Microbiology · Enterobacterales
Salmonella Typhi !
- Bacterium
- Gram-negative
- Contact
- Exam priority: high
A Salmonella that infects only humans and causes typhoid fever: a long high fever from contaminated water or food.
Updated 2026-11-01
Also called: Salmonella enterica serovar Typhi, Salmonella Paratyphi, typhoid bacillus
At a glance
- Gram
- Gram-negative
- Shape
- Rod
- Oxygen
- Facultative anaerobe
- Family
- Enterobacteriaceae
- Stain
- Gram: red rods
Morphology
- Motile Gram-negative rod (flagella, H antigen)
- Non-lactose fermenter; makes H2S (black-centered colonies on XLD)
- Capsule carrying the Vi antigen
Transmission
- Fecal-oral: water or food contaminated with feces or sewage
- Carrier food handlers who do not wash their hands
- Contact (on top of standard) for diapered or incontinent patients
Reservoir: Humans only: patients and chronic carriers (gallbladder)
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
- Vi capsule: resists complement and phagocytosis
- Invades Peyer patches through M cells (type III secretion system)
- Survives inside macrophages and travels to liver, spleen and bone marrow
- Colonizes the gallbladder, especially with stones, creating chronic carriers
- LPS: fever and systemic toxicity
Diseases it causes
- Typhoid and paratyphoid fever (enteric fever) Typhoid (enteric) fever →
Clinical picture
- Fever that climbs step by step over days, headache, malaise and abdominal pain
- Pulse slower than the fever predicts (relative bradycardia)
- Rose spots on the abdomen, hepatosplenomegaly, constipation or diarrhea
- Complications around week three: intestinal bleeding and perforation
Diagnosis
- Blood culture: the key test in week one
- Bone marrow culture: the most sensitive, even after antibiotics
- Stool and urine cultures turn positive later; useful to find carriers
- The Widal test is inaccurate and does not confirm the diagnosis
Treatment
Antibiotics chosen by local resistance: third-generation cephalosporins (ceftriaxone), azithromycin, or fluoroquinolones where they still work; carbapenems for extensively drug-resistant strains. Resistance is common in South Asia, with extensively drug-resistant strains reported in Pakistan.
Drug class only; doses and regimens are set by the clinician from the current guideline.
In Pharmacology:
Prevention
- Safe water and sanitation
- Food-handler hygiene and carrier control
- Vaccines: conjugate (TCV), injectable Vi polysaccharide, and live oral
- For travelers: well-cooked food, no ice or water of doubtful origin
Vaccine: Single-dose typhoid conjugate vaccine (TCV) from 6 months, Vi polysaccharide vaccine and a live oral vaccine. In Colombia it is not in the routine PAI schedule; it is used for travelers and risk groups.
Test yourself
Think of the answer, then tap the question to see it.
What is the only reservoir of Salmonella Typhi?
Humans, including chronic carriers with the bacterium in the gallbladder.
Which test is most useful in the first week of typhoid fever?
Blood culture; bone marrow culture is the most sensitive.
Which classic cardiovascular sign goes with typhoid fever?
Relative bradycardia: a pulse slower than expected for the temperature.
Which serious complication appears around the third week?
Intestinal bleeding or perforation in the ileum, where the Peyer patches are.
Which capsular antigen is the basis of the injectable vaccines?
The Vi antigen.
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: Typhoid (WHO, 2023); About Typhoid Fever and Paratyphoid Fever (CDC, 2024); Fichas y Protocolos (Instituto Nacional de Salud (INS), Colombia, 2026).
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Representative model, for educational purposes only.