to the latter (MICs >0.5 mug/ml). Penicillin G, cephaloridine, minocycline, and clindamycin were also very active in vitro (MICs of 0.03 to 1.0 mug/ml); for a few non-israelii strains the MICs of clindamycin were 2.0 to 8.0 mug/ml. MICs of cephalothin, ampicillin, lincomycin, tetracycline, doxycycline, and chloramphenicol were well within a therapeutic range for all strains of A. israelii and most other species, although the MIC of lincomycin against a few non-israelii strains and of tetracycline and doxycycline against the majority of these strains was 2.0 to 8.0 mug/ml. Oxacillin, dicloxacillin, and cephalexin were less active in vitro, particularly against strains other than A. israelii. Most non-israelii species were not suppressed by 125 mug of metronidazole per ml, which concentration inhibited all strains of A. israelii; otherwise, there were no antimicrobial susceptibility differences among the species tested. Aminoglycoside activity was negligible.