Isolation and characterization of Shiga toxin-producing Escherichia coli strains from raw milk cheeses in France

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Abstract
Aims: To evaluate Shiga toxin-producing Eschericha coli (STEC) prevalence in 1039 French raw milk cheeses including soft, hard, unripened and blue mould cheeses, and to characterize the STEC strains isolated (virulence genes and serotypes). Methods and Results: STEC strains were recovered from cheese samples by colony hybridization. These strains were then serotyped and genetically characterized. These strains (32 STEC) were then recovered from 18 of 136 stx-positive samples: 19 strains had stx2 variant genes stx2vh-a (n = 2), stx2NV206 (n = 2), stx2EDL933 (n = 4) and stx2d (n = 11). Thirty strains had the stx1 gene and one strain, the eae gene. Combinations of stx2 and stx1 genes were present in 17 (81%) of the STEC strains. Nineteen strains belonged to the O6 serogroup and the other strains belonged to the O174, O175, O176, O109, O76, O162 and O22 serogroups in decreasing frequency. Conclusions: No conclusion can be drawn at the moment concerning the potential risk to consumers because the O6:H1 serotype has already been found associated with the haemolytic uremic syndrome and almost no isolate had the eae gene. Significance and Impact of the Study: The large number of STEC strains recovered from the cheese samples evaluated in this study emphasizes the health risks associated with raw milk cheeses. This further emphasizes the immediate need to identify and implement effective pre- and postharvest control methods that decrease STEC carriage by dairy cattle and to eliminate contamination of their cheeses during processing.