• Vol. 44 No. 1
  • 15 January 2015

Anaerobic Bacteraemia Revisited: Species and Susceptibilities



Introduction: This retrospective study was performed to evaluate the frequency of anaerobic bacteraemia over a 10-year period, and to provide updated antibiotic susceptibilities for the more clinically relevant anaerobes causing blood stream infection.

Materials and Methods: Data were retrieved from the laboratory information system for the period 2003 to 2012. During this time, blood cultures were inoculated in Bactec™ Plus vials (BD, USA) and continuously monitored in the Bactec™ 9000 blood culture system (BD, USA). Anaerobic organisms were identified using commercial identification kits, predominantly API 20 A (bioMérieux, France) supplemented with Vitek ANC cards (bioMérieux, France) and AN-Ident discs (Oxoid, United Kingdom). A representative subset of isolates were retrieved from 2009 to 2011 and antimicrobial susceptibilities to penicillin, amoxicillin-clavulanate, clindamycin, imipenem, moxifloxacin, piperacillin-tazobactam and metronidazole were determined using the Etest method.

Results: Anaerobes comprised 4.1% of all positive blood culture with 727 obligate anaerobes recovered over the 10-year period, representing a positivity rate of 0.35%. The only significant change in anaerobe positivity rates occurred between 2003 and 2004, with an increase of 0.2%. The Bacteroides fragilis group (45%) were the predominant anaerobic pathogens, followed by Clostridium species (12%), Propioniobacterium species (11%) and Fusobacterium species (6%). The most active in vitro antibiotics were imipenem, piperacillintazobactam, amoxicillin-clavulanate and metronidazole, with susceptibilities of 95.0%, 93.3%, 90.8% and 90.8% respectively. Resistance was high to penicillin, clindamycin and moxifloxacin. However, there were apparent differences for antibiotic susceptibilities between species.

Conclusion: This study indicates that the anaerobes comprise a small but constant proportion of bloodstream isolates. Antibiotic resistance was high to some antibiotics, but metronidazole, the beta-lactam/beta-lactamase inhibitors and carbapenems retained good in vitro activity.

Over the last 2 decades, the importance of anaerobic bacteraemia has undergone various shifts in opinion. Early studies in the 1970s reported that anaerobes accounted for 2% to 20% of bacteraemia. However, by the mid 1980s, multiple centres reported declining rates of anaerobic bacteraemia, and several authors suggested that the routine inoculation of anaerobic blood cultures should be discontinued. The rationale for this argument was that the inoculation of 2 aerobic blood cultures would increase the yield of obligate aerobes, and that antimicrobial susceptibilities of anaerobes were also predictable. The late 1990s saw several studies reporting a resurgence of anaerobic bacteraemia, although this was also clearly dependent on local epidemiology.

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