Bacillus anthracis: Culture Characteristics, Colony Morphology, and Hallmark Features

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The page below is a sample from the LabCE course Preliminary Identification of the Primary Select Agents of Bioterrorism. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Bacillus anthracis: Culture Characteristics, Colony Morphology, and Hallmark Features

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
  • No hemolysis on SBA (NOTE: Bacillus cereus biovar anthracis is also non-hemolytic, unlike B. cereus which is hemolytic on SBA.)
  • 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
4. CDC/L Stauffer. Image #1898. "This Petri dish culture plate contained a growth medium of sheep blood agar (SBA), which was inoculated with Bacillus anthracis bacteria." PHIL, 2002 public domain. Accessed May 25, 2022. https://phil.cdc.gov/Details.aspx?pid=1898
5. CDC/M Mathias, T Parker. Image#11748. "This image depicts numbers of Bacillus anthracis bacterial colonies, which had been allowed to grow on sheep's blood agar (SBA) for a 24-hour period." PHIL, 2009 public domain. Accessed May 25, 2022. https://phil.cdc.gov/Details.aspx?pid=11748

Sheep blood agar (SBA) inoculated
with Bacillus anthracis bacteria.
Positive tenacity test. (4)
Bacillus anthracis bacterial colonies growing on sheep
blood agar (SBA) after 24 hours of incubation. (5)