A microbiologist is looking at a sheep blood agar (SBA) plate from a patient who might have been exposed to a biological agent. The colonies are nonhemolytic, large, have a ground-glass appearance, and look like a Medusa's head. On Gram stain, the colonies are large gram-positive rods that seem to have central spores. What is the most probable organism?
A) Bacillus anthracis
B) Brucella mallei
C) Burkholderia cepacia
D) Yersinia pestis

Question

Quizplus · Accepted Answer

The ground-glass appearance and Medusa's head morphology are characteristic of Bacillus anthracis, which is a spore-forming, gram-positive rod that grows on sheep blood agar as nonhemolytic colonies. The presence of central spores on the gram stain further supports this identification. Brucella mallei and Yersinia pestis are gram-negative organisms that do not form spores, and Burkholderia cepacia does not typically produce colonies with this morphology on SBA.

A Microbiologist Is Looking at a Sheep Blood Agar (SBA)