Several species of aerobic sporulating bacilli recently isolated from soil, sewage,
manure, and cheese, as well as authentic strains obtained from type culture collections, have been found to exhibit antagonistic activity against unrelated microorganisms. Cultures of these aerobic sporulating bacilli yield an alcohol-soluble, water-insoluble fraction,-
tyrothricin,-which is bactericidal for most Gram-positive and Gram-negative microbial species. Two different crystalline products have been separated from
tyrothricin. One, which may be called
tyrocidine, is bactericidal in vitro for both Gram-positive and Gram-negative species; the other substance,
gramicidin, is effective only against Gram-positive microorganisms. In general,
tyrocidine behaves like a protoplasmic
poison and like other
antiseptics, loses much of its activity in the presence of animal tissues.
Gramicidin on the contrary exerts a much more subtle physiological effect on the susceptible bacterial cells and, when applied locally at the site of the
infection, retains in vivo a striking activity against Gram-positive microorganisms.