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 (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.