Survival and metabolic activity of<i>lux</i>-marked<i>Escherichia coli</i>O157:H7 in different types of milk

Escherichia coli O157:H7 is a potentially lethal pathogen which has been responsible for several outbreaks of milk-borne illness in recent years. The objective of this study was to evaluate the survival and metabolic activity (indexed by bioluminescence) of a chromosomally lux -marked strain of Esch. coli O157:H7 in raw, pasteurized and microfiltered pasteurized milk at 4 and 20°C for up to 14 d. Results showed that the population of Esch. coli O157:H7 and its metabolic activity decreased in all samples during storage at 4°C, with no significant differences in numbers observed between the different milk types; but metabolic activity was significantly higher ( P <0·05) in the microfiltered pasteurized milk than that in raw milk. At 20°C, Esch. coli O157:H7 counts and cell activity peaked at day 2, and then declined progressively. At 20°C, survival and metabolic activity were significantly lower in raw milk compared with pasteurized milk. We conclude that storage temperature is more important in regulating the survival of Esch. coli O157 in contaminated milk than its origin/pre-treatment conditions.