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BACTERIA · Gram-negative coccobacillus

Haemophilus influenzae

Haemophilus influenzae — Gram stain

Stain: Gram stainMorphology: Small pleomorphic gram-negative coccobacilliYield: HIGHDifficulty: EASY
Haemophilus influenzae microscopic image — Gram-negative coccobacillus, Gram stain
Image: Wikimedia Commons · File:Haemophilus_influenzae_Gram.JPG (CC BY-SA)

Key facts

**Pathogenesis**: Requires factors V (NAD+) and X (hematin) — grows on chocolate agar (lysed RBCs release both), satellites around S. aureus. Type b PRP capsule is the major virulence factor of invasive disease; nontypeable strains cause mucosal infections. **Diagnostic clue**: Gram-negative coccobacilli in CSF / epiglottis swab / sputum; positive quellung; thumbprint sign on lateral neck XR in epiglottitis. **Virulence**: PRP capsule, IgA protease, endotoxin.

Boards buzzwords

  • chocolate agar
  • factors V (NAD+) and X (hematin)
  • satellite phenomenon
  • type b capsule
  • thumbprint sign
  • cherry-red epiglottis

Associated diseases

  • Acute epiglottitis (cherry-red epiglottis, drooling, tripod posture)
  • Meningitis (pre-Hib vaccine)
  • Otitis media (nontypeable)
  • Sinusitis
  • COPD exacerbation
  • Pneumonia

Treatment

Ceftriaxone for invasive disease/meningitis; amoxicillin (± clavulanate) for mucosal infections; Hib conjugate vaccine for prevention; rifampin chemoprophylaxis for close contacts of invasive Hib

Related organisms

Listeria monocytogenes
Listeria monocytogenes
Gram-positive rod (facultative intracellular)
Mycobacterium tuberculosis
Mycobacterium tuberculosis
Acid-fast bacillus (mycolic acid cell wall)
Nocardia spp.
Nocardia spp.
Gram-positive filamentous, partially acid-fast
Treponema pallidum
Treponema pallidum
Spirochete
Mycobacterium leprae
Mycobacterium leprae
Acid-fast bacillus (obligate intracellular)
Neisseria meningitidis
Neisseria meningitidis
Gram-negative diplococci