Enumeration of Escherichia coli O157 in Outbreak-Associated Gouda Cheese Made with Raw Milk

In this article, we discuss the enumerative analysis for Escherichia coli O157 in two raw milk Gouda cheese products (A and B), implicated in an outbreak of 29 cases of E. coli O157:H7 illness that occurred across Canada in 2013. Samples were enumerated for E. coli O157 by most probable number (MPN) over a period of 30 to 60 days after the end of the outbreak. Samples (55.55 g) of product A (n = 14) were analyzed at 146 to 180 days postproduction. E. coli O157 was isolated from six samples at 19.9 to 44.6 MPN/kg. The E. coli O157 concentration of product A estimated from the results of all 14 samples was 9.5 MPN/kg. Samples (55.55 g) of product B (n = 20) were analyzed at 133 to 149 days postproduction. E. coli O157 was isolated from four samples at 19.9 MPN/kg. The E. coli O157 concentration of product B estimated from the results of all 20 samples was 3.7 MPN/kg. Analysis of a 305-g sample of product A (n = 1) stored at 4 degrees C until 306 days postproduction revealed that the E. coli O157 concentration had declined to 3.6 MPN/kg. E. coli O157 could not be isolated from 555-g samples of product B (n = 5) after 280 days postproduction. The physicochemical parameters (pH, water activity, percent moisture, and percent salt) of both cheese products were found to be in the normal range for this type of product. The results of this study demonstrate that E. coli O157 could not replicate during storage at 4 degrees C in the products tested but was capable of survival following aging and prolonged storage. This indicates that, if contaminated, the minimum 60-day aging period, which is required for raw milk Gouda cheeses, is not sufficient in all cases to ensure that the product does not contain viable cells of E. coli O157. The results also indicate that samples sizes greater than 100 g may be required to reliably detect E. coli O157 in cheese products associated with outbreaks.