Free-living amoebae !
- Parasites
Water and soil amoebae that rarely infect people, but cause severe meningoencephalitis or keratitis when they do.
Updated 2026-11-01
What they are
Naegleria fowleri, Acanthamoeba and Balamuthia live freely in fresh water, soil and dust and need no host. When they get in through the nose, skin, lungs or eye they can cause almost uniformly fatal brain infections or a sight-threatening keratitis.
What they share
- Free-living in the environment; human infection is accidental.
- Not spread from person to person.
- Diagnosis on CSF, biopsy or corneal scrapings; culture on non-nutrient agar overlaid with E. coli; PCR at reference labs.
Microorganisms in this group
- Naegleria fowleriA warm freshwater amoeba that enters through the nose while swimming and destroys the brain within days. It is very rare but almost always fatal.Exam priority: medium
- Acanthamoeba spp.A water and soil amoeba that infects the cornea of contact-lens wearers and, rarely, the brain of immunocompromised people.Exam priority: medium
Test yourself
Think of the answer, then tap the question to see it.
How does Naegleria fowleri reach the brain?
Contaminated water enters the nose, and the amoeba crosses the nasal mucosa and climbs the olfactory nerves to the brain.
Which free-living amoeba is linked to contact lenses?
Acanthamoeba, the cause of a painful keratitis that can lead to loss of vision.
What medium is used to culture free-living amoebae?
Non-nutrient agar overlaid with Escherichia coli, which the amoebae feed on.
Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: CDC - DPDx - Free Living Amebic Infections (CDC, 2024); Naegleria fowleri Infections | Naegleria fowleri Infection | CDC (CDC, 2025).
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Representative model, for educational purposes only.