Presence and characterization of shiga toxin-producing Escherichia coli and other potentially diarrheagenic E. coli strains in retail meats

Abstract

To determine the presence of Shiga toxin-producing Escherichia coli (STEC) and other potentially diarrheagenic E. coli strains in retail meats, 7,258 E. coli isolates collected by the U.S. National Antimicrobial Resistance Monitoring System (NARMS) retail meat program from 2002 to 2007 were screened for Shiga toxin genes. In addition, 1,275 of the E. coli isolates recovered in 2006 were examined for virulence genes specific for other diarrheagenic E. coli strains. Seventeen isolates (16 from ground beef and 1 from a pork chop) were positive for stx genes, including 5 positive for both stx(1) and stx(2), 2 positive for stx(1), and 10 positive for stx(2). The 17 STEC strains belonged to 10 serotypes: O83:H8, O8:H16, O15:H16, O15:H17, O88:H38, ONT:H51, ONT:H2, ONT:H10, ONT:H7, and ONT:H46. None of the STEC isolates contained eae, whereas seven carried enterohemorrhagic E. coli (EHEC) hlyA. All except one STEC isolate exhibited toxic effects on Vero cells. DNA sequence analysis showed that the stx(2) genes from five STEC isolates encoded mucus-activatable Stx2d. Subtyping of the 17 STEC isolates by pulsed-field gel electrophoresis (PFGE) yielded 14 distinct restriction patterns. Among the 1,275 isolates from 2006, 11 atypical enteropathogenic E. coli (EPEC) isolates were identified in addition to 3 STEC isolates. This study demonstrated that retail meats, mainly ground beef, were contaminated with diverse STEC strains. The presence of atypical EPEC strains in retail meat is also of concern due to their potential to cause human infections.

FIG. 1.

Dendrogram of PFGE profiles obtained by XbaI digestion for 17 STEC isolates from retail meat. Similarities of PFGE profiles were calculated using the Dice algorithm, with a 1.5% tolerance level.

FIG. 2.

Vero cell cytotoxicity of Shiga toxin-producing E. coli isolates from retail meats. The CD₅₀ is expressed as the toxin dilution that causes 50% Vero cell detachment compared with untreated cells (control). The value on the y axis indicates the log of the reciprocal of the CD_50. The data shown are averages for three independent assays. *, strains EDL933 and K-12 served as positive and negative controls, respectively.

FIG. 3.

Phylogenetic trees of stx₁ sequences (A) and stx₂ sequences (B) determined in this study and sequences of previously described stx genes and their variants. stx_1d and stx_2f were not included due to their considerable sequence divergence from classical stx₁ and stx₂, respectively. The horizontal bar indicates 0.001 (A) and 0.002 (B) nucleotide substitution per site. Reference DNA sequences for stx genes were obtained from GenBank and are identified in the phylogenetic trees by their accession numbers.