Persistence of Escherichia coli 0157 : H7 in dairy fermentation systems

We examined (i) the persistence of Escherichia coli O157:H7 as a postpasteu rization contaminant in fermented dairy products; (ii) the ability of E. co li O157:H7 strains with and without the general stress regulatory protein, RpoS, to compete with commercial starter cultures in fermentation systems; and (iii) the survival of E, coli O157:H7 in the yogurt production process. In commercial products inoculated with 10(3) CFU/ml, E, coli O157:H7 was r ecovered for up to 12 days in yogurt (pH 4.0), 28 days in sour cream (pH 4. 3), and at levels >10(2) CFU/ml at 35 days in buttermilk (pH 4.1). For the starter culture competition trials, the relative inhibition of E. coli O157 :H7 in the experimental fermentation systems was, in decreasing order, ther mophilic culture mixture, Lactobacillus delbrueckii subsp. bulgaricus R110 alone, Lactococcus lactis subsp. lactis D280 alone, Lactococcus lactis subs p. cremoris D62 alone, and Streptococcus thermophilus C90 alone showing the least inhibition. Recovery of the rpoS mutant was lower than recovery of i ts wild-type parent by 72 h or earlier in the presence of individual starte r cultures, No E. coil O157:H7 were recovered after the curd formation step in yogurt manufactured with milk inoculated with 10(5) CFU/ml. Our results show that (i) postprocessing entry of E. coli O157:H7 into fermented dairy products represents a potential health hazard; (ii) commercial starter cul tures differ in their ability to reduce E. coli O157:H7 CFU numbers in ferm entation systems; and (iii) the RpoS protein appears to most effectively co ntribute to bacterial survival in the presence of conditions that are moder ately lethal to the cell.