Review: Towards antibacterial strategies: studies on the mechanisms of interaction between antibacterial peptides and model membranes

Lipopolysaccharides (LPSs) play a dual role as inflammation-inducing and as membrane-forming molecules. The former role attracts significantly more attention from scientists, possibly because it is more closely related to sepsis and septic shock. This review aims to focus the reader's attention to the other role, the function of LPS as the major constituent of the outer layer of the outer membrane of Gram-negative bacteria, in particular those of enterobacterial strains. In this function, LPS is a necessary component of the cell envelope and guarantees survival of the bacterial organism. At the same time, it represents the first target for attacking molecules which may either be synthesized by the host's innate or adaptive immune system or administered to the human body. The interaction of these molecules with the outer membrane may not only directly cause the death of the bacterial organism, but may also lead to the release of LPS into the circulation. Here, we review membrane model systems and their application for the study of molecular mechanisms of interaction of peptides such as those of the human complement system, the bactericidal/permeability-increasing protein (BPI), cationic antibacterial peptide 18 kDa (CAP18) as an example of cathelicidins, defensins, and polymyxin B (PMB). Emphasis is on electrical measurements with a reconstitution system of the lipid matrix of the outer membrane which was established in the authors' laboratory as a planar asymmetric bilayer with one leaflet being composed solely of LPS and the other of the natural phospholipid mixture. The main conclusion, which can be drawn from these investigations, is that LPS and in general its negative charges are the dominant determinants for specific peptide—membrane interactions. However, the detailed mechanisms of interaction, which finally lead to bacterial killing, may involve further steps and differ for different antibacterial peptides.

Detection of lipid A by monoclonal antibodies in S-form lipopolysaccharide after acidic treatment of immobilized LPS on Western blot

Lipopolysaccharides (LPSs) were separated by SDS-PAGE, transferred onto polyvinylidene difluoride membranes, and hydrolyzed under acid conditions known to liberate lipid A from a broad variety of bacterial species. Independent of whether bis- or monophosphorylated lipid was set free, it remained bound to the membrane during subsequent immunostaining with monoclonal antibodies (MAbs) specific for the 1,4'-bis- or 4'-monophosphorylated lipid A backbone. Lipid A could be detected in the smooth (S)-form LPS from Enterobacteriaceae (Salmonella enterica, Citrobacter, Escherichia coli, Klebsiella pneumoniae, Serratia marcescens and Yersinia enterocolitica) , Pseudomonas aeruginosa, Chromobacterium violaceum, Acinetobacter, Vibrio cholerae and Legionella pneumophila, thus allowing LPS-phenotype determination. The procedure can be used for the majority of Gram-negative bacteria, since LPSs containing 2,3-diamino-2,3-dideoxy-glucose or glucosamine in the lipid A disaccharide backbone could be detected. It was shown to be sensitive (up to 20 ng of hydrolyzed LPS could be detected), and could be easily performed using isolated LPS or whole-cell lysates with or without prior digestion with proteinase K. In addition to opening several other application possibilities, the procedure may be particularly useful in cases where silver-staining of the O-chain in SDSpolyacrylamide gels fails and specific antibodies against the O-antigen are not available.

Structural and serological characterisation of the O-specific polysaccharide of the lipopolysaccharide from proposed new serotype 029 of Serratia marcescens

The structure of the repeating unit of the O-antigenic polysaccharide from the lipopolysaccharide (LPS) of Serratia marcescens strain 111 was determined by compositional and methylation analyses and NMR spectroscopy as →6)-α-D-Glc p-(1→2)-α-L-Rha p-(1→2)-α-L-Rha p-(1→2)-α-L-Rha p-(1→ (Rha, rhamnose) which represents a new O-antigenic structure in LPS of S. marcescens but is similar to the structure of the repeating unit of the O-antigen of S. marcescens 018 →6)-α-D-Glc pNAc-(1→2)-α-L-Rha p-(1→2)-α-L-Rha p-(1→2)-α-L-Rha p-(1→ (Oxley D., Wilkinson S.G. Structure of a neutral polymer isolated from the lipopolysaccharide of the reference strain for S. marcescens serogroup 018. Carbohydr Res 1989; 195: 111-115). Serological investigations using S. marcescens O- and K-specific ELISA tests and immunoblotting showed that S111 reacted with anti-O18 and anti-K4 sera, and that the anti-S111 serum reacted with serotype strain 018. The K4 reaction was confirmed as capsular by the Quellung reaction. After absorption with the heterologous strain, S. marcescens 018 and strain 111 were shown to be immunologically distinct, probably because of the presence of the GlcNAc residue in 018. We propose to add S111 to the O-serotype strains of S. marcescens as 029.

[Identification and structural analysis of a glycophospholipid component from the venom of ant Paraponera clavata]

The venom of South American ant Paraponera clavata and its low-molecular-mass fraction were shown to possess insectotoxic and pore-forming activities. A number of glycophospholipid components were isolated from this ant venom by means of gel filtration and reversed-phase chromatography. Some of the compounds cause conductivity fluctuations in lipid bilayer membranes within the ranges 3-25 pS and 200-400 pS at concentrations of 10(-6) to 10(-7) M. N-Acetylglucosamine, a fatty acid, and phosphoric acid residues were found in their structures. A full structure, 3-myristoyl-2-acetamido-2-deoxy-alpha-D-glucopyranosyl phosphate, was elucidated for one of the compounds by the use of 1H, 13C, and 31P NMR spectroscopy and mass spectrometry.

Lipopolysaccharides fromSerratia marcescens Possess One or Two 4-Amino-4-deoxy-L-arabinopyranose 1-Phosphate Residues in the Lipid A andD-glycero-D-talo-Oct-2-ulopyranosonic Acid in the Inner Core Region

The carbohydrate backbones of the core-lipid A region were characterized from the lipopolysaccharides (LPSs) of Serratia marcescens strains 111R (a rough mutant strain of serotype O29) and IFO 3735 (a smooth strain not serologically characterized but possessing the O-chain structure of serotype O19). The LPSs were degraded either by mild hydrazinolysis (de-O-acylation) and hot 4 M KOH (de-N-acylation), or by hydrolysis in 2 % aqueous acetic acid, or by deamination. Oligosaccharide phosphates were isolated by high-performance anion-exchange chromatography. Through the use of compositional analysis, electrospray ionization Fourier transform mass spectrometry, and 1H and 13C NMR spectroscopy applying various one- and two-dimensional experiments, we identified the structures of the carbohydrate backbones that contained D-glycero-D-talo-oct-2-ulopyranosonic acid and 4-amino-4-deoxy-L-arabinose 1-phosphate residues. We also identified some truncated structures for both strains. All sugars were D-configured pyranoses and alpha-linked, except where stated otherwise.

The dual role of lipopolysaccharide as effector and target molecule

Lipopolysaccharides (LPS) are major integral components of the outer membrane of Gram-negative bacteria being exclusively located in its outer leaflet facing the bacterial environment. Chemically they consist in different bacterial strains of a highly variable O-specific chain, a less variable core oligosaccharide, and a lipid component, termed lipid A, with low structural variability. LPS participate in the physiological membrane functions and are, therefore, essential for bacterial growth and viability. They contribute to the low membrane permeability and increase the resistance towards hydrophobic agents. They are also the primary target for the attack of antibacterial drugs and proteins such as components of the host's immune response. When set free LPS elicit, in higher organisms, a broad spectrum of biological activities. They play an important role in the manifestation of Gram-negative infection and are therefore termed endotoxins. Physico-chemical parameters such as the molecular conformation and the charges of the lipid A portion, which is responsible for endotoxin-typical biological activities and is therefore termed the 'endotoxic principle' of LPS, are correlated with the biological activity of chemically different LPS.