Haemophilus – summary

• Hemophilus influenzae also called ‘ Pfeiffer bacillus ‘/ influenza bacillus

Morphology & Taxonomy

• Gram -ve coccobacillus
• Non motile
• Non sporing
• Cells from older cultures exhibit pleomorphism
• Capsulated strains are pathogenic
• Classification
  • By Pitman
  • 6 capsular types ‘ a to f ‘
  • Based on capsular polysaccharide
  • 95% of H. influenzae isolates from acute invasive disease belongs to type ‘ b ‘
  • Non capsulated strains are termed – ‘ non typable ‘ since they lack capsular polysaccharide

Culture

• Aerobic & facultative anaerobe
• Fastidious growth requirements
  • X factor, V factor ( present in blood ) are essential for growth
  • X factor – heme and its enzymes, Heat stable
  • V factor – NAD/NADP, Heat labile, synthesized by some fungi & bacteria ( S. aureus ) ; exhibits satellitism – satellite colonies of H. influenzae develop around S. aureus

• Chocolate agar – when Blood agar is heated, factor V is released from RBCs

• Blood agar – streaked with S. aureus
• Nutrient agar – with X & V factors
• Levinthal medium – prepared by boiling & filtering the mixture of blood agar & nutrient broth
  • Capsulated strains form distinct iridiescence
• Fildes agar
  • Best for isolation of H. influenzae

Epidemiology

• Exclusively human pathogen
• Transmission – droplets / direct contact with secretion or fomites
• Colonisation of upper respiratory tract is common but such strains are usually uncapsulated
• Immunity is type specific

Pathogenesis

• Virulence factor – Type b polysaccharide capsule
• Disease due to invasion & hematogenous spread
• Persistent non typable H. influenzae colonisation of lower airway contributes to airway inflammation in adults with COPD – a Hallmark feature

Clinical features

• Meningitis – most serious
  • Primarily affects children < 2 yrs
  • Fever & altered CNS function – most common presentation
  • Most common complication – subdural effusion
• Epiglottitis / croup
  • Life threatening – tracheostomy is often necessary
  • Cellulitis of epiglottis & Supra epiglottic tissue
  • occurs in older children & also in adults
• Cellulitis
  • In young children
  • Most common location – head / neck
  • Associated with bacteremia
• Pneumonia
  • In infants
  • More likely to involve pleura also
• Other – osteomyelitis, Septic arthritis, orbital cellulitis, pericarditis, endophthalmitis, UTI, abscesses, bacteremia

Lab diagnosis

• Microscopy – pleomorphic Gram -ve, bacilli
• Direct antigen detection –
  • By rapid latex agglutination test
  • capsular polysaccharide in CSF & urine
• Culture

Treatment

• Cephalosporins – ceftriaxone or cefotaxime
• In Hib meningitis – glucocorticoids reduces neurological sequelae

Prevention

• Vaccination
  • Three conjugate vaccines
  • Only for Hib
• Chemoprophylaxis
  • Oral Rifampicin

Courtesy of featured image :- CDC/ Sarah Bailey Cutchin