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Prevalence and Characteristics of Shiga Toxin-Producing Escherichia coli in Swiss Raw Milk Cheeses Collected at Producer Level

R. Stephan^*,1, S. Schumacher^*, S. Corti^*, G. Krause, J. Danuser and L. Beutin

^* Institute for Food Safety and Hygiene, University of Zurich, Winterthurerstrasse 272, 8057 Zurich, Switzerland
National Reference Laboratory for Escherichia coli, Centre for Infectiology and Pathogen Characterization, Federal Institute for Risk Assessment, Berlin, Germany
Federal Veterinary Office, Schwarzenburgstrasse 155, 3003 Bern, Switzerland

¹ Corresponding author: stephanr{at}fsafety.uzh.ch

The aim of this study was to describe the prevalence, serotypes, and virulence genes of Shiga toxin-producing Escherichia coli (STEC) isolated from raw milk cheese samples collected at the producer level with the purpose of determining whether raw milk cheeses in Switzer-land represent a potential source of STEC pathogenic for humans. Raw milk cheese samples (soft cheese, n = 52; semihard and hard cheese, n = 744; all produced from Swiss cows’, goats’, and sheep’s milk) collected at the producer level throughout Switzerland within the national sampling plan during the period of March 2006 to December 2007 were analyzed. Of the 432 cheese samples obtained in the year 2006 and the 364 samples obtained in the year 2007, 16 (3.7%) and 23 (6.3%), respectively, were found to be stx positive. By colony dot-blot hybridization, non-O157 STEC strains were isolated from 16 samples. Of the 16 strains, 11 were typed into 7 E. coli O groups (O2, O15, O22, O91, O109, O113, O174), whereas 5 strains were nontypeable (ONT). Among the 16 STEC strains analyzed, stx₁ and stx₂ variants were detected in 1 and 15 strains, respectively. Out of the 15 strains with genes encoding for the Stx2 group, 4 strains were positive for stx₂, 6 strains for stx_2d2, 2 strains for stx_2–O118, 1 strain for stx_2–06, 1 strain for stx_2g, 1 strain for stx₂ and stx_2d2, and 1 strain for stx₂ and stx_2g. Furthermore, 3 STEC strains harbored E-hlyA as a further putative virulence factor. None of the strains tested positive for eae (intimin). Results obtained in this work reinforce the suggestion that semihard raw milk cheese may be a potential vehicle for transmission of pathogenic STEC to humans.

Key Words: raw milk cheese • Shiga toxin-producing Escherichia coli • prevalence • characterization