MICROBIOLOGY

• Small, pleomorphic, intracellular and extracellular fastidious aerobic Gram-negative coccobacillus [Fig 1].
• Culture: poor or no growth on standard media.
  • Media most often employed: Thayer-Martin, BCYE or chocolate agar with the addition of cysteine and/or cysteine (thioglycolate).
    • MacConkey and other conventional media will not yield growth.
  • A slow grower may require CO₂ supplementation.
• Considered an important laboratory hazard to laboratory personnel requiring specialized safety procedures. Notify the lab if considering diagnosis when submitting specimens; higher level labs may be required.
• Recognized subspecies that vary in location and virulence:
  • F. tularensis: distinguished by PCR and the only relatively common human pathogen.
    • F. tularensis subsp. tularensis often are worse than those caused by F. tularensis subsp. holarctica
  • F. novidicida and F. philomiragia
    • A rare disease in humans, mostly immunocompromised.
  • F. hispaniensis
  • F. opportunistica
  • F. salimarina
  • Notes:
    • Biovar tularensis (Type A), typically associated with lagomorphs (hares, rabbits, pikas)
      • Biovar holarctica (B), frequently associated with rodents and aquatic environments
      • Biovar mediasiatica and subspecies novicida
    • Multiple other Francisella species have been isolated from animal and environmental samples.