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
Frequently asked questions
What is Bacitracin (A disk) test?
A presumptive disk concept using a low-dose bacitracin A-disk to distinguish beta-hemolytic Group A streptococcus from other beta-hemolytic streptococci.
What is the high-yield clue for Bacitracin (A disk) test?
Group A strep (S. pyogenes) is bacitracin-sensitive (zone present); Group B strep (S. agalactiae) is bacitracin-resistant.
What is Bacitracin (A disk) test associated with?
Bacitracin (A disk) test is associated with Group A vs Group B beta-hemolytic separation, Study pairing with PYR and CAMP tests, and Beta-hemolysis workup vocabulary.
What is Bacitracin (A disk) test commonly confused with?
Bacitracin (A disk) test is commonly confused with Optochin test and CAMP test.
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.