A microbiologist is examining plates of a lymph node biopsy from a seriously ill patient. The SBA plate shows small, nonhemolytic colonies that have a fried egg appearance. On MacConkey agar, these organisms grow as a small, nonlactose fermenter. On Gram stain, this organism exhibits bipolar staining and plump gram-negative rods. What is the most probable identification?
A) Brucella mallei
B) Bacillus anthracis
C) Yersinia pestis
D) Burkholderia cepacia

Question

Quizplus · Accepted Answer

The small, nonhemolytic colonies that have a fried egg appearance on SBA and small, nonlactose fermenter growth on MacConkey agar are consistent with Yersinia pestis. The bipolar staining and plump gram-negative rods are also characteristic of Yersinia pestis. Brucella mallei and Bacillus anthracis are both gram-positive organisms and would not exhibit the same staining characteristics. Burkholderia cepacia is a gram-negative organism, but it typically grows as a lactose fermenter on MacConkey agar and would not have the same colony morphology on SBA.

A Microbiologist Is Examining Plates of a Lymph Node Biopsy