Undetected Bacillus pseudo-outbreak after renovation work in a teaching hospital.

A 602-bed capacity hospital underwent complete renovation from 1999 to 2004. In April 2005, the Infection Control Team was informed of the occurrence of three consecutive cases of Bacillus cereus bacteremia in a ward for patients with hematologic malignancies. A retrospective analysis of patients with Bacillus isolates was initiated. We found more Bacillus cereus isolates from blood samples in 2004 compare to the preceding years. Swab samples were collected in the particular ward from the surface of a working desk, filter unit of the air-conditioners, entrance of air inlet ducts, exit of the air outlet ducts and three-way valves of the particular ward under the consideration of iatrogenic contamination. Towels and gowns used in the ward were examined. Dens dust was noted in the filter of the air-conditioner and inlets/outlets of the air-ventilation system of the ward. Bacillus cereus was isolated from the dust, and from cleaned towels and gowns. PFGE fingerprinting differed among four patients' sample. We considered the present case as an undetected Bacillus cereus pseudo-outbreak that lasted for about one year after the renovation work of the hospital. We also considered that filters of the HVAC-system and towels and gowns were probable sources of the outbreak.