Is reporting Escherichia coli (E. coli) O157:H7 without Shiga toxin (stx) clinically useful?

Clinical Utility of Reporting E. coli O157:H7 without Shiga Toxin

Reporting E. coli O157:H7 isolates is clinically important regardless of Shiga toxin status, as stx-negative strains can still cause diarrheal illness and retain other virulence factors. 1

Rationale for Reporting O157:H7 Without Shiga Toxin

Clinical Significance

• Stx-negative E. coli O157:H7 strains can cause diarrheal illness, though typically with less severity than Shiga toxin-producing strains 1
• These strains retain other virulence factors, including the locus of enterocyte effacement (LEE) region that contributes to pathogenicity 1
• Studies have documented stx-negative E. coli O157:H7 isolates from patients with diarrhea (4% of sporadic cases) and even hemolytic uremic syndrome (HUS) 2

Diagnostic Algorithm

1. Initial identification: When E. coli O157:H7 is identified through agglutination with O157-specific reagents and biochemical confirmation
2. Preliminary reporting: Report immediately to clinicians before complete confirmation, as this is time-sensitive clinical information 3
3. Shiga toxin testing: Test for Shiga toxin production by EIA or for stx1/stx2 genes by PCR 3
4. Final reporting: Provide complete report including Shiga toxin status to clinician and public health authorities 3

Public Health Implications

• All O157 isolates should be forwarded to public health laboratories regardless of H7 or Shiga toxin status 3
• Prompt reporting enables:
  • Outbreak detection and monitoring
  • Implementation of appropriate infection control measures
  • Molecular characterization through PFGE subtyping 3, 1

Clinical Management Considerations

Patient Care

• The absence of Shiga toxin affects risk assessment:
  • Lower risk of developing HUS compared to Stx-positive strains 2
  • Still requires monitoring for complications and appropriate supportive care 1

Treatment Approach

• Supportive care remains the cornerstone of management 1
• Rehydration therapy based on dehydration severity 1
• Even with stx-negative strains, antibiotics should be used cautiously as:
  • The strain may have lost the stx gene during infection 2
  • The patient may be co-infected with other stx-positive strains

Important Caveats

• Stx-negative E. coli O157:H7 may represent:

  1. Inherently stx-negative strains causing mostly uncomplicated diarrhea, or
  2. Strains that descended from STEC by losing the stx gene during infection, which may still cause severe disease 2

• Diagnostic methods targeting only Shiga toxin will miss stx-negative E. coli O157:H7 variants, potentially leading to underdiagnosis 4

• The CDC and Infectious Diseases Society of America recommend comprehensive testing approaches for all stools from patients with acute community-acquired diarrhea to identify both toxin-positive and toxin-negative strains 1

Conclusion

Reporting E. coli O157:H7 without Shiga toxin has significant clinical utility for individual patient management, infection control, and public health surveillance. The identification of the O157:H7 serotype, regardless of toxin status, provides valuable information for clinical decision-making and public health response.