Lower respiratory tract infections continue to be a major health problem despite advances in antimicrobial therapies. Controversy persists regarding optimal diagnostic approaches and treatment choices for pneumonia.
Pneumonia has classically been considered in terms of the infecting organism (Table 9–8) (eFigures 9–8, 9–9, 9–10, and 9–11). This approach facilitates discussion of characteristic clinical presentations but is a limited guide to patient management since specific microbiologic information is not always available at initial presentation. More recent classification schemes emphasize epidemiologic factors that predict etiology and guide initial therapy. Pneumonia may be classified as community-acquired (CAP) or nosocomial and, within the latter, as hospital-acquired (HAP) or ventilator-associated (VAP). These categories are based on differing settings and infectious agents and require different diagnostic and therapeutic interventions. Anaerobic pneumonia and lung abscess can occur in both hospital and community settings and warrant separate consideration.
++ Table Graphic Jump Location Table 9–8.Characteristics of selected pneumonias. ||Download (.pdf) Table 9–8. Characteristics of selected pneumonias.
|Organism; Appearance on Smear of Sputum ||Clinical Setting ||Complications |
|Streptococcus pneumoniae (pneumococcus). Gram-positive diplococci. ||Chronic cardiopulmonary disease; follows upper respiratory tract infection ||Bacteremia, meningitis, endocarditis, pericarditis, empyema |
|Haemophilus influenzae. Pleomorphic gram-negative coccobacilli. ||Chronic cardiopulmonary disease; follows upper respiratory tract infection ||Empyema, endocarditis |
|Staphylococcus aureus. Plump gram-positive cocci in clumps. ||Residence in long-term care facility, hospital-associated, influenza epidemics, cystic fibrosis, bronchiectasis, injection drug use ||Empyema, cavitation |
|Klebsiella pneumoniae. Plump gram-negative encapsulated rods. ||Alcohol abuse, diabetes mellitus; hospital-associated ||Cavitation, empyema |
|Escherichia coli. Gram-negative rods. ||Hospital-associated; rarely, community-acquired ||Empyema |
|Pseudomonas aeruginosa. Gram-negative rods. ||Hospital-associated; cystic fibrosis, bronchiectasis ||Cavitation |
|Anaerobes. Mixed flora. ||Aspiration, poor dental hygiene ||Necrotizing pneumonia, abscess, empyema |
|Mycoplasma pneumoniae. PMNs and monocytes; no bacteria. ||Young adults; summer and fall ||Skin rashes, bullous myringitis; hemolytic anemia |
Few PMNs; no bacteria.
|Summer and fall; exposure to contaminated construction site, water source, air conditioner; community-acquired or hospital-associated ||Empyema, cavitation, endocarditis, pericarditis |
|Chlamydophila pneumoniae. Nonspecific. ||Clinically similar to M pneumoniae, but prodromal symptoms last longer (up to 2 weeks); sore throat with hoarseness common; mild pneumonia in teenagers and young adults ||Reinfection in older adults with underlying COPD or heart failure may be severe or even fatal |
|Moraxella catarrhalis. Gram-negative diplococci. ||Preexisting lung disease; elderly patients; corticosteroid or immunosuppressive therapy ||Rarely, pleural effusions and bacteremia |
|Pneumocystis jirovecii. Nonspecific. ||AIDS, immunosuppressive or cytotoxic drug therapy, cancer ||Pneumothorax, respiratory failure, ARDS, death |
|SARS-CoV-2. Nonspecific. ||Pandemic. Milder pneumonia (teenagers, young adults); more severe pneumonia (elderly, immunocompromised, multiple comorbidly ill adults) ||Respiratory failure, ARDS, death |++