Abstract
We previously reported that the strains of Escherichia coli O157:H7 (EcO157) that survived longer in austere soil environment lacked expression of curli, a fitness trait linked with intestinal colonization. In addition, the proportion of curli-positive variants of EcO157 decreased with repeated soil exposure. Here we evaluated 84 and 176 clinical strains from outbreaks and sporadic infections in the US, plus 211 animal fecal and environmental strains for curli expression. These shiga-toxigenic strains were from 328 different genotypes, as characterized by multi-locus variable-number tandem-repeat analysis (MLVA). More than half of the fecal strains (human and animal) and a significant proportion of environmental isolates (82%) were found to lack curli expression. EcO157 strains from several outbreaks linked with the consumption of contaminated apple juice, produce, hamburgers, steak, and beef were also found to lack curli expression. Phylogenetic analysis of fecal strains indicates curli expression is distributed throughout the population. However, a significant proportion of animal fecal isolates (84%) gave no curli expression compared to human fecal isolates (58%). In addition, analysis of environmental isolates indicated nearly exclusive clustering of curli expression to a single branch of the minimal spanning tree. This indicates that curli expression depends primarily upon the type of environmental exposure and the isolation source, although genotypic differences also contribute to clonal variation in curli. Furthermore, curli-deficient phenotype appears to be a selective trait for survival of EcO157 in agricultural environments.
Highlights
Shiga-toxigenic Escherichia coli O157:H7 (EcO157) is responsible for over 96,000 diarrheal illnesses and 3200 hospitalizations annually in the United States (Scallan et al, 2011)
Shiga-toxigenic EcO157 strains isolated from infected patients during outbreaks and sporadic cases of infections that occurred during a span of 25 years (1982–2007; Table S1) were evaluated along with environmental isolates to determine if strains surviving various austere environments were phenotypically similar to those responsible for causing sporadic infections and illness outbreaks
Strains grouped into 328 genotypes based on 11-loci multi-locus variable-number tandem-repeat analysis (MLVA) characteristics were evaluated to determine the relationship of illness-associated genotypes and the proportion of curli phenotypes
Summary
Shiga-toxigenic Escherichia coli O157:H7 (EcO157) is responsible for over 96,000 diarrheal illnesses and 3200 hospitalizations annually in the United States (Scallan et al, 2011). EcO157 responsible for 200 infections in a large multi-state outbreak (Mandrell, 2009) related to consumption of baby spinach was traced back to produce grown in central California coast (Jay et al, 2007). EcO157 isolates that are genetically indistinguishable from the 2006 spinach outbreak strain were isolated from feral swine, cattle, surface water, and sediment samples near spinach fields (Jay et al, 2007). These results indicate wide-spread occurrence of this pathogen in numerous habitats, in the vicinity of produce production, each of which may affect survival of EcO157 differently.
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