bacitracin - a polypeptide produced by the growth of a gram-positive, spore-forming organism belonging to the Licheniformis group of Bacillus subtilis. It is a whitish odorless hygroscopic powder, freely soluble in water, and used as an antibiotic for topical application or intramuscular injection; effective against susceptible gram-positive bacteria, gonococci, meningococci, and Entamoeba histolytica.