To determine the roles of Bacillus cereus as a food contaminant and potential pathogen in pasteurized milk, the putrefactive and toxigenic activities of B. cereus in milk were studied at both 25 and 30°C. Vegetative cells of B. cereus rapidly increased to 107-108 CFU/ml and putrefied the milk. We found that B. cereus B-4ac produced a diarrheal enterotoxin during growth in milk and B. cereus NC 7401 produced a vacuolation factor. More than 2 × 107/ml of bacterial cells were required to detect the two toxins produced in the milk. However, no detectable toxin was produced by either strain during incubation in milk at 10deg;C.
Each of fifteen samples of rice-ball and sandwiches from fifteen convenience stores of three different companies was examined for viable bacterial counts, that is coliform group and Staphylococcus aureus immediately after the purchase and on the day of expiration. The viable bacterial counts among the rice-balls with salmon and egg sandwiches at 45 convenience stores showed agreat variation although the stores obtained the products from the same manufacturers. In most samples, there was no problem concerning taste 24 hours after the purchase, but the viable bacterial count after purchase and on the day of expiration was over 105 cfu/g in 22% of the samples. The values of coliform groups after the purchase were less at store C than those at stores A and B, and the count of organisms on the day of expiration was the least at store B. In the test of preservation, the concentration of viable bacterial counts in the foods did not change at 15°C for 25 hours, but increased to 106 cfu/g at 25°C. Growth and enterotoxin A production of Staphylococcus aureus at 25°C for 24 hours showed counts of 107 cfu/g and 40 ng/g in boiled egg, and 108 cfu/g and 20 ng/g in rice, respectively. Staphylococcus aureus was isolated with a count of 102 cfu/g from only one sample of egg sandwiches at store C.
For the surveillance purpose of microbial contamination of foods and utensils, we have devised a new method which is simple, inexpensive, and accurate enough. One of the basic ideas is to see color change of pH indicators in media due to the acid production from lactose by the growth of contaminating Coliform group. This test method was designated as SI-test (Sanitary Indicator test). These media were sensitive enough to detect the viable organisms of less than 10 in the model experiment with the standard strain of E. coli. A good correlation was obtained between viable count of the inoculate and the reaction time (the initiation of color change). This was possible within 24 hrs even when the inoculated media were left to stand at room temperature (around 30°C). Then, these media were subjected to the evaluation test with a total of 152 selected specimens including 90 food samples, 17 water and beverage samples, and 45 swabbed samples. The test was rewarded with satisfactory results promising the high efficiency in practical use. To make this method applicable even in the peripheral laboratories of rural areas, the thik paper disks absorbed with the concentrated media were prepared. The disks can be reconstituted by addition of sterized saline or water before use. When preserved in a refrigerator, the quality of the disks did not change for at least 180 days. In view of these results, we concluded that the methd as devised here is worthy of the practical use in the laboratories of limited facilities and equipments.