Effective Date: 
Mon, 08/12/2013
Fri, 01/05/2018


Purpose and Scope:

The bacitracin susceptibility test is used to distinguish Group A streptococci, from other streptococci. When grown on blood agar, Group A streptococci are sensitive to (killed by) the antibiotic bacitracin. A sterile disk impregnated with bacitracin (also known as “Taxo A disk”) is placed on the first sector of an isolation plate before incubation. A zone of inhibition (area with no growth) will be seen around the disk after incubation if the organism is a Group A beta-hemolytic Streptococcus. Other beta-hemolytic streptococci are resistant to (not killed by) bacitracin. Their colonies will thus grow right up to the disk of bacitracin.


Reagents and Supplies:

  • BBL Taxo A disks (0.04 IU Bacitracin)
  • 5% Sheep Blood  w/TSA plates
  • Sterile inoculating loops
  • 37 C Incubator


Reagent Storage

  • On receipt, store at -20 to +8°C. After use, store vial or cartridge to protect product integrity at 2 to 8°C.
  • Use oldest discs first and discard expired discs. Allow containers to come to room temperature before opening.
  • Return unused discs to the refrigerator when application of discs has been completed.
  • Vials and cartridges from which discs have been frequently removed during one week and discs left out overnight in the laboratory should be discarded, or the discs should be tested for performance with control organisms prior to continued use.

 Quality Control:

 QC is performed weekly by inoculating one half of a blood agar plate with S pyogenes ATCC 19615 and the other half with S agalactiae ATCC 12386. After 24 hr incubation the S pyogenes should show a distinct zone of inhibition while the S agalactiae should not.

Record result in Bacitracin test QC log.



  1. With a sterile inoculating loop, obtain a portion of an isolated colony of the strepcococci being tested.
  2. Streak the plate for isolation.
  3. With sterile forceps, obtain a bacitracin disk and place inon the inoculated agar at the intersection of the primary and secondary streaks. Tap the disk with forceps to ensure adherence to the agar surface.
  4. Incubate the blood agar plate in ambient air at 35°C for 18 to 24 hours.
  5. After the incubation period, examine the blood agar for a zone of growth inhibition around the bacitracin disk.


Interpretation:     Susceptible= any zone of growth inhibition

                             Resistant=no zone of growth inhibition              



BD BBL Taxo Discs for Differentiation of Group A Streptococci Package Insert 8800671JAA(01) Ver. 02/2012

Levinson, M.L and P.F. Frank 1955 Differentiation of Group A from other beta-hemolytic streptococci with bacitracin. J. Bacteriol. 69:284-287