A Pseudo-Outbreak of Clostridium sordellii Related to Cross-Contamination in a Clinical Microbiology Laboratory

Background: Clostridium sordellii is rarely associated with human infections and is a very unusual isolate in clinical cultures.  Infection Control personnel were notified of an unusual increased number of C. sordellii from clinical specimens in April, 2009 and an investigation was begun.

Objective: To investigate the potential sources of the outbreak, consequences for affected patients (pts), and to recommend changes in laboratory practice to prevent recurrence.

Methods: The case definition was any pt with C. sordellii isolated between April 1 and May 30, 2009. Microbiology laboratory records for the past year were searched for further cases. Medical records were reviewed for presence of clinical infection and epidemiologic links. C. sordellii were isolated and identified in clinical specimens using standard microbiological methods. Pulsed field gel electrophoresis (PFGE) was performed on 6 pt isolates, the laboratory ATCC reference strain, and on one isolate from a clinically infected pt isolated ~15 months prior to this outbreak. An investigation of laboratory procedures for handling anaerobic specimens was performed.

Results: Seven clinical isolates of C. sordelli from 6 pts were identified, 4 from wounds and 2 from sinus aspirates.  No further clinical isolates were identified by review of past cultures over the past year. The pts did not share any epidemiological risk factors. None of the pts had clinical symptoms of C. sordellii infection. Two of the 6 pts received antibiotics, one of whom developed diarrhea.  The isolates from 6 pts and the ATCC reference strain had identical PFGE patterns; the prior clinical isolate had a distinct pattern.  Review of laboratory procedures revealed no overt contamination events or breaks in specimen handling protocols. Isolates were not associated with media lot numbers or types of reagents. However, about one month before the first C. sordelli was isolated, the procedure for processing anaerobic cultures was changed. During this time there was also an increase in teaching activities with students handling the reference ATCC strain. Interventions included a thorough disinfection of the anaerobe chamber. Commonly used supplies were replaced. Handling of specimens and controls to minimize release of spores was re-emphasized. The laboratory instituted use of individually packaged anaerobic culture media. No further C. sordellii have been isolated in subsequent 6 months.

Conclusions: A pseudo-outbreak of C. sordellii was temporally related to a change in microbiology laboratory handling of anaerobic specimens, resulting in contamination of clinical specimens with an ATCC  reference strain. Clinicians and epidemiologists must maintain a high degree of suspicion for a pseudo-outbreak when an increase isolation of an uncommon pathogen, such as C. sordellii, is reported.