BACTERIA · Gram-negative coccobacillus
Haemophilus influenzae
Haemophilus influenzae — Gram stain
Stain: Gram stainMorphology: Small pleomorphic gram-negative coccobacilliYield: HIGHDifficulty: EASY
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
Gram-positive rod (facultative intracellular)
Mycobacterium tuberculosis
Acid-fast bacillus (mycolic acid cell wall)
Nocardia spp.
Gram-positive filamentous, partially acid-fast
Treponema pallidum
Spirochete
Mycobacterium leprae
Acid-fast bacillus (obligate intracellular)
Neisseria meningitidis
Gram-negative diplococci