Antimicrobial susceptibility of clinical isolates of anaerobic bacteria in Ontario, 2010-2011.

The local epidemiology of antimicrobial susceptibility patterns in anaerobic bacteria is important in guiding the empiric treatment of infections. However, susceptibility data are very limited on anaerobic organisms, particularly among non-Bacteroides organisms. To determine susceptibility profiles of clinically-significant anaerobic bacteria in Ontario Canada, anaerobic isolates from sterile sites submitted to Public Health Ontario Laboratory (PHOL) for identification and susceptibility testing were included in this study. Using the E-test method, isolates were tested for various antimicrobials including, penicillin, cefoxitin, clindamycin, meropenem, piperacillin-tazobactam and metronidazole. The MIC results were interpreted based on guidelines published by Clinical and Laboratory Standards Institute. Of 2527 anaerobic isolates submitted to PHOL, 1412 were either from sterile sites or bronchial lavage, and underwent susceptibility testing. Among Bacteroides fragilis, 98.2%, 24.7%, 1.6%, and 1.2% were resistant to penicillin, clindamycin, piperacillin-tazobactam, and metronidazole, respectively. Clostridium perfringens was universally susceptible to penicillin, piperacillin-tazobactam, and meropenem, whereas 14.2% of other Clostridium spp. were resistant to penicillin. Among Gram-positive anaerobes, Actinomyces spp., Parvimonas micra and Propionibacterium spp. were universally susceptible to β-lactams. Eggerthella spp., Collinsella spp., and Eubacterium spp. showed variable resistance to penicillin. Among Gram-negative anaerobes, Fusobacterium spp., Prevotella spp., and Veillonella spp. showed high resistance to penicillin but were universally susceptible to meropenem and piperacillin-tazobactam. The detection of metronidazole resistant B. fragilis is concerning as occurrence of these isolates is extremely rare. These data highlight the importance of ongoing surveillance to provide clinically relevant information to clinicians for empiric management of infections caused by anaerobic organisms.