Harvey's Garden

Escherichia coli

3 min read

  • Sorbitol and lactose fermenters (except for EHEC, which does not ferment sorbitol)
  • Form pink colonies on MacConkey agar
  • Form green colonies with a metallic sheen on eosin-methylene blue agar
  • Present the enterobacterial common antigen (ECA) on their surface
  • Produce beta-galactosidase (cleaves lactose into galactose and glucose)
  • Common virulence factors
    • Lipopolysaccharide (LPS)
      • Activates macrophages → widespread release of IL-1, IL-6 & TNF-α
      • Bacteremia & septic shock
    • K1 capsular polysaccharide
    • Shiga toxin
      • Inactivates 60S ribosomal subunit, halting protein synthesis & causing cell death
      • Gastroenteritis (bloody)
    • Heat-stable/heat-labile enterotoxins
      • Promotes fluid & electrolyte secretion from intestinal epithelium
      • Gastroenteritis (watery)
    • P-fimbriae

Key Pathogenic Strains & Associated Diseases

  1. Enterohemorrhagic E. coli (EHEC)

    • Classic Serotype: O157:H7.
    • Source: Undercooked hamburgers, unpasteurized milk/cider, contaminated sprouts or lettuce.
    • Presentation: Hemorrhagic colitis (bloody diarrhea, severe abdominal cramps). Minimal or no fever.
    • Toxin: Shiga-like toxin (Verotoxin).
    • Mechanism: Inactivates the 60S ribosomal subunit, inhibiting protein synthesis and causing cell death.
    • Complication: Hemolytic Uremic Syndrome (HUS), especially in children. Triad of:
      • Microangiopathic hemolytic anemia (schistocytes on blood smear).
      • Thrombocytopenia.
      • Acute kidney injury.
    • Dx: Does not ferment sorbitol (unlike other E. coli). This is tested on Sorbitol-MacConkey (SMAC) agar, where it forms colorless colonies.
    • Tx: Supportive care (hydration) is critical. AVOID ANTIBIOTICS, as they can increase the risk of HUS by inducing toxin release.

  2. Enterotoxigenic E. coli (ETEC)

    • Classic Disease: Traveler’s Diarrhea.
    • Source: Contaminated food or water (“Montezuma’s Revenge”).
    • Presentation: Profuse, watery (non-bloody) diarrhea.
    • Toxins:
      • Heat-Labile Toxin (LT): Increases cAMP (similar to cholera toxin). (“Labile in the Air”).
      • Heat-Stable Toxin (ST): Increases cGMP (“Stable on the Ground”).
    • Tx: Supportive care, but antibiotics (e.g., fluoroquinolones, azithromycin) may be used for severe cases to shorten duration.

  3. Uropathogenic E. coli (UPEC)

    • Diseases: Most common cause of urinary tract infections (UTIs), including cystitis and pyelonephritis.
    • Virulence Factor: P fimbriae allow for attachment to the uroepithelium, preventing it from being flushed out by urine.
    • Tx: Commonly treated with TMP-SMX, nitrofurantoin, or fluoroquinolones, depending on resistance patterns.

  4. Enteroinvasive E. coli (EIEC)

    • Presentation: Causes an invasive, dysentery-like illness with fever and bloody, mucoid diarrhea.
    • Mechanism: Directly invades the intestinal mucosa, causing inflammation and necrosis. No toxin produced.
    • Clinically similar to Shigella.

  5. Enteropathogenic E. coli (EPEC)

    • Classic Disease: Causes diarrhea, primarily in infants and children (Pediatrics), especially in developing countries. Often involved in nursery outbreaks.
    • Mechanism: Adheres to enterocytes and causes effacement (flattening) of microvilli, leading to malabsorption and watery diarrhea.

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