Polymyxin B nonapeptide is a cyclic peptide obtained from Polymyxin B by proteolytic removal of its terminal amino acyl residue[1]. Polymyxin B nonapeptide is less toxic, lacks bactericidal activity, and retains its ability to render gram-negative bacteria susceptible to several antibiotics by permeabilizing their outer membranes[2].

Polymyxin B nonapeptide, a cationic cyclic peptide derived by enzymatic 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 lipopolysaccharide (LPS)[1].

[1]. Tsubery H, et al. Structure-function studies of polymyxin B nonapeptide: implications to sensitization of gram-negative bacteria. J Med Chem. 2000 Aug 10;43(16):3085-92. [2]. Ofek I, et al. Antibacterial synergism of polymyxin B nonapeptide and hydrophobic antibiotics in experimental gram-negative infections in mice. Antimicrob Agents Chemother. 1994 Feb;38(2):374-7.