Bacitracin is a bactericidal drug useful in the treatment of superficial skin infections but too toxic for systemic use. Bacitracin is a polypeptide antibiotic produced by Bacillus subtilis. This drug interferes in the peptidoglycan synthesis of bacteria. The presumptive identification of group A streptococci (GAS) is usually done by testing for sensitivity to bacitracin.
Principle: Bacitracin test is used to determine the effect of a small amount of bacitracin (0.04 IU or 0.05 IU not higher) on an organism. Streptococcus pyogenes (Group A Streptococci) is inhibited by the small amount of bacitracin in the disk; other beta-hemolytic streptococci usually are not. Some laboratories do not recommend the use of 0.04 U bacitracin disk as Lancefield groups C and G streptococci may occasionally also show susceptibility to bacitracin. PYR reaction can confirm the isolate as S. pyogenes as its the only beta-hemolytic streptococci that gives positive PYR reaction.
Procedure of Bacitracin test
- Using an inoculating loop, streak two or three suspect colonies of a pure culture onto a blood agar plate
- Using heated forceps, place a bacitracin disk in the first quadrant (area of heaviest growth). Gently tap the disk to ensure adequate contact with the agar surface.
- Incubate the plate for 18 to 24 hours at 35oC in ambient air.
- Look for zone of inhibition around disk.
Expected results of Bacitracin test:
- Sensitive: Any zone of inhibition around the disk
- Resistant: No zone of inhibition
Quality control organisms used in Bacitracin test:
- Positive: Streptococcus pyogenes
- Negative: Streptococcus agalactiae
- Image 1: sigmaaldrich.com
- Image 2: University of Florida