Phenotypic and Genotypic Characterization of Escherichia coli O157:H7 Harboring stx2 and stx1&stx2 Genes Isolated from Frozen Raw Beef Meat and Beef Liver Samples.

Shiga toxin (Stx) is the main virulence factor of enterohemorrhagic Escherichia coli. Between its two types-Stx1 and Stx2-E. coli O157:H7 produces Stx2, which causes more severe infections compared with Stx1 or both Stx1 and Stx2. In this study, we isolated and identified E. coli O157:H7 from frozen raw beef samples and both Stx1 and Stx2 from frozen raw beef liver samples collected from the Al-Riyadh market in Saudi Arabia. Phenotypic detection was conducted using suitable and specific media-sorbitol MacConkey agar and Salmonella chromogenic agar-with E. coli O157:H7 appearing as colorless and blue-colored colonies, respectively. Furthermore, serological testing using DrySpot E. coli O157 latex agglutination tests was conducted to verify the identification and detection results. A conventional polymerase chain reaction was conducted using specific primers of the genes encoding Stx1 and Stx2 for further verification. The results showed that among the 50 frozen raw beef meat samples, 11 colonies harbored the stx2 gene with an amplicon of 420 bp (22%). Meanwhile, among the 50 frozen raw beef liver samples, 5 samples harbored either both the stx1 and stx2 genes or the stx2 gene alone, with amplicons of 310 and 420 bp, respectively (10%), while 6 samples harbored only the stx2 gene (12%). Overall, E. coli O157:H7 containing Stx2 was prevalent in beef meat, while Stx1 and/or Stx2 were identified in beef liver. Furthermore, E. coli O157:H7 harboring stx2 genes were found to be more resistant to different antibiotic classes (50%) than strains containing stx1 and/or stx2 genes (42.28%). Moreover, doubling time in tryptic soy medium extended to 60 min for strains harboring stx2 alone and 55 min for strains harboring stx1 and/or stx2. In conclusion, E. coli O157:H7 was found to be considerably more multidrug resistant than Stx1 and/or Stx2 with respect to several conditions and circumstances.