Survival of enterohemorrhagic Escherichia coli O157:H7 during the manufacture and curing of cheddar cheese

Abstract

The ability of enterohemorrhagic Escherichia coli O157:H7 to survive a standard Cheddar cheese manufacturing process and subsequent curing was determined. Two treatments with added E. coli O157:H7 were designed with target levels 1 x 103 CFU/ml and 1 CFU/ml of cheese milk. Cheese samples were analyzed for E. coli O157:H7 during manufacture at 14, 28, 42, 60, and 74 days, and at 28-day intervals thereafter until the organism could no longer be detected using direct plating or enrichment in two successive samples. Typical colonies on 3M Petrifilm® E. coli Count Plates were counted as presumptive E. coli O157:H7 and were confirmed with the 3M Petrifilm® Test Kit-HEC for hemorrhagic E. coli O157:H7. When no E. coli O157:H7 were detected in the cheese with the Petrifilm® plates, a 25-g sample of cheese was enriched in modified EC broth with novobiocin to detect viable E. coli O157:H7. Cheese made with 103 CFU/ml of milk showed a 2- log-unit reduction after 60 days of ripening, with viable E. coli O157:H7 still being detected in 25 g of cheese after 158 days. Cheese made with 1 CFU/ml of milk showed a reduction in E. coli O157:H7 to 1 or < 1 CFU/g in 60 days, with no E. coli being detected in 25 g of cheese at 158 days. However, both treatments resulted in the survival of E. coli O157:H7 during manufacture and for more than 60 days of curing at 2.75 to 3.76% salt in the moisture phase.