Paper I
Question
Diarrhaegenic escherichia coli.
Answer
Escherichia coli is normally a harmless commensal of the gut, but several distinct pathotypes, each with characteristic virulence factors, cause diarrhoeal disease by different mechanisms:
- Enterotoxigenic E. coli (ETEC) — the major cause of traveller’s diarrhoea and infantile diarrhoea in developing countries; produces heat-labile toxin (LT, cholera-toxin-like, raises cAMP) and/or heat-stable toxin (ST, raises cGMP), causing profuse watery, non-inflammatory secretory diarrhoea without mucosal invasion.
- Enteropathogenic E. coli (EPEC) — an important cause of infantile diarrhoea in developing countries; adheres to enterocytes and produces characteristic “attaching and effacing” lesions (destruction of microvilli), impairing absorption, without invasion or classical toxin production.
- Enteroinvasive E. coli (EIEC) — closely resembles Shigella both genetically and pathogenically; invades and destroys colonic epithelial cells, causing a dysentery-like illness with fever, abdominal cramps, and blood/mucus in stool.
- Enterohaemorrhagic E. coli (EHEC), notably serotype O157:H7 — produces Shiga-like (Vero) toxin, causing haemorrhagic colitis (bloody diarrhoea) and, in a subset of cases, the serious complication Haemolytic Uraemic Syndrome (HUS) (microangiopathic haemolytic anaemia, thrombocytopenia, acute kidney injury); notably, antibiotic treatment can increase toxin release and HUS risk, so is generally avoided.
- Enteroaggregative E. coli (EAEC) — causes persistent watery diarrhoea, particularly in children and HIV-infected patients, characterized by a “stacked-brick” pattern of bacterial adherence to intestinal epithelium.
Laboratory diagnosis: routine stool culture on MacConkey agar identifies E. coli but cannot distinguish pathotype; specific identification requires detection of virulence genes/toxins by PCR, ELISA (for Shiga toxin, LT/ST), or serotyping (e.g., O157:H7 on sorbitol-MacConkey agar, which EHEC characteristically fails to ferment, unlike most other E. coli).