Aetiology of HAP

Causes of hospital-acquired pneumonia (HAP) include:

• Streptococcus pneumoniaeSuaya, 2021 – part of the usual respiratory flora
• gram-negative bacteria (eg nonmultidrug-resistant [non-MDR] Enterobacterales) – prolonged hospitalisation is a risk factor for colonisation
• more resistant bacteria – recent broad-spectrum antibiotic therapy is a risk factorChang, 2021Costa, 2019Kalil, 2016Torres, 2017
  • resistant gram-negative bacteria (eg multidrug-resistant [MDR] Enterobacterales, Pseudomonas aeruginosa, Acinetobacter species)
  • methicillin-resistant Staphylococcus aureus [MRSA] – in many Australian hospitals, nosocomial MRSA pneumonia is uncommon
• respiratory viruses – can be acquired from other patients, visitors or staffAbbas, 2021
  • severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)
  • influenza
  • respiratory syncytial virus (RSV)
• atypical pathogens¹ – much less common cause of HAP than community-acquired pneumonia (CAP)Hagel, 2019
  • Legionella species – rare, but can be acquired from a contaminated environmental reservoir in the hospital
  • Chlamydophila (Chlamydia) pneumoniae
  • Mycoplasma pneumoniae
• fungal pathogens – rare cause of HAP; most often in patients with immune compromise
  • Aspergillus species
  • Pneumocystis jirovecii.

Although Candida species² can colonise the respiratory tract of patients in hospital, they rarely, if ever, cause HAP.

Children with chronic suppurative lung disease (eg bronchiectasis, cystic fibrosis) may be colonised with P. aeruginosa. For information about managing HAP in children with bronchiectasis, see Management of low- to moderate-severity HAP in patients with COPD and bronchiectasis. For management of HAP in children with cystic fibrosis, see Airway infection and antibiotic therapy in cystic fibrosis.

For aetiology of ventilator-associated pneumonia (VAP), see Ventilator-associated pneumonia.