AKT · Infections/Key pathogens & antimicrobials

Infectious gastroenteritis & diarrhoea

Organism-by-context — bacterial, viral or protozoal

Overview

Acute diarrhoea ± vomiting, usually self-limiting and managed with rehydration. The exam tests organism-by-context: Norovirus/rotavirus (commonest, watery, outbreaks/children); Campylobacter (commonest bacterial — undercooked poultry, bloody, may mimic appendicitis; a/w Guillain–Barré, reactive arthritis); Salmonella/Shigella (bloody, food/travel); E. coli O157 (bloody → HUS, no antibiotics); C. difficile (post-antibiotic, esp. co-amoxiclav/clindamycin/cephalosporins/PPI); Cryptosporidium (watery, immunocompromised/HIV, modified Ziehl–Neelsen); Giardia (prolonged, bloating/malabsorption, travel/hikers); Staph aureus/Bacillus cereus (rapid, preformed toxin — reheated rice for B. cereus). Most need only oral rehydration; antibiotics are the exception.

Recognise

Watery + outbreak/cruise/children → norovirus; bloody + poultry → Campylobacter; bloody + travel → Salmonella/Shigella
Post-antibiotic, offensive, hospital → C. difficile; watery + HIV/immunocompromised → Cryptosporidium; prolonged + malabsorption + travel → Giardia
Rapid onset (1–6 h) after a meal → preformed toxin (S. aureus; B. cereus with reheated rice)

Red flags

E. coli O157 → HUS (AKI + haemolysis + thrombocytopenia) — do NOT give antibiotics/antimotility agents
C. difficile → toxic megacolon/pseudomembranous colitis; dehydration/AKI in the frail; bloody diarrhoea + sepsis

Differentials & how to tell them apart

Inflammatory bowel diseasechronic, relapsing, systemic features — negative cultures (cross-ref gastro)

Appendicitis / surgical abdomenCampylobacter can mimic it; localising signs + imaging distinguish

Antibiotic-associated (C. difficile)recent antibiotics/PPI, offensive stool, toxin-positive

Investigations

Stool culture + PCR (bacterial); stool C. difficile toxin/GDH if post-antibiotic; modified Ziehl–Neelsen for Cryptosporidium; stool microscopy/antigen for Giardia; U&E/renal function; blood film + FBC if HUS suspected.

Management

Oral rehydration for most; oral vancomycin + stop the trigger for C. difficile; targeted antibiotics only in specific organisms

1Rehydrate and identify the organism by context/stool tests. Most cases are self-limiting and need no antibiotics.Gate: E. coli O157:H7 → NO antibiotics/antimotility (HUS risk). C. difficile → STOP the precipitating antibiotic and give oral vancomycin, isolate with contact precautions (spores resist alcohol gel — soap & water).
2Give targeted antibiotics only where indicated (severe/systemic Campylobacter, Shigella, Giardia, enteric fever); notify infectious bloody diarrhoea/food poisoning; watch for HUS (O157) and toxic megacolon (C. difficile).

Oral rehydration (mainstay) — most gastroenteritis is self-limiting — fluids, not antibiotics

Oral vancomycin (C. difficile, first line) — then fidaxomicin; STOP the causative antibiotic; metronidazole no longer first-line

Targeted antibiotic only if indicated — e.g. azithromycin/ciprofloxacin for severe Campylobacter/Shigella/enteric fever; metronidazole for Giardia; nitazoxanide/immune reconstitution for Cryptosporidium

Avoid antibiotics in E. coli O157 — increases HUS risk — supportive only

Key points

Organism-by-context: poultry+bloody → Campylobacter (commonest bacterial); post-antibiotic → C. difficile (oral vancomycin, spores need soap & water); HIV+watery, modified ZN → Cryptosporidium; travel+prolonged+bloating → Giardia; reheated rice → B. cereus; O157 bloody → HUS, NO antibiotics. Most = rehydrate only.

Monitor & prognosis

Hydration/renal function, stool clearance, C. difficile severity (WCC, lactate, colon), HUS surveillance in O157.

Most resolve with rehydration; C. difficile and HUS are the dangerous exceptions.

Source: NICE CKS gastroenteritis / C. difficile; UKHSA (cross-ref gastroenterology)