any of various antibiotics obtained from strains of a soil bacterium (Bacillus polymyxa), esp. effective against Gram-negative bacteria
See polymyxin in American Heritage Dictionary 4
(pŏlˌē-mĭkˈsĭn)
noun
Any of various mainly toxic antibiotics derived from strains of the soil bacterium Bacillus polymyxa and used to treat various infections with gram-negative bacteria.