Is Haemophilus influenzae Part of Normal Flora?
Yes, Haemophilus influenzae colonizes the upper respiratory tract of humans as part of the normal commensal flora, but the distinction between typable (encapsulated) and nontypable (unencapsulated) strains is clinically critical.
Key Distinction: Typable vs. Nontypable Strains
Nontypable (unencapsulated) H. influenzae strains are the predominant normal flora inhabitants:
- Nontypable H. influenzae is a common commensal organism in the human upper respiratory tract and is found in healthy carriers 1, 2
- These unencapsulated strains colonize the nasopharynx and should generally be considered normal flora when isolated from respiratory specimens 3
- H. influenzae colonizes the upper respiratory tract through adherence to epithelial cells via fimbriae 1
Typable (encapsulated) H. influenzae strains are NOT typical normal flora:
- Typable H. influenzae (particularly type b) is found in less than 5% of healthy persons 3
- When typable strains are isolated from sputum, they are more likely to represent true pathogens rather than colonizers 3
- Type b strains account for 98.3% of H. influenzae isolates from blood cultures in patients with pneumonia, despite representing only 1.8% of sputum culture isolates 3
Clinical Implications for Respiratory Specimens
The distinction between colonizer and pathogen is critical in clinical practice:
- When H. influenzae is isolated from respiratory specimens, it may represent either colonizing bacteria or true pathogens of pneumonia 4
- The distinction is facilitated by detecting pathogenic organisms as the dominant flora on direct Gram stain or their recovery in moderate or heavy growth 4
- Potential bacterial pathogens such as S. pneumoniae, S. aureus, and H. influenzae are frequently encountered in respiratory specimens and may represent contaminants that colonize the upper airways 4
Serotyping is recommended to differentiate pathogenic from commensal strains:
- Serotyping of sputum isolates is recommended in patients with pneumonia to separate pathogenic typable strains from nonpathogenic nontypable strains 3
- This distinction is particularly important when deciding whether to treat an isolated organism as a true pathogen versus a colonizer 3
Pathogenic Potential of Normal Flora Strains
Even commensal nontypable strains can cause disease under certain circumstances:
- Nontypable H. influenzae is an important cause of localized respiratory tract disease including otitis media, sinusitis, and pneumonia 2
- The pathogenesis begins with colonization of the nasopharynx, followed by contiguous spread to the middle ear, sinuses, or lungs under appropriate circumstances 2
- Viral upper respiratory infections predispose to secondary bacterial overgrowth by H. influenzae residing in the nasopharynx 4
- Colonization with nontypable H. influenzae increases significantly during concurrent viral infections such as RSV 4
Common Pitfalls to Avoid
Do not automatically dismiss H. influenzae as a contaminant:
- While nontypable strains are normal flora, they can cause significant disease, particularly in patients with COPD, bronchiectasis, or following viral infections 4
- In patients with COPD and FEV1 < 50%, H. influenzae becomes a more significant pathogen 4
Do not assume all H. influenzae isolates are the same:
- The presence of typable H. influenzae (especially type b) in respiratory specimens should raise concern for true infection rather than colonization 3
- Aspirates from the inner channel of bronchoscopes are characteristically contaminated by upper respiratory flora, which includes H. influenzae 4
Consider the clinical context:
- H. influenzae is listed among the most common pathogens in upper and lower respiratory tract infections in travelers and general populations 4
- In critically ill patients, the distinction between colonization and infection requires correlation with clinical findings, imaging, and quantitative cultures 4