Modulation of the hydrophobic domain of polymyxin B nonapeptide: effect on outer-membrane permeabilization and lipopolysaccharide neutralization

Immobilization of Escherichia coli cells by use of the antimicrobial peptide cecropin P1

An immobilization scheme for bacterial cells is described, in which the antimicrobial peptide cecropin P1 was used to trap Escherichia coli K-12 and O157:H7 cells on microtiter plate well surfaces. Cecropin P1 was covalently attached to the well surfaces, and E. coli cells were allowed to bind to the peptide-coated surface. The immobilized cells were detected colorimetrically with an anti-E. coli antibody-horseradish peroxidase conjugate. Binding curves were obtained in which the signal intensities were dependent upon the cell concentration and upon the amount of peptide attached to the well surface. After normalization for the amount of peptide coupled to the surface and the relative binding affinity of the antibody for each strain, the binding data were compared, which indicated that there was a strong preference for E. coli O157:H7 over E. coli K-12. The cells could be immobilized reproducibly at pH values ranging from 5 to 10 and at ionic strengths up to 0.50 M.

Colistin interactions with the mammalian urothelium

Here we describe the effect of colistin on the barrier function of the mammalian urinary bladder epithelium. Addition of colistin to the mucosal solution of the rabbit urinary bladder epithelium (urothelium) resulted in an increase in the transepithelial conductance. The magnitude of the increase in transepithelial conductance was dependent on the membrane voltage, concentration of colistin, and presence of divalent cations in the bath solution. The initial site of action of colistin was at the apical membrane. Colistin increased the membrane conductance only when the apical membrane potential was cell interior negative. The more negative the membrane potential, the larger the conductance increase. The concentration dependence of the conductance increase saturated, suggesting a membrane binding site. Divalent cations decreased the magnitude of the conductance increase. This divalent cation action occurred at two sites: one in competition with colistin for a membrane binding site, and the other by rapidly blocking the induced conductance. At short exposure times, the increase in conductance was reversed by either removing colistin from the bath or changing the voltage so that the apical membrane was cell interior positive. At long exposure times, the increase was only partially reversible by voltage or removal from the bath. This finding suggests that at long exposure times, there is a toxic effect of colistin on the urothelium.

Molecular basis of bacterial outer membrane permeability revisited

Gram-negative bacteria characteristically are surrounded by an additional membrane layer, the outer membrane. Although outer membrane components often play important roles in the interaction of symbiotic or pathogenic bacteria with their host organisms, the major role of this membrane must usually be to serve as a permeability barrier to prevent the entry of noxious compounds and at the same time to allow the influx of nutrient molecules. This review summarizes the development in the field since our previous review (H. Nikaido and M. Vaara, Microbiol. Rev. 49:1-32, 1985) was published. With the discovery of protein channels, structural knowledge enables us to understand in molecular detail how porins, specific channels, TonB-linked receptors, and other proteins function. We are now beginning to see how the export of large proteins occurs across the outer membrane. With our knowledge of the lipopolysaccharide-phospholipid asymmetric bilayer of the outer membrane, we are finally beginning to understand how this bilayer can retard the entry of lipophilic compounds, owing to our increasing knowledge about the chemistry of lipopolysaccharide from diverse organisms and the way in which lipopolysaccharide structure is modified by environmental conditions.

Lipid binding and membrane penetration of polymyxin B derivatives studied in a biomimetic vesicle system

Understanding membrane interactions and cell-wall permeation of Gram-negative bacteria is of great importance, owing to increasing bacterial resistance to existing drugs and therapeutic treatments. Here we use biomimetic lipid vesicles to analyse membrane association and penetration by synthetic derivatives of polymyxin B (PMB), a potent naturally occurring antibacterial cyclic peptide. The PMB analogues studied were PMB nonapeptide (PMBN), in which the hydrophobic alkyl residue was cleaved, PMBN diastereomer containing D-instead of L-amino acids within the cyclic ring (dPMBN) and PMBN where the hydrophobic alkyl chain was replaced with an Ala6 repeat (Ala6-PMBN). Peptide binding measurements, colorimetric transitions induced within the vesicles, fluorescence quenching experiments and ESR spectroscopy were applied to investigate the structural parameters underlying the different membrane-permeation profiles and biological activities of the analogues. The experiments point to the role of negatively charged lipids in membrane binding and confirm the prominence of lipopolisaccharide (LPS) in promoting membrane association and penetration by the peptides. Examination of the lipid interactions of the PMB derivatives shows that the cyclic moiety of PMB is not only implicated in lipid attachment and LPS binding, but also affects penetration into the inner bilayer core. The addition of the Ala6 peptide moiety, however, does not significantly promote peptide insertion into the hydrophobic lipid environment. The data also indicate that the extent of penetration into the lipid bilayer is not related to the overall affinity of the peptides to the membrane.