Community-Acquired Pneumonia: Microbiology, Severity Scoring, and Antibiotic Selection

Community-acquired pneumonia (CAP) is an acute infection of the lung parenchyma that occurs in patients who have not been hospitalized within the past 90 days or resided in a long-term care facility within the past 14 days. CAP represents one of the most common infectious diseases worldwide and remains a leading cause of morbidity and mortality.

[KEY_CONCEPT] CAP differs from healthcare-associated pneumonia (HCAP) and hospital-acquired pneumonia (HAP) based on the setting of acquisition and associated risk factors for multidrug-resistant organisms.

Epidemiology:

• Affects approximately 5-6 million adults annually in the United States
• Hospitalization rate: ~1-2 million cases per year
• Mortality rates vary from <1% in outpatients to 10-15% in hospitalized patients
• Higher mortality in elderly patients (>65 years) and those with comorbidities
• Peak incidence occurs during winter months
• Most common cause of sepsis and septic shock in the community setting

Pathophysiology: Pneumonia develops through several mechanisms:

1. Aspiration of oropharyngeal contents (most common)
2. Inhalation of infected aerosols
3. Hematogenous spread from distant infection sites
4. Direct extension from contiguous infected sites

[CLINICAL_PEARL] The normal lung has robust defense mechanisms including mucociliary clearance, alveolar macrophages, and secretory IgA. CAP typically occurs when these defenses are overwhelmed or compromised.

Host factors that predispose to CAP include:

• Advanced age (>65 years)
• Chronic comorbidities (COPD, heart failure, diabetes)
• Immunosuppression
• Smoking and alcohol abuse
• Recent viral respiratory infection

The microbiology of CAP varies significantly based on patient population, severity of illness, geographic location, and seasonal patterns. Understanding common pathogens is crucial for appropriate empiric antibiotic selection.

[HIGH_YIELD] Typical vs. Atypical Pathogens Classification:

Most Common Pathogens in CAP:

1. Streptococcus pneumoniae (20-30% of cases)

   • Most common bacterial cause across all age groups
   • Encapsulated gram-positive diplococci
   • Risk factors: asplenia, immunocompromise, chronic diseases

2. Mycoplasma pneumoniae (10-20% of cases) [5]

   • Leading cause in young adults and school-aged children
   • Lacks cell wall, resistant to β-lactam antibiotics
   • Associated with extrapulmonary manifestations

3. Chlamydia pneumoniae (5-15% of cases) [5]

   • Obligate intracellular pathogen
   • Often causes mild, persistent cough
   • May coinfect with other pathogens

4. Haemophilus influenzae (5-10% of cases)

   • More common in patients with COPD
   • Both encapsulated and non-encapsulated strains

5. Legionella species (2-8% of cases) [5]

   • Higher prevalence in severe CAP requiring ICU admission
   • Associated with water exposure (cooling towers, hot tubs)
   • Can cause Legionnaire's disease with systemic symptoms

[CLINICAL_PEARL] Atypical pathogens do not have bacterial cell walls and therefore do not respond to β-lactam antibiotics. They require macrolides, fluoroquinolones, or tetracyclines for effective treatment [5].

Viral Pathogens:

• Influenza A and B
• Respiratory syncytial virus (RSV)
• Parainfluenza viruses
• Human metapneumovirus
• SARS-CoV-2 (COVID-19)

Special Considerations:

• Pseudomonas aeruginosa: Risk factors include structural lung disease, recent antibiotic use, hospitalization
• MRSA: Risk factors include prior MRSA infection, injection drug use, necrotizing pneumonia

The clinical presentation of CAP can vary significantly based on the causative pathogen, patient age, and underlying comorbidities. Early recognition and appropriate diagnostic workup are essential for optimal outcomes.

Classic Symptoms:

• Cough (90% of patients) - may be productive or dry
• Fever (80% of patients) - often with chills and rigors
• Dyspnea (70% of patients) - varies with severity
• Chest pain (50% of patients) - often pleuritic
• Sputum production (60% of patients) - purulent or blood-tinged

[HIGH_YIELD] Atypical Presentation Patterns:

Typical Pneumonia (S. pneumoniae):

• Acute onset with high fever
• Productive cough with purulent sputum
• Pleuritic chest pain
• Lobar consolidation on imaging

Atypical Pneumonia (Mycoplasma, Chlamydia): [5]

• Gradual onset over days to weeks
• Dry, nonproductive cough
• Low-grade fever
• Headache and myalgias
• Interstitial pattern on chest imaging

Legionella Pneumonia: [5]

• High fever with relative bradycardia
• Gastrointestinal symptoms (diarrhea, nausea)
• Neurologic symptoms (confusion, headache)
• Hyponatremia common

[CLINICAL_PEARL] Elderly patients may present with atypical symptoms including confusion, falls, failure to thrive, or exacerbation of underlying conditions without classic respiratory symptoms.

Physical Examination Findings:

• Vital signs: Fever, tachypnea, tachycardia, hypoxemia
• Pulmonary: Crackles, bronchial breath sounds, dullness to percussion
• General: Altered mental status (especially elderly), cyanosis

Diagnostic Criteria for CAP:

Clinical Diagnosis Requires:

1. Acute illness with ≥2 of the following: ✓ Fever (>38°C or <36°C) ✓ New cough or change in chronic cough ✓ Purulent sputum production ✓ Dyspnea or tachypnea ✓ Chest pain

PLUS

1. New pulmonary infiltrate on chest imaging

OR

1. Abnormal lung examination findings consistent with pneumonia

Diagnostic Workup:

Essential Studies:

• Chest X-ray - first-line imaging modality
• Complete blood count - assess for leukocytosis or leukopenia
• Basic metabolic panel - evaluate electrolytes, renal function
• Oxygen saturation - assess severity and need for supplemental oxygen

Additional Studies (based on severity):

• Blood cultures (before antibiotics if possible)
• Sputum culture (if good quality specimen available)
• Urinary antigen tests (Legionella, S. pneumoniae)
• PCT or CRP - may help distinguish bacterial from viral etiology
• ABG - if hypoxemic or severe disease
• CT chest - if chest X-ray negative but clinical suspicion high

Accurate severity assessment is crucial for determining the appropriate site of care and guiding initial antibiotic selection. Several validated scoring systems help clinicians stratify risk and predict outcomes in CAP patients.

[KEY_CONCEPT] The primary decision points are: outpatient vs. inpatient treatment, and medical ward vs. ICU admission.

CURB-65 Score: The most widely used severity scoring system internationally.

CURB-65 Risk Stratification:

• 0-1 points: Low risk - outpatient treatment
• 2 points: Intermediate risk - consider hospitalization
• 3-5 points: High risk - hospitalization required, consider ICU

Pneumonia Severity Index (PSI): More complex but potentially more accurate than CURB-65, incorporating 20 variables.

PSI Risk Classes and Recommendations:

Class I: Age <50, no comorbidities → Outpatient Class II: PSI score <70 → Outpatient
Class III: PSI score 71-90 → Brief hospitalization or outpatient Class IV: PSI score 91-130 → Hospitalization Class V: PSI score >130 → Hospitalization, consider ICU

[HIGH_YIELD] ICU Admission Criteria (Severe CAP):

Major Criteria (1 required):

• Invasive mechanical ventilation required
• Septic shock requiring vasopressors

Minor Criteria (≥3 required):

• Respiratory rate ≥30/min
• PaO₂/FiO₂ ratio ≤250
• Multilobar infiltrates
• Confusion/disorientation
• Uremia (BUN ≥20 mg/dL)
• Leukopenia (WBC <4,000/μL)
• Thrombocytopenia (platelets <100,000/μL)
• Hypothermia (core temp <36°C)
• Hypotension requiring aggressive fluid resuscitation

Quick Sequential Organ Failure Assessment (qSOFA): Useful for identifying patients at risk for sepsis:

• Altered mental status (GCS <15)
• Systolic BP ≤100 mmHg
• Respiratory rate ≥22/min

[CLINICAL_PEARL] qSOFA score ≥2 points identifies patients with suspected infection who are at higher risk for poor outcomes and should prompt consideration for sepsis protocols.

Special Populations Requiring Hospitalization:

• Elderly patients (>65 years) with moderate illness
• Significant comorbidities (heart failure, COPD, diabetes, renal disease)
• Immunocompromised patients
• Social factors (inability to take oral medications, lack of support)
• Failed outpatient treatment

Outpatient Treatment Criteria:

• CURB-65 score 0-1
• PSI Class I-III
• Hemodynamically stable
• Adequate oral intake
• Reliable follow-up available
• No significant comorbidities

Empiric antibiotic selection for CAP should be based on patient risk factors, severity of illness, local resistance patterns, and likelihood of specific pathogens. Treatment guidelines emphasize early appropriate therapy to improve outcomes.

[HIGH_YIELD] Empiric Antibiotic Selection Algorithm:

CAP Antibiotic Selection Flowchart:

Outpatient Treatment: ├── No Comorbidities/Risk Factors: │ └── Amoxicillin 1g TID OR │ Doxycycline 100mg BID OR
│ Macrolide (if low S. pneumoniae resistance) │ └── With Comorbidities*: └── Amoxicillin-clavulanate 875/125mg BID OR Cephalexin 500mg QID OR Fluoroquinolone (levofloxacin/moxifloxacin)

Inpatient (Non-ICU): └── Beta-lactam + Macrolide OR Respiratory Fluoroquinolone alone

ICU/Severe CAP: ├── Standard Therapy: │ └── Beta-lactam + (Azithromycin OR Fluoroquinolone) │ └── Pseudomonas Risk: └── Anti-pseudomonal beta-lactam + (Fluoroquinolone OR Aminoglycoside + Macrolide)

*Comorbidities: COPD, diabetes, heart/liver/renal disease, malignancy, asplenia, immunosuppression, recent antibiotic use

Specific Antibiotic Recommendations:

Outpatient Therapy:

Previously Healthy, No Recent Antibiotics:

• Amoxicillin 1g PO TID × 5-7 days (first-line)
• Doxycycline 100mg PO BID × 5-7 days
• Macrolide (azithromycin, clarithromycin) if local S. pneumoniae resistance <25%

With Comorbidities or Recent Antibiotic Use:

• Amoxicillin-clavulanate 875/125mg PO BID × 5-7 days
• Respiratory fluoroquinolone (levofloxacin 750mg daily, moxifloxacin 400mg daily)
• Cephalexin 500mg PO QID (if β-lactam allergy absent)

Inpatient Non-ICU Therapy:

• Ampicillin-sulbactam 3g IV q6h + Azithromycin 500mg IV daily
• Ceftriaxone 1-2g IV daily + Azithromycin 500mg IV daily
• Respiratory fluoroquinolone alone (levofloxacin 750mg IV daily)

[CLINICAL_PEARL] The combination of β-lactam plus macrolide has shown mortality benefit compared to β-lactam monotherapy, likely due to anti-inflammatory effects and atypical pathogen coverage.

ICU/Severe CAP Therapy:

• Ceftriaxone 2g IV daily + Azithromycin 500mg IV daily
• Ampicillin-sulbactam 3g IV q6h + Levofloxacin 750mg IV daily
• Consider MRSA coverage (vancomycin, linezolid) if risk factors present

Pseudomonas aeruginosa Risk Factors:

• Structural lung disease (bronchiectasis, COPD)
• Recent hospitalization or antibiotic use
• Malnutrition or immunosuppression

Anti-Pseudomonal Therapy:

• Piperacillin-tazobactam 4.5g IV q6h + Levofloxacin 750mg IV daily
• Cefepime 2g IV q8h + Azithromycin 500mg IV daily + Tobramycin

Special Considerations:

Legionella Suspected:

• Fluoroquinolone (preferred) or Azithromycin
• Avoid β-lactams (ineffective against Legionella) [5]

MRSA Risk Factors:

• Add Vancomycin 15-20mg/kg IV q8-12h OR Linezolid 600mg IV q12h
• Risk factors: prior MRSA, injection drug use, necrotizing pneumonia

Duration of Therapy:

• Uncomplicated CAP: 5-7 days (minimum 5 days)
• Severe CAP or complications: 7-10 days
• S. aureus or Pseudomonas: 7-10 days
• Legionella: 10-14 days [5]

[HIGH_YIELD] Switch from IV to Oral Therapy Criteria:

• Clinical improvement (decreased fever, improved symptoms)
• Hemodynamically stable
• Functioning GI tract
• Able to take oral medications
• Usually after 2-3 days of IV therapy

While most patients with CAP recover completely with appropriate treatment, complications can occur and significantly impact morbidity and mortality. Early recognition and management of complications are essential for optimal outcomes.

Major Complications of CAP:

Respiratory Complications:

1. Respiratory Failure

   • Most common serious complication
   • May require mechanical ventilation
   • Risk factors: elderly, severe pneumonia, underlying lung disease

2. Pleural Effusion and Empyema

   • Occurs in 20-40% of hospitalized CAP patients
   • Complicated parapneumonic effusion: pH <7.30, glucose <60mg/dL, LDH >1000 U/L
   • Empyema: frank pus in pleural space
   • Management: chest tube drainage, antibiotics, possible surgery

3. Pneumothorax

   • Less common, may occur with necrotizing pneumonia
   • Requires immediate chest tube if tension pneumothorax

Systemic Complications:

1. Sepsis and Septic Shock

   • Leading cause of death in CAP
   • Requires aggressive fluid resuscitation and vasopressor support
   • Early antibiotic administration crucial (within 1 hour)

2. Acute Respiratory Distress Syndrome (ARDS)

   • Severe form of respiratory failure
   • Bilateral pulmonary infiltrates with PaO₂/FiO₂ <300
   • Requires lung-protective ventilation strategies

[CLINICAL_PEARL] Indicators of Treatment Failure:

• Worsening or lack of clinical improvement after 48-72 hours
• Progressive radiographic abnormalities
• Hemodynamic instability
• Development of complications

Cardiovascular Complications:

1. Myocardial Infarction and Arrhythmias

   • CAP increases cardiovascular risk by 2-5 fold
   • Inflammatory response and hypoxemia contribute
   • Consider cardiac evaluation in high-risk patients

2. Heart Failure Exacerbation

   • Common in patients with pre-existing cardiac disease
   • Requires careful fluid management

Neurologic Complications:

1. Meningitis
   • Rare but serious, especially with S. pneumoniae
   • Consider lumbar puncture if altered mental status persists

Prognosis and Mortality:

[HIGH_YIELD] Poor Prognostic Factors:

• Advanced age (>65 years)
• Multiple comorbidities
• Delayed presentation or treatment
• High severity scores (CURB-65 ≥3, PSI Class IV-V)
• Multilobar involvement
• Bacteremia
• Specific pathogens (S. aureus, Gram-negative bacilli)
• Development of complications

Long-term Outcomes:

Post-CAP Syndrome:

• Persistent fatigue, dyspnea, and functional impairment
• May last weeks to months after acute illness
• More common in elderly patients

Cardiovascular Risk:

• Increased risk of MI, stroke, and cardiac death for up to 1 year
• Consider cardiac risk factor modification
• Statin therapy may be beneficial

Follow-up Recommendations:

Post-CAP Follow-up Schedule:

2-3 Days (Outpatients): ├── Clinical response assessment ├── Medication adherence review └── Return precautions

7-10 Days (All Patients): ├── Symptom resolution check ├── Chest X-ray if severe disease └── Switch to oral antibiotics if applicable

6-8 Weeks (High-risk Patients): ├── Chest X-ray to document resolution ├── Consider CT if poor resolution └── Investigate underlying conditions

Prevention Strategies:

• Pneumococcal vaccination (PCV13, PPSV23)
• Annual influenza vaccination
• Smoking cessation
• Management of underlying conditions
• Hand hygiene and respiratory etiquette