Expression of Putative Virulence Factors of Escherichia coli O157:H7 Differs in Bovine and Human Infections

Abstract

Escherichia coli O157:H7 is a commensal organism in cattle, but it is a pathogen in humans. This differential expression of virulence suggests that specific virulence factors are regulated differently in human and bovine hosts. To test this hypothesis, relative real-time reverse transcription-PCR was used to relate the expression of several putative virulence genes (eae, espA, stx 2 , rfbE, ehxA, and iha) to that of the "housekeeping" gene gnd during natural human and experimental bovine infection with E. coli O157:H7. We examined these genes in fecal samples from eight humans and four calves. iha and espA were significantly more expressed in bovine infections. rfbE and ehxA appeared to be more highly expressed in human infections, though these differences did not achieve statistical significance. Our results support the hypothesis that some virulence-associated genes of O157:H7 are differentially expressed in a host-specific manner.Escherichia coli O157:H7, a human pathogen that causes diarrhea, bloody diarrhea, and the hemolytic uremic syndrome (HUS), has acquired multiple virulence factors that might aid its pathogenicity. Principal among these are the phage-encoded Shiga toxins (Stx proteins), which inhibit protein synthesis in eukaryotic cells (41), and the LEE pathogenicity island, which encodes products associated with formation of attaching-and-effacing lesions at the host surface (reviewed in reference 13). E. coli O157:H7 also has a 92-kb nonconjugative plasmid that encodes several putative virulence factors, including enterohemolysin (encoded by ehxA, also termed hlyA [1,5]), a member of the RTX family of exoproteins. E. coli O157:H7 also expresses the IrgA homologue adhesin (Iha), which was recently identified as a virulence factor in uropathogenic E. coli (16,38).Some of these factors can affect animal hosts. For example, E. coli O157:H7 causes attaching-and-effacing lesions on cultured bovine cells (31) and in neonatal calves (8). Despite this, E. coli O157:H7 is not known to cause natural disease in any host except humans. This discrepancy suggests the hypothesis that specific virulence factors of E. coli O157:H7 are regulated differently when the bacteria are in the human rather than the bovine gut. To test this hypothesis, real-time reverse transcription-PCR (RT-PCR) was employed to examine gene expression in fecal samples from children infected with E. coli O157:H7 and from experimentally infected calves. The resulting data support the hypothesis that virulence genes are differentially expressed in the human and bovine hosts.

Escherichia coli O157:H7-secreted cytotoxins are toxic to enterocytes and increase Escherichia coli O157:H7 colonization of jejunum and descending colon in cattle

“…In contrast, Shiga toxins cannot enter the general circulation in cattle as they lack vascular receptors (Valdivieso-Garcia et al 1996) thereby preventing the development of disease. Evidence provided by Rashid et al (2006) indicated that Shiga toxins are equally expressed in EHEC O157:H7-challenged cattle and humans suggesting that these toxins could have an impact on intestinal mucosa from cattle. Shiga toxins and other cytotoxins are toxic to epithelial cell lines, primary cultures of animal cells in vitro and intestinal organ cultures (Konowalchuk et al 1977;Sjogren et al 1994;Bauer and Welch 1996;Valdivieso-Garcia et al 1996;Schuller et al 2004;Aldick et al 2007).…”

“…The current model for EHEC O157:H7 colonization in cattle suggests that the lymphoid-follicle dense mucosa in the terminal rectum is the primary colonization site and it is this colonization that is responsible for persistent shedding (Naylor et al 2003Sheng et al 2004Sheng et al , 2006Lim et al 2007). The basic assumption of this model is that cattle are not adversely affected by EHEC O157:H7 virulence factors, and although a reduction in the expression of some EHEC O157:H7 virulence factors has been demonstrated (Rashid et al 2006), it is not true for all virulence factors. For example, Rashid et al (2006) determined that Shiga toxins were expressed equally in humans and cattle, suggesting that these toxins play a similar role in both systems.…”

“…The basic assumption of this model is that cattle are not adversely affected by EHEC O157:H7 virulence factors, and although a reduction in the expression of some EHEC O157:H7 virulence factors has been demonstrated (Rashid et al 2006), it is not true for all virulence factors. For example, Rashid et al (2006) determined that Shiga toxins were expressed equally in humans and cattle, suggesting that these toxins play a similar role in both systems. Bacterial toxins can have both lethal (Keenen et al 1986;Paton et al 1995;Bauer and Welch 1996;Ferens and Hovde 2000;Schuller et al 2004;Figueiredo et al 2007) and sublethal (Menge et al 2004;Ferens and Hovde 2007) effects on cells that benefit pathogen colonization through a reduction in host defenses or by providing a foothold on the mucosa for replication.…”

Expression of the Escherichia coli IrgA homolog adhesin is regulated by the ferric uptake regulation protein

“…Following preparation of this manuscript, Leveille et al reported that Iha of the uropathogen UCB34 is regulated by iron availability [9]. Finally, we have recently identified iha expression in E. coli O157:H7 in stools that have been freshly shed by infected children and experimentally inoculated cattle [17].…”

A rapid, sensitive method for testing the activity of Escherichia coli 0157:H7 secreted cytotoxins against epithelial cells from the jejunum and descending colon of cattle

“…A number of studies have shown that specific type III effector molecules are required for the formation of classic attaching and effacing lesions in calves (Cookson and Woodward 2003); however, not all colonization is associated with the formation of actin pedestals (Vlisidou et al 2006). In addition, EHEC O157:H7 actin pedestals do not have the same appearance in cattle as those in disease-susceptible animals (Dibb-Fuller et al 2001), and a recent study suggests that this is attributable to lower expression of type III effector molecules in cattle (Rashid et al 2006). In contrast, Rashid et al (2006) determined that Shiga toxin expression is not different for human and cattle systems suggesting that cytotoxins could play a similar role in both systems.…”