Prevalences of Shiga Toxin Subtypes and Selected Other Virulence Factors among Shiga-Toxigenic Escherichia coli Strains Isolated from Fresh Produce

Abstract

b 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, which encodes 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 strains and one O91:H21 strain, which historically have been implicated in illness and therefore may be of concern as well. The ehxA gene, which encodes enterohemolysin, was found in ϳ60% of the isolates, and the saa and subAB genes, which encode STEC agglutinating adhesin and subtilase cytotoxin, respectively, were found in ϳ30% of the isolates. However, the precise roles of these three putative virulence factors in STEC pathogenesis have not yet been fully established. The stx 1a and stx 2a subtypes were present in 22% and 56%, respectively, of the strains overall and were the most common subtypes among produce STEC strains. The stx 2d subtype was the second most common subtype (28% overall), followed by stx 2c (7.5%), and only 2 to 3% of the produce STEC strains had the stx 2e and stx 2g subtypes. Almost half of the produce STEC strains had only 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. Increases in the consumption of fresh produce have resulted in increases in food-borne outbreaks and illness associated with these products, prompting federal agencies to monitor the microbial quality of fresh produce. The FDA has implemented import and domestic compliance programs to check produce samples for the presence of pathogens. Also, the USDA Agricultural Marketing Service initiated the Microbiological Data Program (MDP) in 2001 to conduct microbial surveys of fresh produce samples collected from wholesale distribution centers across the country. On average, 10,000 to 15,000 samples were tested by MDP yearly for the presence of Salmonella, enterotoxigenic Escherichia coli (ETEC), E. coli serotype O157:H7, and other Shiga-toxigenic E. coli (STEC) types. The annual MDP reports (http://www.ams .usda.gov/AMSv1.0/mdp) showed that many of these bacteria can be found in various types of fresh produce. STEC strains are characterized by the production of Shiga toxins (Stx), of which there are two main types, designated Stx1 and Stx2. Within each toxin are many subtypes; currently, there are three known Stx1 (Stx1a, Stx1c, and Stx1d) and seven known Stx2 (Stx2a, Stx2b, Stx2c, Stx2d, Stx2e, Stx2f, and Stx2g) subtypes (1). Some of these subtypes have ...

Molecular Profiling of Shiga Toxin-Producing Escherichia coli and Enteropathogenic E. coli Strains Isolated from French Coastal Environments

“…Finally, the stx 1d subtype dominates (23% of detected stx genes) in STEC strains isolated in our study (from shellfish and freshwater samples). This subtype does not appear to be common in STEC strains (62)(63)(64). However, a proportion of stx 1d variants similar to those observed here has been observed in STEC strains from ruminant stools in India (18.7%) (68).…”

“…These subtypes have frequently been identified in STEC strains from human, animal, environmental, and food samples (62)(63)(64)(65). STEC strains with stx 2a have been found to be associated with several clinical symptoms, such as HUS and HC, whereas STEC strains with stx 1a have been associated mainly with diarrhea without HUS (8).…”

Shiga Toxin-Producing Serogroup O91 Escherichia coli Strains Isolated from Food and Environmental Samples

“…The ehxA gene for enterohemolysin, a putative virulence factor whose role in pathogenicity remains undetermined, was found to be common in 89% of the STEC strains that often cause infections (32) and in about 60% of the STEC strains isolated from fresh produce (22). The ehxA gene was also common in the O91 strains, found in 82% of the O91:H21 strains and 60% of the O91:H14 strains.…”

“…These strains have been isolated from raw milk and cheeses in France (17) and Switzerland (18), from minced beef in France (19), and from cattle feces in Argentina (20), and they are the second most common STEC serogroup found in various types of meats and dairy products in Germany (16). In the United States, STEC O91 strains have been isolated from ground beef (21), from the production environments of other food animals, and also from fresh produce (22). Most studies that have characterized STEC O91 strains have focused on clinical strains (9), so little is known about food and environmental STEC O91 strains.…”

Evaluation of a Loop-Mediated Isothermal Amplification Suite for the Rapid, Reliable, and Robust Detection of Shiga Toxin-Producing Escherichia coli in Produce

“…There are currently 3 stx 1 subtypes (stx 1a , stx 1c , and stx 1d ), 7 stx 2 subtypes (a through g), and about 30 eae subtypes (␣, ␤, ␥, ε, etc.) (37,43). The presence of eae subtypes (primarily ␤, ␥, and ) and certain closely related stx 2 subtypes (stx 2a , stx 2c , and stx 2d ) is strongly associated with severe human illness, such as HUS (36,43).…”