Haemophilus influenzae

Microbiology

• Nontypeable (non-encapsulated) strains are less invasive and cause mucosal infections. (e.g., otitis media, sinusitis, pneumonia).
• The type b strain (encapsulated) can cause more severe infections (e.g., meningitis, epiglottitis), but the incidence of these infections has decreased with routine vaccination.
• The Hib vaccine only has effect on type b strain.
• Growth Requirements:
  • Grows on Chocolate Agar (lysed RBCs).
  • Requires Factor V (NAD+) and Factor X (Hematin).
  • Satellite Phenomenon: Grows on blood agar (complete RBC) only when surrounding colonies of S. aureus (which lyses RBCs to release Factor X and secretes Factor V). t

Virulence factors

• IgA protease
• Capsular polysaccharides (polyribosylribitol phosphate)
  • Contained in Hib vaccine, conjugated to a protein (e.g., diphtheria toxoid)
• Infections are usually caused by unencapsulated strains (vaccine protects against encapsulated H. influenzae type B)