Microbiology Cheat Sheet
Diarrhoea: Secretory vs Invasive
Secretory vs invasive diarrhoea for NEET-PG and FMGE: cholera toxin, ETEC LT and ST, Shigella, EHEC O157:H7, Campylobacter, amoebiasis, and Giardia.
MedNext Academy | 3 min read
Diarrhoea: Secretory vs Invasive
Secretory vs invasive diarrhoea for NEET-PG and FMGE: cholera toxin, ETEC LT and ST, Shigella, EHEC O157:H7, Campylobacter, amoebiasis, and Giardia.
This topic covers the microbial causes of diarrhoea and dysentery, distinguishing secretory (enterotoxin-mediated) from invasive (mucosal invasion) mechanisms and their laboratory diagnosis.
High-yield lines
- Secretory diarrhoea produces watery stool with absent or rare faecal leucocytes, while invasive diarrhoea produces bloody mucoid stool with abundant polymorphs.
- Cholera toxin and the heat-labile toxin of ETEC both activate adenylate cyclase and raise cAMP, whereas the heat-stable toxin of ETEC activates guanylate cyclase and raises cGMP.
- Cholera toxin is an AB5 toxin whose B subunit binds GM1 ganglioside and whose A subunit ADP-ribosylates the Gs-alpha protein, causing CFTR-mediated chloride secretion.
- Vibrio cholerae produces yellow (sucrose-fermenting) colonies on TCBS agar, while Vibrio parahaemolyticus produces blue-green non-fermenting colonies.
- Shigella has the lowest infectious dose (10 to 100 organisms) and spreads intracellularly via IcsA-mediated actin comet tails without extracellular exposure.
- Shiga toxin from S. dysenteriae type 1 and Shiga-like toxins from EHEC O157:H7 both cleave 28S rRNA and can cause haemolytic uraemic syndrome.
- EHEC O157:H7 does not ferment sorbitol and appears colourless on sorbitol-MacConkey (CT-SMAC) agar; antibiotics are contraindicated as they increase Shiga toxin release.
- ETEC is the most common bacterial cause of traveller's diarrhoea, while EPEC is the most common cause of infantile diarrhoea in developing countries.
- Shigella is non-motile, H2S negative, and lysine decarboxylase negative, distinguishing it from motile, H2S positive, lysine decarboxylase positive Salmonella.
- Campylobacter jejuni is the most common antecedent of Guillain-Barre syndrome via molecular mimicry between lipo-oligosaccharide and GM1 gangliosides.
- Entamoeba histolytica causes flask-shaped colonic ulcers, erythrophagocytosis (pathognomonic), and right-lobe amoebic liver abscess with anchovy-sauce pus.
- Giardia is the most common protozoal cause of diarrhoea worldwide, causing malabsorption without invasion and being especially severe in IgA deficiency.
- Rotavirus is the most common cause of severe dehydrating diarrhoea in infants, while norovirus is the leading cause of epidemic gastroenteritis in all ages.
- Clostridioides difficile causes antibiotic-associated pseudomembranous colitis via toxin A and toxin B, and its spores resist alcohol-based hand rubs.
Mapped competency codes
- MI3.1
- MI3.2
Continue into the full chapter
This summary maps to MI3-gastrointestinal-and-hepatobiliary-system.
Frequently Asked Questions
How do LT and ST toxins of ETEC differ mechanistically?
The heat-labile toxin activates adenylate cyclase raising cAMP, while the heat-stable toxin activates guanylate cyclase raising cGMP.
Why are antibiotics contraindicated in EHEC O157:H7 infection?
Antibiotics increase Shiga toxin release and the risk of haemolytic uraemic syndrome.
How is Shigella distinguished from Salmonella biochemically?
Shigella is non-motile, H2S negative, and lysine decarboxylase negative, while Salmonella is motile, H2S positive, and lysine decarboxylase positive.
What colour are Vibrio cholerae colonies on TCBS agar?
Yellow, due to sucrose fermentation, in contrast to the blue-green colonies of Vibrio parahaemolyticus.
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