Scientific Publications by FDA Staff

Appl Environ Microbiol 2013 Nov;79(22):6917-23

The prevalence of Shiga toxin subtypes and selected other virulence factors among Shiga toxigenic Escherichia coli strains isolated from fresh produce.

Feng P, Reddy S

Shiga toxigenic Escherichia coli (STEC) strains were isolated from a variety of fresh produce, but mostly from spinach, with an estimated prevalence rate of 0.5%. A panel of 132 produce STEC strains were characterized for the presence of virulence and putative virulence factor genes and for shiga toxin subtypes. About 9% of the isolates were found to have the eae gene that encodes for the intimin binding protein and most of these belonged to known pathogenic STEC serotypes such as O157:H7 and O26:H11 or to serotypes that reportedly, have caused human illness. Among the eae-negative strains, there were three O113:H21 and one O91:H21 strains, which historically have been implicated in illness and therefore, may be of concern as well. The ehxA gene that encodes for enterohemolysin was found in approximately 60% of the isolates and the saa and subAB genes that encode for STEC agglutinating adhesin and subtilase cytotoxin, respectively, were found in approximately 30% of the isolates. But, the precise role of these three putative virulence factors in STEC pathogenesis has yet been fully established. The stx1a and stx2a subtypes were present in 22% and 56%, respectively, of the strains overall and were the most common subtypes among produce STEC strains. The stx2d subtype was the second most common subtype (28% overall), followed by stx2c (7.5%) and only 2-3% of the produce STEC strains had the stx2e and stx2g subtypes. Almost half of the produce STEC strains only had partial serotypes or were untyped and most of those that were identified, belonged to unremarkable serotypes. Considering the uncertainties of some of these Stx subtypes and putative virulence factors in causing human illness, it is difficult to determine the health risk of many of these produce STEC strains.