Which atypical organisms can cause bacterial pneumonia?

Updated: Sep 30, 2020
  • Author: Justina Gamache, MD; Chief Editor: Guy W Soo Hoo, MD, MPH  more...
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Atypical organisms are generally associated with a milder form of pneumonia, the so-called "walking pneumonia." A feature that makes these organisms atypical is the inability to detect them on Gram stain or to cultivate them in standard bacteriologic media. [23, 3] Atypical organisms include the following:

  • Mycoplasma species: The mycoplasmas are the smallest known free-living organisms in existence. These organisms lack cell walls (and therefore are not apparent after Gram stain) but have protective 3-layered cell membranes.

  • Chlamydophila species (C psittaci, C pneumoniae): Psittacosis, also known as parrot disease or parrot fever, is caused by C psittaci and is associated with the handling of various types of birds.

  • Legionella species: Legionella species are gram-negative bacteria found in freshwater and are known to grow in complex water distribution systems. Institutional water contamination is frequently noted in endemic outbreaks. Legionellapneumophila is the causative agent of the majority of Legionnaires' disease. Other Legionella species are known to infect the lower respiratory system.

  • Coxiella burnetii:C burnetii is the causative agent of Q fever. It is spread from animals to humans; person-to-person transmission is unusual. Animal reservoirs typically include cats, sheep, and cattle.

  • Bordetella pertussis:B pertussis is the agent responsible for pertussis or whooping cough.

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