Which strains of E coli (Escherichia coli) are considered enterohemorrhagic?

Enterohemorrhagic E. coli Strains

The primary enterohemorrhagic E. coli (EHEC) strains include E. coli O157:H7 and six major non-O157 serogroups: O26, O45, O103, O111, O121, and O145. 1

Definition and Classification

Enterohemorrhagic E. coli (EHEC) are a pathogenic subgroup of Shiga toxin-producing E. coli (STEC). While all EHEC are STEC, not all STEC cause human illness. EHEC are specifically defined as those strains that:

• Produce Shiga toxins (Stx1 and/or Stx2)
• Cause human illness (particularly bloody diarrhea and hemolytic uremic syndrome)
• Possess additional virulence factors that enhance colonization and pathogenicity

Major EHEC Strains

O157:H7

• Most common and well-studied EHEC serotype in North America
• Responsible for approximately 73,000 cases annually in the United States 1
• Cannot ferment sorbitol within 24 hours (important laboratory characteristic)
• Has a very low infectious dose (<100 organisms) 1

Major Non-O157 Serogroups

The six most common non-O157 EHEC serogroups in the United States are:

• O26
• O45
• O103
• O111 (also has a low infectious dose like O157) 1
• O121
• O145

These six serogroups account for the majority of reported non-O157 STEC infections in the United States 1.

Other EHEC Strains

• At least 150 STEC serotypes have been associated with human disease 1
• E. coli O104:H4 gained prominence as the cause of a major outbreak in Germany in 2011 2
• Additional serotypes identified in clinical settings include O1:H11, O128:H2, O25:H2, O125:H6, O86:H8, and O18:H15 3

Virulence Factors

The key virulence factors that define EHEC include:

1. Shiga toxins:

   • Stx1: Neutralized by antibodies against Shiga toxin
   • Stx2: Not neutralized by antibodies against Shiga toxin, associated with more severe disease
   • EHEC strains producing only Stx2 are more often associated with HUS than strains producing only Stx1 or both Stx1 and Stx2 1

2. Intimin (encoded by eaeA gene):

   • Facilitates intestinal colonization
   • Present in most clinically significant EHEC strains 3

Clinical Significance

EHEC infections can cause:

• Non-bloody and bloody diarrhea (hemorrhagic colitis)
• Hemolytic uremic syndrome (HUS), a life-threatening condition characterized by:
  • Thrombocytopenia
  • Hemolytic anemia
  • Acute renal failure 1, 4

Approximately 8% of persons diagnosed with O157 STEC infection develop HUS, which is a leading cause of acute renal failure in children 1.

Important Considerations

• EHEC infections are most common in children under 5 years, but can affect all age groups 1
• While more common in summer months, EHEC infections can occur year-round 1
• Prompt diagnosis is critical as antibiotic therapy may increase the risk of HUS, and early volume expansion can improve outcomes 1
• Both O157 and non-O157 EHEC can cause severe disease and HUS 4

Understanding the specific strain of EHEC is important for epidemiological tracking, outbreak investigation, and understanding potential disease severity, but all EHEC infections should be considered potentially serious regardless of serotype.

Human EHEC infections typically result from consumption of contaminated food (especially ground beef, leafy greens, and sprouts), contaminated water, contact with animals or their environment, or person-to-person transmission 1, 2.