gramicidin, antibiotic obtained from the bacterial species Bacillus brevis, which is found in soil. Gramicidin is particularly effective against gram-positive bacteria (see Gram's stain). Because the drug is highly toxic, it cannot be administered internally and so is used only on the skin as a lotion or ointment. It is used primarily in the treatment of infected surface wounds, and in eye, nose, and throat infections. In 1939 the American microbiologist René Dubos isolated the substance tyrothricin and later showed that it was composed of two substances, gramicidin and tyrocidine. These were the first antibiotics to be manufactured commercially.