Structure-function studies of polymyxin B nonapeptide: Implications to sensitization of gram-negative bacteria

Polymyxin B nonapeptide (PMBN), a cationic cyclic peptide derived by enzyma tic processing from the naturally occurring peptide polymyxin B, is able to increase the permeability of the outer membrane of Gram-negative bacteria toward hydrophobic antibiotics probably by binding to the bacterial lipopol ysaccharide (LPS). We have synthesized 11 cyclic analogues of PMBN and eval uated their activities compared to that of PMBN. The synthetic peptides wer e much less potent than PMBN in their capacity to sensitize Escherichia col i and Klebsiella pneumoniae toward novobiocin and to displace dansyl-PMBN f rom Escherichia coli LPS. Moreover, unlike PMBN, none of the analogues were able to inhibit the growth of Pseudomonas aeruginosa. The structural-funct ional features of PMBN were characterized and identified with regard to the ring size, the distance between positive charges and peptide backbone, the chirality of the DPhe-Leu domain, and the nature of the charged groups. Ap parently, the structure of PMBN is highly specific for efficient perturbati on of the outer membrane of Gram-negative bacteria as well as for LPS bindi ng. The present study further increases our understanding of the complex PM BN-LPS and may, potentially, enable the design of compounds having enhanced permeabilization potency of the Gram-negative outer membrane.