This is a differential test used to distinguish between organisms sensitive to the antibiotic bacitracin and those not. Bacitracin is a peptide antibiotic produced by Bacillus subtilis. It inhibits cell wall synthesis and disrupts the cell membrane. This test is commonly used to distinguish between the beta-hemolytic streptococci: Streptococcus agalactiae (bacitracin resistant) and Streptococcus pyogenes (bacitracin sensitive). The plate below was streaked with Streptococcus pyogenes; notice the large zone of inhibition surrounding the disk.
