Microbiology
Gastrointestinal and Hepatobiliary Infections
Gut and liver infections for MBBS: cholera, dysentery, enteric fever, Helicobacter pylori, amoebiasis and viral hepatitis with stool tests, mapped to NMC codes MI3.1 to MI3.8.
MedNext Academy | 3 min read
Gastrointestinal and Hepatobiliary Infections
Gut and liver infections for MBBS: cholera, dysentery, enteric fever, Helicobacter pylori, amoebiasis and viral hepatitis with stool tests, mapped to NMC codes MI3.1 to MI3.8.
This chapter surveys the microbes that infect the gut and hepatobiliary tract. It covers the bacterial, viral and parasitic causes of diarrhoea and dysentery, enteric fever, peptic ulcer disease and the viral hepatitides, alongside the stool and serological tests used to diagnose them.
High-yield: Gastrointestinal and Hepatobiliary Infections
- Vibrio cholerae causes profuse rice-water stool through cholera toxin, which raises cyclic AMP and drives massive fluid secretion.
- Enterotoxigenic Escherichia coli is the leading cause of traveller's diarrhoea through its heat-labile and heat-stable toxins.
- Shigella and enteroinvasive Escherichia coli cause dysentery with blood and mucus by invading the colonic mucosa.
- Salmonella Typhi causes enteric fever with a step-ladder fever, relative bradycardia and rose spots.
- The Widal test detects agglutinating antibodies to Salmonella O and H antigens but has limited specificity.
- Rotavirus is the commonest cause of severe watery diarrhoea in infants and is now covered by an oral vaccine.
- Campylobacter jejuni is a common bacterial diarrhoea and is linked to Guillain-Barre syndrome.
- Clostridioides difficile causes antibiotic-associated pseudomembranous colitis, diagnosed by its toxins A and B.
- Helicobacter pylori is a urease-positive spiral organism causing peptic ulcer disease and gastric malignancy.
- The rapid urease test, urea breath test and stool antigen detect Helicobacter pylori infection.
- Entamoeba histolytica causes amoebic dysentery and can spread to form a right-lobe liver abscess with anchovy-sauce pus.
- Hepatitis A and E spread by the faecal-oral route and cause acute self-limiting hepatitis, with hepatitis E dangerous in pregnancy.
- Hepatitis B and C spread by blood and body fluids and can cause chronic hepatitis, cirrhosis and hepatocellular carcinoma.
- Hanging-drop microscopy shows the darting motility of Vibrio cholerae, aiding rapid presumptive diagnosis.
- Stool culture on selective media such as TCBS for Vibrio and XLD for Salmonella and Shigella guides identification.
Diarrhoea by mechanism
- **Watery, toxin-mediated:** Vibrio cholerae, enterotoxigenic Escherichia coli, rotavirus. Non-inflammatory, no blood.
- **Dysentery, invasive:** Shigella, enteroinvasive Escherichia coli, Entamoeba histolytica. Blood and mucus.
- **Enteric fever:** Salmonella Typhi and Paratyphi. Systemic illness, positive early blood culture.
- **Antibiotic-associated:** Clostridioides difficile with pseudomembranous colitis, toxins A and B.
NMC competencies in this chapter
- **MI3.1:** Microbial causes of diarrhoea, dysentery and food poisoning
- **MI3.2:** Enteric fever: aetiology, pathogenesis and clinical features
- **MI3.3:** Laboratory diagnosis of gastrointestinal infections and stool examination
- **MI3.4:** Helicobacter pylori and acid-peptic disease
- **MI3.5:** Parasitic infections of the gut and hepatobiliary system
- **MI3.6:** Viral hepatitis: types, transmission and serological markers
- **MI3.7:** Specimen collection, transport and culture media selection
- **MI3.8:** Prevention, oral rehydration and clinical correlation
Frequently Asked Questions
How does cholera cause such severe diarrhoea?
Cholera toxin permanently activates adenylate cyclase, raising cyclic AMP in enterocytes, which drives massive secretion of chloride and water and produces the classic rice-water stool.
How is Helicobacter pylori diagnosed?
Non-invasive tests include the urea breath test and stool antigen, while endoscopic biopsy allows the rapid urease test, histology and culture. The organism is urease-positive, which underlies these tests.
Which hepatitis viruses are dangerous in pregnancy?
Hepatitis E carries a high risk of fulminant hepatitis and death in pregnant women, which is why it is emphasised despite the faecal-oral spread it shares with hepatitis A.
What is the difference between watery diarrhoea and dysentery?
Watery diarrhoea is usually toxin-mediated and non-inflammatory with no blood, as in cholera, while dysentery is invasive and inflammatory with blood and mucus, as in Shigella and amoebiasis.
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