A throat swab from a child with sore throat grows beta-haemolytic colonies on blood agar. Bacitracin sensitivity testing shows sensitivity (>10 mm inhibition zone). This feature identifies the organism as:

• A Streptococcus pneumoniae
• B Streptococcus agalactiae (Group B)
• C Enterococcus faecalis
• D Streptococcus pyogenes (Group A Streptococcus) ✓

Explanation

Bacitracin (taxo-A disc) sensitivity is a presumptive test that identifies Group A Streptococcus (S. pyogenes): virtually all GAS strains are inhibited by bacitracin, while Group B (S. agalactiae) and other beta-haemolytic streptococci are resistant. S. pneumoniae shows alpha-haemolysis and is optochin-sensitive. Enterococcus is not beta-haemolytic on routine media. Confirmation of GAS identity requires Lancefield grouping (latex agglutination for Group A antigen).

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.