Paper I
Question
Define food poisoning, Enumerate the organism causing food poisoning. Describe the lab diagnosis (1+3+4)
Answer
Definition: Food poisoning is an acute gastrointestinal illness resulting from ingestion of food or water contaminated with pathogenic microorganisms or their toxins, typically presenting with nausea, vomiting, abdominal cramps, and diarrhoea within hours to a few days of ingestion.
Organisms causing food poisoning:
- Toxin-mediated (pre-formed toxin, rapid onset): Staphylococcus aureus (heat-stable enterotoxin, ~2–6 hours onset); Bacillus cereus (emetic toxin, cereulide, in reheated rice); Clostridium botulinum (botulinum neurotoxin).
- Toxin-mediated (toxin produced in vivo after ingestion): Clostridium perfringens (enterotoxin); Bacillus cereus (diarrhoeal toxin form).
- Infection-type (invasive, longer incubation): Salmonella (non-typhoidal species); Campylobacter jejuni; Vibrio parahaemolyticus; Shigella; E. coli (various pathotypes).
- Viral: Norovirus, Rotavirus.
Laboratory diagnosis:
- Stool culture — on appropriate selective media (e.g., MacConkey, XLD, TCBS) to isolate and identify the causative bacterial pathogen; specimen collection ideally before antibiotic treatment.
- Toxin detection — ELISA for S. aureus enterotoxin, C. perfringens enterotoxin, or C. botulinum toxin (mouse bioassay historically, now largely immunoassay/PCR-based) directly from stool or the implicated food.
- Food sample analysis — culture and toxin testing of the suspected/implicated food item, important for outbreak investigation and confirming the source.
- PCR — increasingly used for rapid, sensitive detection of specific pathogens/toxin genes, including multiplex panels for simultaneous testing of multiple agents.
- Serology — of limited use for acute diagnosis, occasionally used retrospectively in outbreak investigations (e.g., rising antibody titre).
- Blood culture — for cases with systemic/invasive spread (e.g., invasive non-typhoidal Salmonella).
Epidemiological investigation: in outbreak settings, a case-control or cohort study correlating specific food items with illness, combined with laboratory confirmation from stool and food samples, is used to identify the source and vehicle of the outbreak.