Which bacterial pathogens commonly cause community-acquired pneumonia (CAP)?

Updated: Oct 31, 2019
  • Author: Stephanie L Baer, MD; Chief Editor: Michael Stuart Bronze, MD  more...
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Answer

Answer

Typical bacterial pathogens that cause CAP include S pneumoniae, H influenza, and M catarrhalis (Gram stains shown below). The frequency with which CAP is attributable to one of these pathogens varies according to epidemiologic factors (eg, seasonality, patient demographics, exposure history) and the diagnostic testing used. In the past, these organisms had been reported to account for most CAP cases. [37] However, with improvement in diagnostic techniques allowing for better identification of viruses and fastidious bacteria, our understanding of the etiologic agents involved in the development of CAP has evolved. As a result, a smaller percentage of CAP cases are now being attributed to these typical bacterial pathogens. In addition, the possibility of polymicrobial infections has received increased awareness; in recent epidemiologic studies, two pathogens, typically a virus and bacteria combination, were identified in over 30% of cases. [1, 35, 5]

Gram stain showing Streptococcus pneumoniae. Gram stain showing Streptococcus pneumoniae.
Gram stain showing Haemophilus influenzae. Gram stain showing Haemophilus influenzae.

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