Dear Editor,The discovery that the mobile colistin resistance gene(mcr-1)is encoded by plasmids and is prevalent in food animals and human beings worldwide(Hasman et al.,2015;Liu et al.,2015)has challenged greatly our traditional idea that polymyxin(consisting of two isoforms:polymyxin B and polymyxin E(colistin))acts as an ultimate line of refuge in the clinical treatment against the severe infections by the multidrug-resistant Gram-negative pathogens(Nation et al.,2015;Paterson and Harris,2015).To make matters Dear Editor, The discovery that the mobile colistin resistance gene (mcr-1) is encoded by plasmids and is prevalent in food animals and human beings worldwide (Hasman et al., 2015; Liu et al., 2015) has challenged greatly our traditional idea that polymyxin (consisting of two isoforms: polymyxin B and polymyxin E (colistin)) acts as an ultimate line of refuge in the clinical treatment against the severe infections by the multidrug-resistant Gram-negative pathogens (Nation et al., 2015; Paterson and Harris, 2015) .To make matters