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Haemophilus influenzae

H. flu

Morphology: Pleomorphic Gram-negative coccobacillus. Requires factors X (hemin) and V (NAD) — grows on chocolate agar but NOT blood agar (unless satellited around S. aureus). Encapsulated (type b — Hib) and non-typeable (NTHi) forms.

HNPEKGramNegOther

Typical drugs

  1. #1Amoxicillin— First-line for β-lactamase-negative strains (~70% of US isolates). Use higher dose 1 g PO TID for CAP.
  2. #2Amoxicillin-clavulanate— First-line if β-lactamase risk (smoker, COPD, recent abx, hospitalized) — covers BL-positive strains.
  3. #3Ceftriaxone— Hospitalized CAP / meningitis; reliably active even with β-lactamase.
  4. #4Azithromycin— Atypical coverage; H. flu MICs creeping up — less reliable.
  5. #5Doxycycline— Outpatient alternative.

Empiric therapy when resistant

BL-positive (most common): amoxicillin-clavulanate, oral cephalosporin, or ceftriaxone. BLNAR: ceftriaxone, respiratory FQ, or carbapenem. Always covered by IV ceftriaxone — the safe empiric choice.

Resistance mechanisms

  • Enzymatic degradation

    TEM-1 β-lactamase (~25–30% of US isolates)

    Example: Hydrolyzes amoxicillin and ampicillin; amoxicillin-clavulanate or cephalosporins remain active.

  • Target alteration

    Altered penicillin-binding protein (PBP3) — β-lactamase-negative ampicillin-resistant (BLNAR)

    Example: Emerging in Europe and Japan; oral cephalosporins also less reliable. Use ceftriaxone or FQ.

Resistance notes

~25–30% β-lactamase-positive in US — empiric amox-clav over amoxicillin in higher-risk patients (smoker, COPD, recent abx, hospital-acquired). BLNAR less common in US but rising globally.

Common syndromes

Pearls

Most common in COPD / smokers — exacerbation pathogen. Non-typeable strains (NTHi) cause most adult respiratory disease (CAP, bronchitis, sinusitis, otitis). Type b (Hib) historically caused invasive disease (meningitis, epiglottitis) in young children — Hib vaccine has nearly eliminated invasive type b disease; remains important in unvaccinated populations. Otitis media + sinusitis common in children. Epiglottitis classically presents with "three Ds" — drooling, dysphagia, distress (rare in vaccinated era; consider in unvaccinated adults).

References

  • IDSA/ATS Community-Acquired Pneumonia in Adults (2019)
  • AAP Red Book — Haemophilus influenzae (2024)