The pathogenesis of community-acquired pneumonia

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A number of studies have investigated the microbiological pathogenesis of community-acquired pneumonia (CAP) in patients admitted to hospital or, more specifically, to the intensive care unit. However, far fewer studies have sought to identify the cause of lower respiratory tract infections, including CAP, in the community, although what little is known about such mild infections suggests that their microbial aetiology is similar to that of infections in hospitalised patients. The wide variations between studies in the frequency of each microorganism can be explained by several factors, including differences in study populations (e.g. age, risk factors), geographical area, study sample, and microbiological methods. For example, some studies have focused on bacterial agents and others on viruses and intracellular bacteria. Extracellular bacteria, especially Streptococcus pneumoniae, are the most common, followed by Haemophilus influenzae, Staphylococcus aureus and, much less frequently, Gram-negative bacilli. Among atypicals and intracellular bacteria Mycoplasma pneumoniae is the most important, followed in frequency by Legionella sp. and Chlamydia sp. Viruses are involved in 5-20% of cases. Some 41 different prospective studies performed in Europe have established that ∼10 different microbial pathogens are the cause of the vast majority of cases of CAP, with occasional cases due to other rarer pathogens. The frequency of these organisms is similar in most European countries, although there is some geographic variation. Differences in frequency are also apparent according to the severity of illness. Nevertheless, it is generally recognised that S. pneumoniae is the most important causative bacterium in all countries. A relatively recent development among some of the common bacteria has been the appearance and spread of resistance to antibiotics to which they were once sensitive. However, the frequency of such resistance does vary markedly between European countries. Data from North American studies also suggest a major role for Streptococcus pneumoniae and atypicals. Also important are the clinical implications of co-infection on community acquired pneumonia management.

Original languageEnglish
Pages (from-to)80-84
Number of pages5
JournalEuropean Respiratory Review
Issue number91
Publication statusPublished - Dec 2004


  • Aetiology
  • Bacterial resistance
  • Community-acquired pneumonia
  • Diagnostic methods

ASJC Scopus subject areas

  • Pulmonary and Respiratory Medicine


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