Fate of pGFP-bearing Escherichia coli O157 : H7 in ground beef at 2 and 10degrees C and effects of lactate, diacetate, and citrate

Although beef has been implicated in the largest outbreaks of Escherichia c oli O157:H7 infection in the United States, studies on the fate of this pat hogen have been limited. Problems in such studies are associated with detec tion of the pathogen at levels considerably lower than the levels of the co mpeting microorganisms. In the present study, a green fluorescent protein-e xpressing E, coli O157:H7 strain was used, and the stable marker allowed us to monitor the behavior of the pathogen in ground beef stored aerobically from freshness to spoilage at 2 and 10 degrees C, In addition, the effects of sodium salts of lactate (SL) (0.9 and 1.8%), diacetate (SDA) (0.1 and 0. 2%), and buffered citrate (SC) (1 and 2%) and combinations of SL and SDA we re evaluated. SC had negligible antimicrobial activity, and SL delayed micr obial growth, while SDA and SL plus SDA were most inhibitory to the total-a erobe population in the meat. At 2 degrees C, the initial numbers of E. col i O157:H7 (3 and 5 log(10) CFU/g) decreased by similar to 1 log(10) CFU/g w hen spoilage was manifest (>7 log(10) CFU of total aerobes/g), irrespective of the treatment. There was no decline in the numbers of the pathogen duri ng storage at 10 degrees C. Our results showed that the pathogen was resist ant to the salts tested and confirmed that refrigerated meat contaminated w ith the pathogen remains hazardous.