Survival and Growth of Escherichia coli O157:H7 in Unpasteurized and Pasteurized Milk

Escherichia coli O157:H7, which causes hemorrhagic colitis and hemolytic uremic syndrome, has been responsible for several outbreaks associated with consumption of unpasteurized and improperly processed pasteurized milk, and yogurt. Studies were conducted to determine the survival and growth characteristics of this pathogen in unpasteurized milk and pasteurized milk (3.5% fat, 2% fat, skim) at 5,8, 15, and 22°C for up to 28 days. Two levels of inocula (10³ and 10⁵ CFU/ml) of a mixture of five nalidixic acid-resistant E. coli O157:H7 strains were used. E. coli O157:H7 did not grow at 5°C and decreased by 1.6 to 2.0 log CFU/ml in 28 days. Growth occurred at 8°C, with an approximately 1- to 2-log CFU/ml increase within the first 4 days. About a 3- to 5-log CFU/ml increase in E. coli O157:H7 populations was observed at 15°C within the first 3 days. In 3 pasteurized milk samples, E. coli O157:H7 continued to grow to populations of greater than 1.0 × 10⁸ CFU/ml at day 7 and remained constant during the remainder of the incubation period. At 22°C, the pH decreased rapidly to less than 4.0 within 4 days and E. coli O157:H7 decreased to undetectable populations within 14 days. E. coli O157:H7 grew more slowly (P < 0.01) in unpasteurized milk, which had a higher initial microbial population, than in pasteurized milks at 8, 15, or 22°C, likely because of antagonistic activity from preexisting bacteria. No significant differences (P> 0.05) in survival or growth of E. coli O157:H7 were observed among the pasteurized milk samples, regardless of fat concentration, at all temperatures throughout the study. The data indicate that temperature abuse during shipping and handling can result in significant growth of E. coli O157:H7. Holding milk at ≤5°C is recommended to prevent growth of this pathogen.