Lipid Microenvironment Modulates the Pore-Forming Ability of Polymyxin B

The ability of polymyxin B, an antibiotic used to treat infections caused by multidrug-resistant Gram-negative bacteria as a last-line therapeutic option, to form ion pores in model membranes composed of various phospholipids and lipopolysaccharides was studied. Our data demonstrate that polymyxin B predominantly interacts with negatively charged lipids. Susceptibility decreases as follows: Kdo2-Lipid A >> DOPG ≈ DOPS >> DPhPG ≈ TOCL ≈ Lipid A. The dimer and hexamer of polymyxin B are involved in the pore formation in DOPG(DOPS)- and Kdo2-Lipid A-enriched bilayers, respectively. The pore-forming ability of polymyxin B significantly depends on the shape of membrane lipids, which indicates that the antibiotic produces toroidal lipopeptide-lipid pores. Small amphiphilic molecules diminishing the membrane dipole potential and inducing positive curvature stress were shown to be agonists of pore formation by polymyxin B and might be used to develop innovative lipopeptide-based formulations.

How to Combat Gram-Negative Bacteria Using Antimicrobial Peptides: A Challenge or an Unattainable Goal?

Antimicrobial peptides (AMPs) represent a promising and effective alternative for combating pathogens, having some advantages compared to conventional antibiotics. However, AMPs must also contend with complex and specialised Gram-negative bacteria envelops. The variety of lipopolysaccharide and phospholipid composition in Gram-negative bacteria strains and species are decisive characteristics regarding their susceptibility or resistance to AMPs. Such biological and structural barriers have created delays in tuning AMPs to deal with Gram-negative bacteria. This becomes even more acute because little is known about the interaction AMP–Gram-negative bacteria and/or AMPs’ physicochemical characteristics, which could lead to obtaining selective molecules against Gram-negative bacteria. As a consequence, available AMPs usually have highly associated haemolytic and/or cytotoxic activity. Only one AMP has so far been FDA approved and another two are currently in clinical trials against Gram-negative bacteria. Such a pessimistic panorama suggests that efforts should be concentrated on the search for new molecules, designs and strategies for combating infection caused by this type of microorganism. This review has therefore been aimed at describing the currently available AMPs for combating Gram-negative bacteria, exploring the characteristics of these bacteria’s cell envelop hampering the development of new AMPs, and offers a perspective regarding the challenges for designing new AMPs against Gram-negative bacteria.

Non-antibiotic antimicrobial agents to combat biofilm-forming bacteria

Biofilms can be produced by multiple species or by a single strain of bacteria. The biofilm state enhances the resistance of the resident microorganisms to antimicrobial agents by producing extracellular polymeric substances. Typically, antibiotics are used to stop the growth of bacteria, but emerging resistance has limited their effectiveness. Bacteria in biofilms are less susceptible to antibiotics compared with their free-floating state, as biofilms impair antibiotic penetration. To obviate this challenge, non-antibiotic antimicrobial agents are needed. This review describes two classes of these agents, namely antimicrobial nanoparticles and antimicrobial peptides. Applications of these antimicrobials in the food industry and medical applications are discussed, and the directions for future research are highlighted.

HupA, the main undecaprenyl pyrophosphate and phosphatidylglycerol phosphate phosphatase in Helicobacter pylori is essential for colonization of the stomach

The biogenesis of bacterial cell-envelope polysaccharides requires the translocation, across the plasma membrane, of sugar sub-units that are produced inside the cytoplasm. To this end, the hydrophilic sugars are anchored to a lipid phosphate carrier (undecaprenyl phosphate (C₅₅-P)), yielding membrane intermediates which are translocated to the outer face of the membrane. Finally, the glycan moiety is transferred to a nascent acceptor polymer, releasing the carrier in the “inactive” undecaprenyl pyrophosphate (C₅₅-PP) form. Thus, C₅₅-P is generated through the dephosphorylation of C₅₅-PP, itself arising from either de novo synthesis or recycling. Two types of integral membrane C₅₅-PP phosphatases were described: BacA enzymes and a sub-group of PAP2 enzymes (type 2 phosphatidic acid phosphatases). The human pathogen Helicobacter pylori does not contain BacA homologue but has four membrane PAP2 proteins: LpxE, LpxF, HP0350 and HP0851. Here, we report the physiological role of HP0851, renamed HupA, via multiple and complementary approaches ranging from a detailed biochemical characterization to the assessment of its effect on cell envelope metabolism and microbe-host interactions. HupA displays a dual function as being the main C₅₅-PP pyrophosphatase (UppP) and phosphatidylglycerol phosphate phosphatase (PGPase). Although not essential in vitro, HupA was essential in vivo for stomach colonization. In vitro, the remaining UppP activity was carried out by LpxE in addition to its lipid A 1-phosphate phosphatase activity. Both HupA and LpxE have crucial roles in the biosynthesis of several cell wall polysaccharides and thus constitute potential targets for new therapeutic strategies.

Helicobacter pylori colonizes the human’s gastric mucosa and infects around 50% of the world’s population. This pathogen is responsible for chronic gastritis, peptic ulcers and in worst cases leads to gastric cancer. It has been classified as a class I carcinogen by the World Health Organization in 1994. Here, we show that HP0851, renamed HupA, is the major undecaprenyl pyrophosphate (C₅₅-PP) phosphatase (UppP) and the major phosphatidylglycerol phosphate phosphatase (PGPase). This enzyme is also involved in cationic antimicrobial peptide (CAMP) resistance to which H. pylori hupA mutant shows an increased sensitivity (4 fold). This mutant was unable to colonize the stomach in mouse model of infection showing that even if hupA was not essential in vitro, this gene was essential in vivo. Both HupA and LpxE have crucial roles in the biosynthesis of several cell wall polysaccharides and thus constitute potential targets for new therapeutic strategies.

Stress responses as determinants of antimicrobial resistance in Pseudomonas aeruginosa: multidrug efflux and more

Pseudomonas aeruginosa is a notoriously antimicrobial-resistant organism that is increasingly refractory to antimicrobial chemotherapy. While the usual array of acquired resistance mechanisms contribute to resistance development in this organism a multitude of endogenous genes also play a role. These include a variety of multidrug efflux loci that contribute to both intrinsic and acquired antimicrobial resistance. Despite their roles in resistance, however, it is clear that these efflux systems function in more than just antimicrobial efflux. Indeed, recent data indicate that they are recruited in response to environmental stress and, therefore, function as components of the organism's stress responses. In fact, a number of endogenous resistance-promoting genes are linked to environmental stress, functioning as part of known stress responses or recruited in response to a variety of environmental stress stimuli. Stress responses are, thus, important determinants of antimicrobial resistance in P. aeruginosa. As such, they represent possible therapeutic targets in countering antimicrobial resistance in this organism.

Immune-directed support of rich microbial communities in the gut has ancient roots

The animal gut serves as a primary location for the complex host-microbe interplay that is essential for homeostasis and may also reflect the types of ancient selective pressures that spawned the emergence of immunity in metazoans. In this review, we present a phylogenetic survey of gut host-microbe interactions and suggest that host defense systems arose not only to protect tissue directly from pathogenic attack but also to actively support growth of specific communities of mutualists. This functional dichotomy resulted in the evolution of immune systems much more tuned for harmonious existence with microbes than previously thought, existing as dynamic but primarily cooperative entities in the present day. We further present the protochordate Ciona intestinalis as a promising model for studying gut host-bacterial dialogue. The taxonomic position, gut physiology and experimental tractability of Ciona offer unique advantages in dissecting host-microbe interplay and can complement studies in other model systems.

LptE binds to and alters the physical state of LPS to catalyze its assembly at the cell surface

The assembly of lipopolysaccharide (LPS) on the surface of Gram-negative bacterial cells is essential for their viability and is achieved by the seven-protein LPS transport (Lpt) pathway. The outer membrane (OM) lipoprotein LptE and the β-barrel membrane protein LptD form a complex that assembles LPS into the outer leaflet of the OM. We report a crystal structure of the Escherichia coli OM lipoprotein LptE at 2.34 Å. The structure reveals homology to eukaryotic LPS-binding proteins and allowed for the prediction of an LPS-binding site, which was confirmed by genetic and biophysical experiments. Specific point mutations at this site lead to defects in OM biogenesis. We show that wild-type LptE disrupts LPS-LPS interactions in vitro and that these mutations decrease the ability of LptE to disaggregate LPS. Transmission electron microscopic imaging shows that LptE can disrupt LPS aggregates even at substoichiometric concentrations. We propose a model in which LptE functions as an LPS transfer protein in the OM translocon by disaggregating LPS during transport to allow for its insertion into the OM.

The intestinal archaea Methanosphaera stadtmanae and Methanobrevibacter smithii activate human dendritic cells

The methanoarchaea Methanosphaera stadtmanae and Methanobrevibacter smithii are known to be part of the indigenous human gut microbiota. Although the immunomodulatory effects of bacterial gut commensals have been studied extensively in the last decade, the impact of methanoarchaea in human's health and disease was rarely examined. Consequently, we studied and report here on the effects of M. stadtmanae and M. smithii on human immune cells. Whereas exposure to M. stadtmanae leads to substantial release of proinflammatory cytokines in monocyte-derived dendritic cells (moDCs), only weak activation was detected after incubation with M. smithii. Phagocytosis of M. stadtmanae by moDCs was demonstrated by confocal microscopy as well as transmission electronic microscopy (TEM) and shown to be crucial for cellular activation by using specific inhibitors. Both strains, albeit to different extents, initiate a maturation program in moDCs as revealed by up-regulation of the cell-surface receptors CD86 and CD197 suggesting additional activation of adaptive immune responses. Furthermore, M. stadtmanae and M. smithii were capable to alter the gene expression of antimicrobial peptides in moDCs to different extents.

Taken together, our findings strongly argue that the archaeal gut inhabitants M. stadtmanae and M. smithii are specifically recognized by the human innate immune system. Moreover, both strains are capable of inducing an inflammatory cytokine response to different extents arguing that they might have diverse immunomodulatory functions. In conclusion, we propose that the impact of intestinal methanoarchaea on pathological conditions involving the gut microbiota has been underestimated until now.

A helix-PXXP-helix peptide with antibacterial activity without cytotoxicity against MDRPA-infected mice

In response to the growing problem of multidrug-resistant pathogenic microbes, much attention is being paid to naturally occurring and synthetic antimicrobial peptides (AMPs) and the effects of their structural modification. Among these modifications, amino acid substitution is a simple approach to enhancing biological activity and reducing cytotoxicity. An earlier study indicated that HPA3, an analog of HP (2-20) derived from the N-terminus of Helicobacter pylori ribosomal protein L1, forms large pores and shows considerable cytotoxicity. However, HPA3P, in which a proline (Pro) is substituted for glutamic acid (Glu) at position 9 of HPA3, shows markedly less cytotoxicity. This may be attributable to the presence of a Pro-kink into middle of the HPA3P structure within the membrane environment. Unfortunately, HPA3P is not an effective antibacterial agent in vivo. We therefore designed a helix-PXXP-helix structure (HPA3P2), in which Pro was substituted for the Glu and phenylalanine (Phe) at positions 9 and 12 of HPA3, yielding a molecule with a flexible central hinge. As compared to HPA3P, HPA3P3 exhibited dramatically increased antibacterial activity in vivo. ICR mice infected with clinically isolated multidrug-resistant Pseudomonas aeruginosa showed 100% survival when administered one 0.5-mg/kg dose of HPA3P2 or three 0.1-mg/kg doses of HPA3P2. Moreover, in a mouse model of septic shock induced by P. aeruginosa LPS, HPA3P2 reduced production of pro-inflammatory mediators and correspondingly reduced lung (alveolar) and liver tissue damage. The changes in HPA3 behavior with the introduction of Pro likely reflects alterations of the mechanism of action: i) HPA3 forms pores in the bacterial cell membranes, ii) HPA3P permeates the cell membranes and binds to intracellular RNA and DNA, and iii) HPA3P2 acts on the outer cellular membrane component LPS. Collectively, these results suggest HPA3P2 has the potential to be an effective antibiotic for use against multidrug-resistant bacterial strains.

Design of NK-2-derived peptides with improved activity against equine sarcoid cells

Equine sarcoid is a topically accessible model for the evaluation of anticancer peptides acting by physical membrane disruption avoiding the complexity of a systemic application. We aim at evaluating and improving natural peptides for host defence as lead structures, where we focus on the cationic and amphipathic peptide NK-2. Cytotoxicity tests, fluorescence microscopy and a chip-based biosensor, which enabled real-time monitoring of cell metabolism, were applied. Cancer cell killing was dynamic with an initial phase of increased cellular respiration, followed by membrane destruction. NK-2 was substantially improved and shortened. Novel peptides exhibited a fivefold improved activity against sarcoid cells, while haemolysis remained almost unaltered. Similar Zeta potential and similar amount of surface phosphatidylserine of sarcoid and normal skin cells are responsible for a lack of selectivity between these two cell types.

Deciphering the acylation pattern of Yersinia enterocolitica lipid A

Pathogenic bacteria may modify their surface to evade the host innate immune response. Yersinia enterocolitica modulates its lipopolysaccharide (LPS) lipid A structure, and the key regulatory signal is temperature. At 21°C, lipid A is hexa-acylated and may be modified with aminoarabinose or palmitate. At 37°C, Y. enterocolitica expresses a tetra-acylated lipid A consistent with the 3′-O-deacylation of the molecule. In this work, by combining genetic and mass spectrometric analysis, we establish that Y. enterocolitica encodes a lipid A deacylase, LpxR, responsible for the lipid A structure observed at 37°C. Western blot analyses indicate that LpxR exhibits latency at 21°C, deacylation of lipid A is not observed despite the expression of LpxR in the membrane. Aminoarabinose-modified lipid A is involved in the latency. 3-D modelling, docking and site-directed mutagenesis experiments showed that LpxR D31 reduces the active site cavity volume so that aminoarabinose containing Kdo₂-lipid A cannot be accommodated and, therefore, not deacylated. Our data revealed that the expression of lpxR is negatively controlled by RovA and PhoPQ which are necessary for the lipid A modification with aminoarabinose. Next, we investigated the role of lipid A structural plasticity conferred by LpxR on the expression/function of Y. enterocolitica virulence factors. We present evidence that motility and invasion of eukaryotic cells were reduced in the lpxR mutant grown at 21°C. Mechanistically, our data revealed that the expressions of flhDC and rovA, regulators controlling the flagellar regulon and invasin respectively, were down-regulated in the mutant. In contrast, the levels of the virulence plasmid (pYV)-encoded virulence factors Yops and YadA were not affected in the lpxR mutant. Finally, we establish that the low inflammatory response associated to Y. enterocolitica infections is the sum of the anti-inflammatory action exerted by pYV-encoded YopP and the reduced activation of the LPS receptor by a LpxR-dependent deacylated LPS.

Lipopolysaccharide (LPS) is one of the major surface components of Gram-negative bacteria. The LPS contains a molecular pattern recognized by the innate immune system. Not surprisingly, the modification of the LPS pattern is a virulence strategy of several pathogens to evade the innate immune system. Yersinia enterocolitica causes food-borne infections in animals and humans (yersiniosis). Temperature regulates most, if not all, virulence factors of yersiniae including the structure of the LPS lipid A. At 21°C, lipid A is mainly hexa-acylated and may be modified with aminoarabinose or palmitate. In contrast, at 37°C, Y. enterocolitica expresses a unique tetra-acylated lipid A. In this work, we establish that Y. enterocolitica encodes a lipid A deacylase, LpxR, responsible for the lipid A structure expressed by the pathogen at 37°C, the host temperature. Our findings also revealed that the low inflammatory response associated to Y. enterocolitica infections is the sum of the anti-inflammatory action exerted by a Yersinia protein translocated into the cytosol of macrophages and the reduced activation of the LPS receptor complex due to the expression of a LpxR-dependent deacylated LPS.

Bacterial biofilms in the upper airway - evidence for role in pathology and implications for treatment of otitis media

Understanding the nature of the biofilm component in the pathogenesis of otitis media [OM] will likely have a meaningful influence on the development of novel strategies to prevent and/or treat this highly prevalent pediatric disease. The design of vaccine candidates for OM that currently focus on preventing colonization are predicated on the assumption that by reducing the burden of bacteria present in the pediatric nasopharynx, one could reduce or eliminate the likelihood of retrograde ascension of the Eustachian tube by bacteria from the nasopharynx to the middle ear. If effective, this strategy could prevent biofilms from ever forming in the middle ear. Additionally, gaining an improved understanding of the unique properties of bacteria resident within a biofilm and the proteins they express while growing as part of this organized community has the potential to identify novel and perhaps biofilm-specific molecular targets for the design of either therapeutic agents or vaccine candidates for the resolution of existing OM.

Bacterial stress responses as determinants of antimicrobial resistance

Bacteria encounter a myriad of stresses in their natural environments, including, for pathogens, their hosts. These stresses elicit a variety of specific and highly regulated adaptive responses that not only protect bacteria from the offending stress, but also manifest changes in the cell that impact innate antimicrobial susceptibility. Thus exposure to nutrient starvation/limitation (nutrient stress), reactive oxygen and nitrogen species (oxidative/nitrosative stress), membrane damage (envelope stress), elevated temperature (heat stress) and ribosome disruption (ribosomal stress) all impact bacterial susceptibility to a variety of antimicrobials through their initiation of stress responses that positively impact recruitment of resistance determinants or promote physiological changes that compromise antimicrobial activity. As de facto determinants of antimicrobial, even multidrug, resistance, stress responses may be worthy of consideration as therapeutic targets.

Effects of antimicrobial peptides on methanogenic archaea

As members of the indigenous human microbiota found on several mucosal tissues, Methanobrevibacter smithii and Methanosphaera stadtmanae are exposed to the effects of antimicrobial peptides (AMPs) secreted by these epithelia. Although antimicrobial and molecular effects of AMPs on bacteria are well described, data for archaea are not available yet. Besides, it is not clear whether AMPs affect them as the archaeal cell envelope differs profoundly in terms of chemical composition and structure from that of bacteria. The effects of different synthetic AMPs on growth of M. smithii, M. stadtmanae, and Methanosarcina mazei were tested using a microtiter plate assay adapted to their anaerobic growth requirements. All three tested methanoarchaea were highly sensitive against derivatives of human cathelicidin, of porcine lysin, and a synthetic antilipopolysaccharide peptide (Lpep); however, sensitivities differed markedly among the methanoarchaeal strains. The potent AMP concentrations affecting growth were below 10 μM, whereas growth of Escherichia coli WBB01 was not affected at peptide concentrations up to 10 μM under the same anaerobic growth conditions. Atomic force microscopy and transmission electron microscopy revealed that the structural integrity of the methanoarchaeal cells is destroyed within 4 h after incubation with AMPs. The disruption of the cell envelope of M. smithii, M. stadtmanae, and M. mazei within a few minutes of exposure was verified by using LIVE/DEAD staining. Our results strongly suggest that the release of AMPs by eukaryotic epithelial cells is a potent defense mechanism targeting not only bacteria, but also methanoarchaea.

An engineered innate immune defense protects grapevines from Pierce disease

We postulated that a synergistic combination of two innate immune functions, pathogen surface recognition and lysis, in a protein chimera would lead to a robust class of engineered antimicrobial therapeutics for protection against pathogens. In support of our hypothesis, we have engineered such a chimera to protect against the gram-negative Xylella fastidiosa (Xf), which causes diseases in multiple plants of economic importance. Here we report the design and delivery of this chimera to target the Xf subspecies fastidiosa (Xff), which causes Pierce disease in grapevines and poses a great threat to the wine-growing regions of California. One domain of this chimera is an elastase that recognizes and cleaves MopB, a conserved outer membrane protein of Xff. The second domain is a lytic peptide, cecropin B, which targets conserved lipid moieties and creates pores in the Xff outer membrane. A flexible linker joins the recognition and lysis domains, thereby ensuring correct folding of the individual domains and synergistic combination of their functions. The chimera transgene is fused with an amino-terminal signal sequence to facilitate delivery of the chimera to the plant xylem, the site of Xff colonization. We demonstrate that the protein chimera expressed in the xylem is able to directly target Xff, suppress its growth, and significantly decrease the leaf scorching and xylem clogging commonly associated with Pierce disease in grapevines. We believe that similar strategies involving protein chimeras can be developed to protect against many diseases caused by human and plant pathogens.

Helicobacter pylori versus the host: remodeling of the bacterial outer membrane is required for survival in the gastric mucosa

Modification of bacterial surface structures, such as the lipid A portion of lipopolysaccharide (LPS), is used by many pathogenic bacteria to help evade the host innate immune response. Helicobacter pylori, a gram-negative bacterium capable of chronic colonization of the human stomach, modifies its lipid A by removal of phosphate groups from the 1- and 4'-positions of the lipid A backbone. In this study, we identify the enzyme responsible for dephosphorylation of the lipid A 4'-phosphate group in H. pylori, Jhp1487 (LpxF). To ascertain the role these modifications play in the pathogenesis of H. pylori, we created mutants in lpxE (1-phosphatase), lpxF (4'-phosphatase) and a double lpxE/F mutant. Analysis of lipid A isolated from lpxE and lpxF mutants revealed lipid A species with a 1 or 4'-phosphate group, respectively while the double lpxE/F mutant revealed a bis-phosphorylated lipid A. Mutants lacking lpxE, lpxF, or lpxE/F show a 16, 360 and 1020 fold increase in sensitivity to the cationic antimicrobial peptide polymyxin B, respectively. Moreover, a similar loss of resistance is seen against a variety of CAMPs found in the human body including LL37, β-defensin 2, and P-113. Using a fluorescent derivative of polymyxin we demonstrate that, unlike wild type bacteria, polymyxin readily associates with the lpxE/F mutant. Presumably, the increase in the negative charge of H. pylori LPS allows for binding of the peptide to the bacterial surface. Interestingly, the action of LpxE and LpxF was shown to decrease recognition of Helicobacter LPS by the innate immune receptor, Toll-like Receptor 4. Furthermore, lpxE/F mutants were unable to colonize the gastric mucosa of C57BL/6J and C57BL/6J tlr4 -/- mice when compared to wild type H. pylori. Our results demonstrate that dephosphorylation of the lipid A domain of H. pylori LPS by LpxE and LpxF is key to its ability to colonize a mammalian host.

The role of antimicrobial peptides in preventing multidrug-resistant bacterial infections and biofilm formation

Over the last decade, decreasing effectiveness of conventional antimicrobial-drugs has caused serious problems due to the rapid emergence of multidrug-resistant pathogens. Furthermore, biofilms, which are microbial communities that cause serious chronic infections and dental plaque, form environments that enhance antimicrobial resistance. As a result, there is a continuous search to overcome or control such problems, which has resulted in antimicrobial peptides being considered as an alternative to conventional drugs. Antimicrobial peptides are ancient host defense effector molecules in living organisms. These peptides have been identified in diverse organisms and synthetically developed by using peptidomimic techniques. This review was conducted to demonstrate the mode of action by which antimicrobial peptides combat multidrug-resistant bacteria and prevent biofilm formation and to introduce clinical uses of these compounds for chronic disease, medical devices, and oral health. In addition, combinations of antimicrobial peptides and conventional drugs were considered due to their synergetic effects and low cost for therapeutic treatment.

Multiple peptide resistance factor (MprF)-mediated Resistance of Staphylococcus aureus against antimicrobial peptides coincides with a modulated peptide interaction with artificial membranes comprising lysyl-phosphatidylglycerol

Modification of the membrane lipid phosphatidylglycerol (PG) of Staphylococcus aureus by enzymatic transfer of a l-lysine residue leading to lysyl-PG converts the net charge of PG from -1 to +1 and is thought to confer resistance to cationic antimicrobial peptides (AMPs). Lysyl-PG synthesis and translocation to the outer leaflet of the bacterial membrane are achieved by the membrane protein MprF. Consequently, mutants lacking a functional mprF gene are in particular vulnerable to the action of AMPs. Hence, we aim at elucidating whether and to which extent lysyl-PG modulates membrane binding, insertion, and permeabilization by various AMPs. Lysyl-PG was incorporated into artificial lipid bilayers, mimicking the cytoplasmic membrane of S. aureus. Moreover, we determined the activity of the peptides against a clinical isolate of S. aureus strain SA113 and two mutants lacking a functional mprF gene and visualized peptide-induced ultrastructural changes of bacteria by transmission electron microscopy. The studied peptides were: (i) NK-2, an α-helical fragment of mammalian NK-lysin, (ii) arenicin-1, a lugworm β-sheet peptide, and (iii) bee venom melittin. Biophysical data obtained by FRET spectroscopy, Fourier transform infrared spectroscopy, and electrical measurements with planar lipid bilayers were correlated with the biological activities of the peptides. They strongly support the hypothesis that peptide-membrane interactions are a prerequisite for eradication of S. aureus. However, degree and mode of modulation of membrane properties such as fluidity, capacitance, and conductivity were unique for each of the peptides. Altogether, our data support and underline the significance of lysyl-PG for S. aureus resistance to AMPs.

Removal of the outer Kdo from Helicobacter pylori lipopolysaccharide and its impact on the bacterial surface

Helicobacter pylori produces a unique surface lipopolysaccharide (LPS) characterized by strikingly low endotoxicity that is thought to aid the organism in evading the host immune response. This reduction in endotoxicity is predicted to arise from the modification of the Kdo-lipid A domain of Helicobacter LPS by a series of membrane bound enzymes including a Kdo (3-deoxy-d-manno-octulosonic acid) hydrolase responsible for the modification of the core oligosaccharide. Here, we report that Kdo hydrolase activity is dependent upon a putative two-protein complex composed of proteins Hp0579 and Hp0580. Inactivation of Kdo hydrolase activity produced two phenotypes associated with cationic antimicrobial peptide resistance and O-antigen expression. Kdo hydrolase mutants were highly sensitive to polymyxin B, which could be attributed to a defect in downstream modifications to the lipid A 4'-phosphate group. Production of a fully extended O-antigen was also diminished in a Kdo hydrolase mutant, with a consequent increase in core-lipid A. Finally, expression of O-antigen Lewis X and Y epitopes, known to mimic glycoconjugates found on human tissues, was also affected. Taken together, we have demonstrated that loss of Kdo hydrolase activity affects all three domains of H. pylori LPS, thus highlighting its role in the maintenance of the bacterial surface.

Lipopolysaccharide interaction is decisive for the activity of the antimicrobial peptide NK-2 against Escherichia coli and Proteus mirabilis

Phosphatidylglycerol is a widely used mimetic to study the effects of AMPs (antimicrobial peptides) on the bacterial cytoplasmic membrane. However, the antibacterial activities of novel NK-2-derived AMPs could not be sufficiently explained by using this simple model system. Since the LPS (lipopolysaccharide)-containing outer membrane is the first barrier of Gram-negative bacteria, in the present study we investigated interactions of NK-2 and a shortened variant with viable Escherichia coli WBB01 and Proteus mirabilis R45, and with model membranes composed of LPS isolated from these two strains. Differences in net charge and charge distribution of the two LPS have been proposed to be responsible for the differential sensitivity of the respective bacteria to other AMPs. As imaged by TEM (transmission electron microscopy) and AFM (atomic force microscopy), NK-2-mediated killing of these bacteria was corroborated by structural alterations of the outer and inner membranes, the release of E. coli cytoplasma, and the formation of unique fibrous structures inside P. mirabilis, suggesting distinct and novel intracellular targets. NK-2 bound to and intercalated into LPS bilayers, and eventually induced the formation of transient heterogeneous lesions in planar lipid bilayers. However, the discriminative activity of NK-2 against the two bacterial strains was independent of membrane intercalation and lesion formation, which both were indistinguishable for the two LPS. Instead, differences in activity originated from the LPS-binding step, which could be demonstrated by NK-2 attachment to intact bacteria, and to solid-supported LPS bilayers on a surface acoustic wave biosensor.

Role of lipid A acylation in Yersinia enterocolitica virulence

Yersinia enterocolitica is an important human pathogen. Y. enterocolitica must adapt to the host environment, and temperature is an important cue regulating the expression of most Yersinia virulence factors. Here, we report that Y. enterocolitica 8081 serotype O:8 synthesized tetra-acylated lipid A at 37 degrees C but that hexa-acylated lipid A predominated at 21 degrees C. By mass spectrometry and genetic methods, we have shown that the Y. enterocolitica msbB, htrB, and lpxP homologues encode the acyltransferases responsible for the addition of C(12), C(14) and C(16:1), respectively, to lipid A. The expression levels of the acyltransferases were temperature regulated. Levels of expression of msbB and lpxP were higher at 21 degrees C than at 37 degrees C, whereas the level of expression of htrB was higher at 37 degrees C. At 21 degrees C, an lpxP mutant was the strain most susceptible to polymyxin B, whereas at 37 degrees C, an htrB mutant was the most susceptible. We present evidence that the lipid A acylation status affects the expression of Yersinia virulence factors. Thus, expression of flhDC, the flagellar master regulatory operon, was downregulated in msbB and lpxP mutants, with a concomitant decrease in motility. Expression of the phospholipase yplA was also downregulated in both mutants. inv expression was downregulated in msbB and htrB mutants, and consistent with this finding, invasion of HeLa cells was diminished. However, the expression of rovA, the positive regulator of inv, was not affected in the mutants. The levels of pYV-encoded virulence factors Yops and YadA in the acyltransferase mutants were not affected. Finally, we show that only the htrB mutant was attenuated in vivo.

The cost of resistance to colistin in Acinetobacter baumannii: a proteomic perspective

Colistin resistance in Acinetobacter baumannii, a pathogen of clinical concern, was induced in the susceptible strain ATCC 19606 by growth under increasing pressure of the antibiotic, the only drug universally active against multi-resistant clinical strains. In 2-D difference gel electrophoresis (DIGE) experiments, 35 proteins with differences in expression between both phenotypes were identified, most of them appearing as down regulated in the colistin-resistant strain. These include outer membrane (OM) proteins, chaperones, protein biosynthesis factors, and metabolic enzymes, all suggesting substantial loss of biological fitness in the resistant phenotype, as substantiated by complementary experiments in the absence of colistin. Results shed light on the scarcity of widespread clinical outbreaks for resistant phenotypes.

Significance of the cyclic structure and of arginine residues for the antibacterial activity of arenicin-1 and its interaction with phospholipid and lipopolysaccharide model membranes

Arenicin-1 (Ar-1) is a β-sheeted antimicrobial peptide from the marine lugwormArenicola marina. To elucidate the significance of its unique 18-residue cyclic structure and of six cationic arginines for its biological activity and its interaction with biomembranes, we synthesized one linear peptide in which the two cysteines were exchanged for serines (C/S-Ar-1) and a cyclic peptide in which all arginines were replaced by lysines (R/K-Ar-1). We addressed antibacterial and hemolytic activities, the impact of the peptides on bacterial morphology, and their binding to, intercalation into, and permeabilization of model membranes composed of phospholipids or lipopolysaccharide (LPS). In accordance with high salt concentration in sea water, the antibacterial activity of Ar-1 was almost insensitive to high NaCl concentrations. In contrast, the linear derivative lost activity under these conditions against polymyxin B-resistantProteus mirabilis. Ar-1 intercalated into phospholipid and LPS membranes and formed heterogeneous and short-lived lesions. However, when the peptide was present in both membrane leaflets, it formed defined pores. This characteristic was not observed for the linear derivative C/S-Ar-1. Apparently, the disulfide bond provides conforma-tional stability, which has an impact on salt tolerance, prevents fast degradation by trypsin, and is a prerequisite for the formation of structurally defined pores.

Capsule polysaccharide is a bacterial decoy for antimicrobial peptides

Antimicrobial peptides (APs) are important host weapons against infections. Nearly all APs are cationic and their microbicidal action is initiated through interactions with the anionic bacterial surface. It is known that pathogens have developed countermeasures to resist these agents by reducing the negative charge of membranes, by active efflux and by proteolytic degradation. Here we uncover a new strategy of resistance based on the neutralization of the bactericidal activity of APs by anionic bacterial capsule polysaccharide (CPS). Purified CPSs from Klebsiella pneumoniae K2, Streptococcus pneumoniae serotype 3 and Pseudomonas aeruginosa increased the resistance to polymyxin B of an unencapsulated K. pneumoniae mutant. Furthermore, these CPSs increased the MICs of polymyxin B and human neutrophil alpha-defensin 1 (HNP-1) for unencapsulated K. pneumoniae, Escherichia coli and P. aeruginosa PAO1. Polymyxin B or HNP-1 released CPS from capsulated K. pneumoniae, S. pneumoniae serotype 3 and P. aeruginosa overexpressing CPS. Moreover, this material also reduced the bactericidal activity of APs. We postulate that APs may trigger in vivo the release of CPS, which in turn will protect bacteria against APs. We found that anionic CPSs, but not cationic or uncharged ones, blocked the bactericidal activity of APs by binding them, thereby reducing the amount of peptides reaching the bacterial surface. Supporting this, polycations inhibited such interaction and the bactericidal activity was restored. We postulate that trapping of APs by anionic CPSs is an additional selective virulence trait of these molecules, which could be considered as bacterial decoys for APs.

Human antimicrobial proteins effectors of innate immunity

We live in a world populated by an enormous number of micro-organisms. This necessitates the existence of highly effective mechanisms to control microbial growth. Through many research efforts, a chemical defense system based on the production of antimicrobial proteins (AMPs) has been identified. AMPs are endogenous, small proteins exhibiting antimicrobial activity against a wide variety of micro-organisms. The wide distribution of these molecules in the plant and animal kingdom reflects their biological significance. Various human AMPs show a potent effect on pathogenic micro-organisms including antibiotic-resistant bacteria. Thus, there is great interest in understanding the role of AMPs within innate immunity and evaluating their use and/or specific induction to fend off infections. In this review, we provide an overview of the characteristics of human AMPs and discuss examples where AMPs may be involved in the pathogenesis of infectious and inflammatory diseases.

Rationale for the design of shortened derivatives of the NK-lysin-derived antimicrobial peptide NK-2 with improved activity against Gram-negative pathogens

The peptide NK-2 is an effective antimicrobial agent with low hemolytic and cytotoxic activities and is thus a promising candidate for clinical applications. It comprises the alpha-helical, cationic core region of porcine NK-lysin a homolog of human granulysin and of amoebapores of pathogenic amoeba. Here we visualized the impact of NK-2 on Escherichia coli by electron microscopy and used NK-2 as a template for sequence variations to improve the peptide stability and activity and to gain insight into the structure/function relationships. We synthesized 18 new peptides and tested their activities on seven Gram-negative and one Gram-positive bacterial strains, human erythrocytes, and HeLa cells. Although all peptides appeared unordered in buffer, those active against bacteria adopted an alpha-helical conformation in membrane-mimetic environments like trifluoroethanol and negatively charged phosphatidylglycerol (PG) liposomes that mimick the cytoplasmic membrane of bacteria. This conformation was not observed in the presence of liposomes consisting of zwitterionic phosphatidylcholine (PC) typical for the human cell plasma membrane. The interaction was paralleled by intercalation of these peptides into PG liposomes as determined by FRET spectroscopy. A comparative analysis between biological activity and the calculated peptide parameters revealed that the decisive factor for a broad spectrum activity is not the peptide overall hydrophobicity or amphipathicity, but the possession of a minimal positive net charge plus a highly amphipathic anchor point of only seven amino acid residues (two helical turns).

Mechanisms of decreased susceptibility to beta-defensins by Treponema denticola

Treponema denticola, a periodontal pathogen, is relatively resistant to human beta-defensins, which are small cationic antimicrobial peptides produced by a number of cells, including the gingival epithelium. Using two independent methods, we previously demonstrated that T. denticola proteases are not responsible for decreased vulnerability to defensins. In this study, we confirmed that the major outer membrane protease, dentilisin, is not responsible for T. denticola insensitivity to defensins and examined several other possible mechanisms, including reduced binding to the bacterial surface and efflux pump activity. It has been suggested that some bacteria mask their surfaces with serum proteins. T. denticola grown in a serum-free medium did not exhibit increased susceptibility to human beta-defensin 2 and 3 (hbetaD-2 and hbetaD-3, respectively), suggesting that cloaking of the outer surface with host proteins is not involved in defensin resistance. Nonetheless, we demonstrated that T. denticola binds significantly less hbetaD-2 and -3 than susceptible organisms bind, suggesting that the unusual outer membrane composition of T. denticola may discourage cationic peptide binding. Efflux pumps have been shown to mediate resistance to antibiotics and cationic peptides in other bacteria, and their role in T. denticola's relative resistance to beta-defensins was investigated. Three inhibitors of bacterial ATP-binding cassette (ABC) efflux pumps had no effect on T. denticola's susceptibility to hbetaD-2 or -3. In contrast, a proton motive force inhibitor, carbonyl cyanide 3-chlorophenylhydrazone, increased the susceptibility of T. denticola to killing by hbetaD-3, demonstrating a potential role for efflux pumps (other than ABC pumps) in resistance to this peptide. Our data suggest that the combination of decreased defensin binding and efflux of any peptide which enters the cytoplasm may explain T. denticola's relative resistance to human beta-defensins.

Differential expression and antibacterial activity of WFDC10A in the monkey epididymis

The ability of the epididymis to perform its diverse functions stems from its regionalized gene and protein expression patterns. The differences in the gene expression patterns of the caput and cauda regions of the bonnet monkey epididymis were compared using the technique of differential display reverse transcriptase polymerase chain reaction. A transcript showing homology to human whey acidic protein 10 (hWFDC10A) was highly expressed in the monkey caput region. A peptide P2 was designed spanning a region of the monkey WFDC10A (mWFDC10A), which could inhibit the growth of gram-negative bacterial strains of Escherichia coli. P2 could permeabilize the bacterial cell membrane but was unable to permeabilize mammalian cells as evidenced by the lack of hemolysis upon incubation with the peptide. Expression of genes such as mWFDC10A may be essential in providing the first line of defense against microbial infections to the epididymal tract and thus rendering protection to the male gametes sheltered within the epididymis.

Lipopolysaccharide (Endotoxin)-host defense antibacterial peptides interactions: role in bacterial resistance and prevention of sepsis

Lipopolysaccharide (LPS) is the major molecular component of the outer membrane of Gram-negative bacteria and serves as a physical barrier providing the bacteria protection from its surroundings. LPS is also recognized by the immune system as a marker for the detection of bacterial pathogen invasion, responsible for the development of inflammatory response, and in extreme cases to endotoxic shock. Because of these functions, the interaction of LPS with LPS binding molecules attracts great attention. One example of such molecules are antimicrobial peptides (AMPs). These are large repertoire of gene-encoded peptides produced by living organisms of all types, which serve as part of the innate immunity protecting them from pathogen invasion. AMPs are known to interact with LPS with high affinities. The biophysical properties of AMPs and their mode of interaction with LPS determine their biological function, susceptibility of bacteria to them, as well as the ability of LPS to activate the immune system. This review will discuss recent studies on the molecular mechanisms underlying these interactions, their effects on the resistance of the bacteria to AMPs, as well as their potential to neutralize LPS-induced endotoxic shock.