12th January 2024

Background

Pneumonia is inflammation of the lung parenchyma caused by infection, which results in alveoli of the affected areas becoming filled with pus or fluid. In hospital, the diagnosis is made based on symptoms and signs of an acute lower respiratory tract infection and chest X‑ray findings of new shadowing that is not due to another cause. Pneumonia can be classified in multiple ways, including:

• The causative organism which can be bacterial, viral, fungal or parasitic
• The severity of infection, most commonly using the CURB-65 score
• The pattern of consolidation on the chest X-ray. Pneumonia can be split into lobar pneumonia, where inflammation is confined to one lobe, and bronchopneumonia, where there are patchy foci of consolidation with no lobar localisation
• The place where infection occurred, e.g. community-acquired vs hospital-acquired. Pneumonia is defined as hospital-acquired if it occurs >48 hours after hospital admission

Epidemiology

Between 0.5-1% of adults in the United Kingdom (UK) develop community-acquired pneumonia (CAP) every year, with the condition particularly common in the elderly population. Pneumonia carries significant morbidity and mortality; over 100,000 patients per year are admitted to an NHS hospital in England with a primary diagnosis of pneumonia, and approximately 30,000 deaths per year in the UK are due to pneumonia.

Causative organisms

Pneumonia is most commonly caused by bacterial infection. Streptococcus pneumoniae is the most common cause of CAP, accounting for approximately 80% of cases. Other common organisms include Haemophilus influenzae, Moraxella catarrhalis and Staphylococcus aureus, which is classically seen after influenza infection. Klebsiella pneumoniae is a less common cause of pneumonia except in alcoholics and can present with cavitating lesions. “Atypical” causes of pneumonia are those which are challenging to detect through standard bacterial methods and include Legionella pneumophila, Mycoplasma pneumoniae, Chlamydia pneumoniae and Chlamydia psittaci, which is commonly seen in bird owners.

The pathogens typically associated with hospital-acquired pneumonia (HAP) differ from those in CAP, with Gram-negative enterobacteria (e.g. Enterobacter spp, Escherichia coli and Proteus spp) and Staphylococcus aureus being considered the ‘core’ pathogens. Pseudomonas aeruginosa is a cause of severe HAP and is classically seen in patients on mechanical ventilation. Aspiration pneumonia, typically seen in individuals with swallowing difficulties, often has a mixed picture with anaerobic species involved.