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

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 blood agar tends to form small, grayish, smooth colonies with a narrow zone of beta-hemolysis. That appearance is most similar to Streptococcus agalactiae (Group B streptococcus), which also grows as small, gray, beta-hemolytic colonies on blood agar. The other organisms in the list typically show noticeably different colony colors, textures, or hemolysis patterns (for example, pigment production with Rhodococcus, or the dry, irregular look of Corynebacterium). So the closest resemblance is to Group B streptococcus.

Listeria monocytogenes on blood agar tends to form small, grayish, smooth colonies with a narrow zone of beta-hemolysis. That appearance is most similar to Streptococcus agalactiae (Group B streptococcus), which also grows as small, gray, beta-hemolytic colonies on blood agar. The other organisms in the list typically show noticeably different colony colors, textures, or hemolysis patterns (for example, pigment production with Rhodococcus, or the dry, irregular look of Corynebacterium). So the closest resemblance is to Group B streptococcus.