TYROTHRICIN was first discovered by Dubos1 in 1939. He isolated an alcohol-soluble and water-insoluble substance from the autolyzed cultures of an anaerobic, motile, spore-forming bacillus of the soil which had been called Bacillus brevis. This was highly active and bactericidal against various gram-positive organisms, which include Staphylococcus, Stroptococcus and Diplococcus pneumoniae. Dubos named this substance gramicidin. However, further investigation by Dubos and Hotchkiss2 proved that a second substance was also present. This substance had some degree of activity against gram-negative organisms and was named tyrocidine. Because of the two substances the name of the bactericidal extract was then changed to tyrothricin.
Further investigation indicated that tyrothricin contains approximately 85 per cent tyrocidine and 15 per cent gramicidin.3 A single injection of O.1 mg. of tyrocidine administered intraperitoneally is sufficient to protect mice against 10,000 fatal doses of the pneumococcus.4 This is, however, much less active than