The influence of an extracellular polysaccharide, comprised of colanic acid, on the fate of Escherichia coli O157:H7 during processing and storage of stirred yogurt

The goal of this study was to determine the role of an extracellular polysaccharide (EPS), comprised of colanic acid (CA), in assisting the cells of Escherichia coli O157:H7 in surviving the processing and storage of stirred yogurt. A wild-type E. coli O157:H7 strain W6-13, its CA-deficient mutant M4020, and a mixture of the two strains (1:1; WM mixture) were inoculated into pasteurized, homogenized whole milk prior to fermentation or into fermented yogurt before stirring. The yogurts made from the inoculated milk and un-inoculated controls were assayed twice a week for total plate counts, as well as the populations of E. coli O157:H7 and starter cultures on tryptic soy agar (TSA), sorbitol MacConkey agar (SMAC), as well as MRS and M17 agar, respectively, during a three-week storage period at 4 °C or 15 °C. The results indicated that the survival of W6-13 vs. M4020, W6-13 vs. the WM mixture, and M4020 vs. the WM mixture in stirred yogurt as influenced by processing and storage conditions were not statistically different (P>0.05). The stirring step during yogurt processing might have detached the EPS from the surfaces of E. coli O157:H7 cells. The protective effect of the EPS towards the cells of E. coli O15:H7 during subsequent processing and storage was therefore, not observed.