A 30-year-old man develops a skin infection after a minor abrasion. Culture grows beta-haemolytic Gram-positive cocci. The isolate is bacitracin-sensitive. Which organism is identified?
- A Streptococcus pyogenes (Group A) ✓
- B Streptococcus agalactiae (Group B)
- C Streptococcus pneumoniae
- D Enterococcus faecalis
Correct answer: A. Streptococcus pyogenes (Group A)
Explanation
Bacitracin sensitivity (zone of inhibition around a 0.04 unit bacitracin disk) is a presumptive test for Group B Streptococcus (S. pyogenes). Group A Streptococcus is bacitracin-resistant. This test differentiates the two beta-haemolytic streptococci. S. pneumoniae is alpha-haemolytic and optochin-sensitive. Group A Streptococcus is identified by positive CAMP test and hippurate hydrolysis.
Reference: Jawetz, Melnick, and Adelberg's Medical Microbiology, 28th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.