Pseudo-outbreak of Mycobacterium abscessus Infection Caused by laboratory contamination

Abstract

Objective: To investigate the cause(s) of an increased incidence of clinical cultures growing Mycobacterium abscessus at a hospital in Florida.

Design: Outbreak investigation.

Setting: University-affiliated, tertiary-care hospital.

Methods: A site visit was done during the first week of September 2006. We reviewed the medical records of patients from whom M. abscessus was recovered during the period from January 1, 2003, through June 30, 2006. We collected environmental samples from various sites and evaluated specimen processing procedures in the microbiology laboratory. Isolates of M. abscessus recovered from the environment and from 12 randomly selected patients who sought medical care in 2006 were compared by pulsed-field gel electrophoresis (PFGE). Follow-up case surveillance was continued through March 31, 2007.

Results: Specimens from 143 patients obtained from various anatomical sites grew M. abscessus on culture in 2005-2006, compared with specimens from 21 patients in 2003-2004. The 12 isolates from patients that were selected for molecular typing had indistinguishable PFGE patterns. Observations revealed no major breaches in the processing of mycobacterial specimens in the laboratory. Isolates grew only after prolonged incubation (mean +/- SD, 45 +/- 15 days) in test tubes containing diagonally oriented Middlebrook and Cohn 7H10 agar or Lowenstein-Jensen medium. Environmental samples obtained from the inside of the specimen incubator grew M. abscessus on culture. A test tube containing diagonally oriented, uninoculated Middlebrook and Cohn 7H10 agar that was incubated in the same incubator as clinical specimens grew M. abscessus with a PFGE pattern that matched the pattern of the patient isolates. Cases of M. abscessus infection decreased to baseline after the hospital changed suppliers of mycobacterial media and cleaned the incubator.

Conclusions: Although the source was never confirmed, our investigation suggests that this was a pseudo-outbreak of M. abscessus infection that resulted from contamination of mycobacterial cultures during incubation. Our findings emphasize the need for guidance on the disinfection of specimen incubators.