In vitro characterization of Shiga toxin-producing and generic Escherichia coli in respect of cheese production-relevant stresses

Forty-one Escherichia (E.) coli strains from raw milk cheese, including 26 Shiga toxin-producing strains (STEC), and six STEC strains from cattle feces were characterized in respect of cheese production-relevant stress (thermal inactivation, glucose-repressed oxidative acid resistance system, acidic and osmotic stress). Of all 47 E. coli isolates, seven generic E. coli strains showed an increased heat tolerance (55 degrees C, 15 min) and 20 strains (STEC and generic E. coli) exhibited a survival rate of >= 10 % (pH 2.5, 2 h) with induced oxidative acid resistance system. Besides, growth was observed for all E. coli strains at pH or a(w) values commonly found in (semi-hard) cheese (pH = 5.2, a(w) = 0.970), whereas counts tended to decrease at a pH value of 4.5 or an a(w) value of 0.942. Overall, no clear and universal differences between STEC and generic E. coli strains were found. The large strain variations observed in respect of the applied stresses within this strain collection do not indicate a shared feature amongst our E. coli strains that may be beneficial for their survival in raw milk cheese. It remains to be elucidated if there are key factors enabling the survival of E. coli and in particular STEC during production of raw milk cheese or if differences in the production process are more important.