Survival of Enterohemorrhagic Escherichia Coli O157:H7 During the Manufacture and Curing of Cheddar Cheese

The ability of enterohemorrhagic Escherichia coli O157:H7 to survive a standard Cheddar cheese manufacturing process was determined using active lactic mesophilic starter culture and subsequent curing. This strain of E. coli can survive under more adverse intrinsic and extrinsic conditions than other characterized E. coli strains and Salmonella species. Two treatments, each consisting of five replications, were designed utilizing a cocktail of three strains of E. coli O157:H7. The target for treatment one was 1,000 Colony Forming Units per mL (CFU/mL) of cheese milk; treatment two was 1 CFU/mL of cheese milk. The level of salt used in the cheeses (1.09 to 1. 43 % ) was in the lower range of commercial usage to insure a percent salt in the moisture phase which should be favorable for the growth of E. coli O157:H7. Samples of milk, whey, curd, and finished cheese were taken during and after cheese manufacture for composition analysis and enumeration of E. coli O157:H7. 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 E. coli O157:H7 could no longer be detected using direct plating or enrichment in two successive samples. Red colonies producing gas on 3M Petrifilm E. coli Count Plates were counted as presumptive E. coli O157:H7 and were confirmed as such with the 3M Petrifilm Test Kit - HEC for Hemorrhagic E. coli O157:H7. When no CFU/g were detected with the Petrifilm plates, enrichment of 25 g of cheese in modified EC broth with novobiocin was done to detectviable E. coli O157:H7. Results from both treatments indicate that this organism can survive and even grow during the Cheddar cheese manufacturing process. After pressing, the cheeses had 27,000 CFU/g and 5 CFU/g, respectively. Treatment one showed a two log reduction in CFU/g after 60 days of ripening with viable E. coli O157:H7 still being detected in 25 g of cheese after 158 days. Treatment two showed a reduction in E. coli O157:H7 numbers to 1 or/g in 60 days with none being detected in 25 g of cheese at 158 days. However, both treatments permitted the survival of E.coli O157:H7 for more than 60 days of curing at 2. 75 to 3. 76% salt in the moisture phase (SMP)