The antimicrobial peptide-sensing system aps of Staphylococcus aureus

Staphylococcus aureus is a leading cause of hospital-associated and, more recently, community-associated infections caused by highly virulent methicillin-resistant strains (CA-MRSA). S. aureus survival in the human host is largely defined by the ability to evade attacks by antimicrobial peptides (AMPs) and other mechanisms of innate host defence. Here we show that AMPs induce resistance mechanisms in CA-MRSA via the aps AMP sensor/regulator system, including (i) the d-alanylation of teichoic acids, (ii) the incorporation of lysyl-phosphatidylglycerol in the bacterial membrane and a concomitant increase in lysine biosynthesis, and (iii) putative AMP transport systems such as the vraFG transporter, for which we demonstrate a function in AMP resistance. In contrast to the aps system of S. epidermidis, induction of the aps response in S. aureus was AMP-selective due to structural differences in the AMP binding loop of the ApsS sensor protein. Finally, using a murine infection model, we demonstrate the importance of the aps regulatory system in S. aureus infection. This study shows that while significant interspecies differences exist in the AMP-aps interaction, the AMP sensor system aps is functional and efficient in promoting resistance to a variety of AMPs in a clinically relevant strain of the important human pathogen S. aureus.     

Differential Expression of Immune Defences Is Associated with Specific Host-Parasite Interactions in Insects

Recent ecological studies in invertebrates show that the outcome of an infection is dependent on the specific pairing of host and parasite. Such specificity contrasts the long-held view that invertebrate innate immunity depends on a broad-spectrum recognition system. An important question is whether this specificity is due to the immune response rather than some other interplay between host and parasite genotypes. By measuring the expression of putative bumblebee homologues of antimicrobial peptides in response to infection by their gut trypanosome Crithidia bombi, we demonstrate that expression differences are associated with the specific interactions.     

Insect Antiviral Innate Immunity: Pathways,Effectors, and Connections

Insects are infected by a wide array of viruses some of which are insect restricted and pathogenic, and some of which are transmitted by biting insects to vertebrates. The medical and economic importance of these viruses heightens the need to understand the interaction between the infecting pathogen and the insect immune system in order to develop transmission interventions. The interaction of the virus with the insect host innate immune system plays a critical role in the outcome of infection. The major mechanism of antiviral defense is the small, interfering RNA pathway that responds through the detection of virus-derived double-stranded RNA to suppress virus replication. However, other innate antimicrobial pathways such as Imd, Toll, and Jak-STAT and the autophagy pathway have also been shown to play important roles in antiviral immunity. In this review, we provide an overview of the current understanding of the main insect antiviral pathways and examine recent findings that further our understanding of the roles of these pathways in facilitating a systemic and specific response to infecting viruses.     

Differential requirement for c-Jun N-terminal kinase 1 in lung inflammation and host defense

The c-Jun N-terminal kinase (JNK) - 1 pathway has been implicated in the cellular response to stress in many tissues and models. JNK1 is known to play a role in a variety of signaling cascades, including those involved in lung disease pathogenesis. Recently, a role for JNK1 signaling in immune cell function has emerged. The goal of the present study was to determine the role of JNK1 in host defense against both bacterial and viral pneumonia, as well as the impact of JNK1 signaling on IL-17 mediated immunity. Wild type (WT) and JNK1 -/- mice were challenged with Escherichia coli, Staphylococcus aureus, or Influenza A. In addition, WT and JNK1 -/- mice and epithelial cells were stimulated with IL-17A. The impact of JNK1 deletion on pathogen clearance, inflammation, and histopathology was assessed. JNK1 was required for clearance of E. coli, inflammatory cell recruitment, and cytokine production. Interestingly, JNK1 deletion had only a small impact on the host response to S. aureus. JNK1 -/- mice had decreased Influenza A burden in viral pneumonia, yet displayed worsened morbidity. Finally, JNK1 was required for IL-17A mediated induction of inflammatory cytokines and antimicrobial peptides both in epithelial cells and the lung. These data identify JNK1 as an important signaling molecule in host defense and demonstrate a pathogen specific role in disease. Manipulation of the JNK1 pathway may represent a novel therapeutic target in pneumonia.     

Immunity and symbiosis

The invertebrate immune system, which has become a major research focus, shares basic features of innate immunity with vertebrates and men. A special feature apparently found only in invertebrates is their close association with vertically heritable symbiotic microorganisms. The validity of the simple view of symbiosis as a mutually beneficial interaction between two uneven partners mainly improving the nutritional state of the two companions has been challenged, however, as symbiotic interactions might involve more partners, and symbiotic functions of the microorganisms are much more diverse than previously assumed. Likewise, microorganisms considered to be mostly harmful to their hosts have been shown to enhance host fitness under some circumstances. The role of a symbiont itself might change between environments or life stages of the host and symbionts might have features previously thought to be specific for pathogens. Understanding symbiotic interactions requires the comprehension of the cross-talk between the symbiotic companions, and the dissection of how long-lasting infections are established without eliminating the symbiont by host immune responses. Fascinating new findings in this field revealed that symbiosis might contribute to defence against pathogens or natural enemies. New symbiont-based approaches to defeat agricultural pests or pathogen transmission by arthropod vectors are becoming conceivable.     

The Staphylococcus aureus surface protein IsdA mediates resistance to innate defenses of human skin

Resistance to human skin innate defenses is crucial for survival and carriage of Staphylococcus aureus, a common cutaneous pathogen and nasal colonizer. Free fatty acids extracted from human skin sebum possess potent antimicrobial activity against S. aureus. The mechanisms by which S. aureus overcomes this host defense during colonization remain unknown. Here, we show that S. aureus IsdA, a surface protein produced in response to the host, decreases bacterial cellular hydrophobicity rendering them resistant to bactericidal human skin fatty acids and peptides. IsdA is required for survival of S. aureus on live human skin. Reciprocally, skin fatty acids prevent the production of virulence determinants and the induction of antibiotic resistance in S. aureus and other Gram-positive pathogens. A purified human skin fatty acid was effective in treating systemic and topical infections of S. aureus suggesting that our natural defense mechanisms can be exploited to combat drug-resistant pathogens.     

Toll-like receptors are key participants in innate immune responses

During an infection, one of the principal challenges for the host is to detect the pathogen and activate a rapid defensive response. The Toll-like family of receptors (TLRs), among other pattern recognition receptors (PRR), performs this detection process in vertebrate and invertebrate organisms. These type I transmembrane receptors identify microbial conserved structures or pathogen-associated molecular patterns (PAMPs). Recognition of microbial components by TLRs initiates signaling transduction pathways that induce gene expression. These gene products regulate innate immune responses and further develop an antigen-specific acquired immunity. TLR signaling pathways are regulated by intracellular adaptor molecules, such as MyD88, TIRAP/Mal, between others that provide specificity of individual TLR- mediated signaling pathways. TLR-mediated activation of innate immunity is involved not only in host defense against pathogens but also in immune disorders. The involvement of TLR-mediated pathways in auto-immune and inflammatory diseases is described in this review article.     

Survival of Staphylococcus aureus inside neutrophils contributes to infection

Neutrophils have long been regarded as essential for host defense against Staphylococcus aureus infection. However, survival of the pathogen inside various cells, including phagocytes, has been proposed as a mechanism for persistence of this microorganism in certain infections. Therefore, we investigated whether survival of the pathogen inside polymorphonuclear neutrophils (PMN) contributes to the pathogenesis of S. aureus infection. Our data demonstrate that PMN isolated from the site of infection contain viable intracellular organisms and that these infected PMN are sufficient to establish infection in a naive animal. In addition, we show that limiting, but not ablating, PMN migration into the site of infection enhances host defense and that repletion of PMN, as well as promoting PMN influx by CXC chemokine administration, leads to decreased survival of the mice and an increased bacterial burden. Moreover, a global regulator mutant of S. aureus (sar-) that lacks the expression of several virulence factors is less able to survive and/or avoid clearance in the presence of PMN. These data suggest that the ability of S. aureus to exploit the inflammatory response of the host by surviving inside PMN is a virulence mechanism for this pathogen and that modulation of the inflammatory response is sufficient to significantly alter morbidity and mortality induced by S. aureus infection.     

Mapping of interactions between human macrophages and Staphylococcus aureus reveals an involvement of MAP kinase signaling in the host defense

Staphylococcus aureus is a dangerous opportunistic human pathogen that causes serious invasive diseases when it reaches the bloodstream. Recent studies have shown that S. aureus is highly resistant to killing by professional phagocytes and that such cells even provide a favorable environment for intracellular survival of S. aureus. Importantly, the reciprocal interactions between phagocytes and S. aureus have remained largely elusive. Here we have employed kinase profiling to define the nature and time resolution of the human THP-1 macrophage response toward S. aureus and proteomics to identify the response of S. aureus toward macrophages. The results of these studies reveal major macrophage signaling pathways triggered by S. aureus and proteomic signatures of the responses of S. aureus to macrophages. We also identify human proteins bound to S. aureus that have potential roles in bacterial killing and internalization. Most noticeably, our observations challenge the classical concept that macrophage responses are mainly mediated through Toll-like receptor 2 and NF-κB signaling and highlight the important role of the stress-activated MAP kinase signaling in orchestrating the host defense.     

A putative helical cytokine functioning in innate immune signalling in Drosophila melanogaster

In invertebrates and vertebrates, innate immunity is considered the first line of defense mechanism against non-self material. In vertebrates, cytokines play a critical role in innate immune signalling. To date, however, the existence of genes encoding for invertebrate helical cytokines has been anticipated, but never demonstrated. Here, we report the first structural and functional evidence of a gene encoding for a putative helical cytokine in Drosophila melanogaster. Functional experiments demonstrate that its expression, as well as that of the antimicrobial factors defensin and cecropin A1, is significantly increased after immune stimulation. These observations suggest the involvement of helical cytokines in the innate immune response of invertebrates.     

A putative helical cytokine functioning in innate immune signalling in Drosophila melanogaster

In invertebrates and vertebrates, innate immunity is considered the first line of defense mechanism against non-self material. In vertebrates, cytokines play a critical role in innate immune signalling. To date, however, the existence of genes encoding for invertebrate helical cytokines has been anticipated, but never demonstrated. Here, we report the first structural and functional evidence of a gene encoding for a putative helical cytokine in Drosophila melanogaster. Functional experiments demonstrate that its expression, as well as that of the antimicrobial factors defensin and cecropin A1, is significantly increased after immune stimulation. These observations suggest the involvement of helical cytokines in the innate immune response of invertebrates.     

Exploring the applications of invertebrate host-pathogen models for in vivo biofilm infections

In the natural environment, microorganisms exist together in self-produced polymeric matrix biofilms. Often, several species, which can belong to both bacterial and fungal kingdoms, coexist and interact in ways which are not completely understood. Biofilm infections have become prevalent largely in medical settings because of the increasing use of indwelling medical devices such as catheters or prosthetics. These infections are resistant to common antimicrobial therapies because of the inherent nature of their structure. In terms of infectious biofilms, it is important to understand the microbe-microbe interactions and how the host immune system reacts in order to discover therapeutic targets. Currently, single infection immune response studies are thriving with the use of invertebrate models. This review highlights the advances in single microbial-host immune response as well as the promising aspects of polymicrobial biofilm study in five invertebrate models: Lemna minor (duckweed), Arabidopsis thaliana (thale cress), Dictyostelium discoideum (slime mold), Drosophila melanogaster (common fruit fly), and Caenorhabditis elegans (roundworm).     

Characterization of cell membrane parameters of clinical isolates of Staphylococcus aureus with varied susceptibility to alpha-melanocyte stimulating hormone

Staphylococcus aureus (S. aureus), a major human pathogen of hospital and community acquired infections, is becoming resistant to almost all commercially available antibiotics. This has prompted development of antimicrobial peptides as therapeutic options. Alpha melanocyte stimulating hormone (α-MSH) is one such peptide known to possess antimicrobial properties. In the present study, we analyzed the antimicrobial activity of α-MSH against 75 clinical strains of S. aureus including both methicillin susceptible S. aureus (MSSA) and methicillin resistant S. aureus (MRSA) strains. Results of our previous study showed that membrane damage is the major mechanism of staphylocidal activity of α-MSH. In this context, we compared the various bacterial membrane parameters, viz., membrane fluidity, lipid composition, and surface charge of a few selected MSSA and MRSA strains that showed variable susceptibility to the melanocortin peptide. Our results showed that α-MSH killed both type of strains efficiently (≥70% killing in 84% clinical strains after exposure with 6μM of α-MSH for 1h). It was observed that compared to the α-MSH-susceptible strains, the α-MSH-non-susceptible strains had a different membrane order and phospholipid pattern. There was no consistent pattern of cell surface charge to distinguish α-MSH-susceptible strain from a non-susceptible strain. In conclusion, α-MSH possessed potential staphylocidal activity for both against MSSA and MRSA strains. S. aureus strains not susceptible to the peptide exhibited a rigid membrane and a higher amount of the cationic phospholipid as compared to the α-MSH-susceptible strains.     

Autophagy during Mycobacterium tuberculosis infection and implications for future tuberculosis medications

Autophagy is a cellular homeostasis mechanism to eliminate unwanted or excessive organelles, or for the turnover of long-life cytosolic macromolecules. During Mycobacterium tuberculosis infection, autophagy represents not only an antimicrobial mechanism for the clearance of the intracellular pathogen, but also prevents excessive inflammation, avoiding the adverse effects on host. Here we focus on the anti-tuberculosis autophagy and signal pathways involved, and attempt to depict an integrative map of the interaction between autophagy and cytokine, ROS production, vitamin D, and inflammatory response. Novel autophagy-based therapy is also summarized. This integrative insight might add some novel thoughts for better tuberculosis medications.     

Invertebrate models for biomedical research, testing, and education

Invertebrate animals have been used as medicinals for 4,000 years and have served as models for research and teaching since the late 1800s. Interest in invertebrate models has increased over the past several decades as the research community has responded to public concerns about the use of vertebrate animals in research. As a result, invertebrates are being evaluated and recognized as models for many diseases and conditions. Their use has led to discoveries in almost every area of biology and medicine--from embryonic development to aging processes. Species range from terrestrial invertebrates such as nematodes and insects to freshwater and marine life including planarians, crustaceans, molluscs, and many others. The most often used models are the fruit fly Drosophila melanogaster and the minuscule nematode Caenorhabditis elegans. Topics in this article are categorized by biologic system, process, or disease with discussion of associated invertebrate models. Sections on bioactive products discovered from invertebrates follow the models section, and the article concludes with uses of invertebrates in teaching. The models reviewed can serve as references for scientists, researchers, veterinarians, institutional animal care and use committees (IACUCs), and others interested in alternatives to vertebrate animals.     

The pursuit of cryptococcal pathogenesis: heterologous hosts and the study of cryptococcal host-pathogen interactions

Analysis of the molecular mechanisms by which a pathogen interacts with the human host is most commonly performed using a mammalian model of infection. However, several virulence-related genes previously shown to be involved in mammalian infection with Cryptococcus neoformans have also been shown to play a role in the interaction of these pathogens with invertebrates, such as Acanthamoeba castellanii, Caenorhabditis elegans, Dictyostelium discoideum, Drosophila melanogaster and Galleria mellonella. The study of host-pathogen interactions using these model hosts has allowed rapid screening of mutant libraries and can be used for the study of evolutionarily preserved aspects of microbial virulence and host response.     

Staphylococcal resistance to antimicrobial peptides of mammalian and bacterial origin

Antimicrobial host defense peptides, such as defensins, protegrins, and platelet microbicidal proteins are deployed by mammalian skin, epithelia, phagocytes, and platelets in response to Staphylococcus aureus infection. In addition, staphylococcal products with similar structures and activities, called bacteriocins, inhibit competing microorganisms. Staphylococci have developed resistance mechanisms, which are either highly specific for certain host defense peptides or bacteriocins or which broadly protect against a range of cationic antimicrobial peptides. Experimental infection models can be used to study the molecular mechanisms of antimicrobial peptides, the peptide resistance strategies of S. aureus, and the therapeutic potential of peptides in staphylococcal diseases.