Microbiology · Enterobacterales
Shigella !
- Bacterium
- Gram-negative
- Contact
- Exam priority: medium
A bacterium that invades the colon and causes dysentery: bloody diarrhea, fever and straining. A tiny dose is enough to get sick.
Updated 2026-11-01
Also called: Shigella sonnei, Shigella flexneri, Shigella dysenteriae, dysentery bacillus
At a glance
- Gram
- Gram-negative
- Shape
- Rod
- Oxygen
- Facultative anaerobe
- Family
- Enterobacteriaceae
- Stain
- Gram: red rods
Morphology
- Non-motile Gram-negative rod (no flagella)
- Non-lactose fermenter, no H2S: red colonies without black centers on XLD
- Four species: S. sonnei, S. flexneri, S. boydii and S. dysenteriae
Transmission
- Fecal-oral from person to person, through hands, objects, water or food
- Oral-anal sexual contact
- Outbreaks in day care, among travelers and where sanitation is poor
- Contact (on top of standard) for diapered children or incontinent patients
Reservoir: Humans only (and other primates)
Isolation precautions
What the care team adds to standard precautions.
- Contact
- Gown and gloves; dedicated equipment. For C. difficile, soap-and-water hand washing.
Virulence factors
- Very low infectious dose: survives stomach acid
- Enters through M cells, invades colonic epithelium and spreads cell to cell using actin
- Causes ulcers and microabscesses in the colonic mucosa
- Shiga toxin in S. dysenteriae type 1: endothelial damage, hemolytic uremic syndrome
Diseases it causes
- Shigellosis (bacillary dysentery) Bacterial infectious diarrhoea and dysentery →
- Hemolytic uremic syndrome, mainly with S. dysenteriae type 1 Hemolytic uremic syndrome (HUS) →
Clinical picture
- Diarrhea that starts watery and turns to mucus and blood, fever, cramps and tenesmus
- Febrile seizures in young children
- Reactive arthritis days or weeks later, more in HLA-B27 carriers
Diagnosis
- Stool culture on MacConkey, XLD or Hektoen
- Many white and red cells in stool
- Stool PCR panels
Treatment
Rehydration in every case. Antibiotics shorten illness and shedding: azithromycin, fluoroquinolones or ceftriaxone by susceptibility. Resistance to ampicillin and trimethoprim-sulfamethoxazole is common, and extensively drug-resistant strains exist. Antimotility drugs are avoided.
Drug class only; doses and regimens are set by the clinician from the current guideline.
In Pharmacology:
Prevention
- Handwashing with soap and water, especially when changing diapers
- Safe water and sanitation
- Keeping sick food handlers off work
- No licensed vaccine
Test yourself
Think of the answer, then tap the question to see it.
Why does Shigella spread so easily between people?
Its infectious dose is tiny, about 10 to 100 bacteria.
Which lab traits separate Shigella from Salmonella?
Shigella is non-motile, makes no H2S and has no animal reservoir; Salmonella is motile and makes H2S.
Which species makes Shiga toxin and is linked to hemolytic uremic syndrome?
Shigella dysenteriae type 1.
Which joint complication can follow shigellosis?
Reactive arthritis, mainly in people with HLA-B27.
Related
Related conditions
← Back to the group: Enterobacterales
Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Shigellosis (MSD Manual Professional Edition, 2026); About Shigella Infection (CDC, 2024); Fichas y Protocolos (Instituto Nacional de Salud (INS), Colombia, 2026).
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Representative model, for educational purposes only.