What are the common bacterial causes of respiratory tract infections?

Common Bacterial Causes of Respiratory Tract Infections

Streptococcus pneumoniae is the most frequently encountered bacterial pathogen in respiratory tract infections and must always be covered by antibiotic therapy. 1, 2

Primary Bacterial Pathogens by Infection Type

Community-Acquired Pneumonia (CAP)

The bacterial pathogens you must consider, in order of frequency:

• Streptococcus pneumoniae - The dominant pathogen across all severity levels (outpatient to ICU), accounting for 3-30% of identified cases 3, 2
• Haemophilus influenzae - Second most common, particularly in patients with underlying bronchopulmonary disease (3-14% of cases) 3, 4
• Moraxella catarrhalis - Less frequent but important (1-3% of cases) 3
• Staphylococcus aureus - Rare in typical CAP but critical during influenza pandemics with 47% mortality 4
• Legionella pneumophila - Rare overall (0.7-13%) but becomes prominent in severe pneumonia requiring ICU admission 1, 4
• Gram-negative enteric bacilli - Very rare in community settings but associated with worst outcomes 1, 4

Atypical Bacterial Pathogens

These intracellular organisms require different antibiotic coverage:

• Mycoplasma pneumoniae - Most common atypical pathogen (4-39% of cases, higher during epidemics), particularly prevalent in outpatient pneumonia 2, 4
• Chlamydia pneumoniae - Less frequent but clinically significant 1
• Legionella species - Already mentioned above for severe disease 2

Acute Exacerbations of COPD

The bacterial spectrum shifts based on disease severity:

Mild-to-moderate COPD:

• Haemophilus influenzae - Most frequent 1
• Streptococcus pneumoniae - Predominates among bacterial causes 1
• Moraxella catarrhalis - Third most common 1

Severe COPD (FEV₁ <50%):

• Pseudomonas aeruginosa - Becomes prevalent (10-15% of hospitalizations) 3
• Gram-negative organisms - Increase in frequency 3

Upper Respiratory Tract Infections

• Streptococcus pneumoniae - Primary pathogen 5, 6
• Haemophilus influenzae - Key pathogen 5, 6
• Moraxella catarrhalis - Common cause 5, 7
• Streptococcus pyogenes - Predominant in pharyngitis/tonsillitis (15% of cases) 5, 8
• Staphylococcus aureus - Frequent isolate 7

Critical Clinical Context

Severity-Based Pathogen Distribution

ICU-admitted patients with severe CAP:

• S. pneumoniae and L. pneumophila are the leading causes 1
• Consider broader Gram-negative coverage in this population 4

Mixed Infections Are Common

• Polymicrobial infections occur in 6-26% of hospitalized CAP patients 3
• Viral-bacterial co-infections found in 15% of CAP cases, most commonly S. pneumoniae with influenza/parainfluenza, associated with more severe disease and >40% mortality 3, 4
• 35% of patients with determined etiology have two or more pathogens 2, 4

Important Clinical Caveats

Antibiotic Resistance Patterns

Be aware of evolving resistance:

• Penicillin resistance in S. pneumoniae exceeds 20% in many regions, >50% in some Asian countries 9
• Beta-lactamase production is common in H. influenzae and M. catarrhalis 10, 9
• Macrolide resistance reaches 70-80% prevalence in some Asian countries 9
• Multiple drug resistance frequently combines penicillin non-susceptibility with macrolide, TMP-SMX, and tetracycline resistance 9

Diagnostic Limitations

• 40-60% of respiratory infections have no identified pathogen despite appropriate testing, particularly in elderly patients (>70 years) with cardiac or renal comorbidities 3
• Viral pathogens account for 6-61% of lower respiratory tract infections in community studies, with influenza representing 4-30% of cases 3
• Clinical presentation cannot accurately predict microbiological etiology 1

Geographic Considerations

Consider endemic pathogens:

• Burkholderia pseudomallei (melioidosis) in endemic areas, particularly after heavy rainfall, with 70% developing pneumonia 4