17th February 2025

Pathogens

Key respiratory bacterial pathogens

Strep. pneumoniae is a Gram-positive coccus which accounts for the majority of community-acquired pneumonia; around 60%. Infection is particularly severe in asplenic patients and carries a high rate of mortality, necessitating vaccination and prophylactic phenoxymethylpenicillin. Strep. pneumoniae exists as a commensal organism in the nasopharynx in up to 60% of people.

H. influenzae is a Gram-negative bacillus. H. influenzae type B may cause epiglottitis, especially in the unvaccinated. It is a common causative organism in infective exacerbations of COPD. Its main complications can include meningitis and pneumonia.

Staphylococcus aureus  is a Gram-positive coccus. It is highly associated with being a superadded bacterial infection following influenza and can carry a high mortality rate. Pneumonia may also occur following septicaemia in intravenous drug misuse or hospital patients with indwelling lines. The production of the Panton-Valentine leukocidin (PVL) toxin can cause tissue necrosis, cavitation and, rarely, pneumothorax.

Klebsiella pneumoniae is a Gram-negative organism which is now highly drug resistant in the United Kingdom. It may result in an intractable pneumonic infection due to its high resistance to antibiotics. It is especially seen in patients who have impaired resistance to infections including the homeless, the malnourished and those with alcohol misuse. It may also be seen in patients with underlying lung disease.

Pseudomonas aeruginosa is a Gram-negative bacillus common in hospital-acquired pneumonia, aspiration pneumonia and patients who have required endotracheal ventilation. It is drug resistant and requires an antipseudomonal antibiotic such as piperacillin with tazobactam.

Atypical pneumonia

Mycoplasma pneumoniae causes a characteristic flu-like prodrome consisting of headaches, muscle aches and fatigue before progressing into a dry cough and shortness of breath. One may see bilateral peribronchial infiltrates on chest X-ray. It is rarely life threatening. Complications can include a cold, autoimmune haemolytic anaemia, erythema multiforme, transverse myelitis and rarely lymphocytic meningoencephalitis.

Chlamydophilia pscittaci is a zoonotic infection arising from contaminated droplets, particularly from birds, with the main complications including severe pneumonia, fevers and delirium.

Legionella pneumophila is a Gram-negative bacillus found in contaminated water. For this reason, it is especially associated with unclean air conditioners, showers and water storage tanks. Alongside severe pneumonia, it may cause abdominal pain, confusion, hyponatraemia and transaminitis. A urinary antigen test may confirm the diagnosis although direct fluorescent antibody staining on a bronchoalveolar lavage sample is more sensitive.

Coxiella burnetii, the causative agent of Q fever, is another zoonotic infection often obtained from domesticated animals, especially sheep, cattle or goats. It typically causes flu-like symptoms but can progress to an atypical pneumonia with severe respiratory distress. Rarely, hepatitis can be a complication. 

Other respiratory bacterial pathogens

Nocardia are Gram-positive aerobic bacilli found in soil. Whilst the lung is the most common site of infection, dissemination can occur to the CNS, causing abscesses and meningitis or alternatively to the skin and subcutaneous tissues causing soft tissue abscesses. It may also cause endocarditis. Direct smears of abscesses are often positive, whereas cultures seldom are. Extended antibiotic courses and abscess drainage are standard treatments.

Leptospirosis is caused by the bacterium Leptospira. Whilst being more known for causing icterohaemorrhagic fever, it can also cause a diffuse alveolar haemorrhage and subsequent respiratory decompensation through this mechanism. It is particularly associated with sewers and rodents.