Acute Bacterial Pneumonia

Acute infection of the lung parenchyma by bacteria, causing consolidation with alveolar filling by inflammatory exudate. Incidence in adults is about 5 to 11 per 1000 per year, higher in the elderly, with about 20 to 40% needing admission; it is a leading infectious cause of death worldwide.

• Organisms: Streptococcus pneumoniae (the commonest, about 30 to 50% of identified causes), Haemophilus influenzae, Staphylococcus aureus, Klebsiella pneumoniae (alcoholism, diabetes), Moraxella catarrhalis, Pseudomonas aeruginosa (bronchiectasis, cystic fibrosis, hospital exposure), Gram negative enteric bacilli, Legionella pneumophila, anaerobes (aspiration), and atypical organisms (Mycoplasma pneumoniae, Chlamydophila, Coxiella). Respiratory viruses are found in about a third of cases and co-infection occurs.
• In sub-Saharan Africa, HIV infection, tuberculosis and Pneumocystis pneumonia must be considered in every adult with pneumonia, and the burden of Klebsiella, Staphylococcus aureus and non-typhoidal Salmonella is higher.
• Risk factors: age over 65, smoking, alcohol misuse, COPD and asthma, diabetes, heart failure, chronic kidney and liver disease, HIV, malnutrition, immunosuppression, aspiration risk (stroke, seizures, dysphagia, reduced consciousness), indoor air pollution, recent influenza, and absent vaccination.
• Mortality: less than 1% in outpatients, about 5 to 15% in admitted patients, and about 30 to 50% in those needing intensive care.

• Fever (may be absent in the elderly), productive cough with purulent or rust coloured sputum, pleuritic chest pain, dyspnoea, rigors, malaise, headache, and confusion in older patients. Gastrointestinal symptoms occur with Legionella and Mycoplasma.
• Signs: tachypnoea (respiratory rate 20 or more), tachycardia, fever, focal bronchial breathing, dull percussion, increased vocal resonance, coarse crackles, pleural rub, cyanosis, hypotension in sepsis.
• Severity markers: confusion, urea greater than 7 mmol/L, respiratory rate 30 or more, systolic pressure less than 90 mmHg or diastolic 60 mmHg or less, age 65 or more (score one point each; 0 to 1 low risk, 2 admit, 3 or more severe), hypoxaemia (SpO2 less than 92%), multilobar involvement, lactate greater than 2 mmol/L.
• Differential diagnosis: viral pneumonia and influenza, tuberculosis, Pneumocystis pneumonia, pulmonary embolism with infarction, heart failure, lung cancer with post-obstructive pneumonia, organising pneumonia, eosinophilic pneumonia, vasculitis, aspiration pneumonitis, and drug induced pneumonitis.
• Complications: parapneumonic effusion and empyema (about 10 to 20% of admissions), lung abscess, necrotising pneumonia, respiratory failure, ARDS, septic shock, bacteraemia and metastatic infection (meningitis, endocarditis, arthritis), pericarditis, and atrial fibrillation and cardiac events.

• Chest X-ray: new focal or lobar consolidation, air bronchograms, effusion, cavitation. A normal film early does not exclude pneumonia; repeat or use ultrasound or CT when the picture is unclear.
• Severity and oxygenation: pulse oximetry on every patient, arterial blood gas when SpO2 is less than 92% or the patient is severely ill.
• Blood tests: full blood count, urea and electrolytes, creatinine, liver function, CRP, procalcitonin (may help to stop antibiotics early), glucose, lactate in sepsis, HIV test, and blood cultures (two sets) before antibiotics in admitted patients.
• Microbiology in moderate to severe disease: sputum Gram stain and culture, pneumococcal and Legionella urinary antigens, respiratory PCR panel including influenza and SARS-CoV-2, Xpert MTB/RIF on sputum for tuberculosis when cough lasts more than 2 weeks, HIV, weight loss, night sweats, or in endemic areas.
• Pleural fluid when an effusion is more than 10 mm: pH, glucose, LDH, protein, Gram stain, culture; empyema if pus, pH less than 7.2, glucose less than 2.2 mmol/L, or positive culture.
• CT chest for failure to respond within 72 hours, suspected abscess, complications, or non-resolution; bronchoscopy for suspected obstruction, immunosuppression, or refractory disease.

• Give the first antibiotic dose within 1 hour in severe disease or sepsis and within 4 hours in other admitted patients.
• Outpatient (low severity, 0 to 1 point):
  • Amoxicillin 500 mg to 1 g orally three times daily for 5 days.
  • Doxycycline 200 mg orally on day 1 then 100 mg twice daily (total 5 days) if penicillin allergic; or clarithromycin 500 mg twice daily or azithromycin 500 mg once daily for 3 days.
  • Review at 48 to 72 hours, and admit if no improvement.
• Admitted, moderate severity (2 points):
  • Amoxicillin 1 g orally or IV three times daily plus clarithromycin 500 mg twice daily (or doxycycline 100 mg twice daily) for 7 days; switch to oral when stable.
  • Penicillin allergy: levofloxacin 500 mg twice daily (or 750 mg daily) or moxifloxacin 400 mg daily.
• Severe pneumonia (3 or more points, or need for intensive care):
  • Co-amoxiclav 1.2 g IV three times daily (or ceftriaxone 2 g IV daily) plus clarithromycin 500 mg IV or orally twice daily (or azithromycin 500 mg daily) for 7 to 10 days.
  • Penicillin allergy: levofloxacin 750 mg IV daily with a macrolide if Legionella is possible.
  • Add flucloxacillin 2 g IV every 6 hours, and vancomycin 15 to 20 mg/kg IV every 8 to 12 hours or linezolid 600 mg twice daily when Staphylococcus aureus (post-influenza, necrotising) or methicillin resistant Staphylococcus aureus is suspected; treat Staphylococcus aureus for at least 14 days.
  • Pseudomonas risk (bronchiectasis, recent hospital stay, prior isolation): piperacillin tazobactam 4.5 g IV every 6 to 8 hours or meropenem 1 g IV every 8 hours, plus ciprofloxacin 400 mg IV every 8 hours or an aminoglycoside.
  • Legionella: levofloxacin 750 mg daily or azithromycin 500 mg daily for 7 to 14 days (up to 21 days if immunosuppressed).
  • Duration is 5 days if clinically stable by day 3 (afebrile 48 hours, respiratory rate and oxygenation normal), 7 days for moderate to severe, and 10 to 14 days for bacteraemic, necrotising, Staphylococcus aureus or Gram negative disease; empyema and abscess need longer.
• Supportive care:
  • Oxygen to SpO2 94 to 98% (88 to 92% in COPD with carbon dioxide retention); high flow nasal oxygen or non-invasive ventilation for respiratory failure (non-invasive ventilation in COPD with acidosis); intubation for refractory hypoxaemia, exhaustion or shock.
  • IV fluids for hypovolaemia, vasopressors (noradrenaline 0.05 to 0.5 micrograms/kg/min) for septic shock.
  • Hydrocortisone 200 mg/day IV (50 mg every 6 hours) for 4 to 7 days may be used in severe pneumonia with shock or respiratory failure (evidence is moderate; avoid in influenza and suspected fungal infection).
  • Paracetamol 1 g four times daily for fever and pleuritic pain; venous thromboembolism prophylaxis with enoxaparin 40 mg SC daily; early mobilisation; chest physiotherapy for sputum retention; nutrition; smoking cessation (nicotine patch 21 mg/24 h).
• Complications:
  • Parapneumonic effusion greater than 10 mm: diagnostic aspiration; drain complicated effusion or empyema with a chest drain and intrapleural alteplase 10 mg with dornase alfa 5 mg twice daily for 3 days, or video assisted thoracoscopic debridement if the patient is failing.
  • Lung abscess: prolonged antibiotics (4 to 6 weeks, co-amoxiclav 625 mg three times daily or clindamycin 300 to 450 mg three times daily plus metronidazole 400 mg three times daily), with drainage if it does not respond.
• Treat failure at 72 hours: confirm the diagnosis, review cultures, exclude empyema, abscess, resistant organisms, tuberculosis, Pneumocystis pneumonia (co-trimoxazole 120 mg/kg/day in divided doses for 21 days with prednisolone for hypoxaemia), obstruction or non-infective mimics; widen antibiotics.
• Special groups:
  • Children: amoxicillin 40 to 90 mg/kg/day in 2 to 3 divided doses for 5 days (IV ampicillin or ceftriaxone 50 to 100 mg/kg/day if severe); macrolide for atypical infection.
  • Pregnancy: amoxicillin 500 mg to 1 g three times daily plus azithromycin 500 mg daily or erythromycin 500 mg four times daily; avoid doxycycline and fluoroquinolones; treat hypoxaemia promptly (maternal SpO2 greater than 95%).
  • Elderly: lower threshold for admission, watch for delirium and aspiration, and review swallowing.
  • Renal impairment: adjust doses (levofloxacin, vancomycin, aminoglycosides, piperacillin tazobactam).
  • HIV: test for tuberculosis and Pneumocystis pneumonia; give co-trimoxazole prophylaxis 960 mg daily if CD4 count is less than 200; start antiretroviral therapy.
• Prevention: pneumococcal conjugate vaccine (20-valent single dose, or 13-valent followed by 23-valent polysaccharide vaccine) for adults 65 or more and those with chronic disease or HIV; yearly influenza vaccine; smoking cessation; treat aspiration risk.
• Red flags: respiratory rate 30 or more, SpO2 less than 90%, confusion, systolic pressure less than 90 mmHg, lactate greater than 2 mmol/L, multilobar disease, no response at 72 hours, large effusion.
• Follow up: review at 48 to 72 hours for outpatients and at 6 weeks; repeat chest X-ray at 6 weeks for smokers over 50 or with persistent symptoms to exclude malignancy; test for HIV, check vaccination, and screen for underlying lung disease.