Polymyxins

Polymyxins are cyclic peptidal antibiotics that are used when patients develop resistance to conventional drugs. They are produced by Gram-positive bacteria and are most effective against pathological Gram-negative bacteria.

The structures of two widely used polymyxins are shown: polymyxin B (Figure A) and colistin, also known as polymyxin E (Figure B). Both bind to lipopolysaccharides in the outer membranes of Gram-negative bacteria and disrupt both bacterial membranes.

But now some bacteria have evolved to become resistant to polymyxins. Jian-Hua Liu and colleagues at South China Agricultural University (Guangzhou) discovered a gene in an Escherichia coli strain that causes polymyxin resistance. The gene, called mcr-1, operates by a plasmid-mediated mechanism. The authors conclude that their findings “emphasize the urgent need for coordinated global action in the fight against pan–drug-resistant Gram-negative bacteria.”