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Tenacious B. anthracis image courtesy of CDC

B. anthracis image on SBA at 24h courtesy of CDC

Bacillus anthracis

Culture Characteristics:

  • Growth may be noted as soon as eight hours after inoculation and occurs on most routine media, including sheep blood agar (SBA), chocolate agar (CHOC), and routine blood culture media
  • Does not grow on MacConkey (MAC) agar

Colony Morphology on SBA at 35°C, 18-24 hours:

  • Flat or slightly raised, gray to white with a "ground glass" appearance
  • Described as "tenacious" or "sticky" like petroleum jelly, shown in the top image
  • B. anthracis is NOT hemolytic, while B. cereus is hemolytic

Characteristic Features:

  • After 18 hours of incubation on SBA at 35°C, the slightly undulate margin may show curling, displaying a so-called "Medusa head" or described as comma-shaped protrusions, shown in the lower image