Aetiology of CAP in adults

Common pathogens

Less common pathogens

Rare pathogens

Streptococcus pneumoniae

Most common bacterial cause of CAP in adults.

Most common cause of high-severity CAP in adults, CAP in older adults and death from CAP.

For directed therapy regimens, see Streptococcus pneumoniae (pneumococcal) pneumonia.

Respiratory viruses (eg adenovirus, coronavirus, hMPV, influenza virus, parainfluenza virus, RSV)

Respiratory viruses can cause pneumonia as a sole pathogen, or as co-infection with bacterial pathogensJain, 2015.

RSV and hMPV are frequent causes of viral upper respiratory tract infection in the community. Rarely, they cause high-severity bilateral CAP in adults requiring intensive care support; this is more common in older adults.

Consider influenza or coronaviruses (eg SARS-CoV-2) depending on local epidemiology (eg high rates of community transmission). While awaiting the results of investigations, review the need for isolation (eg patients in hospital) and empirical antiviral therapy – see Influenza.

For links to resources on diagnosis and management of SARS-CoV-2, see Diagnosis and management of COVID-19.

Mycoplasma pneumoniae and Chlamydophila (Chlamydia) pneumoniae

Usually cause low- or moderate-severity CAP in adults and are more commonly associated with CAP in young adults. Rarely cause CAP in residents of an aged-care facility.

For directed therapy regimens, see Mycoplasma pneumoniae pneumonia and Chlamydophila (Chlamydia) species pneumonia.

Legionella species

Infection with Legionella species (Legionnaires disease) is acquired from environmental sources.

Patients may present with nonrespiratory symptoms (eg confusion, diarrhoea) and hyponatraemia.

Risk factors for infection include chronic lung disease, smoking, diabetes, advanced kidney disease, cancer, and immune compromise (including chronic corticosteroid use).

L. pneumophila serogroup 1 is the serogroup most associated with outbreaks and causes more severe CAP; L. longbeachae causes sporadic, less severe CAP (eg after exposure to potting mix).

For directed therapy regimens, see Legionella pneumonia in adults.

Moraxella catarrhalis

M. catarrhalis is a gram-negative bacterium that commonly colonises the upper respiratory tract.

CAP due to M. catarrhalis occurs most frequently in older adults, those with cardiopulmonary disease, diabetes or immune compromise.

Haemophilus influenzae

H. influenzae is a gram-negative bacterium that commonly colonises the respiratory tract and predominantly causes CAP in adults with COPD; see Considerations in managing CAP in adults with chronic obstructive pulmonary disease (COPD) or bronchiectasis for more information.

For directed therapy regimens, see Haemophilus influenzae pneumonia.

Staphylococcus aureus

S. aureus is suspected when Gram stain of sputum shows profuse gram-positive cocci in clusters [NB4]. However, identification of S. aureus (including MRSA) by sputum culture may represent colonisation. Consider the clinical context when interpreting the results of investigations; staphylococcal pneumonia is usually a severe and cavitary infection.

For features of staphylococcal pneumonia and directed therapy regimens, see Staphylococcal pneumonia.

Pseudomonas aeruginosa

Identification of P. aeruginosa in sputum does not always indicate infection and may represent colonisation [NB5].

Pseudomonal pneumonia is usually necrotising or destructive.

For directed therapy regimens, see Pseudomonas aeruginosa pneumonia.

Enterobacterales (eg Klebsiella pneumoniae)

Enterobacterales rarely cause CAP.

Identification of enteric gram-negative bacilli in sputum usually reflects colonisation or prior antibiotic use, especially in patients with low- or moderate-severity CAP.

For directed therapy regimens, see Multidrug-resistant Enterobacterales pneumonia or Nonmultidrug-resistant Enterobacterales pneumonia.

Chlamydophila (Chlamydia) psittaci

C. psittaci is a zoonotic pathogen acquired from exposure to infected birds, either directly or from environmental sources (eg dried droppings).

Nonrespiratory signs and symptoms may be prominent.

For directed therapy regimens, see Mycoplasma pneumoniae pneumonia and Chlamydophila (Chlamydia) species pneumonia.

Mycobacterium tuberculosis

Consider tuberculosis in patients with prolonged symptoms, particularly in older adults, patients with immune compromise, and patients who have previously resided in regions with a high prevalence of tuberculosis.

Burkholderia pseudomallei and Acinetobacter baumannii

B. pseudomallei and A. baumannii are gram-negative bacteria that can cause CAP in tropical regions of Australia [NB6].

Risk factors for infection include diabetes, heavy alcohol consumption (including binge drinking), chronic lung or kidney disease, and immunosuppressive therapy (including chronic corticosteroid use)Davis, 2014Smith, 2018.

For directed therapy regimens, see Melioidosis or Acinetobacter baumannii pneumonia in adults.

Coxiella burnetii

C. burnetii is acquired from inhalation of bacteria after contact with animals (eg exposure in rural settings, contact with skins or contaminated dust).

Nonrespiratory signs and symptoms may be prominent.

For directed therapy regimens, see Q fever.