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Microbiology · Intracellular and atypical bacteria

Mycoplasma pneumoniae !

  • Bacterium
  • Does not Gram-stain
  • Droplet
  • Exam priority: medium

A tiny wall-less bacterium that causes «walking pneumonia» in children and young adults; penicillin does not affect it.

Updated 2026-11-01

At a glance

Gram
Does not Gram-stain
Shape
Pleomorphic (no fixed shape); cell with an attachment tip
Oxygen
Facultative aerobe
Family
Mycoplasmataceae
Stain
Does not stain with Gram (no wall)
Size
One of the smallest free-living bacteria

Morphology

  • No cell wall; membrane contains sterols (cholesterol)
  • Tiny «fried-egg» colonies on special media, after weeks

Transmission

  • Respiratory droplets from coughing or sneezing, with close contact
  • Outbreaks in schools, barracks, boarding schools and households

Reservoir: Humans only

Isolation precautions

What the care team adds to standard precautions.

Droplet
Surgical mask within about a metre; single room.

Virulence factors

  • P1 adhesin: attaches to ciliated epithelium and paralyses it
  • CARDS toxin and hydrogen peroxide damage the respiratory epithelium
  • Induces IgM antibodies against red cells (cold agglutinins)

Diseases it causes

  • Atypical («walking») community-acquired pneumonia Community-acquired pneumonia →
  • Tracheobronchitis, pharyngitis; cold-agglutinin haemolytic anaemia; erythema multiforme

Clinical picture

  • Persistent dry cough, headache and low-grade fever; the patient looks better than expected
  • Interstitial infiltrates on X-ray
  • Children under five may present with wheezing or diarrhoea

Diagnosis

  • PCR on a respiratory swab: method of choice
  • IgM serology; cold agglutinins are a classic but non-specific clue
  • Not seen on Gram; culture is slow and rarely used

Treatment

Macrolides (azithromycin) first; doxycycline or respiratory fluoroquinolones as alternatives. Beta-lactams fail because there is no wall. Macrolide resistance is high in some regions, especially Asia.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

Prevention

  • Hand and cough hygiene
  • Droplet precautions in hospital: surgical mask within about a metre

Test yourself

Think of the answer, then tap the question to see it.

Why is Mycoplasma not seen on Gram stain?

Because it has no peptidoglycan cell wall.

Which haematological complication is linked to M. pneumoniae?

Cold-agglutinin haemolytic anaemia (IgM antibodies against the I antigen).

Which isolation precaution does nursing apply for Mycoplasma pneumonia?

Droplet precautions, on top of standard precautions.

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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 (Centers for Disease Control and Prevention (CDC), 2025).

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