Purpose: The objective of this study was to model the effect of temperature, pH and water activity (aw) on the growth/no-growth response of non-O157 STEC.
Methods: A six-serogroup STEC (E. coli O26, O45, O103, O111, O121 and O145; ca. 6 log CFU/ml) cocktail was inoculated into brain heart infusion broth (BHI) at 48 combinations of pH (4.5, 5.0, 5.5, 6.0, 6.5, and 7.0) and aw (0.950, 0.956, 0.963, 0.971, 0.977, 0.984, 0.989, 0.993), and incubated at ten temperatures (6, 9, 12, 18, 24, 30, 37, 41, 45, and 47°C). After 62 days of incubation, growth was determined by turbidity and plating on tryptic soy agar, and the binary data (0 [no growth] or 1 [growth]) were fitted using non-linear logistic regression. The model was validated with independently generated data of the six serogroups of non-O157 STEC, a cocktail of E. coli O157:H7, and two panels of six-serogroup STEC cocktail.
Results: A non-linear, logistic regression model was developed to predict growth/no-growth boundary for non-O157:H7 STEC in a microbiological medium and validated in a beef system. Non-O157 STEC did not grow at temperature ≤ 6°C and ≥ 47°C, and the most tolerance to low pH and aw occurred at 25°C. Although slight difference on growth/no-growth response existed among serogroups of STEC, the developed model can be applied to all the STECs with an average concordance of 92.3%.
Significance: The developed model will be useful to the industry in designing food products with minimal risk of STEC growth.