Is E coli (Escherichia coli) O157:H7 without Shiga toxin still a significant concern?

E. coli O157:H7 Without Shiga Toxin: Clinical Significance

E. coli O157:H7 strains that lack Shiga toxin genes are still clinically significant pathogens that require proper identification, reporting, and management, though they generally pose less risk for severe complications like hemolytic uremic syndrome (HUS) compared to their toxin-producing counterparts.

Pathogenicity and Clinical Relevance

E. coli O157:H7 is traditionally associated with Shiga toxin production, which is the primary virulence factor responsible for severe complications like HUS. However, Shiga toxin-negative variants maintain clinical significance for several reasons:

• Stx-negative E. coli O157:H7 strains can still cause diarrheal illness, though typically with less severity than Shiga toxin-producing strains 1
• These strains may represent either:
  • Inherently Stx-negative strains that cause mostly uncomplicated diarrhea
  • Strains that descended from enterohemorrhagic E. coli O157 by losing the stx gene during infection 1
• Even without Shiga toxin, these organisms retain other virulence factors, including the locus of enterocyte effacement (LEE) region that enables intimate attachment to intestinal epithelial cells 2

Diagnostic Considerations

The CDC and Infectious Diseases Society of America recommend comprehensive testing approaches:

• All stools from patients with acute community-acquired diarrhea should be cultured for O157 STEC on selective and differential agar AND simultaneously tested for non-O157 STEC with assays that detect Shiga toxins or their encoding genes 3
• This dual approach is critical because:
  1. It helps identify both toxin-positive and toxin-negative strains
  2. It prevents missing clinically significant infections
  3. It enables proper epidemiological tracking

Public Health Implications

Stx-negative E. coli O157:H7 has important public health implications:

• These strains have been implicated in outbreaks of diarrheal illness 1
• The proportion of Stx-negative strains is significantly higher among sorbitol-fermenting, nonmotile E. coli O157 isolates (12.7%) than among non-sorbitol-fermenting E. coli O157:H7 isolates (0.8%) 1
• Proper identification and reporting are essential for outbreak detection and control 4

Management Considerations

When E. coli O157:H7 is identified, regardless of Shiga toxin status:

• Prompt reporting to public health authorities is essential 3
• For patients with diarrhea:
  • Evaluate for dehydration, which increases risk of serious outcomes 4
  • Provide appropriate rehydration therapy based on dehydration severity
  • Monitor for complications, though these are less likely with Stx-negative strains

Pitfalls and Caveats

Important considerations to avoid mismanagement:

1. Diagnostic pitfall: Relying solely on Shiga toxin testing without culture may miss Stx-negative E. coli O157:H7 infections
2. Classification confusion: Stx-negative E. coli O157:H7 may be misclassified as atypical enteropathogenic E. coli (aEPEC) due to the presence of eae gene but lack of stx genes, despite being genetically similar to STEC 2
3. Dynamic nature: The Stx-encoding bacteriophage can be lost during infection or laboratory culturing, potentially leading to false-negative results for Shiga toxin 2
4. Outbreak potential: Despite lacking Shiga toxin, these strains have been implicated in outbreaks and should not be dismissed as clinically insignificant 1

Conclusion

While E. coli O157:H7 strains lacking Shiga toxin generally cause less severe disease than their toxin-producing counterparts, they remain clinically significant pathogens that can cause diarrheal illness and outbreaks. Proper laboratory identification, reporting, and management are essential to ensure appropriate patient care and public health response.