bacterial pneumonia
Summary
Bacterial pneumonia is an infection of the lung parenchyma caused by bacteria, leading to alveolar consolidation with inflammatory exudate. It presents with fever, productive cough, pleuritic chest pain, and dyspnea, and is diagnosed via clinical findings plus chest X-ray showing lobar or bronchopneumonic infiltrates. Common pathogens vary by setting (community vs. hospital) and patient risk factors, guiding empiric antibiotic therapy.
Detail
Bacterial pneumonia results from bacterial invasion of the alveoli, triggering an inflammatory response with neutrophil infiltration, fibrin deposition, and exudate filling the alveolar spaces—classically progressing through stages of lobar pneumonia: congestion, red hepatization, gray hepatization, and resolution. Pathogens gain access via aspiration, inhalation, or hematogenous spread, often following viral URIs or in patients with impaired mucociliary clearance (e.g., smokers, COPD, immunosuppression).
Community-acquired pneumonia (CAP) is most commonly caused by Streptococcus pneumoniae (most common overall), followed by Haemophilus influenzae, Mycoplasma pneumoniae (atypical, walking pneumonia in young adults), Chlamydophila pneumoniae, and Legionella pneumophila (atypical, associated with water sources, GI symptoms, hyponatremia). Klebsiella pneumoniae is classically seen in alcoholics and diabetics, causing currant-jelly sputum and often affecting the right upper lobe. Staphylococcus aureus pneumonia can follow influenza infection and is associated with cavitary lesions and empyema; MRSA is a concern in severe cases.
Hospital-acquired pneumonia (HAP) and ventilator-associated pneumonia (VAP) are more likely caused by gram-negative rods (Pseudomonas aeruginosa, Klebsiella, E. coli) and MRSA, reflecting nosocomial flora and prior antibiotic exposure.
Clinical presentation includes sudden-onset fever, chills, productive cough with purulent sputum, pleuritic chest pain, tachypnea, and dyspnea. Typical pneumonia (S. pneumoniae) presents acutely with lobar consolidation, while atypical pneumonia (Mycoplasma, Chlamydophila, Legionella) presents more insidiously with dry cough, headache, and extrapulmonary symptoms, showing patchy interstitial infiltrates ("walking pneumonia").
Diagnosis involves chest X-ray (lobar consolidation, air bronchograms, or patchy infiltrates), sputum Gram stain and culture, blood cultures in severe cases, and clinical scoring systems (CURB-65, PSI) to determine severity and disposition (outpatient vs. inpatient vs. ICU).
Treatment is empiric based on setting: outpatient CAP typically treated with macrolides or doxycycline; inpatient CAP with beta-lactam plus macrolide or respiratory fluoroquinolone; HAP/VAP requires broader gram-negative and MRSA coverage (e.g., piperacillin-tazobactam or cefepime plus vancomycin).
Complications include parapneumonic effusion, empyema, lung abscess (especially with aspiration or S. aureus/Klebsiella), and sepsis. Vaccination (pneumococcal conjugate/polysaccharide vaccines) is key prevention, especially in elderly, immunocompromised, and chronic disease patients.
Sources
- First Aid for the USMLE Step 1
- Robbins and Cotran Pathologic Basis of Disease
- Harrison's Principles of Internal Medicine
- UpToDate: Community-Acquired Pneumonia in Adults
Reviewed by AnkiBoss editorial — medical student review. Information here is for study reference only and is not medical advice. Spotted an error? Let us know.