Lab method
Bacitracin (A disk) test
A-disk: Group A strep sensitive, Group B resistant
bass-ih-TRAY-sin
High-yield clue
Group A strep (S. pyogenes) is bacitracin-sensitive (zone present); Group B strep (S. agalactiae) is bacitracin-resistant.
Overview
A presumptive disk concept using a low-dose bacitracin A-disk to distinguish beta-hemolytic Group A streptococcus from other beta-hemolytic streptococci.
Classification
- Presumptive disk concept
- Beta-hemolytic cocci
- Zone-of-inhibition readout
Lab & identification clues
- A-disk zone = presumptive Group A (S. pyogenes)
- No zone = Group B and most others
- Pairs with PYR for Group A confirmation
- Read on blood agar
Associations
- Group A vs Group B beta-hemolytic separation
- Study pairing with PYR and CAMP tests
- Beta-hemolysis workup vocabulary
Commonly confused with
- Optochin test
- CAMP test
Your notes
Original concept summary for coursework. Sources checked: OpenStax Microbiology 2e and NCBI Bookshelf Medical Microbiology; reviewed 2026-06. Describes vocabulary and interpretation concepts only; not a lab protocol and not for handling specimens or identifying patient isolates.