Shigella and Bacillary Dysentery
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Direct answer
Shigella is a non-motile, usually lactose-negative Gram-negative bacillus that invades the colonic epithelium and causes bacillary dysentery. As few as 10 to 100 organisms can cause disease, so spread through contaminated water, food and hands is efficient in crowded settings. Shiga toxin from some strains adds the risk of haemolytic uraemic syndrome, which makes fluid support and antibiotic choice important.
What you must remember
- Morphology: non-motile, non-capsulated, non-spore-forming and oxidase-negative Gram-negative bacillus.
- Species: S. dysenteriae, S. flexneri, S. boydii and S. sonnei, with S. sonnei common in developed countries and S. flexneri common in much of the developing world.
- Pathogenesis: invades the colonic mucosa, multiplies inside epithelial cells and spreads laterally, causing ulceration.
- Shiga toxin: made mainly by S. dysenteriae type 1, it cleaves 28S rRNA of the 60S ribosomal subunit and halts protein synthesis.
- Clinical picture: fever, cramping abdominal pain, tenesmus and frequent small stools of blood and mucus, with the left colon most affected.
- Complications: haemolytic uraemic syndrome after Shiga toxin exposure, and reactive arthritis linked to HLA-B27.
- Diagnosis: stool culture on MacConkey agar, where lactose-negative colonies are colourless, with confirmation by biochemical tests.
- Treatment: oral or intravenous rehydration first, then an antibiotic such as ciprofloxacin or azithromycin per local susceptibility; antimotility drugs are avoided.
How Shigella produces dysentery
Shigella needs only a small inoculum, because the organism survives gastric acid and then targets the colon. The steps below follow the sequence from ingestion to the bloody stool.
- Survival: the bacillus tolerates gastric acid unusually well for an enteric pathogen, so enough organisms reach the colon to start infection.
- Entry: bacteria cross the epithelium through M cells and then invade enterocytes from the basolateral side.
- Escape: the organism lyses the phagocytic vacuole and multiplies freely in the cytoplasm.
- Spread: actin-based movement driven by IcsA pushes bacteria into neighbouring cells, so the infection moves laterally.
- Inflammation: neutrophils pour into the mucosa and ulceration follows, producing the bloody, mucoid stool that distinguishes dysentery from the watery diarrhoea of cholera.
- Toxin effect: in S. dysenteriae type 1, Shiga toxin injures the colon and the endothelial cells of the kidney, which can lead to haemolytic uraemic syndrome in children.
Where the exam tries to catch you
Shigella questions usually test species identity, motility or the handling of bloody diarrhoea.
- Trap: assuming Shigella is motile because it is a Gram-negative rod Correct: Shigella is non-motile, which separates it from Salmonella and E. coli on motility testing.
- Trap: choosing S. dysenteriae as the commonest species in India Correct: S. flexneri is the species most often isolated in India, while S. dysenteriae type 1 is the epidemic, toxin-producing species.
- Trap: giving loperamide for bloody diarrhoea Correct: antimotility agents can prolong invasive dysentery, so antibiotics and fluids are the right approach.
Frequently asked questions
Which Shigella species causes epidemic dysentery with Shiga toxin?
Shigella dysenteriae type 1 is the species responsible for epidemic dysentery, and its strains produce the potent Shiga toxin. This toxin explains the systemic complications, including haemolytic uraemic syndrome, that can follow infection with this species.
Why is Shigella identified on MacConkey agar?
MacConkey agar is a selective and differential medium on which Shigella forms colourless colonies, because most species do not ferment lactose. Sonnei is a late lactose fermenter and can give pink colonies after prolonged incubation, so lactose testing must be read with care in Shigella sonnei infection.
What is the link between Shigella and reactive arthritis?
Reactive arthritis can follow Shigella infection, typically appearing a few weeks after the diarrhoea has settled. It is associated with the HLA-B27 haplotype and is immune-mediated, so joint cultures are usually negative.
Should every child with bloody diarrhoea due to Shigella receive an antibiotic?
Antibiotics are recommended for proven or strongly suspected Shigella dysentery, mainly to shorten illness and reduce transmission, per current guidance. Ciprofloxacin or azithromycin are commonly used, with susceptibility testing because fluoroquinolone resistance is increasing. Oral rehydration remains the foundation of care whatever antibiotic is chosen.
How does Shiga toxin cause haemolytic uraemic syndrome?
Shiga toxin injures endothelial cells in small vessels, which produces microangiopathic haemolytic anaemia, thrombocytopenia and acute kidney injury. Haemolytic uraemic syndrome after Shigella infection needs supportive care, careful fluid balance and close monitoring of renal function.