Evaluation of serotyping, biotyping, plasmid banding pattern analysis, and HEp-2 vacuolation factor assay in the epidemiological investigation of Bacillus cereus emetic-syndrome food poisoning

To assess the value of the plasmid banding patterns, the vacuolation factor (VF) assay, biotyping, and serological typing as epidemiological markers for strains of Bacillus cereus causing emetic-syndrome illness, 43 isolates from five outbreaks and an additional 76 strains isolated in food-poisoning outbreaks caused by other enteric pathogens were examined by these techniques, and the results were compared. Thirty-eight (88%) of the 43 outbreak strains produced vacuolation responses in HEp-2 cells and were all starch-hydrolysis negative. The other 76 strains associated with outbreaks caused by other food-poisoning bacteria gave all negative VF production results except four strains, and 56 (74%) of these strains produced positive reactions in starch hydrolysis tests. Starch hydrolysis emerged as a convenient screen for VF production, because no starch hydrolysis-positive strains produced VF. With the exception of one isolate, all 38 VF-positive isolates from emtic-syndrome outbreaks were serotype H.1. Isolates from four of the five outbreaks revealed identical plasmid banding patterns in each outbreak, whereas only three of eight serotype H. 1 strains from the fifth outbreak exhibited indistinguishable plasmid banding patterns. These results suggest that the plasmid banding pattern analysis may be of value in discriminating between isolates of the same serotype, and establishing if an outbreak arises from a common food source. In conclusion, the vacuolation factor assay combined with the plasmid banding patterns proved to be a valuable tool for the epidemiological investigation of emetic-syndrome outbreaks caused by B. cereus. Moreover, these methods are particularly useful for laboratories that do not have ready access to serotyping facilities.