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Abstract
Summary
A total of 450 samples of milk and some milk products including raw milk (75), kareish, Damietta cheese (55 samples each), Hard cheese, (70), cooking Butter, Yoghurt and Ice cream (55 samples each) were collected randomly from different localities in Assiut Governorate. The samples were examined bacteriologically for the isolation and identification of enterohaemorrhagic E.coli (serovar O157:H7).
E.coli O157:H7 was isolated from 1 (1.33%), 1 (1.8%), 3 (4.3%),
2 (3.6%), 2 (3.6%) and 1 (1.8%) of the examined raw milk, kareish cheese, hard cheese, butter, yoghurt and ice cream samples, respectively. However, the pathogen could not be detected in the examined Damietta cheese samples. The frequency distribution of the isolated E.coli O157:H7 strains, among the examined E.coli strains, were 5.56, 5.56, 8.82, 6.25, 25 and 7.69%, in raw milk, kareish cheese, hard cheese, butter, yoghurt and ice cream samples, respectively.
Polymerase chair reaction (PCR) proved to be rapid, accurate, sensitive and specific method for the detection of E.coli O157:H7 in regard to conventional, biochemical and serological methods, it reduced the time of identification to 6-8 h.
Additionally, the ability of E.coli O157:H7 to grow and survive during manufacture and storage of Damietta cheese made from milk containing 5 and 10% added sodium chloride was evaluated. Damietta cheese were laboratory prepared from pasteurized milk and inoculated with the previously isolated and identified E.coli O157:H7 to yield a concentration of 1012 cfu/g before addition of rennet. Then stored at room temperature (352C) in their salted whey and examined periodically for E.coli O157:H7 count, aerobic plate count, pH value, salt and moisture contents. The obtained results reveal that the pathogen undergo sudden decline during curd formation, from initial count of 7x1012 cfu/g. to reach 4x106 and 2x105/g of the examined curds containing 5 and 10% salts, respectively. Then cell population show substantial increase in number, then decreased but was able to survive until the end of the 3rd and 4th week of storage in cheese containing 5 and 10% salt, respectively. There was a decrease in pH value and moisture content of cheeses, while the salt contents (in water phase) increased gradually during the storage period.
Effect of freezing and deep-freezing on the survival of E.coli O157:H7 in ice cream was estimated, where ice cream were laboratory prepared and inoculated with the previously isolated and identified E.coli O157:H7, to yield a concentration of 105 cfu/g. The results showed that, at freezing temperature the cell populations were decreased in the first two days then returned to increase in the 3rd day, then started to decrease until the end of the experiment to 2.2x103 cfu/g. While, at deep freezing temperature the cell population were decreased at the first day only, and then show gradual increase in the 2nd, 3rd days and 1st week, then undergo gradual decrease till the end of the experiment to reach 2x103 cfu/g.
The effect of low pH developed during manufacture and refrigerated storage of yoghurt on the growth and survival of E.coli O157:H7 was also studied, where yoghurt was laboratory prepared, inoculated with the previously isolated and identified E.coli O157:H7 to yield a concentration of 109 cfu/g, E.coli O157:H7 counts and pH value were determined daily. The results indicated that E.coli O157:H7 was able to survive low pH developed during refrigerated storage of yoghurt, where its count show transient increase in the first day of storage, then undergo gradual decrease to reach 1x104 cfu/g. with pH value of 4.55 at the 9th day of storage. The decrease in count was parallel with lowering of pH value along the storage period.
The public health importance of E.coli O157:H7 as well as the recommended sanitary measures were discussed.