Survival and growth of Escherichia coli O157:H7 in roast beef and salami after exposure to an alkaline cleaner

Abstract

Survival and growth of wild-type (EDL 933) and rpoS-deficient (FRIK 816-3) strains of Escherichia coli O157:H7 after exposure to an alkaline cleaner for 2 min and inoculating into roast beef (pH 6.3) and hard salami (pH 4.9) at low (0.003 to 0.52 CFU/g) and high (0.69 to 31.5 CFU/g) populations were determined. Roast beef was stored at 4 and 12°C; salami was stored at 4, 12, and 20°C. At 4°C, untreated cells of both strains showed greater reductions in populations in salami than in roast beef during a 21-day storage period. Populations of treated and untreated cells recovered from roast beef and salami stored at 4°C on tryptic soy agar were significantly (P ≤ 0.05) higher than on sorbitol MacConkey agar, indicating that a portion of the cells was injured. Treated and untreated cells grew in roast beef at 12°C. Growth of treated cells of the FRIK 816-3 strain in roast beef at 12°C was significantly slower than that of the EDL 933 strain. Populations of both strains decreased at different rates in salami stored at different temperatures (20°C > 12°C > 4°C). E. coli O157:H7 strain EDL 933 grew more rapidly at 20°C in a slurry (pH 5.97) prepared from stored salami (17 days at 20°C) on which Penicillium chrysogenum had grown than in a slurry (5.23) prepared from salami showing no mold growth. Within 2 to 3 days, populations were ca. 3 log CFU/ml higher in slurry made from infected salami than in control salami. Results indicate that treatment of E. coli O157: H7 with an alkaline cleaner for 2 min does not impair resuscitation and growth of surviving cells in roast beef at 12°C. Cross protection of cells exposed to an alkaline cleaner against subsequent stress conditions imposed by roast beef and salami stored at 4°C was not evident in either of the test strains.