What causes community-acquired pneumonia (CAP)?

Updated: Jul 22, 2021
  • Author: Folusakin O Ayoade, MD; Chief Editor: Thomas E Herchline, MD  more...
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A number of pathogens can give rise to CAP, generally categorized into typical and atypical pathogens. A third category seen are respiratory viruses. Overall, the most common causes are Streptococcus pneumoniae and respiratory viruses. However, in a large portion of the population, around 62%, no pathogen is detected despite extensive microbiologic evaluation. [1]

Typical bacterial pathogens that cause CAP include Streptococcus pneumoniae (penicillin-sensitive/resistant strains), Haemophilus influenzae (ampicillin-sensitive/resistant strains), Moraxella catarrhalis (all strains penicillin-resistant), Staphylococcus aureus, Group A streptococci, aerobic gram-negative bacteria (eg, Enterobacteriaceae such as Klebsiella spp or Escherichia coli) and microaerophilic bacteria and anaerobes. CAP is usually acquired via inhalation or aspiration of a pulmonary pathogen into a lung segment or lobe. Less commonly, CAP results from secondary bacteremia from a distant source, such as Escherichia coli urinary tract infection and/or bacteremia. Aspiration pneumonia is the only form of CAP typically caused by polymicrobic infection (eg, aerobic/anaerobic oral organisms). Staphylococcal aureus may cause CAP in patients with influenza. Pseudomonas aeruginosa is a cause of CAP in patients with bronchiectasis or cystic fibrosis. [1]

Atypical pathogen CAP manifests a variety of pulmonary and extrapulmonary findings (eg, CAP plus diarrhea). Atypical bacteria are defined as bacteria with intrinsic resistance to beta-lactams and their inability to be visualized on Gram stain or cultured using traditional techniques. They can be divided into those caused by either zoonotic or nonzoonotic atypical pathogens. Zoonotic atypical CAP pathogens include Chlamydophila (Chlamydiapsittaci (psittacosis), Coxiella burnetii (Q fever), and Francisella tularensis (tularemia). Nonzoonotic atypical CAP pathogens include Mycoplasma pneumoniaeLegionella species, and Chlamydia pneumoniae. [1]

Respiratory viruses include: Influenza A and B viruses, Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), other coronaviruses (eg, Middle East respiratory syndrome CoV, severe acute respiratory syndrome CoV, CoV-229E, CoV-NL63, CoV-OC43, CoV-HKU1), rhinoviruses, parainfluenza viruses, adenoviruses, respiratory syncytial virus, human metapneumovirus and human bocaviruses. [1]

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