Haemophilus influenzae is one of those Step bugs that shows up everywhere—peds ear infections, epiglottitis vignettes, COPD exacerbations—and the questions usually hinge on a few high-yield “tells”. Here’s a quick mnemonic you can picture in your head and recall in seconds.
The Visual Mnemonic: “HI loves XV + a Pink Capsule + Daycare”
Picture a big “HI” (for Haemophilus influenzae) walking into a daycare carrying two vitamin bottles labeled X and V, wearing a bright pink capsule jacket, then coughing on kids who suddenly develop ear pain and a swollen cherry-red epiglottis.
One-liner (the Step takeaway)
“H. influenzae is a small gram-negative coccobacillus that needs factors X (hemin) and V (NAD⁺), often has a type b capsule, and causes otitis media, sinusitis, epiglottitis, and meningitis—preventable with the Hib vaccine.”
Decode the Mnemonic (What each piece means)
“HI” = Haemophilus influenzae
- Gram-negative coccobacillus
- Often described as pleomorphic (variable shapes)
“Loves XV” = Requires Factors X and V
This is the classic lab hook.
| Factor | What it is | Why it matters |
|---|---|---|
| X | Hemin | Required for growth |
| V | NAD⁺ | Required for growth |
High-yield lab associations:
- Chocolate agar: supports growth because RBCs are lysed → releases X and V
- Satellite phenomenon: grows near Staphylococcus aureus on blood agar because Staph releases V factor (NAD⁺) and lyses RBCs to free X
“Pink capsule jacket” = Type b polysaccharide capsule
- The most tested virulence factor is the type b capsule
- Capsule is polyribosylribitol phosphate (PRP) → anti-phagocytic
- The most feared invasive disease is in unvaccinated children
High-Yield Clinical Syndromes (the ones NBME loves)
1) Epiglottitis
Clues:
- Fever, sore throat, drooling, dysphagia
- Tripod positioning
- Stridor
- “Thumbprint sign” on lateral neck X-ray
Testable management pearl:
- Secure the airway first (do not agitate the child)
- Typical antibiotics: ceftriaxone/cefotaxime (often plus anti-staph coverage depending on context)
2) Meningitis (especially infants/unvaccinated)
- Think: unvaccinated child with fever, neck stiffness, altered mental status
- Historically a major cause of pediatric meningitis before widespread vaccination
3) Otitis media + Sinusitis (nontypeable strains)
- Common in kids; also a frequent cause of acute bacterial sinusitis
- “Nontypeable” = no capsule → tends to cause mucosal infections rather than invasive disease
4) COPD exacerbations / Bronchitis
- In adults with chronic lung disease, H. influenzae is a classic cause of acute exacerbations of COPD
Vaccine & Immunology: the fast facts you actually need
Hib vaccine (type b)
- Conjugate vaccine: PRP capsule + carrier protein
- Why conjugate matters: turns a polysaccharide antigen into a T-cell–dependent response → better class switching and memory, effective in infants
What it prevents best:
- Invasive Hib disease (e.g., meningitis, epiglottitis)
What it does NOT reliably prevent:
- Mucosal infections caused by nontypeable (non-encapsulated) strains (e.g., some otitis media cases)
Rapid Recall Box (copy/paste for your notes)
- Gram-negative coccobacillus
- Needs X (hemin) + V (NAD⁺) → chocolate agar, satellites near Staph aureus
- Type b capsule (PRP) → invasive disease (meningitis, epiglottitis)
- Hib conjugate vaccine prevents invasive disease
- Key clinical: epiglottitis (thumbprint, drooling, tripod), otitis media, sinusitis, COPD exacerbations