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Microbiology · Other Gram-negative rods

Haemophilus influenzae !

  • Bacterium
  • Gram-negative
  • Droplet
  • Exam priority: medium

A throat bacterium. Type b caused meningitis and epiglottitis in children before the vaccine; capsule-free strains cause ear and sinus infections.

Updated 2026-11-01

Also called: Hib, Haemophilus influenzae type b

At a glance

Gram
Gram-negative
Shape
Pleomorphic coccobacillus
Oxygen
Facultative anaerobe
Family
Pasteurellaceae
Stain
Gram: small red coccobacilli

Morphology

  • Small Gram-negative coccobacillus of variable shape
  • Needs factor X (hemin) and factor V (NAD): grows on chocolate agar
  • On blood agar it grows only around S. aureus colonies, which release factor V (satellitism)
  • Six capsular types (a to f); type b is the most virulent. Nontypeable strains have no capsule

Transmission

  • Respiratory droplets and contact with secretions; healthy carriers are the usual source
  • Birth canal to newborn
  • Droplet (on top of standard) for invasive Hib disease until 24 hours of antibiotics

Reservoir: Humans only (nasopharynx)

Isolation precautions

What the care team adds to standard precautions.

Droplet
Surgical mask within about a metre; single room.

Virulence factors

  • Type b polyribosyl-ribitol-phosphate (PRP) capsule: blocks phagocytosis and allows bloodstream invasion
  • IgA protease: mucosal colonization
  • Lipooligosaccharide: inflammation
  • Some strains make beta-lactamase

Diseases it causes

Clinical picture

  • Epiglottitis: high fever, drooling, muffled voice, painful swallowing and tripod posture; lateral neck film shows the "thumb sign"
  • Also Hib cellulitis, septic arthritis and bacteremia
  • Purulent conjunctivitis from nontypeable strains

Diagnosis

  • Culture of CSF, blood or secretions on chocolate agar
  • Gram stain: Gram-negative coccobacilli
  • Capsular antigen or PCR in CSF
  • In epiglottitis, secure the airway before examining the throat

Treatment

Invasive disease (meningitis, epiglottitis): third-generation cephalosporins such as ceftriaxone. Otitis and sinusitis: amoxicillin, or amoxicillin with clavulanic acid when beta-lactamase is present. Contacts of an invasive case get rifampicin chemoprophylaxis according to local rules.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

Prevention

  • Hib conjugate vaccine (PRP linked to a carrier protein)
  • Rifampicin chemoprophylaxis for selected contacts
  • Droplet precautions in invasive disease

Vaccine: The Hib conjugate vaccine is in Colombia's PAI within the pentavalent (DTP, hepatitis B and Hib) or hexavalent vaccine, with a series in the first months of life and a booster. It does not protect against nontypeable strains.

Test yourself

Think of the answer, then tap the question to see it.

Which two growth factors does Haemophilus influenzae need?

Factor X (hemin) and factor V (NAD).

What is Haemophilus satellitism?

On blood agar it grows only around S. aureus colonies, which release factor V and lyse red cells.

Which Hib component is used in the conjugate vaccine?

The capsular polysaccharide polyribosyl-ribitol-phosphate (PRP), linked to a carrier protein.

Which classic radiographic sign is seen in epiglottitis?

The "thumb sign": a swollen epiglottis on a lateral neck film.

Why does the Hib vaccine not prevent most Haemophilus ear infections?

Those infections come from nontypeable strains, which lack the type b capsule the vaccine targets.

Add to my cards (5)

← Back to the group: Other Gram-negative rods

Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: Chapter 8: Haemophilus influenzae (CDC, 2024); About Haemophilus influenzae Disease (CDC, 2026).

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Representative model, for educational purposes only.