Is Haemophilus influenzae (H. influenzae) part of the normal flora of the upper respiratory tract?

Haemophilus influenzae as Part of the Normal Flora

Yes, Haemophilus influenzae is a normal colonizer of the upper respiratory tract in humans. This organism naturally colonizes the human upper respiratory tract and is transmitted person-to-person through respiratory droplets or direct contact with respiratory secretions 1.

Characteristics of H. influenzae Colonization

H. influenzae exists in two main forms:

1. Encapsulated (typeable) strains:

   • Six distinct capsular types (a, b, c, d, e, f)
   • Type b historically caused severe invasive disease before widespread vaccination
   • Less common as normal flora

2. Unencapsulated (nontypeable) strains:

   • More commonly found as part of normal flora
   • Primarily responsible for mucosal infections rather than invasive disease
   • Comprise the majority of colonizing strains in healthy individuals 2

Colonization Patterns

• H. influenzae primarily colonizes the nasopharynx in healthy individuals 3
• Studies have demonstrated that the prevalence of H. influenzae carriage has decreased among preschool-aged children from 2-7% in the pre-vaccine era to <1% in the vaccine era 1
• Nontypeable H. influenzae strains can persist in the upper respiratory tract as commensals without causing disease 4

Pathogenic Potential

While H. influenzae is part of the normal flora, it can also act as a pathogen under certain conditions:

• Contiguous spread: Can spread from the upper respiratory tract to cause sinusitis, otitis media, and pneumonia 5
• Invasive disease: Primarily caused by encapsulated type b strains (Hib), though now rare in countries with Hib vaccination 2
• Common respiratory infections: Nontypeable strains are important pathogens in:
  • Acute otitis media
  • Sinusitis
  • Conjunctivitis-otitis syndrome
  • Pneumonia (especially in developing countries) 5

Factors Affecting Colonization and Disease

• Immune status: Persons with certain immunocompromising conditions are at increased risk for invasive disease 1
• Age: Younger children are more susceptible to colonization and disease
• Bacterial factors: H. influenzae produces factors that inhibit ciliary activity of human epithelial cells 2
• Vaccine impact: Hib vaccination has dramatically reduced carriage and disease due to type b strains 1

Clinical Relevance

Understanding H. influenzae as part of the normal flora has important implications:

• It explains why this organism is a common cause of respiratory infections
• It informs antibiotic selection for respiratory infections (amoxicillin or amoxicillin-clavulanate are often recommended) 1
• It highlights the importance of distinguishing between colonization and infection
• It underscores why nontypeable strains remain important pathogens despite Hib vaccination 5

In summary, H. influenzae, particularly nontypeable strains, is a common component of the normal upper respiratory tract flora in humans, but can become pathogenic under certain conditions, causing a range of respiratory infections through contiguous spread from its colonization site.