Thermodynamic analysis of the lipopolysaccharide-dependent resistance of gram-negative bacteria against polymyxin B

A molecular overview of the polymyxin-LPS interaction in the context of its mode of action and resistance development

With the rising incidences of antimicrobial resistance (AMR) and the diminishing options of novel antimicrobial agents, it is paramount to decipher the molecular mechanisms of action and the emergence of resistance to the existing drugs. Polymyxin, a cationic antimicrobial lipopeptide, is used to treat infections by Gram-negative bacterial pathogens as a last option. Though polymyxins were identified almost seventy years back, their use has been restricted owing to toxicity issues in humans. However, their clinical use has been increasing in recent times resulting in the rise of polymyxin resistance. Moreover, the detection of "mobile colistin resistance (mcr)" genes in the environment and their spread across the globe have complicated the scenario. The mechanism of polymyxin action and the development of resistance is not thoroughly understood. Specifically, the polymyxin-bacterial lipopolysaccharide (LPS) interaction is a challenging area of investigation. The use of advanced biophysical techniques and improvement in molecular dynamics simulation approaches have furthered our understanding of this interaction, which will help develop polymyxin analogs with better bactericidal effects and lesser toxicity in the future. In this review, we have delved deeper into the mechanisms of polymyxin-LPS interactions, highlighting several models proposed, and the mechanisms of polymyxin resistance development in some of the most critical Gram-negative pathogens.

Polymyxin B-Enriched Exogenous Lung Surfactant: Thermodynamics and Structure

The use of an exogenous pulmonary surfactant (EPS) to deliver other relevant drugs to the lungs is a promising strategy for combined therapy. We evaluated the interaction of polymyxin B (PxB) with a clinically used EPS, the poractant alfa Curosurf (PSUR). The effect of PxB on the protein-free model system (MS) composed of four phospholipids (diC16:0PC/16:0-18:1PC/16:0-18:2PC/16:0-18:1PG) was examined in parallel to distinguish the specificity of the composition of PSUR. We used several experimental techniques (differential scanning calorimetry, small- and wide-angle X-ray scattering, small-angle neutron scattering, fluorescence spectroscopy, and electrophoretic light scattering) to characterize the binding of PxB to both EPS. Electrostatic interactions PxB-EPS are dominant. The results obtained support the concept of cationic PxB molecules lying on the surface of the PSUR bilayer, strengthening the multilamellar structure of PSUR as derived from SAXS and SANS. A protein-free MS mimics a natural EPS well but was found to be less resistant to penetration of PxB into the lipid bilayer. PxB does not affect the gel-to-fluid phase transition temperature, Tm, of PSUR, while Tm increased by ∼+ 2 °C in MS. The decrease of the thickness of the lipid bilayer (dL) of PSUR upon PxB binding is negligible. The hydrophobic tail of the PxB molecule does not penetrate the bilayer as derived from SANS data analysis and changes in lateral pressure monitored by excimer fluorescence at two depths of the hydrophobic region of the bilayer. Changes in dL of protein-free MS show a biphasic dependence on the adsorbed amount of PxB with a minimum close to the point of electroneutrality of the mixture. Our results do not discourage the concept of a combined treatment with PxB-enriched Curosurf. However, the amount of PxB must be carefully assessed (less than 5 wt % relative to the mass of the surfactant) to avoid inversion of the surface charge of the membrane.

A Comparison between SARS-CoV-2 and Gram-Negative Bacteria-Induced Hyperinflammation and Sepsis

Sepsis is a life-threatening condition caused by the body's overwhelming response to an infection, such as pneumonia or urinary tract infection. It occurs when the immune system releases cytokines into the bloodstream, triggering widespread inflammation. If not treated, it can lead to organ failure and death. Unfortunately, sepsis has a high mortality rate, with studies reporting rates ranging from 20% to over 50%, depending on the severity and promptness of treatment. According to the World Health Organization (WHO), the annual death toll in the world is about 11 million. One of the main toxins responsible for inflammation induction are lipopolysaccharides (LPS, endotoxin) from Gram-negative bacteria, which rank among the most potent immunostimulants found in nature. Antibiotics are consistently prescribed as a part of anti-sepsis-therapy. However, antibiotic therapy (i) is increasingly ineffective due to resistance development and (ii) most antibiotics are unable to bind and neutralize LPS, a prerequisite to inhibit the interaction of endotoxin with its cellular receptor complex, namely Toll-like receptor 4 (TLR4)/MD-2, responsible for the intracellular cascade leading to pro-inflammatory cytokine secretion. The pandemic virus SARS-CoV-2 has infected hundreds of millions of humans worldwide since its emergence in 2019. The COVID-19 (Coronavirus disease-19) caused by this virus is associated with high lethality, particularly for elderly and immunocompromised people. As of August 2023, nearly 7 million deaths were reported worldwide due to this disease. According to some reported studies, upregulation of TLR4 and the subsequent inflammatory signaling detected in COVID-19 patients "mimics bacterial sepsis". Furthermore, the immune response to SARS-CoV-2 was described by others as "mirror image of sepsis". Similarly, the cytokine profile in sera from severe COVID-19 patients was very similar to those suffering from the acute respiratory distress syndrome (ARDS) and sepsis. Finally, the severe COVID-19 infection is frequently accompanied by bacterial co-infections, as well as by the presence of significant LPS concentrations. In the present review, we will analyze similarities and differences between COVID-19 and sepsis at the pathophysiological, epidemiological, and molecular levels.

Structure-Activity Relationships of Low Molecular Weight Alginate Oligosaccharide Therapy against Pseudomonas aeruginosa

Low molecular weight alginate oligosaccharides have been shown to exhibit anti-microbial activity against a range of multi-drug resistant bacteria, including Pseudomonas aeruginosa. Previous studies suggested that the disruption of calcium (Ca2+)-DNA binding within bacterial biofilms and dysregulation of quorum sensing (QS) were key factors in these observed effects. To further investigate the contribution of Ca2+ binding, G-block (OligoG) and M-block alginate oligosaccharides (OligoM) with comparable average size DPn 19 but contrasting Ca2+ binding properties were prepared. Fourier-transform infrared spectroscopy demonstrated prolonged binding of alginate oligosaccharides to the pseudomonal cell membrane even after hydrodynamic shear treatment. Molecular dynamics simulations and isothermal titration calorimetry revealed that OligoG exhibited stronger interactions with bacterial LPS than OligoM, although this difference was not mirrored by differential reductions in bacterial growth. While confocal laser scanning microscopy showed that both agents demonstrated similar dose-dependent reductions in biofilm formation, OligoG exhibited a stronger QS inhibitory effect and increased potentiation of the antibiotic azithromycin in minimum inhibitory concentration and biofilm assays. This study demonstrates that the anti-microbial effects of alginate oligosaccharides are not purely influenced by Ca2+-dependent processes but also by electrostatic interactions that are common to both G-block and M-block structures.

Affinity-Guided Isolation and Identification of Procyanidin B2 from Mangosteen (Garcinia mangostana L.) Rinds and its In Vitro LPS Binding and Neutralization Activities

Garcinia mangostana L. (mangosteen) is a tropical fruit that has been used for medicinal purposes in Southeast Asia for centuries. With an interest in its applications to treat infection, we sought to investigate the bioactive constituents of mangosteen and identified the phenolic compound procyanidin B2 from the mangosteen pericarp by examining lipopolysaccharide (LPS) binding capacity. The LPS binding and neutralization activities of procyanidin B2 were determined by a combination of biophysical and in silico techniques. The affinity of procyanidin B2 to LPS was 1.61 × 10^-5 M. Procyanidin B2 significantly neutralized LPS and selectively inhibited the LPS-induced release of tumor necrosis factor (TNF)-α from RAW264.7 cells in a dose-dependent manner. Binding thermodynamics revealed favorable hydrogen bonding and hydrophobic interactions between procyanidin B2 and LPS. Molecular simulations suggested that hydrogen bonding and hydrophobic interactions were involved in the binding process. These findings have, for the first time, shed light on the anti-inflammatory properties of procyanidin B2 through LPS binding and neutralization and provided a promising lead for the development of antiendotoxin agents.

Dual-site recognition of Pseudomonas aeruginosa using polymyxin B and bacteriophage tail fiber protein

As one of the top three opportunistic pathogens, Pseudomonas aeruginosa (P. aeruginosa) has long accounted for hospital-acquired infections with high risk of death. In this work, a fluorescent method based on a dual-site recognition mode was developed for rapid assay of P. aeruginosa. Employing its strong binding capability towards lipid A on the outer membrane of Gram-negative bacteria, polymyxin B acted as one recognition element for P. aeruginosa. To overcome the poor binding specificity of polymyxin B, a recombinant bacteriophage tail fiber protein was expressed and employed as a species-specific recognition element for the target pathogen. Thus a dual-site recognition mode was developed for specific assay of P. aeruginosa species by using fluorescein isothiocyanate as a fluorescent probe. The target pathogen can be assayed within a broad dynamic range from 2.0 × 10³ CFU mL^-1 to 2.0 × 10⁷ CFU mL^-1. Due to the ideal specificity of tail fiber protein, the method is capable of excluding the interference from other Gram-negative bacteria and all Gram-positive bacteria. It has been employed for assaying P. aeruginosa in various types of sample matrixes inclusive of lake water, physiological saline injection, human urine and milk. The acceptable assay results demonstrate its promising prospect for practical application in various areas such as environmental hygiene, medical diagnosis, as well as drug and food safety.

Anti-Infective and Anti-Inflammatory Mode of Action of Peptide 19-2.5

The polypeptide Pep19-2.5 (Aspidasept^®) has been described to act efficiently against infection-inducing bacteria by binding and neutralizing their most potent toxins, i.e., lipopolysaccharides (LPS) and lipoproteins/peptides (LP), independent of the resistance status of the bacteria. The mode of action was described to consist of a primary Coulomb/polar interaction of the N-terminal region of Pep19-2.5 with the polar region of the toxins followed by a hydrophobic interaction of the C-terminal region of the peptide with the apolar moiety of the toxins. However, clinical development of Aspidasept as an anti-sepsis drug requires an in-depth characterization of the interaction of the peptide with the constituents of the human immune system and with other therapeutically relevant compounds such as antibiotics and non-steroidal anti-inflammatory drugs (NSAIDs). In this contribution, relevant details of primary and secondary pharmacodynamics, off-site targets, and immunogenicity are presented, proving that Pep19-2.5 may be readily applied therapeutically against the deleterious effects of a severe bacterial infection.

Antimicrobial Peptides and Copper(II) Ions: Novel Therapeutic Opportunities

The emergence of new pathogens and multidrug resistant bacteria is an important public health issue that requires the development of novel classes of antibiotics. Antimicrobial peptides (AMPs) are a promising platform with great potential for the identification of new lead compounds that can combat the aforementioned pathogens due to their broad-spectrum antimicrobial activity and relatively low rate of resistance emergence. AMPs of multicellular organisms made their debut four decades ago thanks to ingenious researchers who asked simple questions about the resistance to bacterial infections of insects. Questions such as "Do fruit flies ever get sick?", combined with pioneering studies, have led to an understanding of AMPs as universal weapons of the immune system. This review focuses on a subclass of AMPs that feature a metal binding motif known as the amino terminal copper and nickel (ATCUN) motif. One of the metal-based strategies of hosts facing a pathogen, it includes wielding the inherent toxicity of copper and deliberately trafficking this metal ion into sites of infection. The sudden increase in the concentration of copper ions in the presence of ATCUN-containing AMPs (ATCUN-AMPs) likely results in a synergistic interaction. Herein, we examine common structural features in ATCUN-AMPs that exist across species, and we highlight unique features that deserve additional attention. We also present the current state of knowledge about the molecular mechanisms behind their antimicrobial activity and the methods available to study this promising class of AMPs.

The Beauty of Asymmetric Membranes: Reconstitution of the Outer Membrane of Gram-Negative Bacteria

The architecture of the lipid matrix of the outer membrane of Gram-negative bacteria is extremely asymmetric: Whereas the inner leaflet is composed of a phospholipid mixture, the outer leaflet is built up by glycolipids. For most Gram-negative species, these glycolipids are lipopolysaccharides (LPS), for a few species, however, glycosphingolipids. We demonstrate experimental approaches for the reconstitution of these asymmetric membranes as (i) solid supported membranes prepared by the Langmuir-Blodgett technique, (ii) planar lipid bilayers prepared by the Montal-Mueller technique, and (iii) giant unilamellar vesicles (GUVs) prepared by the phase transfer method. The asymmetric GUVs (aGUVs) composed of LPS on one leaflet are shown for the first time. They are characterized with respect to their phase behavior, flip-flop of lipids and their usability to investigate the interaction with membrane active peptides or proteins. For the antimicrobial peptide LL-32 and for the bacterial porin OmpF the specificity of the interaction with asymmetric membranes is shown. The three reconstitution systems are compared with respect to their usability to investigate domain formation and interactions with peptides and proteins.

Effect of sodium deoxycholate sulfate on outer membrane permeability and neutralization of bacterial lipopolysaccharides by polymyxin B formulations

We demonstrated binding interactions of polymyxin B (PMB), PMB formulations in the mole ratios of 1:2 and 1:3 of PMB:sodium deoxycholate sulfate (SDCS) and a commercial PMB formulation (CPMB) with lipopolysaccharides (LPS). The 1:2 PMB formulation (78.5-135.2 nM) exhibited a lower number of binding sites to the tested LPS compared to CPMB (112.6-140.9 nM) whereas 1:3 PMB formulation exhibited a higher number of binding sites (143.9-340.2 nM). Similarly, in the presence of LPS, the 1:2 PMB formulation (163.8-221.4 nm) exhibited smaller particle sizes compared to PMB, CPMB and 1:3 PMB formulation (248.8-603.5 nm). Molecular docking simulation suggested that the fatty acyl tails of LPS wrap together to produce a pseudo-globular structure of PMB-LPS complex, and among those 1:2 PMB formulation formed a more stable structure. The primary forces behind this complex are hydrogen bonds and salt bridges among the LPS, PMB, and SDCS. This study revealed that the PMB, CPMB, and PMB formulations inserted into the LPS micelles to disrupt the LPS membrane, whereas the SDCS may induce aggregation. The 1:2 PMB formulation also had higher bacterial uptake than other PMB formulations. The 1:2 PMB formulation neutralized the LPS micelles and was effective against Escherichia coli and Pseudomonas aeruginosa.

Infrared spectroscopy with multivariate analysis to interrogate the interaction of whole cells and secreted soluble exopolimeric substances of Pseudomonas veronii 2E with Cd(II), Cu(II) and Zn(II)

Extracellular polymeric substances (EPS) are bacterial products associated to cell wall or secreted to the liquid media that form the framework of microbial mats. These EPS contain functional groups as carboxyl, amino, hydroxyl, phosphate and sulfhydryl, able to interact with cations. Thus, EPS may be considered natural detoxifying compounds of metal polluted waters and wastewaters. In this work Attenuated Total Reflectance-Fourier Transform Infrared spectroscopy (ATR-FTIR) in combination with multivariate analysis (Principal Component Analysis-PCA-) were used to study the interaction of Cd(II), Cu(II) and Zn(II) and Pseudomonas veronii 2E cells, including bound EPS and cell wall, and its different soluble EPS fractions, previously characterized as Cd(II) ligands of moderate strength. Amino groups present in exopolysaccharide fraction were responsible for Zn(II) and Cu(II) complexation, while carboxylates chelated Cd(II). In lipopolysaccharide fraction, phosphoryl and carboxyl sites were involved in Cd(II) and Cu(II) binding, while Zn(II) interacted with amino groups. Similar results were obtained from cells. These studies confirmed that FTIR-PCA is a rapid analytical tool to provide valuable information regarding the functional groups in biomolecules related to metal interaction. Moreover, a discrimination and identification of functional groups present in both EPS and cells that interacted with Cd(II), Zn(II) and Cu(II) was demonstrated.

Binding interactions of bacterial lipopolysaccharides to polymyxin B in an amphiphilic carrier 'sodium deoxycholate sulfate'

This work presents the outcomes of a comparative study of molecular interactions of polymyxin B (PMB) and F12 and F13 formulations in the mole ratios of 1:2 and 1:3 of PMB:sodium deoxycholate sulfate (SDCS), respectively, and a commercial PMB formulation (CPMB) with lipopolysaccharides (LPS). Several spectroscopic and interfacial studies were performed to obtain LPS-peptide interactions at a molecular level. The fluorescence titrimetry method revealed that the F12 formulation (325 nM) exhibited a lower number of binding sites to the LPS compared to CPMB and F13 as well as PMB alone (537 nM). Similarly, in the presence of LPS, the F12 formulation (88 nm) exhibited smaller particle sizes in the dynamic light scattering study compared to PMB (116 nm), CPMB, and the F13 formulation. An interfacial study and circular dichroism spectroscopy revealed PMB and CPMB insertion into the LPS micelles to destabilize and disrupt the LPS membrane, whereas the F12 and F13 formulations may induce pseudo-aggregation. The NMR and IR studies showed that the presence of SDCS, the hydrophobicity of PMB increased by hydrogen bonding and electrostatic interactions and formed stabilized PMB-SDCS micelles. The PMB-SDCS formulation is likely to release PMB for easy penetration into the lipid membrane and cause disruption of the complex LPS micelles. Furthermore, the PMB-SDCS formulations neutralized and detoxified the LPS micelles with minimal toxicity to normal kidney tubular cells as well as an immortalised kidney cell line. The antimicrobial properties of PMBloaded SDCS nanomicelles were effective against a resistant strain of Pseudomonas aeruginosa.

Inhibition of Lipopolysaccharide- and Lipoprotein-Induced Inflammation by Antitoxin Peptide Pep19-2.5

The most potent cell wall-derived inflammatory toxins (“pathogenicity factors”) of Gram-negative and -positive bacteria are lipopolysaccharides (LPS) (endotoxins) and lipoproteins (LP), respectively. Despite the fact that the former signals via toll-like receptor 4 (TLR4) and the latter via TLR2, the physico-chemistry of these compounds exhibits considerable similarity, an amphiphilic molecule with a polar and charged backbone and a lipid moiety. While the exterior portion of the LPS (i.e., the O-chain) represents the serologically relevant structure, the inner part, the lipid A, is responsible for one of the strongest inflammatory activities known. In the last years, we have demonstrated that antimicrobial peptides from the Pep19-2.5 family, which were designed to bind to LPS and LP, act as anti-inflammatory agents against sepsis and endotoxic shock caused by severe bacterial infections. We also showed that this anti-inflammatory activity requires specific interactions of the peptides with LPS and LP leading to exothermic reactions with saturation characteristics in calorimetry assays. Parallel to this, peptide-mediated neutralization of LPS and LP involves changes in various physical parameters, including both the gel to liquid crystalline phase transition of the acyl chains and the three-dimensional aggregate structures of the toxins. Furthermore, the effectivity of neutralization of pathogenicity factors by peptides was demonstrated in several in vivo models together with the finding that a peptide-based therapy sensitizes bacteria (also antimicrobial resistant) to antibiotics. Finally, a significant step in the understanding of the broad anti-inflammatory function of Pep19-2.5 was the demonstration that this compound is able to block the intracellular endotoxin signaling cascade.

Lipopolysaccharide of Klebsiella pneumoniae attenuates immunity of Caenorhabditis elegans and evades by altering its supramolecular structure

Given the prominence of lipopolysaccharide (LPS) in the pathogenesis of Gram-negative bacteria, investigations at the molecular level in in vivo conditions are in dire need to understand its role in provoking infection.

Inhibitory Effects of Antimicrobial Peptides on Lipopolysaccharide-Induced Inflammation

Antimicrobial peptides (AMPs) are usually small molecule peptides, which display broad-spectrum antimicrobial activity, high efficiency, and stability. For the multiple-antibiotic-resistant strains, AMPs play a significant role in the development of novel antibiotics because of their broad-spectrum antimicrobial activities and specific antimicrobial mechanism. Besides broad-spectrum antibacterial activity, AMPs also have anti-inflammatory activity. The neutralization of lipopolysaccharides (LPS) plays a key role in anti-inflammatory action of AMPs. On the one hand, AMPs can readily penetrate the cell wall barrier by neutralizing LPS to remove Gram-negative bacteria that can lead to infection. On the contrary, AMPs can also inhibit the production of biological inflammatory cytokines to reduce the inflammatory response through neutralizing circulating LPS. In addition, AMPs also modulate the host immune system by chemotaxis of leukocytes, to promote immune cell proliferation, epithelialization, and angiogenesis and thus play a protective role. This review summarizes some recent researches about anti-inflammatory AMPs, with a focus on the interaction of AMPs and LPS on the past decade.

Electrostatic modification of the lipopolysaccharide layer: competing effects of divalent cations and polycationic or polyanionic molecules

The outer membrane (OM) of Gram-negative bacteria is asymmetrical with its outer layer mainly populated with polyanionic lipopolysaccharide (LPS). Much empirical evidence shows how OM permeability can be altered electrostatically: if Mg(2+) or divalent cations are required for the integrity of the OM, antimicrobial peptides (AMPs) or ethylene-diaminetetraacetic acid (EDTA) can permeabilize it. Using a coarse-grained model of the outer LPS layer, in which the layer is viewed as forming discrete binding sites for opposite charges, we study how the LPS layer can be modified electrostatically. In particular, we capture systematically ion-pairing and lateral-charge correlations on the LPS layer. Our results offer a clear picture of (competitive) ion binding onto the LPS layer and its impact on the lateral packing of LPS molecules, similarly to what has been seen in experiments: divalent cations such as Mg(2+) not only neutralize the LPS layer but also make its planar charge distribution heterogeneous, thus tightening the LPS layer; on the other hand, polycationic AMPs or polyanionic EDTA can displace Mg(2+) ions from the LPS layer and counteract the favorable effect of Mg(2+). Our result will be useful for clarifying to what extent OM permeability can be modified electrostatically.

Mechanism of Hbγ-35-induced an increase in the activation of the human immune system by endotoxins

Endotoxins (LPS) are highly potent immune stimulatory molecules and are mainly known for triggering Gram-negative sepsis. However, besides their toxic effects, this stimulatory function may be advantageous, for example when used as an adjuvant during vaccination. Thus, there is always a narrow range between the useful wake-up of the immune system and its overwhelming reaction, which can lead to diseases like sepsis. This raises the question of which conformational properties are responsible for making the LPS aggregates more or less potent. As described previously, the size, type and form of LPS aggregates play a major role in their immune stimulatory activity. In this study we investigate the role of these parameters. On the one hand, we use a peptide (Pep19-2.5; Aspidasept) that causes a change of the LPS aggregate structure into a less toxic state; on the other hand, we use a potent immune stimulating peptide (Hbγ-35), leading to higher toxicity. We have found opposing effects on LPS aggregate conformations allowing a better understanding of the processes of immune stimulation.

Cysteine deleted protegrin-1 (CDP-1): anti-bacterial activity, outer-membrane disruption and selectivity

BACKGROUND

Protegin-1 (PG-1: RGGRLCYCRRRFCVCVGR-amide) assumes a rigid β-hairpin like structure that is stabilized by two disulfide bridges between Cys6-Cys15 and Cys8-Cys13. Previous studies, employing linear analogs of PG-1, with Cys to Ala mutations or modified Cys, have demonstrated that the disulfide bridges are critical for the broad spectrum and salt resistant antimicrobial activity of PG-1.

METHODS

In order to understand structural and functional roles of disulfide bonds in protegrins, we have synthesized a Cys deleted variant of PG-1 or CDP-1, RGGRLYRRRFVVGR-amide, and two of its analogs, RR11, RLYRRRFVVGR-amide, and LR10, LYRRRFVVGR-amide, containing deletion of residues at the N-terminus. These peptides have been characterized for bactericidal activity and mode of action in lipopolysaccharide (LPS) using optical spectroscopy, ITC and NMR.

RESULTS

Antibacterial activity, against Gram-negative and Gram-positive strains, of the three peptides follows the order: CDP-1>RR11>LR10. LR10 displays only limited activity toward Gram-negative strains. CDP-1 demonstrates efficient membrane permeabilization and high-affinity interactions with LPS. CDP-1 and RR11 both assume β-hairpin like compact structures in complex with LPS, whereas LR10 adopts an extended conformation in LPS. In zwitterionic DPC micelles CDP-1 and the truncated analog peptides do not adopt folded conformations.

MAJOR CONCLUSIONS

Despite the absence of stabilizing disulfide bridges CDP-1 shows broad-spectrum antibacterial activity and assumes β-hairpin like structure in complex with LPS. The β-hairpin structure may be essential for outer membrane permeabilization and cell killing.

Modulating potency: Physicochemical characteristics are a determining factor of TLR4-agonist nanosuspension activity

Activity of adjuvanted vaccines is difficult to predict in vitro and in vivo. The wide compositional and conformational range of formulated adjuvants, from aluminum salts to oil-in-water emulsions, makes comparisons between physicochemical and immunological properties difficult. Even within a formulated adjuvant class, excipient selection and concentration can alter potency and physicochemical properties of the mixture. Complete characterization of physicochemical properties of adjuvanted vaccine formulations and relationship to biological response is necessary to move beyond a guess-and-check paradigm toward directed development. Here we present a careful physicochemical characterization of a two-component nanosuspension containing synthetic TLR-4 agonist glucopyranosyl lipid adjuvant (GLA) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) at various molar ratios. Physicochemical properties were compared with potency, as measured by stimulation of cytokine production in human whole blood. We found a surprising, nonlinear relationship between physicochemical properties and GLA-DPPC ratios that corresponded well with changes in biological activity. We discuss these data in light of the current understanding of TLR4 activation and the conformation-potency relationship in development of adjuvanted vaccines.

Resurrecting inactive antimicrobial peptides from the lipopolysaccharide trap

Host defense antimicrobial peptides (AMPs) are a promising source of antibiotics for the treatment of multiple-drug-resistant pathogens. Lipopolysaccharide (LPS), the major component of the outer leaflet of the outer membrane of Gram-negative bacteria, functions as a permeability barrier against a variety of molecules, including AMPs. Further, LPS or endotoxin is the causative agent of sepsis killing 100,000 people per year in the United States alone. LPS can restrict the activity of AMPs inducing aggregations at the outer membrane, as observed for frog AMPs, temporins, and also in model AMPs. Aggregated AMPs, "trapped" by the outer membrane, are unable to traverse the cell wall, causing their inactivation. In this work, we show that these inactive AMPs can overcome LPS-induced aggregations while conjugated with a short LPS binding β-boomerang peptide motif and become highly bactericidal. The generated hybrid peptides exhibit activity against Gram-negative and Gram-positive bacteria in high-salt conditions and detoxify endotoxin. Structural and biophysical studies establish the mechanism of action of these peptides in LPS outer membrane. Most importantly, this study provides a new concept for the development of a potent broad-spectrum antibiotic with efficient outer membrane disruption as the mode of action.

Biophysical mechanisms of the neutralization of endotoxins by lipopolyamines

Endotoxins (lipopolysaccharides, LPS) are one of the strongest immunostimulators in nature, responsible for beneficial effects at low, and pathophysiological effects at high concentrations, the latter frequently leading to sepsis and septic shock associated with high mortality in critical care settings. There are no drugs specifically targeting the pathophysiology of sepsis, and new therapeutic agents are therefore urgently needed. The lipopolyamines are a novel class of small molecules designed to sequester and neutralize LPS. To understand the mechanisms underlying the binding and neutralization of LPS toxicity, we have performed detailed biophysical analyses of the interactions of LPS with candidate lipopolyamines which differ in their potencies of LPS neutralization. We examined gel-to-liquid crystalline phase behavior of LPS and of its supramolecular aggregate structures in the absence and presence of lipopolyamines, the ability of such compounds to incorporate into different membrane systems, and the thermodynamics of the LPS:lipopolyamine binding. We have found that the mechanisms which govern the inactivation process of LPS obey similar rules as found for other active endotoxin neutralizers such as certain antimicrobial peptides.

Surface changes and polymyxin interactions with a resistant strain of Klebsiella pneumoniae

This study examines the interaction of polymyxin B and colistin with the surface and outer membrane components of a susceptible and resistant strain of Klebsiella pneumoniae. The interaction between polymyxins and bacterial membrane and isolated LPS from paired wild type and polymyxin-resistant strains of K. pneumoniae were examined with N-phenyl-1-naphthylamine (NPN) uptake, fluorometric binding and thermal shift assays, lysozyme and deoxycholate sensitivity assays, and by (1)H NMR. LPS from the polymyxin-resistant strain displayed a reduced binding affinity for polymyxins B and colistin in comparison with the wild type LPS. The outer membrane NPN permeability of the resistant strain was greater compared with the susceptible strain. Polymyxin exposure enhanced the permeability of the outer membrane of the wild type strain to lysozyme and deoxycholate, whereas polymyxin concentrations up to 32 mg/ml failed to permeabilize the outer membrane of the resistant strain. Zeta potential measurements revealed that mid-logarithmic phase wild type cells exhibited a greater negative charge than the mid-logarithmic phase-resistant cells. Taken together, our findings suggest that the resistant derivative of K. pneumoniae can block the electrostatically driven first stage of polymyxin action, which thereby renders the hydrophobically driven second tier of polymyxin action on the outer membrane inconsequential.

Preclinical investigations reveal the broad-spectrum neutralizing activity of peptide Pep19-2.5 on bacterial pathogenicity factors

Bacterial infections are known to cause severe health-threatening conditions, including sepsis. All attempts to get this disease under control failed in the past, and especially in times of increasing antibiotic resistance, this leads to one of the most urgent medical challenges of our times. We designed a peptide to bind with high affinity to endotoxins, one of the most potent pathogenicity factors involved in triggering sepsis. The peptide Pep19-2.5 reveals high endotoxin neutralization efficiency in vitro, and here, we demonstrate its antiseptic/anti-inflammatory effects in vivo in the mouse models of endotoxemia, bacteremia, and cecal ligation and puncture, as well as in an ex vivo model of human tissue. Furthermore, we show that Pep19-2.5 can bind and neutralize not only endotoxins but also other bacterial pathogenicity factors, such as those from the Gram-positive bacterium Staphylococcus aureus. This broad neutralization efficiency and the additive action of the peptide with common antibiotics makes it an exceptionally appropriate drug candidate against bacterial sepsis and also offers multiple other medication opportunities.

Insight into the antimicrobial activities of coprisin isolated from the dung beetle, Copris tripartitus, revealed by structure-activity relationships

The novel 43-residue, insect defensin-like peptide coprisin, isolated from the dung beetle, Copris tripartitus, is a potent antibiotic with bacterial cell selectivity, exhibiting antimicrobial activities against Gram-positive and Gram-negative bacteria without exerting hemolytic activity against human erythrocytes. Tests against Staphylococcus aureus using fluorescent dye leakage and depolarization measurements showed that coprisin targets the bacterial cell membrane. To understand structure-activity relationships, we determined the three-dimensional structure of coprisin in aqueous solution by nuclear magnetic resonance spectroscopy, which showed that coprisin has an amphipathic α-helical structure from Ala(19) to Arg(28), and β-sheets from Gly(31) to Gln(35) and Val(38) to Arg(42). Coprisin has electropositive regions formed by Arg(28), Lys(29), Lys(30), and Arg(42) and ITC results proved that coprisin and LPS have electrostatically driven interactions. Using measurements of nitric oxide release and inflammatory cytokine production, we provide the first verification of the anti-inflammatory activity and associated mechanism of an insect defensin, demonstrating that the anti-inflammatory actions of the defensin-like peptide, coprisin, are initiated by suppressing the binding of LPS to toll-like receptor 4, and subsequently inhibiting the phosphorylation of p38 mitogen-activated protein kinase and nuclear translocation of NF-kB. In conclusion, we have demonstrated that an amphipathic helix and an electropositive surface in coprisin may play important roles in its effective interaction with bacterial cell membranes and, ultimately, in its high antibacterial activity and potent anti-inflammatory activity. In addition to elucidating the antimicrobial action of coprisin, this work may provide insight into the mechanism of action of insect defense systems.

Lipopolysaccharide neutralization by antimicrobial peptides: a gambit in the innate host defense strategy

Antimicrobial peptides (AMPs) are nowadays understood as broad multifunctional tools of the innate immune system to fight microbial infections. In addition to its direct antimicrobial action, AMPs can modulate the host immune response by promoting or restraining the recruitment of cells and chemicals to the infection focus. Binding of AMPs to lipopolysaccharide is a critical step for both their antimicrobial action and their immunomodulatory properties. On the one hand, removal of Gram-negative bacteria by AMPs can be an effective strategy to prevent a worsened inflammatory response that may lead to septic shock. On the other hand, by neutralizing circulating endotoxins, AMPs can successfully reduce nitric oxide and tumor necrosis factor-α production, hence preventing severe tissue damage. Furthermore, AMPs can also interfere with the Toll-like receptor 4 recognition system, suppressing cytokine production and contributing to modulate the inflammatory response. Here, we review the immune system strategies devised by AMPs to avoid an exacerbated inflammatory response and thus prevent a fatal end to the host.

Biophysical mechanisms of endotoxin neutralization by cationic amphiphilic peptides

Bacterial endotoxins (lipopolysaccharides (LPS)) are strong elicitors of the human immune system by interacting with serum and membrane proteins such as lipopolysaccharide-binding protein (LBP) and CD14 with high specificity. At LPS concentrations as low as 0.3 ng/ml, such interactions may lead to severe pathophysiological effects, including sepsis and septic shock. One approach to inhibit an uncontrolled inflammatory reaction is the use of appropriate polycationic and amphiphilic antimicrobial peptides, here called synthetic anti-LPS peptides (SALPs). We designed various SALP structures and investigated their ability to inhibit LPS-induced cytokine secretion in vitro, their protective effect in a mouse model of sepsis, and their cytotoxicity in physiological human cells. Using a variety of biophysical techniques, we investigated selected SALPs with considerable differences in their biological responses to characterize and understand the mechanism of LPS inactivation by SALPs. Our investigations show that neutralization of LPS by peptides is associated with a fluidization of the LPS acyl chains, a strong exothermic Coulomb interaction between the two compounds, and a drastic change of the LPS aggregate type from cubic into multilamellar, with an increase in the aggregate sizes, inhibiting the binding of LBP and other mammalian proteins to the endotoxin. At the same time, peptide binding to phospholipids of human origin (e.g., phosphatidylcholine) does not cause essential structural changes, such as changes in membrane fluidity and bilayer structure. The absence of cytotoxicity is explained by the high specificity of the interaction of the peptides with LPS.

Peptide-based treatment of sepsis

Sepsis (blood poisoning) is a severe infectious disease with high mortality, and no effective therapy is actually known. In the case of Gram-negative bacteria, endotoxins (lipopolysaccharides) are known to be responsible for the strong inflammation reaction leading to the systemic infection. Peptides based on endotoxin-binding domains of human or animal proteins represent a promising approach in sepsis research. Although so far no medicament is available, the progress in recent years might lead to a breakthrough in this field. In this review, recent investigations are summarised, which may lead to an understanding of the mechanisms of action of peptides to suppress the inflammation reaction in vitro and in vivo (animal models) and thus may allow the development of effective anti-septic drugs.

Design, synthesis, and evaluation of a new fluorescent probe for measuring polymyxin-lipopolysaccharide binding interactions

Fluorescence assays employing semisynthetic or commercial dansyl-polymyxin B have been widely employed to assess the affinity of polycations, including polymyxins, for bacterial cells and lipopolysaccharide (LPS). The five primary γ-amines on diaminobutyric acid residues of polymyxin B are potentially derivatized with dansyl-chloride. Mass spectrometric analysis of the commercial product revealed a complex mixture of di- or tetra-dansyl-substituted polymyxin B. We synthesized a mono-substituted fluorescent derivative, dansyl[Lys]¹polymyxin B₃. The affinity of polymyxin for purified gram-negative LPS and whole bacterial cells was investigated. The affinity of dansyl[Lys]¹polymyxin B₃ for LPS was comparable to polymyxin B and colistin, and considerably greater (K(d)<1 μM) than for whole cells (K(d)∼6-12μM). Isothermal titration calorimetric studies demonstrated exothermic enthalpically driven binding between both polymyxin B and dansyl[Lys]¹polymyxin B₃ to LPS, attributed to electrostatic interactions. The hydrophobic dansyl moiety imparted a greater entropic contribution to the dansyl[Lys]¹polymyxin B₃-LPS reaction. Molecular modeling revealed a loss of electrostatic contact within the dansyl[Lys]¹polymyxin B₃-LPS complex due to steric hindrance from the dansyl[Lys]¹ fluorophore; this corresponded with diminished antibacterial activity (MIC≥16μg/mL). Dansyl[Lys]¹polymyxin B₃ may prove useful as a screening tool for drug development.

Electrostatic binding of substituted metal phthalocyanines to enterobacterial cells: its role in photodynamic inactivation

The effect of ionic substituents in zinc and aluminum phthalocyanine molecules and of membrane surface charge on the interaction of dyes with artificial membranes and enterobacterial cells, as well as on photosensitization efficiency was studied. It has been shown that increasing the number of positively charged substituents enhances the extent of phthalocyanine binding to Escherichia coli cells. This, along with the high quantum yield of singlet oxygen generation, determines efficient photodynamic inactivation of Gram-negative bacteria by zinc and aluminum octacationic phthalocyanines. The effect of Ca2+ and Mg2+ cations and pH on photodynamic inactivation of enterobacteria in the presence of octacationic zinc phthalocyanine has been studied. It has been shown that effects resulting in lowering negative charge on outer membrane protect bacteria against photoinactivation, which confirms the crucial role in this process of the electrostatic interaction of the photosensitizer with the cell wall. Electrostatic nature of binding is consistent with mainly electrostatic character of dye interactions with artificial membranes of different composition. Lower sensitivity of Proteus mirabilis to photodynamic inactivation, compared to that of E. coli and Salmonella enteritidis, due to low affinity of the cationic dye to the cells of this species, was found.

Design and characterization of a broad -spectrum bactericidal acyl-lysyl oligomer

Previously characterized chemical mimics of host defense peptides belonging to the oligo-acyl-lysyl (OAK) family have so far failed to demonstrate broad-spectrum antibacterial potency combined with selectivity toward host cells. Here, we investigated OAK sequences and characterized a promising representative, designated C(12)K-3beta(10), with broad-spectrum activity (MIC(90) = 6.2 microM) and low hemotoxicity (LC(50) > 100 microM). Whereas C(12)K-3beta(10) exerted an essentially bactericidal effect, E. coli bacteria were killed faster than S. aureus (minutes versus hours). Mechanistic studies addressing this difference revealed that unlike E. coli, S. aureus bacteria undergo a transient rapid bactericidal stage that over time converts to a bacteriostatic effect. This behavior was dictated by interactions with cell wall-specific components. Preliminary efficacy studies in mice using the thigh infection model demonstrated the OAK's ability to significantly affect bacterial viability upon single-dose systemic treatment (2 mg/kg).

NMR structure of pardaxin, a pore-forming antimicrobial peptide, in lipopolysaccharide micelles: mechanism of outer membrane permeabilization

Lipopolysaccharide (LPS), the major constituent of the outer membrane of Gram-negative bacteria, is an important element against permeability of bactericidal agents, including antimicrobial peptides. However, structural determinants of antimicrobial peptides for LPS recognition are not clearly understood. Pardaxins (Pa1, Pa2, Pa3, and Pa4) are a group of pore-forming bactericidal peptides found in the mucous glands of sole fishes. Despite having a low net positive charge, pardaxins contain a broad spectrum of antibacterial activities. To elucidate the structural basis of LPS interactions of pardaxins, herein, we report the first three-dimensional structure of Pa4 bound to LPS micelles. The binding kinetics of Pa4 with LPS is estimated using [(15)N-Leu-19] relaxation dispersion NMR experiments. LPS/Pa4 interactions are further characterized by a number of biophysical methods, including isothermal titration calorimetry, (31)P NMR, saturation transfer difference NMR, dynamic light scattering, and IR spectroscopy. In the LPS-Pa4 complex, Pa4 adopts a unique helix-turn-helix conformation resembling a "horseshoe." Interestingly, the LPS-bound structure of Pa4 shows striking differences with the structures determined in lipid micelles or organic solvents. Saturation transfer difference NMR identifies residues of Pa4 that are intimately associated with LPS micelles. Collectively, our results provide mechanistic insights into the outer membrane permeabilization by pardaxin.

Designed beta-boomerang antiendotoxic and antimicrobial peptides: structures and activities in lipopolysaccharide

Lipopolysaccharide (LPS), an integral part of the outer membrane of Gram-negative bacteria, is involved in a variety of biological processes including inflammation, septic shock, and resistance to host-defense molecules. LPS also provides an environment for folding of outer membrane proteins. In this work, we describe the structure-activity correlation of a series of 12-residue peptides in LPS. NMR structures of the peptides derived in complex with LPS reveal boomerang-like beta-strand conformations that are stabilized by intimate packing between the two aromatic residues located at the 4 and 9 positions. This structural feature renders these peptides with a high ability to neutralize endotoxicity, >80% at 10 nM concentration, of LPS. Replacements of these aromatic residues either with Ala or with Leu destabilizes the boomerang structure with the concomitant loss of antiendotoxic and antimicrobial activities. Furthermore, the aromatic packing stabilizing the beta-boomerang structure in LPS is found to be maintained even in a truncated octapeptide, defining a structured LPS binding motif. The mode of action of the active designed peptides correlates well with their ability to perturb LPS micelle structures. Fourier transform infrared spectroscopy studies of the peptides delineate beta-type conformations and immobilization of phosphate head groups of LPS. Trp fluorescence studies demonstrated selective interactions with LPS and the depth of insertion into the LPS bilayer. Our results demonstrate the requirement of LPS-specific structures of peptides for endotoxin neutralizations. In addition, we propose that structures of these peptides may be employed to design proteins for the outer membrane.

A survey of the year 2007 literature on applications of isothermal titration calorimetry

Elucidation of the energetic principles of binding affinity and specificity is a central task in many branches of current sciences: biology, medicine, pharmacology, chemistry, material sciences, etc. In biomedical research, integral approaches combining structural information with in-solution biophysical data have proved to be a powerful way toward understanding the physical basis of vital cellular phenomena. Isothermal titration calorimetry (ITC) is a valuable experimental tool facilitating quantification of the thermodynamic parameters that characterize recognition processes involving biomacromolecules. The method provides access to all relevant thermodynamic information by performing a few experiments. In particular, ITC experiments allow to by-pass tedious and (rarely precise) procedures aimed at determining the changes in enthalpy and entropy upon binding by van't Hoff analysis. Notwithstanding limitations, ITC has now the reputation of being the "gold standard" and ITC data are widely used to validate theoretical predictions of thermodynamic parameters, as well as to benchmark the results of novel binding assays. In this paper, we discuss several publications from 2007 reporting ITC results. The focus is on applications in biologically oriented fields. We do not intend a comprehensive coverage of all newly accumulated information. Rather, we emphasize work which has captured our attention with originality and far-reaching analysis, or else has provided ideas for expanding the potential of the method.

Alexidine and chlorhexidine bind to lipopolysaccharide and lipoteichoic acid and prevent cell activation by antibiotics

OBJECTIVES

Many antibiotics used to treat infections cause release of immunostimulatory cell wall components from bacteria. Therefore, a combination of antimicrobial and endotoxin-neutralizing activity is desired to prevent inflammation induced by destroyed bacteria. Chlorhexidine and alexidine are amphipathic bisbiguanides and could neutralize bacterial membrane components as stimulators of Toll-like receptors (TLRs).

METHODS

Binding of chlorhexidine and alexidine to lipopolysaccharide (LPS) and lipoteichoic acid (LTA) was determined by fluorescence displacement assay and isothermal calorimetric titration. Neutralization of the biological effect of LPS and LTA on TLR-activated cellular activation was determined by NF-kappaB reporter luciferase activation on cells transfected with specific TLRs and NO production of murine macrophages in the presence of isolated agonists and antibiotic-treated bacteria.

RESULTS

Alexidine and chlorhexidine bind not only to LPS but also to LTA from Gram-positive bacteria. Alexidine has a higher affinity than chlorhexidine for both compounds. Calorimetric titration shows an initial endothermic contribution indicating participation of hydrophobic interactions in LPS binding, while binding to LTA displayed initial exothermic contribution. Both compounds prevent cell activation of TLR4 and TLR2 by LPS and LTA, respectively. The addition of both compounds suppressed NO production by macrophages in the presence of bacteria treated with different types of antibiotics.

CONCLUSIONS

Chlorhexidine and alexidine suppress bacterial membrane-induced cell activation at concentrations two orders of magnitude lower than that used in topical applications. Combining biocides with different types of antibiotics prevented macrophage activation in the presence of bacteria and demonstrated the potential of chlorhexidine and alexidine to suppress inflammatory responses caused by activation of TLRs.

Lipopolysaccharide, a key molecule involved in the synergism between temporins in inhibiting bacterial growth and in endotoxin neutralization

Lipopolysaccharide (LPS) is the major structural component of the outer membrane of Gram-negative bacteria and shields them from a variety of host defense factors, including antimicrobial peptides (AMPs). LPS is also recognized by immune cells as a pathogen-associated molecular pattern and stimulates them to secrete pro-inflammatory cytokines that, in extreme cases, lead to a harmful host response known as septic shock. Previous studies have revealed that a few isoforms of the AMP temporin, produced within the same frog specimen, can synergize to overcome bacterial resistance imposed by the physical barrier of LPS. Here we found that temporins can synergize in neutralizing the LPS-induced macrophage activation. Furthermore, the synergism between temporins, to overcome the protective function of LPS as well as its endotoxic effect, depends on the length of the polysaccharide chain of LPS. Importantly, mode of action studies, using spectroscopic and thermodynamic methods, have pointed out different mechanisms underlying the synergism of temporins in antimicrobial and anti-endotoxin activities. To the best of our knowledge, such a dual synergism between isoforms of AMPs from the same species has not been observed before, and it might explain the ability of such amphibians to resist a large repertoire of microorganisms.

Physico-chemical and biophysical study of the interaction of hexa- and heptaacyl lipid A from Erwinia carotovora with magainin 2-derived antimicrobial peptides

The neutralization of endotoxin structures such as the active 'endotoxic principle' lipid A by suitable compounds has been shown to be a key step in the treatment of infectious diseases, in particular in the case of Gram-negative bacteria which frequently may lead to the septic shock syndrome. An effective antimicrobial peptide, originally found in the skin of an African frog, is magainin 2. Here, the interaction of magainin 2-amide and a peptide derived thereof, M2V, with chemically defined and homogeneous hexaacyl and heptaacyl lipids A isolated from LPS of Erwinia carotovora, was investigated. By using Fourier-transform infrared spectroscopy, the gel to liquid crystalline phase transition of the acyl chains of lipid A and the conformation of their phosphate groups due to peptide binding was investigated. The former parameter was also determined by using differential scanning calorimetry. The electrophoretic mobility of lipid A aggregates under the influence of the peptides was studied to determine the Zeta potential, and small-angle X-ray scattering was applied for the elucidation of the types of aggregate structures in the absence and presence of the peptides. The lipid A-induced cytokine production in human mononuclear cells shows that the ability of the two peptides to inhibit a tumor necrosis factor-alpha production correlates with characteristic changes of the biophysical parameters. These are much stronger expressed for the peptide M2V than for magainin 2-amide, which apparently is connected with the higher number of positive as well as more hydrophobic amino acids, leading to a stronger amphiphilicity necessary to neutralize the amphiphilic lipid A aggregates.

Polymyxin B induces lysis of marine pseudoalteromonads

Polymyxin B (PMB) is a cationic antibiotic that interacts with the envelopes of gram-negative bacterial cells. The therapeutic use of PMB was abandoned for a long time due to its undesirable side effects; however, the spread of resistance to currently used antibiotics has forced the reevaluation of PMB for clinical use. Previous studies have used enteric bacteria to examine the mode of PMB action, resulting in a somewhat limited understanding of this process. This study examined the effects of PMB on marine pseudoalteromonads and demonstrates that the frequently accepted view that "what is true for Escherichia coli is true for all bacteria" does not hold true. We show here that in contrast to the growth inhibition observed for enteric bacteria, PMB induces lysis of pseudoalteromonads, which is not prevented by high concentrations of divalent cations. Furthermore, we demonstrate that a high membrane voltage is required for the interaction of PMB with the cytoplasmic membranes of pseudoalteromonads, further elucidating the mechanisms by which PMB interacts with the cell envelopes of those gram-negative bacteria.

Bacterial lipid composition and the antimicrobial efficacy of cationic steroid compounds (Ceragenins)

Ceragenins are cationic bile salt derivatives having antimicrobial activity. The interactions of several ceragenins with phospholipid bilayers were tested in different systems. The ceragenins are capable of forming specific associations with several phospholipid species that may be involved with their antimicrobial action. Their antimicrobial activity is lower in bacteria that have a high content of phosphatidylethanolamine. Gram negative bacteria with a high content of phosphatidylethanolamine exhibit sensitivity to different ceragenins that corresponds to the extent of interaction of these compounds with phospholipids, including the ability of different ceragenins to induce leakage of aqueous contents from phosphatidylethanolamine-rich liposomes. A second class of bacteria having cell membranes composed largely of anionic lipids and having a low content of phosphatidylethanolamine are very sensitive to the action of the ceragenins but they exhibit similar minimal inhibitory concentrations with most of the ceragenins and for different strains of bacteria. Although Gram negative bacteria generally have a high content of phosphatidylethanolamine, there are a few exceptions. In addition, a mutant strain of Escherichia coli has been made that is essentially devoid of phophatidylethanolamine, although 80% of the lipid of the wild-type strain is phosphatidylethanolamine. Furthermore, certain Gram positive bacteria are also exceptions in that they can have a high content of phosphatidylethanolamine. We find that the antimicrobial action of the ceragenins correlates better with the content of phosphatidylethanolamine in the bacterial membrane than whether or not the bacteria has an outer membrane. Thus, the bacterial lipid composition can be an important factor in determining the sensitivity of bacteria to antimicrobial agents.