What is the most likely diagnosis?

Listeria monocytogenes is the correct answer. The motility agar is showing motility at the top of the tube, but not deeper; typical of this catalase-positive, gram positive bacillus. Streptococcus agalactiae would be catalase negative and a coccus. Erysipelothrix rhusiopathiae would be H2S-positive and catalase negative. Escherichia coli is a gram negative bacillus.

This Gram-positive bacillus grew as a diffusely beta-hemolytic colony from a newborn. It was catalase positive and had tumbling motility on a hanging drop preparation. This is how it appeared on triple sugar iron agar and motility medium.

What is the most likely diagnosis?
A . Listeria monocytogenes
B. Streptococcus agalactiae
C. Erysipelothrix rhusiopathiae
D. Escherichia coli

View Answer

Answer: A