- Community-acquired pneumonia (CAP) may be caused by a variety of bacterial pathogens that can be classified as either Gram-positive, Gram-negative, or atypical.1
- Streptococcus pneumoniae is the most frequently isolated bacterial pathogen, detected in 5–15% of cases of patients who require hospitalization.1,2
Various pathogens, including many different bacteria, can cause CAP. Streptococcus pneumoniae is the most frequently isolated bacterial pathogen, detected in up to 10–15% of cases of patients who require hospitalization. 1,2 Bacterial pathogens that cause CAP can be classified as either Gram-positive, Gram-negative or atypical.
Streptococcus pneumoniae is the most commonly identified cause of CAP, still implicated in 5–15% of cases requiring hospitalization in the US, despite declining in frequency.1,2 Drug-resistant Streptococcus pneumoniae has been classified by CDC as a serious threat based on the current rates of resistance to commonly used antibiotics.3
Staphylococcus aureus is an important cause of CAP. Methicillin-resistant Staphylococcus aureus (MRSA) can occur in the
setting of recent or concurrent influenza infection and may cause severe disease in previously healthy patients.4
Haemophilus influenzae and Klebsiella pneumoniae often cause CAP in patients with underlying structural lung disease such as chronic obstructive pulmonary disease (COPD) or bronchiectasis.1 Patients with alcohol use disorders are also at risk of infection with Klebsiella pneumoniae.5
Approximately one third of H. influenza bacteria produce β-lactamase (conferring resistance to amoxicillin, penicillin, aminopenicillin, and first generation cephalosporins).1
The atypical bacteria are intracellular pathogens that are difficult to detect with routine cultures and do not respond to treatment with β-lactam antibiotics.1,6Mycoplasma pneumoniae and Chlamydophila pneumoniae are associated with milder disease that is often self-limiting in younger adults and children.1,7
CAP caused by Legionella spp. may be severe and associated with outbreaks from contaminated water sources.1
The frequency at which specific pathogens are identified as the cause of CAP has shifted over time. In the US there has been a decline in the prevalence of bacterial pathogens such as S. pneumoniae, due in part to widespread use of pneumococcal vaccination. In contrast, the detection of respiratory viruses has increased. Most importantly, recent studies failed to establish an etiologic diagnosis in more than 50% patients with CAP.1,3
- Musher DM, Thorner AR. Community-acquired pneumonia. N Engl J Med. 2014;371(17):1619-1628.
- Jain S, Self WH, Wunderink RG, et al. Community-Acquired Pneumonia Requiring Hospitalization among U.S. Adults. N Engl J Med. 2015;373(5):415-427.
- Centers for Disease Control and Prevention. Antibiotic resistance threats in the United States 2019. Available from: https://www.cdc.gov/ drugresistance/pdf/threats-report/2019-ar-threats-report-508.pdf (accessed October 09 2020).
- Mulcahy ME, McLoughlin RM. Staphylococcus aureus and Influenza A Virus: Partners in Coinfection. mBio. 2016;7(6).
- Samuelson DR, Shellito JE, Maffei VJ, et al. Alcohol-associated intestinal dysbiosis impairs pulmonary host defense against Klebsiella pneumoniae. PLoS Pathog. 2017;13(6):e1006426.
- Lynch JP, III. Pneumonia: Atypical. Encyclopedia of Respiratory Medicine 2006:410-417.
- Musher DM, Abers MS, Bartlett JG. Evolving understanding of the causes of pneumonia in adults, with special attention to the role of Pneumococcus. Clin Infect Dis. 2017;65(10):1736-1744.