Rhamnolipid-induced shedding of flagellin from Pseudomonas aeruginosa provokes hBD-2 and IL-8 response in human keratinocytes

Antibiofilm and wound healing efficacy of rhamnolipid biosurfactant against pathogenic bacterium Staphylococcus aureus

The present study evaluates the in-vitro antibiofilm activity against the biofilm formed by Staphylococcus aureus, and the wound-healing efficacy of two different types of rhamnolipids produced by Pseudomonas aeruginosa strain JS29 in S.aureus infected wounds. The biosurfactant production was carried out in a mineral salt medium supplemented with 2 % Glucose and 2 % Glycerol individually and thus were designated as RL-Glu and RL-Gly respectively. 0.5 mg/ml of RL-Glu and RL-Gly demonstrated 90 % growth inhibition of S. aureus while exhibiting bactericidal activity at 4 mg/ml of RL-Glu and 1 mg/ml of RL-Gly. Both types of rhamnolipid cause changes in membrane permeability leading to pathogens' non-viability. 90 % inhibition of biofilm formation by S. aureus was observed at 2 mg/ml of RL-Glu and 0.5 mg/ml of RL-Gly, while 0.5 mg/ml of both rhamnolipid disrupted 90 % of the preformed biofilm. 0.5 mg/ml of RL-Glu and RL-Gly decreases the production of exopolysaccharides and also causes structural alteration. 0.5 mg/ml of RL-Glu and RL-Gly were found to exhibit effective wound healing efficacy in S. aureus infected wounds within 7 days of treatment. Histopathological studies of wound sites revealed efficient wound management by both the rhamnolipid. LCMS and GCMS characterization of the biosurfactant revealed that JS29 produces different rhamnolipid congeners when grown on different carbon sources, thereby influencing the antimicrobial, antibiofilm, and wound healing efficacy of rhamnolipid.

The power of patterns: new insights into pattern-triggered immunity

The plant immune system features numerous immune receptors localized on the cell surface to monitor the apoplastic space for danger signals from a broad range of plant colonizers. Recent discoveries shed light on the enormous complexity of molecular signals sensed by these receptors, how they are generated and removed to maintain cellular homeostasis and immunocompetence, and how they are shaped by host-imposed evolutionary constraints. Fine-tuning receptor sensing mechanisms at the molecular, cellular and physiological level is critical for maintaining a robust but adaptive host barrier to commensal, pathogenic, and symbiotic colonizers alike. These receptors are at the core of any plant-colonizer interaction and hold great potential for engineering disease resistance and harnessing beneficial microbiota to keep crops healthy.

Deficient Pseudomonas aeruginosa in MlaA/VacJ outer membrane lipoprotein shows decrease in rhamnolipids secretion, motility, and biofilm formation, and increase in fluoroquinolones susceptibility and innate immune response

Pseudomonas aeruginosa, a Gram-negative bacterium that causes severe hospital acquired infections poses threat by its ability for adaptation to various growth modes and environmental conditions and by its intrinsic resistance to antibiotics. The latter is mainly due to the outer membrane (OM) asymmetry which is maintained by the Mla pathway resulting in the retrograde transport of glycerophospholipids from the OM to the inner membrane. It comprises six Mla proteins, including MlaA, an OM lipoprotein involved in the removal of glycerophospholipids mislocalized at the outer leaflet of OM. To investigate the role of P. aeruginosa OM asymmetry especially MlaA, this study investigated the effect of mlaA deletion on (i) the susceptibility to antibiotics, (ii) the secretion of virulence factors, the motility, biofilm formation, and (iii) the inflammatory response. mlaA deletion in P. aeruginosa ATCC27853 results in phenotypic changes including, an increase in fluoroquinolones susceptibility and in PQS (Pseudomonas Quinolone Signal) and TNF-α release and a decrease in rhamnolipids secretion, motility and biofilm formation. Investigating how the mlaA knockout impacts on antibiotic susceptibility, bacterial virulence and innate immune response will help to elucidate the biological significance of the Mla system and contribute to the understanding of MlaA in P. aeruginosa OM asymmetry.

Can mesenchymal stem/stromal cells and their secretomes combat bacterial persisters?

The increasing number of life-threatening infections caused by persister bacteria is associated with various issues, including antimicrobial resistance and biofilm formation. Infections due to persister cells are often difficult to suppress without the use of last-resort antibiotics. Throughout the world, bacterial persistence and resistance create an unmet clinical demand for the exploration of newly introduced therapeutic approaches. Mesenchymal stem / stromal cells (MSCs) have an antimicrobial activity to protect against bacterial infections, including those caused by bacterial persisters. MSCs have substantial potential to secrete antimicrobial peptides (AMPs), including cathelicidin, beta-defensins, lipocalin-2, hepcidin, indoleamine 2,3-dioxygenase (IDO), cysteine proteases, and inducible nitric oxide synthases (iNOS). MSCs possess the potential to contribute to innate immunity by regulating the immune response. Recently, MSCs and their secreted components have been reported to improve antimicrobial activity. Bactericidal activity by MSCs and their secretomes has been shown to be mediated in part by the secretion of AMPs. Even though they were discovered more than 80 years ago, therapeutic options for persisters are restricted, and there is an urgent need for alternative treatment regimens. Hence, this review intends to critically assess the current literature on the effects of MSCs and their secretomes on persister bacteria. MSCs and their secretome-based therapies could be preferred as an up-and-coming approach to reinforce the antimicrobial efficiency in persister infections.

Regulation of Host Defense Peptide Synthesis by Polyphenols

The rise of antimicrobial resistance has created an urgent need for antibiotic-alternative strategies for disease control and prevention. Host defense peptides (HDPs), which have both antimicrobial and immunomodulatory properties, are an important component of the innate immune system. A host-directed approach to stimulate the synthesis of endogenous HDPs has emerged as a promising solution to treat infections with a minimum risk for developing antimicrobial resistance. Among a diverse group of compounds that have been identified as inducers of HDP synthesis are polyphenols, which are naturally occurring secondary metabolites of plants characterized by the presence of multiple phenol units. In addition to their well-known antioxidant and anti-inflammatory activities, a variety of polyphenols have been shown to stimulate HDP synthesis across animal species. This review summarizes both the in vitro and in vivo evidence of polyphenols regulating HDP synthesis. The mechanisms by which polyphenols induce HDP gene expression are also discussed. Natural polyphenols warrant further investigation as potential antibiotic alternatives for the control and prevention of infectious diseases.

A Systematic Review of Keratinocyte Secretions: A Regenerative Perspective

Cell regenerative therapy is a modern solution for difficult-to-heal wounds. Keratinocytes, the most common cell type in the skin, are difficult to obtain without the creation of another wound. Stem cell differentiation towards keratinocytes is a challenging process, and it is difficult to reproduce in chemically defined media. Nevertheless, a co-culture of keratinocytes with stem cells usually achieves efficient differentiation. This systematic review aims to identify the secretions of normal human keratinocytes reported in the literature and correlate them with the differentiation process. An online search revealed 338 references, of which 100 met the selection criteria. A total of 80 different keratinocyte secretions were reported, which can be grouped mainly into cytokines, growth factors, and antimicrobial peptides. The growth-factor group mostly affects stem cell differentiation into keratinocytes, especially epidermal growth factor and members of the transforming growth factor family. Nevertheless, the reported secretions reflected the nature of the involved studies, as most of them focused on keratinocyte interaction with inflammation. This review highlights the secretory function of keratinocytes, as well as the need for intense investigation to characterize these secretions and evaluate their regenerative capacities.

Bacterial rhamnolipids and their 3-hydroxyalkanoate precursors activate Arabidopsis innate immunity through two independent mechanisms

Plant innate immunity is activated upon perception of invasion pattern molecules by plant cell-surface immune receptors. Several bacteria of the genera Pseudomonas and Burkholderia produce rhamnolipids (RLs) from l-rhamnose and (R)-3-hydroxyalkanoate precursors (HAAs). RL and HAA secretion is required to modulate bacterial surface motility, biofilm development, and thus successful colonization of hosts. Here, we show that the lipidic secretome from the opportunistic pathogen Pseudomonas aeruginosa, mainly comprising RLs and HAAs, stimulates Arabidopsis immunity. We demonstrate that HAAs are sensed by the bulb-type lectin receptor kinase LIPOOLIGOSACCHARIDE-SPECIFIC REDUCED ELICITATION/S-DOMAIN-1-29 (LORE/SD1-29), which also mediates medium-chain 3-hydroxy fatty acid (mc-3-OH-FA) perception, in the plant Arabidopsis thaliana HAA sensing induces canonical immune signaling and local resistance to plant pathogenic Pseudomonas infection. By contrast, RLs trigger an atypical immune response and resistance to Pseudomonas infection independent of LORE. Thus, the glycosyl moieties of RLs, although abolishing sensing by LORE, do not impair their ability to trigger plant defense. Moreover, our results show that the immune response triggered by RLs is affected by the sphingolipid composition of the plasma membrane. In conclusion, RLs and their precursors released by bacteria can both be perceived by plants but through distinct mechanisms.

Effects of Rhamnolipids on Growth Performance, Immune Function, and Cecal Microflora in Linnan Yellow Broilers Challenged with Lipopolysaccharides

This present study aimed to investigate the effects of rhamnolipids (RLS) on the growth performance, intestinal morphology, immune function, short-chain fatty acid content, and microflora community in broiler chickens challenged with lipopolysaccharides (LPS). A total of 450 broiler chickens were randomly allocated into three groups: basal diet with no supplement (NCO), basal diet with bacitracin (ANT), and basal diet with rhamnolipids (RLS). After 56 d of feeding, 20 healthy broilers were selected from each group, with half being intraperitoneally injected with lipopolysaccharides (LPS) and the other half with normal saline. Treatments with LPS were labelled LPS-NCO, LPS-ANT, and LPS-RLS, whereas treatments with normal saline were labelled NS-NCO, NS-ANT, and NS-RLS. LPS-challenged birds had lower jejunal villus height and higher crypt depth than unchallenged birds. LPS-RLS broilers had increased jejunal villus height and villus height/crypt depth ratio (V/C) but lower crypt depth than LPS-NCO. Dietary supplementation with RLS reduced the LPS-induced immunological stress. Compared with LPS-NCO, birds in LPS-RLS had lower concentrations of IL-1β, IL-6, and TNF-α. In LPS-challenged broilers, RLS and ANT increased the concentrations of IgA, IgM, and IgY compared with LPS-NCO. In LPS treatments, RLS enhanced the contents of acetic acid, butyrate, isobutyric acid, isovalerate, and valerate more than LPS-NCO birds. High-throughput sequencing indicated that RLS supplementation led to changes in the cecal microbial community of broilers. At the species level, Clostridium-sp-Marseille-p3244 was more abundant in NS-RLS than in NS-NCO broilers. In summary, RLS improved the growth performance and relative abundance of cecal microbiota and reduced the LPS-induced immunological stress in broiler chickens.

Surfaceome and Exoproteome Dynamics in Dual-Species Pseudomonas aeruginosa and Staphylococcus aureus Biofilms

Bacterial biofilms are an important underlying cause for chronic infections. By switching into the biofilm state, bacteria can evade host defenses and withstand antibiotic chemotherapy. Despite the fact that biofilms at clinical and environmental settings are mostly composed of multiple microbial species, biofilm research has largely been focused on single-species biofilms. In this study, we investigated the interaction between two clinically relevant bacterial pathogens (Staphylococcus aureus and Pseudomonas aeruginosa) by label-free quantitative proteomics focusing on proteins associated with the bacterial cell surfaces (surfaceome) and proteins exported/released to the extracellular space (exoproteome). The changes observed in the surfaceome and exoproteome of P. aeruginosa pointed toward higher motility and lower pigment production when co-cultured with S. aureus. In S. aureus, lower abundances of proteins related to cell wall biosynthesis and cell division, suggesting increased persistence, were observed in the dual-species biofilm. Complementary phenotypic analyses confirmed the higher motility and the lower pigment production in P. aeruginosa when co-cultured with S. aureus. Higher antimicrobial tolerance associated with the co-culture setting was additionally observed in both species. To the best of our knowledge, this study is among the first systematic explorations providing insights into the dynamics of both the surfaceome and exoproteome of S. aureus and P. aeruginosa dual-species biofilms.

Pseudomonas aeruginosa infection stimulates mitogen-activated protein kinases signaling pathway in human megakaryocytes

Pseudomonas aeruginosa is a major cause of nosocomial infections and contributes to higher mortality in hospitalized individuals. Infection by P. aeruginosa triggers host immune response through activation of pathogen recognition receptors, which are present in innate cells. Several studies have reported the mechanism of P. aeruginosa induced innate immunity in multiple cell types. But so far there is no reports on response of megakaryocytes to P. aeruginosa infection. Hence, our aim was to investigate the precise role and signaling mechanism of megakaryocytes during P. aeruginosa infection. In this study, we used Mo7e cells as representatives of human megakaryocyte and found that P. aeruginosa infection induces cytotoxicity in these cells. We further demonstrated that P. aeruginosa infection modulates p38 and extracellular signal regulated kinase pathways in Mo7e cells. Protein expression profiling in P. aeruginosa lipopolysaccharide-treated Mo7e cells revealed upregulation of importin subunit β and downregulation of metabolic enzymes. Our results suggest that P. aeruginosa infection regulates mitogen-activated protein kinases signaling pathway and importin in Mo7e cells and that this is a potential mechanism for nuclear translocation of nuclear factor binding near the κ light-chain gene in B cells and c-Jun N-terminal kinases to induce cell cytotoxicity.

The potential for immunoglobulins and host defense peptides (HDPs) to reduce the use of antibiotics in animal production

Innate defense mechanisms are aimed at quickly containing and removing infectious microorganisms and involve local stromal and immune cell activation, neutrophil recruitment and activation and the induction of host defense peptides (defensins and cathelicidins), acute phase proteins and complement activation. As an alternative to antibiotics, innate immune mechanisms are highly relevant as they offer rapid general ways to, at least partially, protect against infections and enable the build-up of a sufficient adaptive immune response. This review describes two classes of promising alternatives to antibiotics based on components of the innate host defense. First we describe immunoglobulins applied to mimic the way in which they work in the newborn as locally acting broadly active defense molecules enforcing innate immunity barriers. Secondly, the potential of host defense peptides with different modes of action, used directly, induced in situ or used as vaccine adjuvants is described.

Rhamnolipids, Microbial Virulence Factors, in Alzheimer's Disease

Alzheimer's disease (AD) has been attributed to chronic bacterial infections. The recognition of human microbiota as a substantial contributor to health and disease is relatively recent and growing. During evolution, mammals live in a symbiotic state with myriads of microorganisms that survive at a diversity of tissue micro-surroundings. Microbes produce a plethora of secretory products [amyloids, lipopolysaccharides, virulence factors rhamnolipids (RLs), toxins, and a great number of neuroactive compounds]. The contribution of infectious microbial components to the pathophysiology of the human central nervous system including AD is considered potentially substantial, but the involvement of the RLs has never been reported. Here, RLs were isolated from serum and identified through various conventional methods including the colorimetric orcinol method, thin-layer chromatography, attenuated total reflection Fourier transform infrared (ATR-FTIR), and dot blot using antibodies against RLs. Dot blot demonstrated elevated RL levels in sera of AD patients compared to controls (p = 0.014). Moreover, ELISA showed similarly elevated RL levels in cerebrospinal fluid of both AD (0.188 versus 0.080) (p = 0.04) and mild cognitive impairment (0.188 versus 0.129) (p = 0.088) patients compared to healthy, and are well-correlated with the AD stages severity assessed using the Mini-Mental State Examination. These results provide conclusive evidence for the newly-reported implication of RLs in AD, adding it to the list of bacterial components, opening new avenues for AD investigation. Moreover, they strengthen and vindicate the divergence of research toward the exploration of bacterial involvement in AD generation and progression.

Local Burn Injury Promotes Defects in the Epidermal Lipid and Antimicrobial Peptide Barriers in Human Autograft Skin and Burn Margin: Implications for Burn Wound Healing and Graft Survival

Burn injury increases the risk of morbidity and mortality by promoting severe hemodynamic shock and risk for local or systemic infection. Graft failure due to poor wound healing or infection remains a significant problem for burn subjects. The mechanisms by which local burn injury compromises the epithelial antimicrobial barrier function in the burn margin, containing the elements necessary for healing of the burn site, and in distal unburned skin, which serves as potential donor tissue, are largely unknown. The objective of this study was to establish defects in epidermal barrier function in human donor skin and burn margin, to identify potential mechanisms that may lead to graft failure and/or impaired burn wound healing. In this study, we established that epidermal lipids and respective lipid synthesis enzymes were significantly reduced in both donor skin and burn margin. We further identified diverse changes in the gene expression and protein production of several candidate skin antimicrobial peptides (AMPs) in both donor skin and burn margin. These results also parallel changes in cutaneous AMP activity against common burn wound pathogens, aberrant production of epidermal proteases known to regulate barrier permeability and AMP activity, and greater production of proinflammatory cytokines known to be induced by AMPs. These findings suggest that impaired epidermal lipid and AMP regulation could contribute to graft failure and infectious complications in subjects with burn or other traumatic injury.

Lifestyle alters GUT-bacteria function: Linking immune response and host

Microbiota in human is a "mixture society" of different species (i.e. bacteria, viruses, funguses) populations with a different way of relationship classification to Human. Human GUT serves as the host of the majority of different bacterial populations (GUT flora, more than 500 species), which are with us ("from the beginning") in an innate manner known as the commensal (no harm to each other) and symbiotic (mutual benefit) relationship. A homeostatic balance of host-bacteria relationship is very important and vital for a normal health process. However, this beneficial relationship and delicate homeostatic state can be disrupted by the imbalance of microbiome-composition of gut microbiota, expressing a pathogenic state. A strict homeostatic balance of microbiome-composition strongly depends on several factors; 1- lifestyle, 2- geography, 3- ethnicities, 4- "mom" as prime of the type of bacterial colonization in infant and 5- the disease. With such diversity in individuals combined with huge number of different bacterial species and their interactions, it is wise to perform an in-depth systems biology (e.g. genomics, proteomics, glycomics, and etcetera) analysis of personalized microbiome. Only in this way, we are able to generate a map of complete GUT microbiota and, in turn, to determine its interaction with host and intra-interaction with pathogenic bacteria. A specific microbiome analysis provides us the knowledge to decipher the nature of interactions between the GUT microbiota and the host and its response to the invading bacteria in a pathogenic state. The GUT-bacteria composition is independent of geography and ethnicity but lifestyle well affects GUT-bacteria composition and function. Microbiome knowledge obtained by systems biology also helps us to change the behavior of GUT microbiota in response to the pathogenic microbes as protection. Functional microbiome changes in response to environmental factors will be discussed in this review.

Potential applications of biosurfactant rhamnolipids in agriculture and biomedicine

Rhamnolipids have recently emerged as promising bioactive molecules due to their novel structures, diverse and versatile biological functions, lower toxicity, higher biodegradability, as well as production from renewable resources. The advantages of rhamnolipids make them attractive targets for research in a wide variety of applications. Especially rhamnolipids are likely to possess potential applications of the future in areas such as biomedicine, therapeutics, and agriculture. The purpose of this mini review is to provide a comprehensive prospective of biosurfactant rhamnolipids as potential antimicrobials, immune modulators, and virulence factors, and anticancer agents in the field of biomedicine and agriculture that may meet the ever-increasing future pharmacological treatment and food safety needs in human health.

Rhamnolipids form drug-loaded nanoparticles for dermal drug delivery

Bacterial biosurfactants are nature's strategy to solubilize and ingest hydrophobic molecules and nutrients using a fully biodegradable transport system. Eight structurally defined rhamnolipids were selected and investigated as potential drug carrier systems. Depending on the molecular structures defining their packing parameters, the rhamnolipids were found to form spherical nanoparticles with precisely defined average sizes between 5 and 100nm, low polydispersity, and stability over a broad concentration range as revealed from dynamic light scattering and electron microscopy. As rhamnolipids were tolerated well by the human skin, rhamnolipid nanoparticles were considered for dermal drug delivery and thus loaded with hydrophobic drug molecules. Using the drug model, Nile red, dexamethasone, and tacrolimus nanoparticles charged with up to 30% drug loading (w/w) were obtained. Nanoparticles loaded with Nile red were investigated for dermal drug delivery in a Franz cell using human skin. Fluoresence microscopy of skin slices indicated the efficient penetration of the model drug into human skin, both into the stratum corneum and although to a lesser extent into the lower epidermis. Rhamnolipid nanocarriers were found to be non-toxic to primary human fibroblasts in a proliferation assay and thus are considered candidates for the dermal delivery of drugs.

Differential regulation of interleukin-8 and human beta-defensin 2 in Pseudomonas aeruginosa-infected intestinal epithelial cells

Background

The human opportunistic pathogen, Pseudomonas aeruginosa (P. aeruginosa) carries the highest case fatality rate of all gram-negative infections. Unfortunately, antimicrobial therapy has not been demonstrated to improve clinical outcome and the emergence of multidrug resistant P. aeruginosa has become a major concern in the hospital setting. Fever and diarrhea are the two most common initial symptoms in P. aeruginosa sepsis in previously healthy infants and children. This implies that intestinal epithelial cells in first contact with the pathogen may play an important role in innate immunity to P. aeruginosa infection. Human beta–defensins-2 (hBD-2) and interleukin-8 (IL-8) are crucial for host defense at mucosa but IL-8 may give rise to characteristic pathology of colitis.

Results

Pseudomonas aeruginosa strain PAO1 was used to infect SW480, an intestinal epithelial cell. IL-8 and hBD-2 mRNA expression and protein secretion were then assessed in SW480 cells using RT-PCR and enzyme-linked immunosorbent assay (ELISA), respectively. Intracellular signaling pathways and nucleotide-binding oligomerization domain (NOD) 1 protein expression were analyzed by Western blot in SW480 cells in the presence or absence of inhibitors or transfected with siRNA.

We demonstrate that prolonged infection by P. aeruginosa results in suppression of IL-8 but enhancement of hBD-2, either protein secretion and mRNA expression, in SW480 cells. Inhibitors of ERK suppressed but inhibitor of PI3K enhanced P. aeruginosa-induced IL-8 mRNA expression in SW480 cells while both signaling had no effect on P. aeruginosa-induced hBD-2 expression in SW480 cells. On the other hand, NOD 1 was illustrated to get involved in P. aeruginosa-induced hBD-2 mRNA expression and protein production in SW480 cells.

Conclusions

The P. aeruginosa-induced antimicrobial peptide in IECs continuously protect the host against prolonged infection, while modulation of proinflammatory responses prevents the host from the detrimental effects of overwhelming inflammation. Thus, P. aeruginosa-induced innate immunity in IECs represents a host protective mechanism, which may provide new insight into the pathogenesis of inflammatory bowel diseases.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-014-0275-6) contains supplementary material, which is available to authorized users.

Host-induced bacterial cell wall decomposition mediates pattern-triggered immunity in Arabidopsis

Peptidoglycans (PGNs) are immunogenic bacterial surface patterns that trigger immune activation in metazoans and plants. It is generally unknown how complex bacterial structures such as PGNs are perceived by plant pattern recognition receptors (PRRs) and whether host hydrolytic activities facilitate decomposition of bacterial matrices and generation of soluble PRR ligands. Here we show that Arabidopsis thaliana, upon bacterial infection or exposure to microbial patterns, produces a metazoan lysozyme-like hydrolase (lysozyme 1, LYS1). LYS1 activity releases soluble PGN fragments from insoluble bacterial cell walls and cleavage products are able to trigger responses typically associated with plant immunity. Importantly, LYS1 mutant genotypes exhibit super-susceptibility to bacterial infections similar to that observed on PGN receptor mutants. We propose that plants employ hydrolytic activities for the decomposition of complex bacterial structures, and that soluble pattern generation might aid PRR-mediated immune activation in cell layers adjacent to infection sites.

Epithelial uptake of flagella initiates proinflammatory signaling

The airway epithelium serves multiple roles in the defense of the lung. Not only does it act as a physical barrier, it acts as a distal extension of the innate immune system. We investigated the role of the airway epithelium in the interaction with flagella, an important virulence factor of the pathogen Pseudomonas aeruginosa, a cause of ventilator associated pneumonia and significant morbidity and mortality in patients with cystic fibrosis. Flagella were required for transmigration across polarized airway epithelial cells and this was a direct consequence of motility, and not a signaling effect. Purified flagella did not alter the barrier properties of the epithelium but were observed to be rapidly endocytosed inside epithelial cells. Neither flagella nor intact P. aeruginosa stimulated epithelial inflammasome signaling. Flagella-dependent signaling required dynamin-based uptake as well as TLR5 and primarily led to the induction of proinflammatory (Tnf, Il6) as well as neutrophil (Cxcl1, Cxcl2, Ccl3) and macrophage (Ccl20) chemokines. Although flagella are important in invasion across the epithelial barrier their shedding in the airway lumen results in epithelial uptake and signaling that has a major role in the initial recruitment of immune cells in the lung.

Microbial recognition and evasion of host immunity

Plants are able to detect microbes by pattern recognition receptors in the host cells that, upon recognition of the enemy, activate effective immune responses in the invaded tissue. Recognition of microbes occurs by common conserved structures called microbe-associated molecular patterns (MAMPs). Plant pathogens and beneficial soil-borne microbes live in close contact with their host. Hence, prevention of the host's defence programme is essential for their survival. Active suppression of host defences by microbial effector proteins is a well-known strategy employed by many successful plant-associated microbes. Evasion of host immune recognition is less well studied but is emerging as another important strategy. Escape from recognition by the host's immune system can be caused by alterations in the structure of the recognized MAMPs, or by active intervention of ligand-receptor recognition. This paper reviews the structure and recognition of common MAMPs and the ways that plant-associated microbes have evolved to prevent detection by their host.

Antimicrobial peptide elicitors: New hope for the post-antibiotic era

Antimicrobial peptides or host defense peptides are fundamental components of human innate immunity. Recent and growing evidence suggests they have a role in a broad range of diseases, including cancer, allergies and susceptibility to infection, including HIV/AIDS. Antimicrobial peptide elicitors (APEs) are physical, biological or chemical agents that boost human antimicrobial peptide expression. The current knowledge of APEs and their potential use in the treatment of human infectious diseases are reviewed, and a classification system for APEs is proposed. The efficient use of APEs in clinical practice could mark the beginning of the urgently needed post-antibiotic era, but further trials assessing their efficacy and safety are required.

Human beta-defensin 3 induces maturation of human langerhans cell-like dendritic cells: an antimicrobial peptide that functions as an endogenous adjuvant

Human beta-defensins (hBDs) are antimicrobial peptides that have an important role in innate immune responses at epithelial barriers such as the skin. However, the role that hBDs have in initiating cellular immune responses that contribute to antigen-specific adaptive immunity is not well understood. Here we show that one member of the hBD family, hBD3, can induce maturation and T-helper type 1 skewing function in human Langerhans cell-like dendritic cells (LC-DCs). Specifically, hBD3 potently induces phenotypic maturation of LC-DCs, including increased expression of CCR7, which mediates functional chemotactic responses to CCL19 and CCL21. hBD3-stimulated LC-DCs induce strong proliferation of and IFN-γ secretion by naive human T cells. hBD3 also induces phenotypic maturation of primary human skin-migratory DCs derived from human skin explants. These results suggest an important role for hBD3 in inducing DC activation, migration, and polarization. Thus, hBD3 contributes to the integration of innate and adaptive immune responses in the skin, and may be a useful adjuvant for skin immunization and an important factor in the pathophysiology of inflammatory skin diseases.

Pseudomonas aeruginosa-derived rhamnolipids subvert the host innate immune response through manipulation of the human beta-defensin-2 expression

Pseudomonas aeruginosa is a well-known cause of infections especially in compromised patients. To neutralize this pathogen, the expression of antimicrobial factors in epithelial cells is crucial. In particular the human beta-defensin hBD-2 is especially active against P. aeruginosa. In this study, we identified rhamnolipids in P. aeruginosa culture supernatants that are able to prevent the pathogen-induced hBD-2 response in keratinocytes. The presence of rhamnolipids within the host cells and inhibition assays suggest that calcium-regulated pathways and protein kinase C activation are impaired by rhamnolipids. In consequence, the induction of hBD-2 in keratinocytes by P. aeruginosa-derived flagellin as well as the host's own hBD-2 mediator interleukin IL-1β is inhibited. Strikingly, rhamnolipids did not affect the release of the proinflammatory mediator interleukin IL-8 by flagellin. Thus, in addition to their function in establishment and persistence of P. aeruginosa infections, rhamnolipids can be engaged by P. aeruginosa for a targeted attenuation of the innate immunity to manage its survival and colonization on compromised epithelia.

The surfactant of Legionella pneumophila Is secreted in a TolC-dependent manner and is antagonistic toward other Legionella species

When Legionella pneumophila grows on agar plates, it secretes a surfactant that promotes flagellum- and pilus-independent "sliding" motility. We isolated three mutants that were defective for surfactant. The first two had mutations in genes predicted to encode cytoplasmic enzymes involved in lipid metabolism. These genes mapped to two adjacent operons that we designated bbcABCDEF and bbcGHIJK. Backcrossing and complementation confirmed the importance of the bbc genes and suggested that the Legionella surfactant is lipid containing. The third mutant had an insertion in tolC. TolC is the outer membrane part of various trimolecular complexes involved in multidrug efflux and type I protein secretion. Complementation of the tolC mutant restored sliding motility. Mutants defective for an inner membrane partner of TolC also lacked a surfactant, confirming that TolC promotes surfactant secretion. L. pneumophila (lspF) mutants lacking type II protein secretion (T2S) are also impaired for a surfactant. When the tolC and lspF mutants were grown next to each other, the lsp mutant secreted surfactant, suggesting that TolC and T2S conjoin to mediate surfactant secretion, with one being the conduit for surfactant export and the other the exporter of a molecule that is required for induction or maturation of surfactant synthesis/secretion. Although the surfactant was not required for the extracellular growth, intracellular infection, and intrapulmonary survival of L. pneumophila, it exhibited antimicrobial activity toward seven other species of Legionella but not toward various non-Legionella species. These data suggest that the surfactant provides L. pneumophila with a selective advantage over other legionellae in the natural environment.

Antimicrobial peptides in healthy skin and atopic dermatitis

Antimicrobial Peptides and Proteins (AMPs) represent effector molecules of the innate defense system in all organisms. AMPs are either constitutively or inducibly produced mainly by various epithelial cells, including keratinocytes. This report reviews our current knowledge about the major yet known keratinocyte-derived AMPs, its role in healthy skin and atopic dermatitis.

Flagellin delivery by Pseudomonas aeruginosa rhamnolipids induces the antimicrobial protein psoriasin in human skin

The opportunistic pathogen Pseudomonas aeruginosa can cause severe infections in patients suffering from disruption or disorder of the skin barrier as in burns, chronic wounds, and after surgery. On healthy skin P. aeruginosa causes rarely infections. To gain insight into the interaction of the ubiquitous bacterium P. aeruginosa and healthy human skin, the induction of the antimicrobial protein psoriasin by P. aeruginosa grown on an ex vivo skin model was analyzed. We show that presence of the P. aeruginosa derived biosurfactant rhamnolipid was indispensable for flagellin-induced psoriasin expression in human skin, contrary to in vitro conditions. The importance of the bacterial virulence factor flagellin as the major inducing factor of psoriasin expression in skin was demonstrated by use of a flagellin-deficient mutant. Rhamnolipid mediated shuttle across the outer skin barrier was not restricted to flagellin since rhamnolipids enable psoriasin expression by the cytokines IL-17 and IL-22 after topical application on human skin. Rhamnolipid production was detected for several clinical strains and the formation of vesicles was observed under skin physiological conditions. In conclusion we demonstrate herein that rhamnolipids enable the induction of the antimicrobial protein psoriasin by flagellin in human skin without direct contact of bacteria and responding cells. Hereby, human skin might control the microflora to prevent colonization of unwanted microbes in the earliest steps before potential pathogens can develop strategies to subvert the immune response.

Rhamnolipid biosurfactants as new players in animal and plant defense against microbes

Rhamnolipids are known as very efficient biosurfactant molecules. They are used in a wide range of industrial applications including food, cosmetics, pharmaceutical formulations and bioremediation of pollutants. The present review provides an overview of the effect of rhamnolipids in animal and plant defense responses. We describe the current knowledge on the stimulation of plant and animal immunity by these molecules, as well as on their direct antimicrobial properties. Given their ecological acceptance owing to their low toxicity and biodegradability, rhamnolipids have the potential to be useful molecules in medicine and to be part of alternative strategies in order to reduce or replace pesticides in agriculture.

The role of keratinocytes in defense against infection

PURPOSE OF REVIEW

The discovery of increasing numbers of epithelial antimicrobial peptides (AMPs), cytokines that specifically induce AMPs in epithelial cells, and mechanisms of its regulation point toward a central role of the keratinocyte as effector cell of the epithelial innate defense system. This review summarizes recent progress in understanding the keratinocyte's role in combating infection that can help to understand why skin is usually covered with microbes but normally not infected.

RECENT FINDINGS

The AMP LL-37 has been identified as regulator of keratinocyte apoptosis. The hypothesis of a direct defense function of keratinocytes, combating bacterial, fungal, virus and parasite infection, is strengthened. The discovery of an IL-22-producing T-lymphocyte subpopulation implicates a role in AMP induction of keratinocytes. Multiple studies are adding to our understanding of how skin keratinocytes are interacting in skin barrier defects and with the microflora. Although in atopic patients AMP production is not generally impaired, in hyperIgE syndrome a lack of Th17 cytokines causes local Staphylococcus aureus infection due to a defective keratinocyte defense system. Ultraviolet radiation induces AMPs and thus may have beneficial effects to combat skin infection.

SUMMARY

There is better understanding of the keratinocyte's role in the skin's innate defense system, and these data can help to generate therapeutics that can activate this defense system.

Human gammadelta T cells produce the protease inhibitor and antimicrobial peptide elafin

Human gammadelta T cells rapidly secrete pro-inflammatory cytokines in response to T cell receptor-dependent recognition of pyrophosphates produced by many bacteria and parasites. In further support of an important role of gammadelta T cells in the immune defence against infection, human gammadelta T cells have been shown to produce the antimicrobial peptide LL37/cathelicidin. In this study, we have investigated whether gammadelta T cells can produce additional antimicrobial peptides. To this end, we have screened human gammadelta T cell clones by RT-PCR for mRNA expression of a broad range of antimicrobial peptides. While alpha-defensins were absent and beta-defensins (HBD1) present only in rare gammadelta T cell clones, elafin mRNA was induced by supernatant of Pseudomonas aeruginosa grown under static conditions. Elafin is a protease inhibitor that also displays antimicrobial activity. Constitutive intracellular expression of elafin was demonstrated by flow cytometry and Western blot analysis. Furthermore, trappin-2 (pre-elafin) could be immunoprecipitated in cell lysates but also in the supernatant of gammadelta T cells stimulated by Ps. aeruginosa supernatant. Taken together, our studies reveal a novel effector function of gammadelta T cells which might be important for local immune defence.

Injury is a major inducer of epidermal innate immune responses during wound healing

We examined the importance of injury for the epidermal innate immune response in human skin wounds. We found that injury, independent of infiltrating inflammatory cells, generated prominent chemotactic activity toward neutrophils in injured skin because of IL-8 production. Furthermore, injury was a major inducer of the expression of antimicrobial (poly)peptides (AMPs) in skin wounds. In human skin, these injury-induced innate immune responses were mediated by activation of the epidermal growth factor receptor (EGFR). Consequently, inhibition of the EGFR blocked both the chemotactic activity generated in injured skin and the expression of the majority of the AMPs. The importance of injury was confirmed in mouse experiments in vivo, in which injury independent of infection was a potent inducer of AMPs in skin wounds. To our knowledge, these data thereby provide a previously unreported molecular link between injury and neutrophil accumulation and identify the molecular background for the vast expression of IL-8 and AMPs in wounded epidermis. Conceptually, these data show that the growth factor response elicited by injury is important for the recruitment of neutrophils in skin wounds.