Detection of Shiga Toxin-ProducingEscherichia coliSerotypes O26:H11, O103:H2, O111:H8, O145:H28, and O157:H7 in Raw-Milk Cheeses by Using Multiplex Real-Time PCR
ABSTRACTShiga toxin (Stx)-producingEscherichia coli(STEC) strains are a diverse group of food-borne pathogens with various levels of virulence for humans. In this study, we describe the use of a combination of multiple real-time PCR assays for the screening of 400 raw-milk cheeses for the five main pathogenic STEC serotypes (O26:H11, O103:H2, O111:H8, O145:H28, and O157:H7). The prevalences of samples positive forstx, intimin-encoding gene (eae), and at least one of the five O group genetic markers were 29.8%, 37.3%, and 55.3%, respectively. The H2, H7, H8, H11, and H28fliCalleles were highly prevalent and could not be used as reliable targets for screening. Combinations ofstx,eaevariants, and O genetic markers, which are typical of the five targeted STEC serotypes, were detected by real-time PCR in 6.5% of the cheeses (26 samples) and includedstx-wzxO26-eae-β1(4.8%; 19 samples),stx-wzxO103-eae-ε (1.3%; five samples),stx-ihp1O145-eae-γ1(0.8%; three samples), andstx-rfbEO157-eae-γ1(0.3%; one sample). Twenty-eight immunomagnetic separation (IMS) assays performed on samples positive for these combinations allowed the recovery of seveneaeβ1-positive STEC O26:H11 isolates, whereas no STEC O103:H2, O145:H28, or O157:H7 strains could be isolated. Threestx-negative andeaeβ1-positiveE. coliO26:[H11] strains were also isolated from cheeses by IMS. Colony hybridization allowed us to recover STEC fromstx-positive samples for 15 out of 45 assays performed, highlighting the difficulties encountered in STEC isolation from dairy products. The STEC O26:H11 isolates shared the same virulence genetic profile as enterohemorrhagicE. coli(EHEC) O26:H11, i.e., they carried the virulence-associated genes EHEC-hlyA,katP, andespP, as well as genomic O islands 71 and 122. Except for one strain, they all contained thestx1variant only, which was reported to be less frequently associated with human cases thanstx2. Pulsed-field gel electrophoresis (PFGE) analysis showed that they displayed high genetic diversity; none of them had patterns identical to those of human O26:H11 strains investigated here.