A microbiologist notices that a throat swab isolate of Streptococcus pyogenes is resistant to bacitracin on disc testing. Which additional test should confirm the provisional identification of this organism?
- A Optochin sensitivity (>14 mm zone on blood agar)
- B Latex agglutination for Lancefield group A-specific carbohydrate antigen ✓
- C CAMP test positivity
- D Bile solubility (organism lyses in 10% sodium deoxycholate)
Explanation
Bacitracin sensitivity is used as a presumptive test for S. pyogenes (Group A Streptococcus): a zone >10 mm around a 0.04-unit bacitracin disc strongly suggests Group A, but up to 5–10% of Group A strains and some Group C/G strains may give ambiguous results. Definitive identification requires Lancefield grouping by latex agglutination, which detects the group A-specific rhamnose-N-acetylglucosamine carbohydrate antigen. Optochin sensitivity identifies S. pneumoniae. Bile solubility also identifies S. pneumoniae. CAMP test identifies Group B Streptococcus (S. agalactiae).
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.