Tyrothricin »Tyrothricin is an antibacterial substance produced by the growth of Bacillus brevisDubos (Fam.Bacteriaceae).It consists principally of gramicidin and tyrocidine,the tyrocidine usually being present as the hydrochloride.It contains not less than 900µg and not more than 1400µg of tyrothricin per mg. Packaging and storage— Preserve in tight containers. USP Reference standards á11ñ USP Gramicidin RS. Identification— Add about 5mg to 5mLof p-dimethylaminobenzaldehyde TS,and shake for 2minutes.Add 2drops of sodium nitrite solution (1in 150)and 5mLof water,and mix:a blue color is produced. Loss on drying á731ñ Dry about 100mg,accurately weighed,in a capillary-stoppered bottle in vacuum at a pressure not exceeding 5mm of mercury at 60for 3hours:it loses not more than 5.0%of its weight. Assay— Proceed as directed for gramicidin under Antibiotics—Microbial Assays á81ñ,using a suitable,accurately weighed portion of Tyrothricin dissolved quantitatively in alcohol to yield a stock solution of convenient concentration.Dilute an accurately measured volume of this solution quantitatively and stepwise with alcohol to obtain a Test Dilutionhaving a concentration of tyrothricin assumed to be 5.0times the median dose level of the gramicidin Standard.Multiply the result of the gramicidin assay by 5.0to obtain the amount of tyrothricin,in µg,in each mLof the Test Dilution. Auxiliary Information— Staff Liaison:William W.Wright,Ph.D.,Scientific Fellow Expert Committee:(PA7)Pharmaceutical Analysis 7 USP28–NF23Page 2004 Phone Number:1-301-816-8335