Behaviour of the Shiga toxin-producing Escherichia coli O157:H7 and O26:H11 in dairy products, in static and dynamic conditions for predictive model validation

Abstract

In this study, the kinetics of two Shiga-toxin-producing Escherichia coli (STEC) strains (one O157:H7 strain and one O26:H11 strain) were investigated in different dairy matrices (milk supplemented or not with NaCl, mascarpone and raw milk cheese). Significant differences in the growth parameters of these two strains occurred in stressful conditions and in raw milk cheese. The O157:H7 strain showed better growth abilities at a_w 0.973 in milk supplemented with NaCl and at T = 8.0 and 8.8 °C in mascarpone. However, during raw-milk cheese making, it was the O26:H11 strain that showed better growth capacities compared to the O157:H7 one. The latter result is consistent with previous studies on STEC in raw milk cheeses. A probabilistic model was developed for the growth and non-growth of generic E. coli. This model, developed from literature data and expert opinion, incorporates the effects of temperature, pH, a_w and undissociated lactic acid in the aqueous phase. Strain variability was incorporated by using statistic distributions instead of single values for the model parameters. The maximum specific growth rates of the two STEC strains in milk and mascarpone (under static conditions) fall within the prediction intervals of this generic model. During the making of raw milk cheese (dynamic conditions), the incorporation of the inhibitory effects of indigenous lactic acid bacteria (Jameson effect) improves model predictions and STEC counts are within or very close to the predicted upper and lower concentrations. During ripening and storage (140 days), inactivation of STEC occurs (with observed decreases in counts between 3.7 and 5.5 log₁₀ CFU/g). The survival data generated were used to refine an existing model developed for non-thermal inactivation of pathogenic E. coli in meat products. Further model validation is necessary to combine this inactivation model with the developed growth and growth/no growth model to describe the behaviour of pathogenic E. coli throughout the making and ripening of raw milk cheese.