Resistance patterns in clinical isolates of pathogenic Actinomyces species

Objectives: Actinomyces spp. are commensals that may occasionally invade deep tissue structures, causing difficult-to-treat and disfiguring lesions. Information on antimicrobial resistance patterns is limited to observations from two previous studies. Therefore, we examined antimicrobial resistance patterns in clinical isolates of Actinomyces spp.

Methods: In this retrospective assessment of antimicrobial resistance patterns, we identified 392 Actinomyces spp. at a tertiary care centre from January 2008 to December 2014. MICs of various antimicrobial agents, including ampicillin/sulbactam, meropenem, clindamycin, metronidazole and vancomycin for anaerobic actinomycetes, were obtained by Etest. For aerobic actinomycetes, imipenem, cefotaxime, amikacin, linezolid, moxifloxacin, trimethoprim/sulfamethoxazole and clarithromycin were tested. MIC results were interpreted based on guidelines published by the CLSI (formerly NCCLS).

Results: Actinomyces meyeri was predominantly isolated and accounted for 34% of all Actinomyces spp. identified, followed by Actinomyces turicensis with 23%. Actinomyces neuii is considered to be a rare Actinomyces sp., but accounted for 8% of isolates. Antimicrobial susceptibility testing of isolates showed that the Actinomyces spp. were almost uniformly susceptible to β-lactam antimicrobials (with and without β-lactamase inhibitors), carbapenems, tetracyclines and vancomycin. In contrast, Actinomyces spp. isolates were almost uniformly resistant to metronidazole.

Conclusions: β-Lactam antimicrobial agents remain the first choice, whereas metronidazole should be avoided, in the treatment of actinomycosis. Reasonable alternatives for treatment are tetracyclines and carbapenems.