Spirochetes !
- Bacteria
Long, spiral bacteria that move like a corkscrew: syphilis, leptospirosis, Lyme disease and relapsing fever.
Updated 2026-11-01
What they are
Spirochetes are thin, helical bacteria with internal flagella (endoflagella) between the wall and the outer membrane, which make them spin. They are so thin that Gram stain barely shows them: they are seen with dark-field microscopy, silver stains or, for Borrelia, on a blood smear. The medical genera are Treponema, Leptospira and Borrelia.
What they share
- Spiral shape with endoflagella
- Gram-negative-type outer membrane, but they stain poorly with Gram
- Diagnosis is usually serological or molecular
- The Jarisch-Herxheimer reaction may follow the first antibiotic dose
Microorganisms in this group
- Treponema pallidumThe syphilis spirochete: it spreads through sex and from mother to baby in pregnancy; penicillin cures it.Exam priority: high
- Leptospira interrogansA spirochete from the urine of rats and other animals; caught from water or mud, especially after floods, it can harm kidneys and liver.Exam priority: high
- Borrelia burgdorferiThe spirochete of Lyme disease, spread by Ixodes ticks in North America and Europe; it is not endemic in Colombia.Exam priority: medium
- Borrelia recurrentis and tick-borne relapsing fever borreliaeBorreliae spread by lice or soft ticks that cause fevers that come and go for weeks.For completeness
Test yourself
Think of the answer, then tap the question to see it.
Which technique shows T. pallidum in chancre fluid?
Dark-field microscopy (or direct immunofluorescence); Gram stain fails because it is too thin.
Which spirochete is acquired from water contaminated with rat urine?
Leptospira interrogans, which enters through broken skin or mucous membranes.
What is the Jarisch-Herxheimer reaction?
Fever, chills and myalgia in the first hours of treating a spirochetal infection, caused by bacterial lysis; it is not an allergy.
Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Syphilis - STI Treatment Guidelines (Centers for Disease Control and Prevention (CDC), 2021); About Leptospirosis | Leptospirosis | CDC (Centers for Disease Control and Prevention (CDC), 2024); About Tick and Louse-borne Relapsing Fevers (Centers for Disease Control and Prevention (CDC), 2024).
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Representative model, for educational purposes only.