A fluorescent BODIPY FL derivative of polymyxin B is now available from Molecular Probes. Polymyxin B is a cyclic cationic peptide antibiotic that binds to anionic lipids. This antibiotic binds to the lipopolysaccharide (LPS) component of the outer membrane of gram-negative bacteria and increases the permeability of the outer membrane to lysozyme and hydrophobic compounds.