Behavioral Patterns of the Isolated Bacillus cereus Strains from Milk and Some Milk Products in Yoghurt and Damietta Cheese

Abstract

The Present study was undertaken to assess the prevalence of B. cereus in milk and some milk Products and its survival in yoghurt and Damietta cheese. A total of 100 samples of raw milk, Pasteurized milk, Damietta cheese and yoghurt (25 for each) were collected from supermarkets in Assiut city and were examined for isolation of B. cereus. It was isolated from 24, 32, 68 and 20% of the tested samples, respectively. All the isolated strains were confirmed positive for B. cereus except one strain for each raw milk and Damietta cheese. 14.7%, 85.3, 50.0 and 76.5% of the confirmed strains were carried Ces, Nhe, Hbl and cytK genes. Concerning its survival in yoghurt, there was a significant difference between the effect of yoghurt and acidophilus yoghurt on B. cereus growth, where the organism remained viable to the 9^th day in yoghurt and failed to be isolated at the 5^th day in yoghurt with probiotics. In case of Damietta cheese, 2 and 5% salt were more favorable for the pathogen growth, while 10% salt was inconvenient for its growth. In addition, the inhibitory effect of thyme essential oil was more obvious than the effect of rosemary EO, where the bacterial population was decreased to a count of 2.05 and 2.46 log cfu/ g. at the 2^nd week in case of addition of 1.65% thyme EO and could not be detected at the 4^th week, whereas the organism could survive to the 6^th week in case of rosemary. In conclusion, strict hygienic measures must be implemented during the manufacture of dairy products and addition of biopreservatives to control B. cereus growth is substantial.