A throat swab from a student with exudative tonsillitis grows beta-haemolytic streptococci. A bacitracin (0.04 unit) disc placed on the lawn produces a zone of inhibition. This result supports identification as:

• A Streptococcus bovis group
• B Streptococcus agalactiae (Lancefield group B)
• C Streptococcus pyogenes (Lancefield group A) ✓
• D Viridans streptococci

Explanation

Susceptibility to low-concentration bacitracin (0.04 units) is a rapid presumptive marker of Streptococcus pyogenes, distinguishing it from other beta-haemolytic streptococci such as group B, which are bacitracin resistant. Definitive confirmation requires Lancefield grouping or PYR testing. Viridans streptococci are alpha-haemolytic, so they cannot grow with beta-haemolysis as described here.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.