Antibiotics including the polymixins and gramicidin act by interfering with the functioning of the bacterial cell membrane by increasing its permeability. Gramicidin is one of a family of cyclic decapeptides active against Gram-positive bacteria. Polymixins have a smaller peptide ring attached to a peptide chain ending with a branched fatty acid. They act specifically against Gram-negative bacteria, although chemically modified derivatives do have a broader spectrum of activity. These antibiotics are toxic to humans and are now rarely used in clinical practice.
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Chemical structure of a polymixin
(for polymixin B, the fatty acid is 6-methyloctanoic acid)
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Chemical structure of gramicidin
The broad-spectrum drug metronidazole, an anti-anaerobic and anti-protozoal agent, acts primarily to inhibit DNA gyrase. It probably also affects bacterial cell membranes as a secondary target.
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