Beta-hemolytic streptococcus with Lancefield Group B. Which organism is it?

• A. Streptococcus pyogenes
• B. Streptococcus agalactiae
• C. Streptococcus pneumoniae
• D. Streptococcus mutans

Answer: (B)

Lancefield grouping classifies beta-hemolytic streptococci by a specific C carbohydrate in the cell wall, and Group B corresponds to Streptococcus agalactiae. So when the clue is a beta-hemolytic streptococcus with Lancefield Group B, the organism that fits is Streptococcus agalactiae. This species is classically associated with neonatal infections and can colonize the vaginal tract, but the key point here is its Group B designation. The other organisms listed are not Group B: the one that is Group A is also beta-hemolytic but belongs to a different Lancefield group; Streptococcus pneumoniae is typically alpha-hemolytic rather than beta-hemolytic; and Streptococcus mutans is a viridans group streptococcus not characterized by beta-hemolysis.