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Infectious Disease and Microbiology – Pneumonia
Pneumonia is defined as inflammation of the pulmonary parenchyma caused by an infectious agent. Pneumonia can be categorized according to the clinical setting in which it occurs. These categories include community-acquired pneumonia (CAP), which may be typical or atypical; pneumonia occurring in nursing-home residents; nosocomial pneumonia, including hospital-acquired and ventilator-associated pneumonia; pneumonia in immunocompromised hosts; and aspiration pneumonia.
Pneumonia is a major cause of morbidity and mortality worldwide. Approximately 4 million patients develop pneumonia annually in the United States. Community-acquired pneumonia affects about 12 per 1,000 inhabitants each year, with the highest incidence occurring at the extremes of age. Nosocomial pneumonia develops in approximately 0.5–1.5% of hospitalized patients and is particularly common among mechanically ventilated patients, especially those ventilated for prolonged periods. CAP is more common in children and adults older than 60 years.
Several important risk factors predispose individuals to pneumonia. These include alcoholism, aspiration, endotracheal intubation, immunosuppression, age greater than 65 years, hospitalization or nursing-home residence, particularly ICU stay, and underlying pulmonary diseases such as COPD and cystic fibrosis. Preventive measures include influenza and pneumococcal vaccination and minimizing the duration of endotracheal intubation whenever possible.
The pathophysiology of pneumonia involves activation of the host inflammatory response. Inflammatory mediators and chemokines released by macrophages and recruited neutrophils increase alveolar-capillary permeability, leading to alveolar filling with fluid and inflammatory cells. This results in infiltrates, rales, hemoptysis, hypoxemia, decreased lung compliance, and dyspnea. Increased respiratory drive, airway secretions, and infection-related bronchospasm further contribute to respiratory symptoms.
The etiology of pneumonia varies depending on the clinical setting. In community-acquired pneumonia, Streptococcus pneumoniae remains the most common pathogen. Other important organisms include Mycoplasma pneumoniae, Haemophilus influenzae, Chlamydia pneumoniae, and Staphylococcus aureus, particularly following influenza infection. Community-acquired MRSA strains may cause necrotizing or cavitary pneumonia. Viral pathogens such as hantaviruses, metapneumoviruses, and coronaviruses including SARS are also implicated. Polymicrobial infections are common, and Coxiella burnetii is an important atypical pathogen.
Nosocomial pneumonia is usually caused by gram-negative bacilli such as multidrug-resistant Pseudomonas, Klebsiella, Acinetobacter, and Xanthomonas species. MRSA is also common. Anaerobic bacteria and Legionella species may occasionally be involved, particularly in outbreaks related to contaminated water supplies.
In immunocompromised hosts, the responsible pathogens depend on the nature of the immune defect. Patients with humoral immune deficiencies are susceptible to S. pneumoniae, H. influenzae, S. aureus, and Neisseria species. Granulocyte dysfunction predisposes to gram-negative bacilli and Aspergillus infections. Cellular immune deficiencies increase susceptibility to fungal infections such as Pneumocystis jiroveci, Candida, and Cryptococcus; parasitic infections such as Toxoplasma and Strongyloides; mycobacterial infections; and viral infections including CMV and herpesviruses.
Patients with community-acquired pneumonia often report a recent upper respiratory tract infection. Symptoms commonly include fever, cough, sputum production, pleuritic chest pain, dyspnea, chills, sweats, fatigue, headache, myalgias, and arthralgias. Elderly patients may present atypically with confusion or altered mental status. Nosocomial pneumonia should be suspected in hospitalized patients after more than 48 hours of admission.
Physical examination commonly reveals tachypnea and tachycardia. Tactile fremitus may be increased or decreased. Percussion may demonstrate dullness due to consolidation or pleural fluid. Crackles, bronchial breath sounds, and pleural friction rubs may be present. Severe disease may manifest with hypotension or evidence of organ failure.
Initial laboratory evaluation includes complete blood count, pulse oximetry or arterial blood gas measurements, inflammatory markers such as C-reactive protein, and microbiologic cultures obtained from uncontaminated respiratory specimens. Chest radiography is the primary imaging study. If the initial chest radiograph is negative despite strong clinical suspicion, repeat imaging or chest CT may be necessary. Pneumatoceles suggest S. aureus infection, while upper-lobe cavitary lesions raise suspicion for tuberculosis. CT scanning may help identify post-obstructive pneumonia due to tumors or foreign bodies.
Additional diagnostic procedures may be required in severely ill or diagnostically challenging cases. These include bronchoscopy with bronchoalveolar lavage, transbronchial biopsy, lung biopsy, PCR testing, urinary antigen testing for S. pneumoniae and Legionella, and quantitative cultures. Concurrent endocarditis or meningitis should be excluded when clinically suspected.
The differential diagnosis includes infectious conditions such as bronchitis, empyema, lung abscess, and acute exacerbations of chronic bronchitis, as well as noninfectious conditions such as congestive heart failure, pulmonary embolism, lung cancer, and lymphoma.
Empiric antimicrobial therapy depends on the severity and setting of pneumonia. Outpatients with community-acquired pneumonia who are previously healthy and have not received recent antibiotics may be treated with azithromycin, clarithromycin, or doxycycline. Patients with comorbidities or recent antibiotic exposure should receive either a respiratory fluoroquinolone or a beta-lactam combined with a macrolide. Hospitalized non-ICU patients are generally treated with a respiratory fluoroquinolone or a beta-lactam plus macrolide. ICU patients require broader therapy with a beta-lactam plus azithromycin or a fluoroquinolone. If Pseudomonas is suspected, antipseudomonal agents are required, and linezolid should be added when CA-MRSA is suspected.
Treatment duration for CAP is generally at least 5 days, although many patients require 7–10 days of therapy. Longer treatment courses are necessary for infections due to Legionella, Coxiella burnetii, or S. aureus. Nosocomial pneumonia without MDR risk factors may be treated with ceftriaxone, respiratory fluoroquinolones, ampicillin-sulbactam, or ertapenem. Patients at risk for multidrug-resistant organisms require broad-spectrum antipseudomonal beta-lactams combined with aminoglycosides or fluoroquinolones and MRSA coverage with vancomycin or linezolid.
Supportive management includes adequate hydration, oxygen therapy, and assisted ventilation when necessary. Early administration of empiric antibiotics is critical and should ideally occur within 4 hours of hospital arrival. Severity assessment tools such as CURB-65 and the Pneumonia Severity Index help determine the need for hospitalization.
Patients should be monitored closely for oxygen saturation and clinical response. Discharge is appropriate once the patient is clinically improved, afebrile for at least 72 hours, hemodynamically stable, and able to tolerate oral antibiotics.
Radiographic abnormalities may take 4–12 weeks to resolve. Persistent or recurrent pneumonia should prompt evaluation for underlying malignancy or structural lung disease. Prognosis is generally excellent in outpatients with CAP, with mortality rates below 1%. Mortality rises to approximately 10% among hospitalized patients and is significantly higher in ventilator-associated pneumonia.
Complications include treatment failure due to resistant organisms, inadequate therapy, or noninfectious mimics; metastatic infection; complicated pleural effusions; and lung abscess formation.
Pneumonia is defined as inflammation of the pulmonary parenchyma caused by an infectious agent. Pneumonia can be categorized according to the clinical setting in which it occurs. These categories include community-acquired pneumonia (CAP), which may be typical or atypical; pneumonia occurring in nursing-home residents; nosocomial pneumonia, including hospital-acquired and ventilator-associated pneumonia; pneumonia in immunocompromised hosts; and aspiration pneumonia.
Pneumonia is a major cause of morbidity and mortality worldwide. Approximately 4 million patients develop pneumonia annually in the United States. Community-acquired pneumonia affects about 12 per 1,000 inhabitants each year, with the highest incidence occurring at the extremes of age. Nosocomial pneumonia develops in approximately 0.5–1.5% of hospitalized patients and is particularly common among mechanically ventilated patients, especially those ventilated for prolonged periods. CAP is more common in children and adults older than 60 years.
Several important risk factors predispose individuals to pneumonia. These include alcoholism, aspiration, endotracheal intubation, immunosuppression, age greater than 65 years, hospitalization or nursing-home residence, particularly ICU stay, and underlying pulmonary diseases such as COPD and cystic fibrosis. Preventive measures include influenza and pneumococcal vaccination and minimizing the duration of endotracheal intubation whenever possible.
The pathophysiology of pneumonia involves activation of the host inflammatory response. Inflammatory mediators and chemokines released by macrophages and recruited neutrophils increase alveolar-capillary permeability, leading to alveolar filling with fluid and inflammatory cells. This results in infiltrates, rales, hemoptysis, hypoxemia, decreased lung compliance, and dyspnea. Increased respiratory drive, airway secretions, and infection-related bronchospasm further contribute to respiratory symptoms.
The etiology of pneumonia varies depending on the clinical setting. In community-acquired pneumonia, Streptococcus pneumoniae remains the most common pathogen. Other important organisms include Mycoplasma pneumoniae, Haemophilus influenzae, Chlamydia pneumoniae, and Staphylococcus aureus, particularly following influenza infection. Community-acquired MRSA strains may cause necrotizing or cavitary pneumonia. Viral pathogens such as hantaviruses, metapneumoviruses, and coronaviruses including SARS are also implicated. Polymicrobial infections are common, and Coxiella burnetii is an important atypical pathogen.
Nosocomial pneumonia is usually caused by gram-negative bacilli such as multidrug-resistant Pseudomonas, Klebsiella, Acinetobacter, and Xanthomonas species. MRSA is also common. Anaerobic bacteria and Legionella species may occasionally be involved, particularly in outbreaks related to contaminated water supplies.
In immunocompromised hosts, the responsible pathogens depend on the nature of the immune defect. Patients with humoral immune deficiencies are susceptible to S. pneumoniae, H. influenzae, S. aureus, and Neisseria species. Granulocyte dysfunction predisposes to gram-negative bacilli and Aspergillus infections. Cellular immune deficiencies increase susceptibility to fungal infections such as Pneumocystis jiroveci, Candida, and Cryptococcus; parasitic infections such as Toxoplasma and Strongyloides; mycobacterial infections; and viral infections including CMV and herpesviruses.
Patients with community-acquired pneumonia often report a recent upper respiratory tract infection. Symptoms commonly include fever, cough, sputum production, pleuritic chest pain, dyspnea, chills, sweats, fatigue, headache, myalgias, and arthralgias. Elderly patients may present atypically with confusion or altered mental status. Nosocomial pneumonia should be suspected in hospitalized patients after more than 48 hours of admission.
Physical examination commonly reveals tachypnea and tachycardia. Tactile fremitus may be increased or decreased. Percussion may demonstrate dullness due to consolidation or pleural fluid. Crackles, bronchial breath sounds, and pleural friction rubs may be present. Severe disease may manifest with hypotension or evidence of organ failure.
Initial laboratory evaluation includes complete blood count, pulse oximetry or arterial blood gas measurements, inflammatory markers such as C-reactive protein, and microbiologic cultures obtained from uncontaminated respiratory specimens. Chest radiography is the primary imaging study. If the initial chest radiograph is negative despite strong clinical suspicion, repeat imaging or chest CT may be necessary. Pneumatoceles suggest S. aureus infection, while upper-lobe cavitary lesions raise suspicion for tuberculosis. CT scanning may help identify post-obstructive pneumonia due to tumors or foreign bodies.
Additional diagnostic procedures may be required in severely ill or diagnostically challenging cases. These include bronchoscopy with bronchoalveolar lavage, transbronchial biopsy, lung biopsy, PCR testing, urinary antigen testing for S. pneumoniae and Legionella, and quantitative cultures. Concurrent endocarditis or meningitis should be excluded when clinically suspected.
The differential diagnosis includes infectious conditions such as bronchitis, empyema, lung abscess, and acute exacerbations of chronic bronchitis, as well as noninfectious conditions such as congestive heart failure, pulmonary embolism, lung cancer, and lymphoma.
Empiric antimicrobial therapy depends on the severity and setting of pneumonia. Outpatients with community-acquired pneumonia who are previously healthy and have not received recent antibiotics may be treated with azithromycin, clarithromycin, or doxycycline. Patients with comorbidities or recent antibiotic exposure should receive either a respiratory fluoroquinolone or a beta-lactam combined with a macrolide. Hospitalized non-ICU patients are generally treated with a respiratory fluoroquinolone or a beta-lactam plus macrolide. ICU patients require broader therapy with a beta-lactam plus azithromycin or a fluoroquinolone. If Pseudomonas is suspected, antipseudomonal agents are required, and linezolid should be added when CA-MRSA is suspected.
Treatment duration for CAP is generally at least 5 days, although many patients require 7–10 days of therapy. Longer treatment courses are necessary for infections due to Legionella, Coxiella burnetii, or S. aureus. Nosocomial pneumonia without MDR risk factors may be treated with ceftriaxone, respiratory fluoroquinolones, ampicillin-sulbactam, or ertapenem. Patients at risk for multidrug-resistant organisms require broad-spectrum antipseudomonal beta-lactams combined with aminoglycosides or fluoroquinolones and MRSA coverage with vancomycin or linezolid.
Supportive management includes adequate hydration, oxygen therapy, and assisted ventilation when necessary. Early administration of empiric antibiotics is critical and should ideally occur within 4 hours of hospital arrival. Severity assessment tools such as CURB-65 and the Pneumonia Severity Index help determine the need for hospitalization.
Patients should be monitored closely for oxygen saturation and clinical response. Discharge is appropriate once the patient is clinically improved, afebrile for at least 72 hours, hemodynamically stable, and able to tolerate oral antibiotics.
Radiographic abnormalities may take 4–12 weeks to resolve. Persistent or recurrent pneumonia should prompt evaluation for underlying malignancy or structural lung disease. Prognosis is generally excellent in outpatients with CAP, with mortality rates below 1%. Mortality rises to approximately 10% among hospitalized patients and is significantly higher in ventilator-associated pneumonia.
Complications include treatment failure due to resistant organisms, inadequate therapy, or noninfectious mimics; metastatic infection; complicated pleural effusions; and lung abscess formation.
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