The description of "Medusa head" colonies on solid agar is most characteristic of which organism?

• A. Bacillus anthracis
• B. Enterococcus faecalis
• C. Staphylococcus saprophyticus
• D. Streptococcus agalactiae

Answer: (A)

Medusa head colonies are a classic clue for Bacillus anthracis. On solid agar (especially blood agar), this organism forms large, nonhemolytic colonies with radiating, hairlike filaments that extend outward from the center, giving a distinctive medusa-like appearance. That radiating fringe is a growth pattern specific enough to stand out against other Gram-positive organisms in the options.

beyond colony shape, Bacillus species are Gram-positive rods that form spores; B. anthracis is notable for being nonmotile and encapsulated, which helps distinguish it from others in the genus and from the cocci listed. The other organisms—Enterococcus faecalis, Staphylococcus saprophyticus, and Streptococcus agalactiae—do not produce this medusa head morphology; their colonies are typically smooth, not radiating and hairlike.