In vitro antibacterial activity of gramicidin and tyrothricin.

815 recent clinical isolates of different species representing pathogenic or saprophytic constituents of human microbial flora were submitted to agar dilution tests. In concentrations above 64 micrograms/ml tyrothricin is in general equally as gramicidin or slightly more effective whereas below 8 micrograms/ml gramicidin is mostly more effective than tyrothricin. Of 401 streptococci all were inhibited by tyrothricin and gramicidin in concentrations up to 96 micrograms/ml; staphylococci were more resistant. The pathogenic species of 130 strains of gram-negative cocci were more susceptible than the saprophytic species. Haemophilus strains were mostly resistant to gramicidin and to a lesser extent to tyrothricin. Enterobacteriaceae were totally resistant to both antibiotics.