Occurrence and diversity of Shiga toxin-producing Escherichia coli (STEC) in Italian Alpine raw milk cheeses and their development in the earlier stages of different cheese-making processes

Shiga toxin-producing Escherichia coli (STEC) are foodborne pathogens implicated in diseases including hemorrhagic colitis and hemolytic uremic syndrome. In the last years, an increasing number of STEC infections associated with the consumption of raw milk cheeses have been reported, contributing to raise the public awareness. The aim of this study was to assess the occurrence of STEC biotypes in raw milk cheeses from Italian Alpine region and evaluate the influence of different scalding temperatures on their development. Six out 82 (7.3%) cheeses led to the isolation of E. coli biotypes with different virulence traits ( stx1 , stx2 and eae genes). To evaluate the impact of scalding temperatures on STEC growth, three E. coli biotypes were spiked into milk (<50 CFU/mL) according to the FAO and WHO recommendation. An increase in STEC counts of 3 log occurred in uncooked and semi-cooked cheeses (scalding temperature of 38 and 45 °C) while in the cooked-cheeses (54 and 56 °C) E. coli content does not exceed the 2.5 log 10 CFU/g. These findings showed that raw milk cheese safety is strictly related to their production technology, highlighting the importance of the control measures at farm and dairy level to preserve the safety of these products. • Raw milk cheeses might merit concern regarding the risk of STEC infection. • 7.3% of raw milk cheeses from Alpine region of Italy were positive for STEC. • STEC isolates were able to growth in the early stages of cheese-making process. • Cheese-making process of uncooked and semi-cooked cheeses support the STEC growth. • Higher scalding temperatures (56 °C) hamper the STEC development.