Tracking laboratory contamination by using a Bacillus cereus pseudoepidemic as an example

From 1 March to 31 May 1990, Bacillus cereus was recovered from 24 of 5,534 (0.49%) blood cultures and 22 of 1,088 (2.02%) other body fluid cultures. The rarity of this organism as a pathogen and comparison with previous baseline rates led to the conclusion that it was a pseudoepidemic involving some form of culture contamination. Generalized precautions taken without specific knowledge of the contaminant source reduced the recovery rate of the organism. Recovery rates for the organism returned to normal baseline prevalence after environmental cultures and epidemiological analysis led to the sterilization of a contaminated water bath used for boiling thioglycollate media. The problems encountered in this investigation are examined, and a systematic approach to clinical laboratory epidemiology is outlined.