Use of pulsed field gel electrophoresis genetic typing for tracing contamination with virulent Escherichia coli O157:H7 in beef-cattle producing farms

Abstract

E. coli O157:H7 is an important causative agent of a variety of foodborne infections including hemorrhagic colitis (HC), hemolytic uremic syndrome (HUS) and thrombotic thrombocytopenic purpura (TTP) that may lead to renal failure in humans. The main source of infection is contaminated food of animal origin and a number of animal species especially cattle have been associated with this pathogen. Contamination usually occurs when faeces of animals get into contact with carcass during slaughtering. The aim of the present study was to determine the virulence profiles and genetic relationships of E. coli O157:H7 strains isolated from cattle some commercial farms in the North West Province, South Africa. A total of 260 faecal samples were collected from eight farms over a six-month period and assessed for the presence of E. coli O157:H7 through PCR analysis. Bacteria 16S rRNA gene fragments were amplified as an internal control while the rfbO157 and flicH7 gene PCR were used to confirm identities of isolates. STEC virulence genes stx1, stx2, eaeA and the hlyA were detected using PCR and the genetic relatedness of the isolates was determined using Pulse Field Gel electrophoresis (PFGE). A total of 69 (26.5%) E. coli O157:H7 isolates were detected in samples obtained from all the eight farms. A large proportions of the E. coli isolates possessed the stx1 (66.7%) and stx2 (97.1%) genes respectively. In addition, other accessory virulence genes eaeA and hlyA were detected in 30 (43.5%) and 43 (62.3%) of the E. coli isolates respectively. Pulsed-Filed Gel Electrophoresis (PFGE) analysis grouped these isolates into 6 clusters and the largest cluster contained 17 isolates originating from different farms. These results provide valid evidence that cattle harbour virulent and genetically similar E. coli strains particularly those belonging to the serotype O157 and therefore these animals may serve as possible source for zoonotic transfer of these pathogens to humans. The implication is that these isolates may have severe public health consequences on consumers.