Which organism is bacitracin sensitive, beta-hemolytic, encapsulated with hyaluronic acid, and produces streptolysin O?

• A. Staphylococcus aureus
• B. Streptococcus pneumoniae
• C. Staphylococcus epidermidis
• D. Streptococcus pyogenes

Answer: (D)

This set of traits points to Streptococcus pyogenes (group A streptococcus). Bacitracin sensitivity helps distinguish group A from other beta-hemolytic streptococci, such as group B which is resistant. The beta-hemolysis on blood agar reflects complete lysis of red cells, a classic feature of group A strep. The hyaluronic acid capsule is a distinctive virulence factor of S. pyogenes, helping it avoid phagocytosis and giving it a capsule that resembles human connective tissue. Streptolysin O is a toxin produced by this organism that damages host cells, and it’s a characteristic finding for S. pyogenes. Taken together, these features uniquely identify Streptococcus pyogenes, whereas the other organisms listed lack one or more of these specific traits.