Escherichia coli Pathotypes
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Direct answer
Escherichia coli pathotypes are strains grouped by virulence mechanism and clinical syndrome, with six main groups: ETEC, EPEC, EHEC, EIEC, EAEC and UPEC. Enterotoxigenic E. coli causes watery travellers' diarrhoea, enterohaemorrhagic E. coli produces Shiga toxin and can cause haemolytic uraemic syndrome, and uropathogenic E. coli is the commonest cause of urinary tract infection. Identification relies on virulence genes or specific assays rather than on routine biochemistry.
What you must remember
- ETEC: produces heat-labile and heat-stable enterotoxins, causing watery diarrhoea in travellers and young children in endemic areas.
- EPEC: causes attaching and effacing lesions through the eae gene and intimin, a classic cause of infant diarrhoea, without Shiga toxin.
- EHEC: produces Shiga toxin, with serotype O157:H7 as the classic example, causing bloody diarrhoea and haemolytic uraemic syndrome.
- EIEC: invades the colonic epithelium in a manner similar to Shigella, causing dysentery, and is Sereny test positive.
- EAEC: adheres in an aggregative pattern, producing a stacked-brick appearance on HEp-2 cells and persistent diarrhoea.
- UPEC: uses P fimbriae to attach to the urinary epithelium and causes cystitis and pyelonephritis.
- Sorbitol MacConkey: O157:H7 does not ferment sorbitol, so its colonies stay colourless while most E. coli turn pink.
Comparing the six pathotypes
Students meet E. coli in diarrhoeal, urinary and neonatal questions, and the pathotype is inferred from the syndrome and the virulence determinant. The table compares the six groups by mechanism, disease and laboratory clue.
| Pathotype | Key mechanism | Typical disease | Laboratory clue |
|---|---|---|---|
| ETEC | Heat-labile and heat-stable enterotoxins | Watery travellers' diarrhoea | Enterotoxin genes on PCR |
| EPEC | Attaching and effacing lesions, eae gene | Infantile diarrhoea | No Shiga toxin |
| EHEC | Shiga toxin, mainly O157:H7 | Bloody diarrhoea and HUS | O157 is sorbitol-negative |
| EIEC | Epithelial invasion, Shigella-like | Dysentery | Sereny test positive |
| EAEC | Aggregative adherence | Persistent diarrhoea | Stacked-brick pattern on HEp-2 cells |
| UPEC | P fimbriae binding urinary epithelium | Cystitis and pyelonephritis | Commonest urinary pathogen |
Most pathotypes are identified by gene detection or specific assays rather than routine culture, because their biochemical profiles overlap with commensal E. coli. Clinicians therefore match the syndrome to the likely pathotype first and then confirm it where the laboratory allows. Shiga toxin-producing strains need careful handling, since antibiotics are generally avoided in suspected EHEC infection.
Where the exam tries to catch you
Pathotype questions reward linking the syndrome to the virulence mechanism rather than to the species name alone.
- Trap: giving antibiotics for every bloody diarrhoea from E. coli Correct: antibiotics are generally avoided in suspected EHEC infection because they may increase the risk of haemolytic uraemic syndrome.
- Trap: choosing EPEC for watery travellers' diarrhoea Correct: ETEC is the classic cause of travellers' diarrhoea, while EPEC is linked to infant diarrhoea.
- Trap: calling EIEC a variant of EHEC Correct: EIEC is a distinct pathotype that invades the colon like Shigella and does not share the Shiga toxin mechanism of EHEC.
Frequently asked questions
Which E. coli pathotype causes haemolytic uraemic syndrome?
Shiga toxin-producing enterohaemorrhagic E. coli, mainly serotype O157:H7, causes haemolytic uraemic syndrome. It produces bloody diarrhoea, and the Shiga toxin damages renal endothelium, leading to acute kidney injury.
Why does enterohaemorrhagic E. coli O157 appear colourless on sorbitol MacConkey agar?
Enterohaemorrhagic E. coli O157:H7 does not ferment sorbitol, so its colonies stay colourless on sorbitol MacConkey agar. Most other E. coli ferment sorbitol and produce pink colonies, which makes this medium useful for screening stool samples.
What is the Sereny test used for in E. coli?
The Sereny test is a guinea pig keratoconjunctivitis test that identifies invasive enteroinvasive E. coli. A positive result confirms the ability of EIEC to invade epithelial cells in a manner similar to Shigella.
Which E. coli pathotype is the commonest cause of urinary tract infection?
Uropathogenic E. coli is the commonest cause of urinary tract infection, using P fimbriae to attach to the urinary epithelium. Its adhesins allow ascending infection of the bladder and kidneys, leading to cystitis and pyelonephritis.
Why do enterotoxigenic E. coli cause watery diarrhoea?
Enterotoxigenic E. coli produces heat-labile and heat-stable enterotoxins that stimulate fluid secretion in the small intestine. The result is watery diarrhoea without invasion of the mucosa.