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polymyxin pol·y·myx·in (pŏl'ē-mĭk'sĭn)
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.
any of five polypeptide antibiotics derived from various species of the soil bacterium Bacillus that are active against gram-negative bacteria such as Escherichia coli and Pseudomonas aeruginosa. Polymyxins disrupt the cell membranes of bacteria, destroying their ability to function as osmotic barriers.