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F. tularensis CHOC 48h image courtesy of CDC

F. tularensis CHOC 72h image courtesy of CDC

Francisella tularensis

Culture Characteristics:

  • Slow growing and fastidious
  • Growth of visible colonies on agar may require two to five days
  • Growth is stimulated by CO2
  • Will grow initially on sheep's blood agar (SBA), but growth is poor or absent on subculture
  • Prefers cystein-enriched media such as chocolate (CHOC), Thayer-Martin (TM), buffered charcoal-yeast extract (BCYE), and thioglycollate (THIO)
  • Growth is slow in broth, so blood cultures should be held up to three weeks if F. tularensis is suspected

Colony Morphology on SBA at 35oC:

  • Can grow poorly or not at all on SBA
  • Tiny, pin-point, translucent colonies after 18-24 hours
  • Difficult to see individual colonies in growth that is less than 24 hours
  • Gray-white, opaque colonies less than 1 mm after 48 hours
  • No hemolysis

Colony Morphology on CHOC at 35oC:

  • Pin-point, gray-white, opaque growth after 24 hours
  • 1-2 mm, gray-white, smooth, shiny growth after 48 hours
  • Individual colonies exhibit mature growth at 3 days in CO2 environment and have a greenish appearance