Overview
Shigella, named after Kiyoshi Shiga who first isolated it, is the causative genus of bacillary dysentery — a diarrhoeal disease characterised by frequent, scanty, blood- and mucus-containing stools. Humans are the only natural host, and the infective dose is remarkably low, as few as 10–100 organisms.
This lesson covers the morphology, cultural and biochemical characteristics, classification into four species, toxin production, and laboratory diagnosis of Shigella, which remains an important cause of dysentery worldwide, particularly in areas with poor sanitation.
Learning Objectives
After this lesson you will be able to…- Describe the morphological and cultural characteristics of Shigella species.
- Classify Shigella species based on biochemical and serological characteristics.
- Describe the laboratory diagnosis of shigellosis.
Clinical Story
Why This MattersA 3-year-old child in a crowded refugee camp develops frequent, scanty stools mixed with blood and mucus, accompanied by abdominal cramps and painful straining (tenesmus). Given the extremely low infective dose of Shigella and the risk of rapid person-to-person spread in crowded conditions, the outbreak investigation team urgently needs the lab to isolate and identify the species from a fresh stool sample to guide both treatment and public health response.
Core Concepts
Shigellae are short, Gram-negative rods that are non-motile, non-sporing and non-capsulated. They are aerobes and facultative anaerobes, growing over 10–40°C (optimum 37°C) and pH 7.4 on ordinary media. Deoxycholate Citrate Agar (DCA) is a useful selective medium, while growth is inhibited on Wilson and Blair's bismuth sulphite medium (unlike Salmonella, which grows well on it).
Shigella are not specially resistant organisms — they are killed at 56°C in one hour and by 1% phenol in 30 minutes. They remain viable in ice for 1–6 months and survive in moist environments, but die within hours in faeces due to the acidity produced by co-existing coliform growth.
Shigella are MR positive and reduce nitrates to nitrites. Catalase is produced by all species except Sh. dysenteriae type I. Glucose is fermented with acid production but no gas. Mannitol fermentation is the key classification criterion, dividing Shigella into mannitol-fermenting and mannitol-non-fermenting species.
Sh. dysenteriae (Subgroup A) — mannitol non-fermenting, ten serotypes, indole negative, always catalase negative; type 1 produces a toxin with neurotoxic, enterotoxic and cytotoxic activity. Sh. flexneri (Subgroup B) — mannitol fermenting, biochemically heterogeneous and antigenically complex. Sh. boydii (Subgroup C) — named after Boyd. Sh. sonnei (Subgroup D) — ferments lactose and sucrose late, indole negative, causes the mildest form of bacillary dysentery.
Shigella causes bacillary dysentery via ingestion; the minimum infective dose is remarkably low (10–100 bacilli) because the organisms can survive gastric acidity. Humans are the only natural host. Incubation period is short (1–7 days, usually 48 hours). Clinical features include frequent, scanty, loose stools with blood and mucus, abdominal cramps and tenesmus. Complications include arthritis, toxic neuritis, conjunctivitis, parotitis, intussusception, and haemolytic uraemic syndrome — collectively termed shigellosis.
Diagnosis is made by isolating bacilli from faeces. Fresh stool should be inoculated without delay or transported in a medium such as Sachs' buffered glycerol saline (pH 7.0–7.4); mucus-containing portions of the stool give the best yield. MacConkey and DCA plates are inoculated and, after overnight incubation at 37°C, examined for non-lactose-fermenting colonies, which are then tested for motility (negative) and full biochemical reactions to confirm and speciate the isolate.
Laboratory Principle
Shigella identification hinges on two complementary biochemical distinctions from other Enterobacteriaceae: it is non-motile (unlike the actively motile Salmonella and most Escherichia coli), and it produces acid without gas from glucose fermentation, reflecting a purely fermentative rather than mixed gas-forming metabolism. Mannitol fermentation further sub-classifies the genus into four species groups, while selective media such as DCA exploit the organism's relative tolerance of bile salts compared to normal flora, and its intolerance of bismuth sulphite (unlike Salmonella) helps exclude it on Wilson-Blair medium — together enabling a stepwise, cost-effective identification pathway in the routine laboratory.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| Deoxycholate Citrate Agar (DCA) | Selective medium | Selective isolation of Shigella from stool | 2–8°C |
| MacConkey Agar | Bile salts, lactose | Screening for non-lactose-fermenting colonies | 2–8°C |
| Sachs' Buffered Glycerol Saline | pH 7.0–7.4 transport medium | Transport of stool specimens | Room temperature, sealed |
| Mannitol Fermentation Medium | Mannitol sugar broth | Key classification test (fermenter vs non-fermenter) | 2–8°C |
| Kovac's Reagent | p-DMAB reagent | Indole test | 2–8°C, protect from light |
| Shigella Group-Specific Antisera | Polyvalent and monovalent | Serological confirmation and speciation | 2–8°C |
Step-by-Step Procedure
Collect mucus-containing portions of fresh faeces without delay; if delay is unavoidable, use Sachs' buffered glycerol saline transport medium.
Streak onto MacConkey and DCA plates; incubate at 37°C overnight.
Look for non-lactose-fermenting (pale) colonies on both media, distinguishing Shigella from lactose-fermenting normal flora.
Confirm non-motility, a key feature distinguishing Shigella from motile Salmonella and E. coli.
Test MR (positive), nitrate reduction (positive), catalase (positive except Sh. dysenteriae type 1), glucose fermentation (acid, no gas), and mannitol fermentation for species classification.
Perform slide agglutination with polyvalent and monovalent group-specific antisera to confirm species (Sh. dysenteriae, flexneri, boydii, or sonnei).
Flow Diagram
Quality Control
Include known Shigella flexneri and Shigella sonnei reference strains as positive controls, and a motile, lactose-fermenting E. coli as a negative control, with every new batch of DCA and MacConkey media, confirming expected selectivity and colony morphology.
Participate in EQAS programmes for enteric pathogen identification, particularly important for outbreak surveillance given Shigella's very low infective dose and rapid person-to-person transmission potential in crowded or poor-sanitation settings.
Reference Values
Key Diagnostic Parameters⚠️ Reference ranges may vary between laboratories. Always apply your laboratory's established reference intervals.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Non-motile, non-lactose-fermenting bacilli, MR+, mannitol non-fermenting | Suggestive of Sh. dysenteriae (Subgroup A) | Confirm serologically; monitor for toxin-related complications |
| Non-motile, mannitol-fermenting colonies, biochemically heterogeneous | Suggestive of Sh. flexneri (Subgroup B) | Confirm serologically; common cause of endemic dysentery |
| Non-motile, late lactose/sucrose fermenter, indole negative | Suggestive of Sh. sonnei (Subgroup D) | Confirm serologically; typically causes mildest dysentery |
| Bloody, mucus-laden stool with tenesmus in a young child | Clinical picture consistent with bacillary dysentery (shigellosis) | Stool culture, supportive care, and appropriate antibiotics if indicated |
Common Errors & How to Avoid Them
Cause: Shigella die within hours in faeces due to acidity produced by co-existing coliforms, so delayed transport significantly reduces the chance of a positive culture.
Prevention: Process fresh stool without delay or transport promptly in Sachs' buffered glycerol saline to preserve organism viability.
Cause: Sampling only formed, non-mucoid stool may miss the small numbers of Shigella present, since mucus-containing portions have the highest yield.
Prevention: Always select visibly mucus-containing or bloody portions of the stool sample for inoculation.
Cause: Rare non-motile E. coli strains can superficially resemble Shigella on selective media.
Prevention: Always confirm with the full biochemical panel (including mannitol fermentation and serological testing) rather than motility testing alone.
Laboratory Tips from the Bench
Because Shigella has such a low infective dose (as few as 10 organisms), always handle stool specimens and cultures in a biosafety cabinet with strict hand hygiene to prevent laboratory-acquired infection.
When investigating a suspected shigellosis outbreak, prioritise fast turnaround and immediate plating of fresh specimens over batching samples, since organism viability drops quickly in stool at room temperature.
Remember the four Shigella subgroups by their letter-species pairing: A = dysenteriae (most severe, toxin-producing), B = flexneri, C = boydii, D = sonnei (mildest) — 'A is for the most Aggressive disease.'
Important Notes
Shigella requires only 10–100 organisms to cause infection — far lower than most other enteric pathogens — making it highly transmissible in settings with poor sanitation, overcrowding, or inadequate hand hygiene, such as refugee camps, childcare centres and institutions.
Sh. dysenteriae type 1 produces a potent toxin (Shiga toxin) with neurotoxic, enterotoxic and cytotoxic activity — the same toxin family later found in Shiga-toxin-producing E. coli (STEC), reflecting shared evolutionary toxin genes.
Interactive Quiz
Test Your KnowledgeFlashcards
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Clinical Case Study
Apply Your Knowledge2-day history of frequent, scanty stools containing visible blood and mucus, associated with abdominal cramping and painful straining (tenesmus). Several other children in the same camp section report similar symptoms.
The clinical picture of bloody, mucoid, scanty stools with tenesmus in a crowded setting, combined with non-motile, non-lactose-fermenting, mannitol-negative colonies on DCA, strongly suggests Shigella dysenteriae infection — the most severe Shigella subgroup, capable of producing a potent toxin and posing an outbreak risk given the crowded living conditions.
- →Non-motility and mannitol fermentation pattern together help rapidly narrow Shigella speciation.
- →The low infective dose of Shigella makes outbreak control in crowded settings a public health priority.
- →Sh. dysenteriae type 1's toxin can cause severe systemic and neurological complications beyond simple dysentery.
Frequently Asked Questions
Shigella can survive the acidic environment of the stomach more effectively at lower numbers, meaning far fewer organisms (as few as 10) are needed to establish infection compared to Salmonella, which typically requires a much larger inoculum.
Wilson-Blair (bismuth sulphite) medium is formulated to select for Salmonella, which tolerates and can even use bismuth sulphite, while Shigella lacks this tolerance and is inhibited — making DCA the preferred selective medium for Shigella instead.
No — Sh. dysenteriae, particularly type 1 with its potent Shiga toxin, causes the most severe disease with the highest complication risk, while Sh. sonnei typically causes the mildest form of bacillary dysentery.
Quick Revision
10-Minute ReviewKey Takeaways
- Shigella causes bacillary dysentery, with humans as the only natural host.
- Non-motility and mannitol fermentation pattern are the key laboratory differentiators.
- The extremely low infective dose makes Shigella highly transmissible in crowded, poor-sanitation settings.
- Sh. dysenteriae type 1's toxin can cause severe systemic complications beyond typical dysentery.
- Prompt specimen processing is essential, as Shigella loses viability quickly in stool.
Competency Checklist
Track Your MasteryReferences
- National Institute of Open Schooling. Microbiology — Lesson 24: Shigella.
- Forbes BA, Sahm DF, Weissfeld AS. Bailey & Scott's Diagnostic Microbiology.
- World Health Organization. Guidelines for the control of shigellosis.