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[Microbiology](https://durgical.com/en/microbiology/) · [Bacteria](https://durgical.com/en/microbiology/#d-bacteria)

# 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

## Key points

- Atypical pneumonia: Mycoplasma, Chlamydia pneumoniae, C. psittaci, Legionella, Coxiella
- Chlamydia: infectious elementary body and dividing reticulate body
- Fever, headache and rash after a tick bite: doxycycline is started without waiting for the lab
- Coxiella needs no vector: it is inhaled in dust from animal births

## Microorganisms in this group

- [**Chlamydia trachomatis** The most common sexually transmitted bacterium: often silent but can cause infertility; it also causes trachoma in the eyes. Exam priority: high](https://durgical.com/en/microbiology/chlamydia-trachomatis/)
- [**Mycoplasma pneumoniae** A tiny wall-less bacterium that causes «walking pneumonia» in children and young adults; penicillin does not affect it. Exam priority: medium](https://durgical.com/en/microbiology/mycoplasma-pneumoniae/)
- [**Chlamydia pneumoniae / Chlamydia psittaci** Two chlamydiae that cause atypical pneumonia: one passes between people by coughing; the other comes from birds such as parrots and pigeons. Exam priority: medium](https://durgical.com/en/microbiology/chlamydia-pneumoniae-chlamydia-psittaci/)
- [**Rickettsia rickettsii / Rickettsia typhi** Bacteria that live inside blood vessels; spread by ticks or fleas, they cause fever with spots that can be fatal. Exam priority: medium](https://durgical.com/en/microbiology/rickettsia-rickettsii-rickettsia-typhi/)
- [**Mycoplasma genitalium / Ureaplasma** Genital mycoplasmas: M. genitalium causes persistent urethritis and is increasingly drug-resistant; Ureaplasma is usually a coloniser. Exam priority: medium](https://durgical.com/en/microbiology/mycoplasma-genitalium-ureaplasma/)
- [**Coxiella burnetii** A very hardy bacterium inhaled in dust from goat, sheep and cattle births; it causes Q fever and sometimes endocarditis. For completeness](https://durgical.com/en/microbiology/coxiella-burnetii/)
- [**Ehrlichia / Anaplasma** Tick-borne bacteria that live inside white blood cells; they cause fever with low white cells and platelets. For completeness](https://durgical.com/en/microbiology/ehrlichia-anaplasma/)
- [**Orientia tsutsugamushi** A bacterium spread by mite larvae (chiggers) that causes scrub typhus, with a black scab at the bite site. For completeness](https://durgical.com/en/microbiology/orientia-tsutsugamushi/)

## 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.

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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](https://www.cdc.gov/mycoplasma/about/index.html) (Centers for Disease Control and Prevention (CDC), 2025); [About Rocky Mountain Spotted Fever | Rocky Mountain Spotted Fever (RMSF) | CDC](https://www.cdc.gov/rocky-mountain-spotted-fever/about/index.html) (Centers for Disease Control and Prevention (CDC), 2024); [About Q fever | Q Fever | CDC](https://www.cdc.gov/q-fever/about/index.html) (Centers for Disease Control and Prevention (CDC), 2024).

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/microbiology/groups/intracellular-and-atypical-bacteria/