Intracellular and atypical bacteria !
- Bacteria
Bacteria that live inside cells or lack a wall: Chlamydia, Rickettsia, Coxiella, Ehrlichia and Mycoplasma.
Updated 2026-11-01
What they are
This group gathers bacteria that routine methods do not show or grow: the obligate intracellular ones (Chlamydia, Rickettsia, Orientia, Ehrlichia, Anaplasma, Coxiella) and the mycoplasmas, which have no cell wall. Beta-lactams therefore fail, and drugs that work inside cells, such as tetracyclines and macrolides, are used.
What they share
- They do not grow on routine agar: diagnosis by PCR or serology
- Beta-lactams ineffective (no wall, or hidden inside cells)
- Doxycycline is the core drug for rickettsiae, Ehrlichia and Coxiella
- Several are spread by ticks, fleas, lice or mites
Microorganisms in this group
- Chlamydia trachomatisThe most common sexually transmitted bacterium: often silent but can cause infertility; it also causes trachoma in the eyes.Exam priority: high
- Mycoplasma pneumoniaeA tiny wall-less bacterium that causes «walking pneumonia» in children and young adults; penicillin does not affect it.Exam priority: medium
- Chlamydia pneumoniae / Chlamydia psittaciTwo chlamydiae that cause atypical pneumonia: one passes between people by coughing; the other comes from birds such as parrots and pigeons.Exam priority: medium
- Rickettsia rickettsii / Rickettsia typhiBacteria that live inside blood vessels; spread by ticks or fleas, they cause fever with spots that can be fatal.Exam priority: medium
- Mycoplasma genitalium / UreaplasmaGenital mycoplasmas: M. genitalium causes persistent urethritis and is increasingly drug-resistant; Ureaplasma is usually a coloniser.Exam priority: medium
- Coxiella burnetiiA very hardy bacterium inhaled in dust from goat, sheep and cattle births; it causes Q fever and sometimes endocarditis.For completeness
- Ehrlichia / AnaplasmaTick-borne bacteria that live inside white blood cells; they cause fever with low white cells and platelets.For completeness
- Orientia tsutsugamushiA bacterium spread by mite larvae (chiggers) that causes scrub typhus, with a black scab at the bite site.For completeness
Test yourself
Think of the answer, then tap the question to see it.
Why does amoxicillin not treat Mycoplasma pneumonia?
Mycoplasma has no peptidoglycan wall, the target of beta-lactams.
Which antibiotic is used for rickettsial disease, even in children?
Doxycycline, because the risk of death without treatment far outweighs the risk of tooth staining.
Which bacterium of this group spreads without a vector, by inhalation?
Coxiella burnetii, found in placentas, birth fluids and manure of goats, sheep and cattle.
Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: About Mycoplasma pneumoniae Infection | M. pneumoniae | CDC (Centers for Disease Control and Prevention (CDC), 2025); About Rocky Mountain Spotted Fever | Rocky Mountain Spotted Fever (RMSF) | CDC (Centers for Disease Control and Prevention (CDC), 2024); About Q fever | Q Fever | CDC (Centers for Disease Control and Prevention (CDC), 2024).
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Representative model, for educational purposes only.