DETECTION OF Escherichia coli IN DAIRY AND MILK-RELATED PRODUCTS USING DIRECT PLATING AND THE 3-STAGE METHOD

Abstract

Escherichia coli is a frequent foodborne contaminant that negatively affects the quality and safety of dairy products. The present study aimed to examine the presence of E. coli in dairy and milk-related products using two detection methods namely, "direct plating" and "3-stage method". Sixty-nine samples of yoghurt, Domiati cheese, Kariesh cheese, UHT milk, dried skim milk, dried ice-cream, dried whey proteins, creamchantee, infant milk formulas, and "sahlab" were collected from 3 Egyptian governorates and examined for the presence of E. coli. A total of 250 suspected E. coli isolates could be cultured from these samples and subjected to biochemical identification that characterized 35 of these isolates as potential E. coli cultures. Further biochemical testing of these 35 cultures using the Microbact MGNB-12A miniaturized identification system confirmed their belonging to the E. coli species. These isolates were variably recovered from yoghurt (8 isolates), Kariesh cheese (12 isolates), UHT milk (1 isolate), dried ice-cream (2 isolates), creamchantee (1 isolate), infant milk formulas (5 isolates), and sahlab (6 isolates). The 3-stage method showed higher efficiency in detecting E. coli in these products than direct plating. This was attributed to the ability of the 3-stage method to detect injured cells generated on exposure to sub-lethal stress factors including those applied during food manufacture and preservation. The 3-stage method outlined in this study is, therefore, recommended for reliable detection of E. coli in dairy and milk-related products.

DEVELOPING AN EFFECTIVE THIN AGAR LAYER METHOD FOR THE DETECTION OF INJURED CELLS OF Escherichia coli IN MILK

“…Seven isolates of E. coli recovered from dairy and milk-related products by Yassin et al (2011) were used in this study. E. coli isolates were grown on tryptone soy agar (TSA) (Oxoid, Basingstoke, UK).…”

“…Previous work in our laboratory has compared the efficacy of direct plating on a selective, differential medium with that of a 3-stage method to detect E. coli in 69 samples of dairy and milk-related products (Yassin et al, 2011). The 3-stage method showed higher efficiency in detecting E. coli in these products than direct plating.…”