Listeria monocytogenes colonies on SBA plate most closely resemble colonies of which organism?

• A. Corynebacterium diphtheriae
• B. Streptococcus agalactiae
• C. Streptococcus bovis group
• D. Rhodococcus equi

Answer: (B)

Listeria monocytogenes on sheep blood agar tends to form small, smooth, gray-white colonies with a narrow zone of beta-hemolysis. That same beta-hemolytic, small, translucent colony appearance is most characteristic of Streptococcus agalactiae (group B Streptococcus), making them the closest match in colony morphology. To sort them out, rely on other tests: Listeria is catalase positive and motile at room temperature (tumbling motility), whereas Streptococcus agalactiae is catalase negative, nonmotile, and typically hippurate positive. The other organisms described have distinct colony textures or pigments (for example, Rhodococcus equi can be pigmented; Corynebacterium diphtheriae often has a dry, irregular colony texture; Streptococcus bovis group lacks the classic beta-hemolytic appearance seen with group B Streptococcus).