Common Virulence Genes of E. coli
The most important virulence genes in pathogenic E. coli are the Shiga toxin genes (stx1 and stx2), intimin (eaeA), and enterohemorrhagic E. coli hemolysin (EHEC-hlyA), which together determine the pathogenic potential and clinical severity of infections.
Major Virulence Factors
Shiga Toxin Genes
- stx1 and stx2 are the primary virulence determinants responsible for severe clinical manifestations including hemorrhagic colitis and hemolytic-uremic syndrome (HUS) 1, 2
- Multiple Shiga toxin subtypes exist: stx2c, stx2d, stx2e, and stx2f, with stx2 being significantly more frequent in eae-positive strains and associated with more severe disease 1, 3
- Among human STEC isolates, 78.3% carry at least one stx gene 1
Intimin (eaeA)
- The eaeA gene encodes intimin, which mediates attaching and effacing adherence to intestinal epithelium 1, 2
- Present in 62.6-74.4% of STEC strains from humans 1, 3
- Multiple intimin subtypes exist (alpha1, beta1, gamma1, epsilon, theta, eta), with different distributions among pathogenic strains 3
- eae-positive STEC infections are significantly more common in children under 6 years of age compared to adults 3
Enterohemorrhagic E. coli Hemolysin (EHEC-hlyA)
- EHEC-hlyA is present in 71.3% of STEC isolates overall 1
- Shows strong association with eae-positive strains (96.2% prevalence) compared to eae-negative strains (65.2%) 3
- Serves as a key marker to differentiate highly pathogenic EHEC from less pathogenic STEC when combined with eae 2, 4
Pathotype-Specific Patterns
Enterohemorrhagic E. coli (EHEC)
- All O157:H7 isolates carry EHEC-hlyA, eae, and one or both stx genes, making this combination the hallmark of highly pathogenic strains 2
- The combination of EHEC-hlyA and eae genes serves as a reliable marker to differentiate EHEC from less pathogenic STEC 2
Extraintestinal Pathogenic E. coli (ExPEC)
- K1 capsular polysaccharide gene is found in 88% of neonatal meningitis strains 5
- ChuA (hemin receptor) is present in 95% of K1 strains 5
- P, S, and F1C fimbriae genes are characteristic of extraintestinal E. coli 5
- Aerobactin, alpha-hemolysin, and cytotoxic necrotizing factor genes are associated with K1 and related extraintestinal strains 5
Clinical Significance
Disease Severity Correlation
- stx2-positive strains, particularly when combined with eae, are associated with more severe clinical outcomes including bloody diarrhea (14.3% of cases) and HUS (3.5% of cases) 3
- eae-negative STEC infections predominate in adults and generally cause less severe disease 3
Common Pitfalls
- Not all STEC strains carry all virulence genes; approximately 21.7% of E. coli isolates lack stx genes entirely 1
- Some strains (18.2%) may carry none of the major virulence genes despite being serotype O157:H7 4
- K1 strains constitute a heterogeneous group and can occasionally carry virulence genes typical of intestinal E. coli, complicating classification 5