The effect of salt concentration and pH on the survival and growth of E. coli O157:H7 in white cheese and trypticase soy broth

Abstract

The survival and growth of E. coli O157:H7 in white cheese during storage at 7 oC and the effect of NaCl concentration (0.5 to 3%) and pH (3.5 to 6.5) in trypticase soy broth on the survival and growth of E. coli O157:H7 at 7 oC and 20 oC was investigated. The number of E. coli O157:H7 was determined by surface plating decimal dilutions of the artificially contaminated white cheese after 0, 5, 10, 15 and 20 days storage at 7 oC. Trypticase soy broth of pH 3.5, 5 and 6.5 or with a salt concentration of 0.5%, 2% and 3% was inoculated with 105 E. coli O157:H7 and examined after 0, 1 and 5 days storage at 7oC and 20 oC on Fluorocult E. coli O157:H7 agar plates following incubation in air at 37 oC. E. coli O157:H7 survived in the white cheese at 7oC and was reduced by 1 log after 10 days and then remained at the same value up to day 20. In the same time the pH fell from 4.02 to 3.35 after 10 days and then slightly increased. The concentration of salt increased from 0.52% to 0.73%. At a pH level of 3.5 in the trypticase soy broth at 7oC E. coli O157:H7 survived but did not grow and the number of pathogens was reduced by 1 log after 5 days. At pH 5 and 6.5 the number of E. coli O157:H7 in the trypticase soy broth increased at both 7 oC and 20 oC. The number of E. coli O157:H7 in the broth at pH 5 kept at 7oC, increased by 1.7 log, after one day and 3.5 log after five days. At pH 6.5 in the trypticase soy broth kept at 20 oC the populations of E. coli O157:H7, increased by 3.5 log, after one day and 4.5 after five days. The organism grew (up to 2 log increase) in the trypticase soy broth containing 0.5%, 2% and 3% of salt at 20oC but did not grow at 7 oC and decreased by 0.5 log, at 0.5% salt, 1 log at 2% salt and 2 log at 3% salt, after five days. This study indicates that E. coli O157:H7 may persist in white cheese. If this pathogen should contaminate low salt products with a pH greater than 4.0, the survival could be sufficient to cause illness. The risk of an outbreak is highly dependent on pH, salt content and storage temperature of the white cheese, but since inactivation is slow and marginal for this type of cheese, the initial level of contamination is critical in determining the safety of white cheese.