Fate of Nonpathogenic and Enteropathogenic Escherichia coli During the Manufacture of Colby-like Cheese

Abstract

Pasteurized whole milk was artificially contaminated with 100 to 1000 Escherichia coli/ml and was used to manufacture Colby-like cheese. Some cheeses were made so their composition differed from that of normal Colby cheese. Cheeses were cut in half and stored at 3°C and 10°C. E. coli was enumerated by surface-plating of samples on Trypticase Soy Agar (TSA) with an overlay of Violet Red Bile Agar (VRB). E. coli increased by 100 to 1000-fold, depending on the strain, to about 1 × 106/g of curd, in most instances, by the end of the cook (3.5–3.9 h). After this point numbers of E. coli in cheeses generally decreased over a period of weeks. One strain of enteropathogenic E. coli (EEC) could not be detected after 4 weeks, and another (in all but one instance) after 6 weeks. However, EEC in one lot of cheese persisted at numbers in excess of 1 × 103/g after 12 weeks of refrigerated storage. EEC survived at low levels (<350/g) for many weeks in one instance. Cheeses of poor quality (high moisture and pH) were made to assess the effects of improper manufacture on survival of E. coli. In these cheeses, E. coli eventually reached numbers in excess of 1 × 108/g and persisted for many weeks at high numbers. Survival of E. coli in Colby-like cheese appeared to be influenced by pH, salt and temperature; pH seemed to have the greatest effect on survival of the bacterium.