Aetiology

A single pathogen is identified in 85% of patients where an aetiology is found. However, the true frequency of polymicrobial CAP is not clear.

Streptococcus pneumoniae is the most frequently identified pathogen (39%). Chlamydophilia pneumonia is identified in 13% of hospitalised patients but the incidence in community cases is unknown.

Mycoplasma pneumoniae occurs in epidemics spanning three winters every four years and therefore its incidence is variable.

Although legionella species and staphylococcus aureus are identified more frequently in patients managed on the ICU, streptocoocus pneumoniae is still the most frequent isolated organism in this setting.

Gram-negative enteric organisms, chlamydophila psittaci and coxiella burnetii are uncommon causes of CAP. Legionella and mycoplasma species are less commonly isolated in elderly patients with CAP but otherwise the causative organisms have a similar frequency as those found in younger patients.

Pooled data of causative organisms in UK patients diagnosed with CAP [1]