17th February 2025

Bacterial respiratory infections

Overview of bacterial respiratory infections

Bacterial pathogens constitute a majority of respiratory infections that require medical intervention and treatment. The different bacterial pathogens often result in specific clinical courses. 

The vast majority of community-acquired pneumonia is caused by Streptococcus pneumoniae and Haemophilus influenzae. Atypical pneumonia is caused by pathogens that may be harder to detect, such as Legionella, Mycoplasma or Chlamydia, and are associated with specific causes. Hospital-acquired pneumonia is defined as pneumonia starting from 72 hours after admission to hospital and is very often bacterial. Hospital-acquired pneumonia is associated with a worse outcome than community-acquired pneumonia. Ventilator-acquired pneumonia is common in patients who are on ventilators for longer than 72 hours; this carries a poor prognosis.

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.

Grading pneumonia

The CURB-65 score is a screening tool used to assess the severity of pneumonia. It uses the following criteria:

• C – Confusion
• U – Urea > 7.0
• R – Respiratory rate > 30/minute
• B – Systolic blood pressure < 90 or diastolic blood pressure < 60 mgHg
• 65 – Age 65 or older 

Scores ≥3 are at highest risk with a mortality of  >15% and require hospital admission and consideration of ICU.

Investigating pneumonia

Patients with bacterial pneumonia often present with a productive cough, shortness of breath and fever. They may be tachypnoeic with localised crackles with dullness to percussion on auscultation, reflecting underlying consolidation. Other symptoms or signs typical of any severe infection, such as tachycardia and reduced urine output, may also be present.

Initial investigations

A full history, examination, bloods and a chest X-ray would be expected as a minimum. Blood cultures and sputum cultures may be useful, especially for atypical organisms. Urine samples may be used to look for pneumococcal or Legionella antigen; PCR is necessary for confirmation of Mycoplasma.

If unable to achieve samples, a bronchoalveolar lavage may help. If there is an effusion, a pleural aspirate may offer a diagnosis. A pH <7.2 is diagnostic of a complicated parapneumonic effusion. If the patient requires a pleural tap, the triangle of safety offers borders which minimise the risk of iatrogenic harm: lateral border of the pectoralis major, lateral border of the latissimus dorsi and line of the fifth intercostal space.

Further investigations

A CT thorax can be used in severe cases or to assess for cavitation or abscess. A pulmonary abscess could be from septic emboli from endocarditis and an echocardiogram would be recommended. 

Recurrent or atypical bacterial pneumonia should prompt consideration of an underlying immunodeficiency such as HIV or cancer, including myeloma. Alternatively, recurrent bacterial infections in a young person with worsening respiratory function may prompt testing for cystic fibrosis or alpha-1 antitrypsin deficiency-related COPD.

On discharge, patients should be followed up with a 6 week post-discharge chest X-ray to assess for resolution and possible underlying causes of pneumonia, including malignancy.

Managing bacterial pneumonia

Management of pneumonia includes oxygen therapy and IV crystalloid fluids if the patient is septic. Chest physiotherapy may be used to increase expectoration and alveolar recruitment. 

Non-invasive ventilation (NIV) is not typically indicated in pneumonia but may be undertaken with expert supervision on a case-specific basis. Worsening oxygenation despite appropriate therapy will necessitate intubation and ventilation. The positive end-expiratory pressure (PEEP) from mechanical ventilation can increase oxygenation by increasing alveolar recruitment.  

Antibiotics will be required to treat pneumonia, with IV administration recommended for a CURB-65 score ≥2. Aspiration pneumonia requires an antibiotic with Pseudomonas cover. Atypical pneumonias should be covered with a fluoroquinolone or macrolide antibiotic. In cases of recurrent lower respiratory tract infections due to COPD, a prophylactic antibiotic such as azithromycin may be indicated.

Complications of bacterial pneumonia

In severe cases of pneumonia a septic picture may emerge, which can include an acute kidney injury, disseminated intravascular coagulation and a high risk of mortality.

Other complications include syndrome of inappropriate antidiuretic hormone, pulmonary abscesses, parapneumonic effusions and empyema. Rarely, often in vulnerable patients prone to self neglect, a long-standing empyema may dissect from the pleural space and invade through the chest wall, which is described as empyema necessitans.

References

• Chapman S. Oxford handbook of respiratory medicine. Oxford, United Kingdom: Oxford University Press; 2014
• H. Ticona J, M. Zaccone V, M. McFarlane I. Community-Acquired Pneumonia: A Focused Review. American Journal of Medical Case Reports. 2020 Nov 4;9(1):45–52
• Noviello S, Huang D. The Basics and the Advancements in Diagnosis of Bacterial Lower Respiratory Tract Infections. Diagnostics. 2019 Apr 3;9(2):37
• Ramirez JA. Overview of community-acquired pneumonia in adults [Internet]. File TM, Bond S, editors. UpToDate. 2022 [accessed 13/08/2022]. Available from: https://www.uptodate.com/contents/overview-of-community-acquired-pneumonia-in-adults
• Cunha BA. The atypical pneumonias: clinical diagnosis and importance. Clinical Microbiology and Infection. 2006;12(3):12–24
• National Institute for Health and Care Excellence. Pneumonia (hospital-acquired): antimicrobial prescribing [Internet]. NICE. 2019 [accessed 13/08/2022]. Available from: https://www.nice.org.uk/guidance/ng139
• Davies HE, Davies RJO, Davies CWH. Management of pleural infection in adults: British Thoracic Society pleural disease guideline 2010. Thorax [Internet][accessed 18/08/2022]. 2010 Aug 1;65(Suppl 2):ii41–53. Available from: https://thorax.bmj.com/content/65/Suppl_2/ii41#T1
• Kinnear W. Non-invasive ventilation in acute respiratory failure. Thorax. 2002 Mar 1;57(3):192–211
• Najib Rahman, Hunt I, Fergus Gleeson, Maskell N. ABC of pleural diseases. Hoboken, New Jersey: Wiley Blackwell; 2018
• Bourke SJ, Burns GP. Lecture notes. Respiratory medicine. Chichester, West Sussex; Hoboken, Nj: John Wiley & Sons, Ltd; 2015