MATERIALS AND METHODS
Bacterial cultures. The clinical stool specimens were collected from patients in food poisoning outbreaks, transported to the laboratory, and analyzed in less than 8 h according to standard procedures (6). The specimens were added to 100 ml of alkaline peptone water and incubated at 37°C for 7 to 8 h. A loopful of the enrichment culture was streaked on thiosulfate-citrate-bile salt-sucrose agar (Difco Laboratories, Detroit, Mich.) and incubated at 37°C for 18 to 24 h. The