Haemophilus influenzae
Also known as: H. influenzae, Hib
Haemophilus influenzae is a small gram-negative coccobacillus that causes respiratory infections, otitis media, epiglottitis, and meningitis. Despite the name, it is a bacterium and does not cause influenza.
H. influenzae is a fastidious gram-negative coccobacillus that grows only on chocolate agar, because it requires two accessory growth factors released when red blood cells are lysed: factor X (hemin) and factor V (NAD⁺). On blood agar it will grow only as satellite colonies near a Staphylococcus aureus streak, which supplies factor V — a classic laboratory identification finding. The organism was misnamed after Richard Pfeiffer isolated it from influenza patients in 1892 and mistook it for the cause of the disease.
Strains split into encapsulated and unencapsulated forms, and the distinction drives the disease pattern. Encapsulated type b (Hib) carries a polyribosylribitol phosphate capsule that resists phagocytosis and causes invasive disease: meningitis in unvaccinated young children, acute epiglottitis with the characteristic thumbprint sign on lateral neck radiograph, septic arthritis, and pneumonia. Nontypeable (unencapsulated) strains rarely cause invasive disease in healthy hosts but are a leading cause of otitis media, sinusitis, conjunctivitis, and acute exacerbations of COPD. All strains produce IgA protease, which cleaves mucosal IgA and aids colonization.
The Hib conjugate vaccine, which links the capsular polysaccharide to a protein carrier so infants can mount a T cell–dependent response, is given in infancy and has made invasive Hib disease rare where vaccination is routine. Invasive disease is treated with ceftriaxone; rifampin is used for prophylaxis of close contacts. Patients with functional or surgical asplenia — including those with sickle cell disease — are at heightened risk from encapsulated organisms and require vaccination against H. influenzae type b, pneumococcus, and meningococcus.
USMLE Step 1 tests the growth requirements, the capsule and IgA protease virulence factors, the Hib disease spectrum, and the asplenia connection. The CCMA exam covers H. influenzae among common respiratory and CNS infections, and the PTCE covers the Hib conjugate vaccine within vaccine types and immunization schedules.
Key takeaways
- H. influenzae is a gram-negative coccobacillus requiring factors X and V, so it grows on chocolate agar.
- Encapsulated type b strains cause invasive disease: meningitis, epiglottitis, septic arthritis, and pneumonia.
- Nontypeable strains cause otitis media, sinusitis, and COPD exacerbations.
- The Hib conjugate vaccine given in infancy has made invasive Hib disease uncommon.
- Asplenic patients, including those with sickle cell disease, are especially vulnerable to encapsulated organisms.