Use of pulsed-field gel electrophoresis to investigate a pseudo-outbreak of Bacillus cereus in a pediatric unit.

Bacillus cereus is a well-known cause of food poisoning. It also causes rare systemic infections, usually in immunocompromised patients. Dissemination of this species in hospitals had been reported. Most of these episodes were pseudo-outbreaks and were usually secondary to equipment or environmental contamination. We report here on the use of pulsed-field gel electrophoresis (PFGE) to analyze a pseudo-outbreak of B. cereus in a pediatric unit. Different restriction endonucleases had been tested, and SmaI was found to give the best result for PFGE. Among the 26 clinical isolates of B. cereus and the type strain of the species, 15 distinct PFGE patterns were distinguished. PFGE after DNA macrorestriction with SmaI could clearly differentiate between the epidemiologically related isolates and the unrelated isolates. Because the same epidemic strain of B. cereus was isolated from the settle plates which were exposed near the outlet of the ventilation system, the source of this pseudo-outbreak was suspected to be the unit's air filtration system. This is one of the first reports of the application of PFGE to the study of B. cereus, and this method is useful for epidemiological investigation.