Wall teichoic acids of Staphylococcus aureus limit recognition by the drosophila peptidoglycan recognition protein-SA to promote pathogenicity

MprF-mediated immune evasion is necessary for Lactiplantibacillus plantarum resilience in the Drosophila gut during inflammation

Multiple peptide resistance factor (MprF) confers resistance to cationic antimicrobial peptides (AMPs) in several pathogens, thereby enabling evasion of the host immune response. The role of MprF in commensals remains, however, uncharacterized. To close this knowledge gap, we used a common gut commensal of animals, Lactiplantibacillus plantarum, and its natural host, the fruit fly Drosophila melanogaster, as an experimental model to investigate the role of MprF in commensal-host interactions. The L. plantarum ΔmprF mutant that we generated exhibited deficiency in the synthesis of lysyl-phosphatidylglycerol (Lys-PG), resulting in increased negative cell surface charge and increased susceptibility to AMPs. Susceptibility to AMPs had no effect on ΔmprF mutant's ability to colonize guts of uninfected flies. However, we observed significantly reduced abundance of the ΔmprF mutant after infection-induced inflammation in the guts of wild-type flies but not of flies lacking AMPs. Additionally, we found that the ΔmprF mutant compared to wild-type L. plantarum induces a stronger intestinal immune response in flies due to the increased release of immunostimulatory peptidoglycan fragments, indicating an important role of MprF in promoting host tolerance to commensals. Our further analysis suggests that MprF-mediated lipoteichoic acid modifications are involved in host immunomodulation. Overall, our results demonstrate that MprF, besides its well-characterized role in pathogen immune evasion and virulence, is also an important commensal resilience factor.

Drosophila melanogaster as an organism model for studying cystic fibrosis and its major associated microbial infections

Cystic fibrosis (CF) is a human genetic disease caused by mutations in the cystic fibrosis transmembrane conductance regulator gene that encodes a chloride channel. The most severe clinical manifestation is associated with chronic pulmonary infections by pathogenic and opportunistic microbes. Drosophila melanogaster has become the invertebrate model of choice for modeling microbial infections and studying the induced innate immune response. Here, we review its contribution to the understanding of infections with six major pathogens associated with CF (Staphylococcus aureus, Pseudomonas aeruginosa, Burkholderia cepacia, Mycobacterium abscessus, Streptococcus pneumoniae, and Aspergillus fumigatus) together with the perspectives opened by the recent availability of two CF models in this model organism.

Controlling biofilm and virulence properties of Gram-positive bacteria by targeting wall teichoic acid and lipoteichoic acid

Wall teichoic acid (WTA) and lipoteichoic acid (LTA) are structural components of Gram-positive bacteria's peptidoglycan and cell membrane, which are mostly anionic glycopolymers. WTA confers numerous physiological, virulence, and pathogenic features to bacterial pathogens. It controls cell shape, cell division, and the localisation of autolytic enzymes and ion homeostasis. In the context of virulence and pathogenicity, it aids bacterial cell attachment and colonisation and protects against the host defence system and antibiotics. Having such a broad function in pathogenic bacteria's lifecycle, WTA/LTA become one of the potential targets for antibacterial agents to reduce bacterial infection in the host. The number of reports for targeting the WTA/LTA pathway has risen, mostly by focusing on three distinct targets: antivirulence targets, β-lactam potentiator targets, and essential targets. The current review looked at the role of WTA/LTA in biofilm development and virulence in a range of Gram-positive pathogenic bacteria. Furthermore, alternate strategies, such as the application of natural and synthetic compounds that target the WTA/LTA pathway, have been thoroughly discussed. Moreover, the application of nanomaterials and a combination of drugs have also been discussed as a viable method for targeting the WTA/LTA in numerous Gram-positive bacteria. In addition, a future perspective for controlling bacterial infection by targeting the WTA/LTA is proposed.

Structural and functional analysis of the D-alanyl carrier protein ligase DltA from Staphylococcus aureus Mu50

D-Alanylation of the teichoic acids of the Gram-positive bacterial cell wall plays crucial roles in bacterial physiology and virulence. Deprivation of D-alanine from the teichoic acids of Staphylococcus aureus impairs biofilm and colony formation, induces autolysis and ultimately renders methicillin-resistant S. aureus highly susceptible to antimicrobial agents and host defense peptides. Hence, the D-alanylation pathway has emerged as a promising antibacterial target against drug-resistant S. aureus. D-Alanylation of teichoic acids is mediated via the action of four proteins encoded by the dlt operon, DltABCD, all four of which are essential for the process. In order to develop novel antimicrobial agents against S. aureus, the D-alanyl carrier protein ligase DltA, which is the first protein in the D-alanylation pathway, was focused on. Here, the crystal structure of DltA from the methicillin-resistant S. aureus strain Mu50 is presented, which reveals the unique molecular details of the catalytic center and the role of the P-loop. Kinetic analysis shows that the enantioselectivity of S. aureus DltA is much higher than that of DltA from other species. In the presence of DltC, the enzymatic activity of DltA is increased by an order of magnitude, suggesting a new exploitable binding pocket. This discovery may pave the way for a new generation of treatments for drug-resistant S. aureus.

Horizontal transfer and evolution of wall teichoic acid gene cassettes in Bacillus subtilis

Background: Wall teichoic acid (WTA) genes are essential for production of cell walls in gram-positive bacteria and necessary for survival and variability in the cassette has led to recent antibiotic resistance acquisition in pathogenic bacteria. 

Methods: Using a pan-genome approach, we examined the evolutionary history of WTA genes in  Bacillus subtilis ssp.  subtilis. 

Results: Our analysis reveals an interesting pattern of evolution from the type-strain WTA gene cassette possibly resulting from horizontal acquisition from organisms with similar gene sequences. The WTA cassettes have a high level of variation which may be due to one or more independent horizontal transfer events during the evolution of  Bacillus subtilis ssp.  subtilis. This swapping of entire WTA cassettes and smaller regions within the WTA cassettes is an unusual feature in the evolution of the  Bacillus subtilis genome and highlights the importance of horizontal transfer of gene cassettes through homologous recombination within  B. subtilis or other bacterial species. 

Conclusions: Reduced sequence conservation of these WTA cassettes may indicate a modified function like the previously documented WTA ribitol/glycerol variation. An improved understanding of high-frequency recombination of gene cassettes has ramifications for synthetic biology and the use of  B. subtilis in industry.

Drosophila versus Mycobacteria: A model for mycobacterial host-pathogen interactions

Animal models have played an essential role in understanding the host-pathogen interactions of pathogenic mycobacteria, including the Mycobacterium tuberculosis and emerging nontuberculous mycobacteria (NTM) species such as M. avium and M. abscessus. Drosophila melanogaster has become a well-established model for the study of innate immunity and is increasingly being used as a tool to study host-pathogen interactions, in part due to its genetic tractability. The use of D. melanogaster has led to greater understanding of the role of the innate immune system in response to mycobacterial infection, including in vitro RNAi screens and in vivo studies. These studies have identified processes and host factors involved in mycobacterial infection, such as those required for cellular entry, those required to control or resist non-pathogenic mycobacteria, or factors that become dysregulated as a result of mycobacterial infection. Developments in genetic tools for manipulating mycobacterial genomes will allow for more detailed studies into how specific host and pathogen factors interact with one another by using D. melanogaster; however, the full potential of this model has not yet been reached. Here we provide an overview of how D. melanogaster has been used to study mycobacterial infection and discuss the current gaps in our understanding.

The fliK Gene Is Required for the Resistance of Bacillus thuringiensis to Antimicrobial Peptides and Virulence in Drosophila melanogaster

Antimicrobial peptides (AMPs) are essential effectors of the host innate immune system and they represent promising molecules for the treatment of multidrug resistant microbes. A better understanding of microbial resistance to these defense peptides is thus prerequisite for the control of infectious diseases. Here, using a random mutagenesis approach, we identify the fliK gene, encoding an internal molecular ruler that controls flagella hook length, as an essential element for Bacillus thuringiensis resistance to AMPs in Drosophila. Unlike its parental strain, that is highly virulent to both wild-type and AMPs deficient mutant flies, the fliK deletion mutant is only lethal to the latter's. In agreement with its conserved function, the fliK mutant is non-flagellated and exhibits highly compromised motility. However, comparative analysis of the fliK mutant phenotype to that of a fla mutant, in which the genes encoding flagella proteins are interrupted, indicate that B. thuringiensis FliK-dependent resistance to AMPs is independent of flagella assembly. As a whole, our results identify FliK as an essential determinant for B. thuringiensis virulence in Drosophila and provide new insights on the mechanisms underlying bacteria resistance to AMPs.

Wall teichoic acids: physiology and applications

Wall teichoic acids (WTAs) are charged glycopolymers containing phosphodiester-linked polyol units and represent one of the major components of Gram-positive cell envelope. WTAs have important physiological functions in cell division, gene transfer, surface adhesion, drug resistance and biofilm formation, and are critical virulence factors and vital determinants in mediating cell interaction with and tolerance to environmental factors. Here, we first briefly introduce WTA structure, biosynthesis and its regulation, and then summarize in detail four major physiological roles played by WTAs, i.e. WTA-mediated resistance to antimicrobials, virulence to mammalian cells, interaction with bacteriolytic enzymes and regulation of cell metabolism. We also review the applications of WTAs in these fields that are closely related to the human society, including antibacterial drug discovery targeting WTA biosynthesis, development of vaccines and antibodies regarding WTA-mediated pathogenicity, specific and sensitive detection of pathogens in food using WTAs as a surface epitope and regulation of WTA-related pathways for efficient microbial production of useful compounds. We also point out major problems remaining in these fields, and discuss some possible directions in the future exploration of WTA physiology and applications.

Accessibility to Peptidoglycan Is Important for the Recognition of Gram-Positive Bacteria in Drosophila

In Drosophila, it is thought that peptidoglycan recognition proteins (PGRPs) SA and LC structurally discriminate between bacterial peptidoglycans with lysine (Lys) or diaminopimelic (DAP) acid, respectively, thus inducing differential antimicrobial transcription response. Here, we find that accessibility to PG at the cell wall plays a central role in immunity to infection. When wall teichoic acids (WTAs) are genetically removed from S. aureus (Lys type) and Bacillus subtilis (DAP type), thus increasing accessibility, the binding of both PGRPs to either bacterium is increased. PGRP-SA and -LC double mutant flies are more susceptible to infection with both WTA-less bacteria. In addition, WTA-less bacteria grow better in PGRP-SA/-LC double mutant flies. Finally, infection with WTA-less bacteria abolishes any differential activation of downstream antimicrobial transcription. Our results indicate that accessibility to cell wall PG is a major factor in PGRP-mediated immunity and may be the cause for discrimination between classes of pathogens.

Drosophila as a Model Organism in Host-Pathogen Interaction Studies

Owing to the genetic similarities and conserved pathways between a fruit fly and mammals, the use of the Drosophila model as a platform to unveil novel mechanisms of infection and disease progression has been justified and widely instigated. Gaining proper insight into host-pathogen interactions and identifying chief factors involved in host defense and pathogen virulence in Drosophila serves as a foundation to establish novel strategies for infectious disease prevention and control in higher organisms, including humans.

A comparative genomics approach for identifying host-range determinants in Streptococcus thermophilus bacteriophages

Comparative genomics has proven useful in exploring the biodiversity of phages and understanding phage-host interactions. This knowledge is particularly useful for phages infecting Streptococcus thermophilus, as they constitute a constant threat during dairy fermentations. Here, we explore the genetic diversity of S. thermophilus phages to identify genetic determinants with a signature for host specificity, which could be linked to the bacterial receptor genotype. A comparative genomic analysis was performed on 142 S. thermophilus phage genomes, 55 of which were sequenced in this study. Effectively, 94 phages were assigned to the group cos (DT1), 36 to the group pac (O1205), six to the group 5093, and six to the group 987. The core genome-based phylogeny of phages from the two dominating groups and their receptor binding protein (RBP) phylogeny corresponded to the phage host-range. A role of RBP in host recognition was confirmed by constructing a fluorescent derivative of the RBP of phage CHPC951, followed by studying the binding of the protein to the host strain. Furthermore, the RBP phylogeny of the cos group was found to correlate with the host genotype of the exocellular polysaccharide-encoding operon. These findings provide novel insights towards developing strategies to combat phage infections in dairies.

Methods for the study of innate immunity in Drosophila melanogaster

From flies to humans, many components of the innate immune system have been conserved during metazoan evolution. This foundational observation has allowed us to develop Drosophila melanogaster, the fruit fly, into a powerful model to study innate immunity in animals. Thanks to an ever-growing arsenal of genetic tools, an easily manipulated genome, and its winning disposition, Drosophila is now employed to study not only basic molecular mechanisms of pathogen recognition and immune signaling, but also the nature of physiological responses activated in the host by microbial challenge and how dysregulation of these processes contributes to disease. Here, we present a collection of methods and protocols to challenge the fly with an assortment of microbes, both systemically and orally, and assess its humoral, cellular, and epithelial response to infection. Our review covers techniques for measuring the reaction to microbial infection both qualitatively and quantitatively. Specifically, we describe survival, bacterial load, BLUD (a measure of disease tolerance), phagocytosis, melanization, clotting, and ROS production assays, as well as efficient protocols to collect hemolymph and measure immune gene expression. We also offer an updated catalog of online resources and a collection of popular reporter lines and mutants to facilitate research efforts. This article is categorized under: Technologies > Analysis of Cell, Tissue, and Animal Phenotypes.

Revisiting Anti-tuberculosis Therapeutic Strategies That Target the Peptidoglycan Structure and Synthesis

Tuberculosis (TB), which is caused by Mycobacterium tuberculosis (Mtb), is one of the leading cause of death by an infectious diseases. The biosynthesis of the mycobacterial cell wall (CW) is an area of increasing research significance, as numerous antibiotics used to treat TB target biosynthesis pathways of essential CW components. The main feature of the mycobacterial cell envelope is an intricate structure, the mycolyl-arabinogalactan-peptidoglycan (mAGP) complex responsible for its innate resistance to many commonly used antibiotics and involved in virulence. A hallmark of mAGP is its unusual peptidoglycan (PG) layer, which has subtleties that play a key role in virulence by enabling pathogenic species to survive inside the host and resist antibiotic pressure. This dynamic and essential structure is not a target of currently used therapeutics as Mtb is considered naturally resistant to most β-lactam antibiotics due to a highly active β-lactamase (BlaC) that efficiently hydrolyses many β-lactam drugs to render them ineffective. The emergence of multidrug- and extensive drug-resistant strains to the available antibiotics has become a serious health threat, places an immense burden on health care systems, and poses particular therapeutic challenges. Therefore, it is crucial to explore additional Mtb vulnerabilities that can be used to combat TB. Remodeling PG enzymes that catalyze biosynthesis and recycling of the PG are essential to the viability of Mtb and are therefore attractive targets for novel antibiotics research. This article reviews PG as an alternative antibiotic target for TB treatment, how Mtb has developed resistance to currently available antibiotics directed to PG biosynthesis, and the potential of targeting this essential structure to tackle TB by attacking alternative enzymatic activities involved in Mtb PG modifications and metabolism.

Unprotonated Short-Chain Alkylamines Inhibit Staphylolytic Activity of Lysostaphin in a Wall Teichoic Acid-Dependent Manner

Lysostaphin (Lst) is a potent bacteriolytic enzyme that kills Staphylococcus aureus, a common bacterial pathogen of humans and animals. With high activity against both planktonic cells and biofilms, Lst has the potential to be used in industrial products, such as commercial cleansers, for decontamination. However, Lst is inhibited in the presence of monoethanolamine (MEA), a chemical widely used in cleaning solutions and pharmaceuticals, and the underlying mechanism of inhibition remains unknown. In this study, we examined the cell binding and killing capabilities of Lst against S. aureus ATCC 6538 in buffered salt solution with MEA at different pH values (7.5 to 10.5) and discovered that only the unprotonated form of MEA inhibited Lst binding to the cell surface, leading to low Lst activity, despite retention of its secondary structure. This reduced enzyme activity could be largely recovered via a reduction in wall teichoic acid (WTA) biosynthesis through tunicamycin treatment, indicating that the suppression of Lst activity was dependent on the presence and amount of WTA. We propose that the decreased cell binding and killing capabilities of Lst are associated with the influence of uncharged MEA on the conformation of WTA. A similar effect was confirmed with other short-chain alkylamines. This study offers new insight into the impact of short-chain alkylamines on both Lst and WTA structure and function and provides guidance for the application of Lst in harsh environments.IMPORTANCE Lysostaphin (Lst) effectively and selectively kills Staphylococcus aureus, the bacterial culprit of many hospital- and community-acquired skin and respiratory infections and food poisoning. Lst has been investigated in animal models and clinical trials, industrial formulations, and environmental settings. Here, we studied the mechanistic basis of the inhibitory effect of alkylamines, such as monoethanolamine (MEA), a widely used chemical in commercial detergents, on Lst activity, for the potential incorporation of Lst in disinfectant solutions. We have found that protonated MEA has little influence on Lst activity, while unprotonated MEA prevents Lst from binding to S. aureus cells and hence dramatically decreases the enzyme's bacteriolytic efficacy. Following partial removal of the wall teichoic acid, an important component of the bacterial cell envelope, the inhibitory effect of unprotonated MEA on Lst is reduced. This phenomenon can be extended to other short-chain alkylamines. This mechanistic report of the impact of alkylamines on Lst functionality will help guide future applications of Lst in disinfection and decontamination of health-related commercial products.

The Toll pathway underlies host sexual dimorphism in resistance to both Gram-negative and Gram-positive bacteria in mated Drosophila

BACKGROUND

Host sexual dimorphism is being increasingly recognized to generate strong differences in the outcome of infectious disease, but the mechanisms underlying immunological differences between males and females remain poorly characterized. Here, we used Drosophila melanogaster to assess and dissect sexual dimorphism in the innate response to systemic bacterial infection.

RESULTS

We demonstrated sexual dimorphism in susceptibility to infection by a broad spectrum of Gram-positive and Gram-negative bacteria. We found that both virgin and mated females are more susceptible than mated males to most, but not all, infections. We investigated in more detail the lower resistance of females to infection with Providencia rettgeri, a Gram-negative bacterium that naturally infects D. melanogaster. We found that females have a higher number of phagocytes than males and that ablation of hemocytes does not eliminate the dimorphism in resistance to P. rettgeri, so the observed dimorphism does not stem from differences in the cellular response. The Imd pathway is critical for the production of antimicrobial peptides in response to Gram-negative bacteria, but mutants for Imd signaling continued to exhibit dimorphism even though both sexes showed strongly reduced resistance. Instead, we found that the Toll pathway is responsible for the dimorphism in resistance. The Toll pathway is dimorphic in genome-wide constitutive gene expression and in induced response to infection. Toll signaling is dimorphic in both constitutive signaling and in induced activation in response to P. rettgeri infection. The dimorphism in pathway activation can be specifically attributed to Persephone-mediated immune stimulation, by which the Toll pathway is triggered in response to pathogen-derived virulence factors. We additionally found that, in absence of Toll signaling, males become more susceptible than females to the Gram-positive Enterococcus faecalis. This reversal in susceptibility between male and female Toll pathway mutants compared to wildtype hosts highlights the key role of the Toll pathway in D. melanogaster sexual dimorphism in resistance to infection.

CONCLUSION

Altogether, our data demonstrate that Toll pathway activity differs between male and female D. melanogaster in response to bacterial infection, thus identifying innate immune signaling as a determinant of sexual immune dimorphism.

Wall teichoic acid is an essential component of Staphylococcus aureus for the induction of human dendritic cell maturation

Staphylococcus aureus is a Gram-positive pathogen that can cause chronic skin inflammation, pneumonia, and septic shock. The immunomodulatory functions of wall teichoic acid (WTA), a glycopolymer abundantly expressed on the Gram-positive bacterial cell wall, are poorly understood. Here, we investigated the role of WTA in the phenotypic and functional activation of human monocyte-derived dendritic cells (DCs) treated with ethanol-killed S. aureus. WTA-deficient S. aureus mutant (ΔtagO) exhibited attenuated binding and internalization to DCs compared to the wild-type. ΔtagO induced lower expression of maturation markers on and cytokines in DCs than the wild-type S. aureus. Furthermore, autologous human peripheral blood mononuclear cells cocultured with ΔtagO-treated DCs exhibited a marked reduction in T cell proliferative activity, the expression of activation markers, and the production of cytokines compared to the wild-type S. aureus-stimulated DCs. Collectively, these results suggest that WTA is an important cell wall component of S. aureus for the induction of DC maturation and activation.

Wall Teichoic Acids Are Involved in the Medium-Induced Loss of Function of the Autolysin CD11 against Clostridium difficile

Bacterial lysins are potent antibacterial enzymes with potential applications in the treatment of bacterial infections. Some lysins lose activity in the growth media of target bacteria, and the underlying mechanism remains unclear. Here we use CD11, an autolysin of Clostridium difficile, as a model lysin to demonstrate that the inability of this enzyme to kill C. difficile in growth medium is not associated with inhibition of the enzyme activity by medium, or the modification of the cell wall peptidoglycan. Rather, wall teichoic acids (WTAs) appear to prevent the enzyme from binding to the cells and cleaving the cell wall peptidoglycan. By partially blocking the biosynthetic pathway of WTAs with tunicamycin, cell binding improved and the lytic efficacy of CD11 was significantly enhanced. This is the first report of the mechanism of lysin inactivation in growth medium, and provides insights into understanding the behavior of lysins in complex environments, including the gastrointestinal tract.

The Drosophila CD36 Homologue croquemort Is Required to Maintain Immune and Gut Homeostasis during Development and Aging

Phagocytosis is an ancient mechanism central to both tissue homeostasis and immune defense. Both the identity of the receptors that mediate bacterial phagocytosis and the nature of the interactions between phagocytosis and other defense mechanisms remain elusive. Here, we report that Croquemort (Crq), a Drosophila member of the CD36 family of scavenger receptors, is required for microbial phagocytosis and efficient bacterial clearance. Flies mutant for crq are susceptible to environmental microbes during development and succumb to a variety of microbial infections as adults. Crq acts parallel to the Toll and Imd pathways to eliminate bacteria via phagocytosis. crq mutant flies exhibit enhanced and prolonged immune and cytokine induction accompanied by premature gut dysplasia and decreased lifespan. The chronic state of immune activation in crq mutant flies is further regulated by negative regulators of the Imd pathway. Altogether, our data demonstrate that Crq plays a key role in maintaining immune and organismal homeostasis.

The Lantibiotic NAI-107 Efficiently Rescues Drosophila melanogaster from Infection with Methicillin-Resistant Staphylococcus aureus USA300

We used the fruit fly Drosophila melanogaster as a cost-effective in vivo model to evaluate the efficacy of novel antibacterial peptides and peptoids for treatment of methicillin-resistant Staphylococcus aureus (MRSA) infections. A panel of peptides with known antibacterial activity in vitro and/or in vivo was tested in Drosophila Although most peptides and peptoids that were effective in vitro failed to rescue lethal effects of S. aureus infections in vivo, we found that two lantibiotics, nisin and NAI-107, rescued adult flies from fatal infections. Furthermore, NAI-107 rescued mortality of infection with the MRSA strain USA300 with an efficacy equivalent to that of vancomycin, a widely applied antibiotic for the treatment of serious MRSA infections. These results establish Drosophila as a useful model for in vivo drug evaluation of antibacterial peptides.

Wall teichoic acids prevent antibody binding to epitopes within the cell wall of Staphylococcus aureus

Staphylococcus aureus is a Gram-positive bacterial pathogen that produces a range of infections including cellulitis, pneumonia, and septicemia. The principle mechanism in antistaphylococcal host defense is opsonization with antibodies and complement proteins, followed by phagocytic clearance. Here we use a previously developed technique for installing chemical epitopes in the peptidoglycan cell wall to show that surface glycopolymers known as wall teichoic acids conceal cell wall epitopes, preventing their recognition and opsonization by antibodies. Thus, our results reveal a previously unrecognized immunoevasive role for wall teichoic acids in S. aureus: repulsion of peptidoglycan-targeted antibodies.

Analysis of Cell Wall Teichoic Acids in Staphylococcus aureus

Most bacterial cells are surrounded by a surface composed mainly of peptidoglycan (PGN), a glycopolymer responsible for ensuring the bacterial shape and a telltale molecule that betrays the presence of bacteria to the host immune system. In Staphylococcus aureus, as in most gram-positive bacteria, peptidoglycan is concealed by covalently linked molecules of wall teichoic acids (WTA)-phosphate rich molecules made of glycerol and ribitol phosphates which may be tailored by different amino acids and sugars.In order to analyze and compare the composition of WTA produced by different S. aureus strains, we describe methods to: (1) quantify the total amount of WTA present at the bacterial cell surface, through the determination of the inorganic phosphate present in phosphodiester linkages of WTA; (2) identify which sugar constituents are present in the assembled WTA molecules, by detecting the monosaccharides, released by acid hydrolysis, through an high-performance anion exchange chromatography analysis coupled with pulsed amperometric detection (HPAEC-PAD) and (3) compare the polymerization degree of WTA found at the cell surface of different S. aureus strains, through their different migration in a polyacrylamide gel electrophoresis (PAGE).

18 GHz electromagnetic field induces permeability of Gram-positive cocci

The effect of electromagnetic field (EMF) exposures at the microwave (MW) frequency of 18 GHz, on four cocci, Planococcus maritimus KMM 3738, Staphylococcus aureus CIP 65.8(T), S. aureus ATCC 25923 and S. epidermidis ATCC 14990(T), was investigated. We demonstrate that exposing the bacteria to an EMF induced permeability in the bacterial membranes of all strains studied, as confirmed directly by transmission electron microscopy (TEM), and indirectly via the propidium iodide assay and the uptake of silica nanospheres. The cells remained permeable for at least nine minutes after EMF exposure. It was shown that all strains internalized 23.5 nm nanospheres, whereas the internalization of the 46.3 nm nanospheres differed amongst the bacterial strains (S. epidermidis ATCC 14990(T) ~  0%; Staphylococcus aureus CIP 65.8(T) S. aureus ATCC 25923, ~40%; Planococcus maritimus KMM 3738, ~ 80%). Cell viability experiments indicated that up to 84% of the cells exposed to the EMF remained viable. The morphology of the bacterial cells was not altered, as inferred from the scanning electron micrographs, however traces of leaked cytosolic fluids from the EMF exposed cells could be detected. EMF-induced permeabilization may represent an innovative, alternative cell permeability technique for applications in biomedical engineering, cell drug delivery and gene therapy.

Staphylococcus aureus Survives with a Minimal Peptidoglycan Synthesis Machine but Sacrifices Virulence and Antibiotic Resistance

Many important cellular processes are performed by molecular machines, composed of multiple proteins that physically interact to execute biological functions. An example is the bacterial peptidoglycan (PG) synthesis machine, responsible for the synthesis of the main component of the cell wall and the target of many contemporary antibiotics. One approach for the identification of essential components of a cellular machine involves the determination of its minimal protein composition. Staphylococcus aureus is a Gram-positive pathogen, renowned for its resistance to many commonly used antibiotics and prevalence in hospitals. Its genome encodes a low number of proteins with PG synthesis activity (9 proteins), when compared to other model organisms, and is therefore a good model for the study of a minimal PG synthesis machine. We deleted seven of the nine genes encoding PG synthesis enzymes from the S. aureus genome without affecting normal growth or cell morphology, generating a strain capable of PG biosynthesis catalyzed only by two penicillin-binding proteins, PBP1 and the bi-functional PBP2. However, multiple PBPs are important in clinically relevant environments, as bacteria with a minimal PG synthesis machinery became highly susceptible to cell wall-targeting antibiotics, host lytic enzymes and displayed impaired virulence in a Drosophila infection model which is dependent on the presence of specific peptidoglycan receptor proteins, namely PGRP-SA. The fact that S. aureus can grow and divide with only two active PG synthesizing enzymes shows that most of these enzymes are redundant in vitro and identifies the minimal PG synthesis machinery of S. aureus. However a complex molecular machine is important in environments other than in vitro growth as the expendable PG synthesis enzymes play an important role in the pathogenicity and antibiotic resistance of S. aureus.

Peptidoglycan forms the stress-bearing sacculus that prevents lysis of bacteria due to turgor pressure. The integrity of peptidoglycan is therefore essential for bacterial survival and its synthesis is the target of many important antibiotics, such as penicillin. The final steps of peptidoglycan synthesis are catalyzed by penicillin-binding proteins, enzymes that are proposed to work in multi-enzyme complexes. We show that seven of the nine genes encoding peptidoglycan synthesis enzymes can be deleted from the Staphylococcus aureus genome without affecting normal growth and cell morphology in vitro, identifying the minimal peptidoglycan synthesis machinery of this organism. Identification of minimal machineries is key for synthetic biology efforts towards the design of systems with reduced complexity. However, the non-essential peptidoglycan synthetic proteins are important for survival of S. aureus in more challenging environments, such as in the presence of antibiotics that target cell wall synthesis or within the host, as shown by the inability of the mutant strain to establish a successful infection and kill Drosophila flies.

Recognition, survival and persistence of Staphylococcus aureus in the model host Tenebrio molitor

The degree of specificity of any given immune response to a parasite is governed by the complexity and variation of interactions between host and pathogen derived molecules. Here, we assess the extent to which recognition and immuno-resistance of cell wall mutants of the pathogen Staphylococcus aureus may contribute to establishment and maintenance of persistent infection in the model insect host, Tenebrio molitor. The cell surface of S. aureus is decorated with various molecules, including glycopolymers such as wall teichoic acid (WTA). WTA is covalently bound to peptidoglycan (PGN) and its absence has been associated with increased recognition of PGN by host receptors (PGRPs). WTA is also further modified by other molecules such as D-alanine (D-alanylation). Both the level of WTA expression and its D-alanylation were found to be important in the mediation of the host-parasite interaction in this model system. Specifically, WTA itself was seen to influence immune recognition, while D-alanylation of WTA was found to increase immuno-resistance and was associated with prolonged persistence of S. aureus in T. molitor. These results implicate WTA and its D-alanylation as important factors in the establishment and maintenance of persistent infection, affecting different critical junctions in the immune response; through potential evasion of recognition by PGRPs and resistance to humoral immune effectors during prolonged exposure to the immune system. This highlights a mechanism by which specificity in this host-parasite interaction may arise.

Cell separation in Vibrio cholerae is mediated by a single amidase whose action is modulated by two nonredundant activators

Synthesis and hydrolysis of septal peptidoglycan (PG) are critical processes at the conclusion of cell division that enable separation of daughter cells. Cleavage of septal PG is mediated by PG amidases, hydrolytic enzymes that release peptide side chains from the glycan strand. Most gammaproteobacteria, including Escherichia coli, encode several functionally redundant periplasmic amidases. However, members of the Vibrio genus, including the enteric pathogen Vibrio cholerae, encode only a single PG amidase, AmiB. Here, we show that V. cholerae AmiB is crucial for cell division and growth. Genetic and biochemical analyses indicated that AmiB is regulated by two activators, EnvC and NlpD, at least one of which is required for AmiB's localization to the cell division site. Localization of the activators (and thus of AmiB) is dependent upon the cell division protein FtsN. These factors mediate septal PG cleavage in E. coli as well; however, their precise roles vary between the two organisms in a number of ways. Notably, even though V. cholerae EnvC and NlpD appear to be functionally redundant under most growth conditions tested, NlpD is specifically required for intestinal colonization in the infant mouse model of cholera and for V. cholerae resistance against bile salts, perhaps due to environmental regulation of AmiB or its activators. Collectively, our findings reveal that although the cellular components that enable cleavage of septal PG appear to be generally conserved between E. coli and V. cholerae, they can be combined into diverse functional regulatory networks.

Increased survival of experimentally evolved antimicrobial peptide-resistant Staphylococcus aureus in an animal host

Antimicrobial peptides (AMPs) have been proposed as new class of antimicrobial drugs, following the increasing prevalence of bacteria resistant to antibiotics. Synthetic AMPs are functional analogues of highly evolutionarily conserved immune effectors in animals and plants, produced in response to microbial infection. Therefore, the proposed therapeutic use of AMPs bears the risk of 'arming the enemy': bacteria that evolve resistance to AMPs may be cross-resistant to immune effectors (AMPs) in their hosts. We used a panel of populations of Staphylococcus aureus that were experimentally selected for resistance to a suite of individual AMPs and antibiotics to investigate the 'arming the enemy' hypothesis. We tested whether the selected strains showed higher survival in an insect model (Tenebrio molitor) and cross-resistance against other antimicrobials in vitro. A population selected for resistance to the antimicrobial peptide iseganan showed increased in vivo survival, but was not more virulent. We suggest that increased survival of AMP-resistant bacteria almost certainly poses problems to immune-compromised hosts.

Subterfuge and sabotage: evasion of host innate defenses by invasive gram-positive bacterial pathogens

The development of a severe invasive bacterial infection in an otherwise healthy individual is one of the most striking and fascinating aspects of human medicine. A small cadre of gram-positive pathogens of the genera Streptococcus and Staphylococcus stand out for their unique invasive disease potential and sophisticated ability to counteract the multifaceted components of human innate defense. This review illustrates how these leading human disease agents evade host complement deposition and activation, impede phagocyte recruitment and activation, resist the microbicidal activities of host antimicrobial peptides and reactive oxygen species, escape neutrophil extracellular traps, and promote and accelerate phagocyte cell death through the action of pore-forming cytolysins. Understanding the molecular basis of bacterial innate immune resistance can open new avenues for therapeutic intervention geared to disabling specific virulence factors and resensitizing the pathogen to host innate immune clearance.

The medium is the message: interspecies and interkingdom signaling by peptidoglycan and related bacterial glycans

Peptidoglycan serves as a key structure of the bacterial cell by determining cell shape and providing resistance to internal turgor pressure. However, in addition to these essential and well-studied functions, bacterial signaling by peptidoglycan fragments, or muropeptides, has been demonstrated by recent work. Actively growing bacteria release muropeptides as a consequence of cell wall remodeling during elongation and division. Therefore, the presence of muropeptide synthesis is indicative of growth-promoting conditions and may serve as a broadly conserved signal for nongrowing cells to reinitiate growth. In addition, muropeptides serve as signals between bacteria and eukaryotic organisms during both pathogenic and symbiotic interactions. The increasingly appreciated role of the microbiota in metazoan organisms suggests that muropeptide signaling likely has important implications for homeostatic mammalian physiology.

Bacterial autolysins trim cell surface peptidoglycan to prevent detection by the Drosophila innate immune system

Bacteria have to avoid recognition by the host immune system in order to establish a successful infection. Peptidoglycan, the principal constituent of virtually all bacterial surfaces, is a specific molecular signature recognized by dedicated host receptors, present in animals and plants, which trigger an immune response. Here we report that autolysins from Gram-positive pathogenic bacteria, enzymes capable of hydrolyzing peptidoglycan, have a major role in concealing this inflammatory molecule from Drosophila peptidoglycan recognition proteins (PGRPs). We show that autolysins trim the outermost peptidoglycan fragments and that in their absence bacterial virulence is impaired, as PGRPs can directly recognize leftover peptidoglycan extending beyond the external layers of bacterial proteins and polysaccharides. The activity of autolysins is not restricted to the producer cells but can also alter the surface of neighboring bacteria, facilitating the survival of the entire population in the infected host.

DOI:http://dx.doi.org/10.7554/eLife.02277.001

While most bacteria are harmless, some can cause diseases as varied as food poisoning and meningitis, so our immune system has developed various ways of detecting and eliminating bacteria and other pathogens. Receptor proteins belonging to the immune system detect molecules that give away the presence of the bacteria and trigger an immune response targeted at the invading pathogen.

Peptidoglycan is one telltale molecule that betrays the presence of bacteria. Peptidoglycan is found in the bacterial cell wall, and for many years it was assumed that the immune system detected stray fragments of peptidoglycan that were accidentally shed by the bacteria. However, it was later shown that the immune system could, under certain conditions, detect peptidoglycan when it is still part of the cell wall. This raised an interesting question: do bacteria use other methods to stop peptidoglycan being detected by the immune system?

Now, Atilano, Pereira et al. have found that enzymes called autolysins can conceal bacteria from the receptor proteins that detect peptidoglycan. These enzymes are needed to break the bonds within the peptidoglycan present in the rigid bacterial cell wall to allow the bacteria to grow and divide. ‘Knocking out’ the genes for autolysins allowed the receptor proteins from the fruit fly, Drosophila, to bind to the bacteria; however, the mutant bacteria were able to evade the immune system after they had been treated with the purified enzymes.

Atilano, Pereira et al. suggest that the autolysins trim the exposed ends of the peptidoglycan molecules on the surface of the cell wall, which could otherwise be detected by the host. The experiments also show that bacterial pathogens—including a strain of MRSA—with mutations that knock out autolysin activity trigger a stronger immune response in fruit flies, and are therefore less able to infect this host. Autolysins also help to conceal Streptococcus pneumoniae—a bacterial pathogen that is a common cause of pneumonia and infant deaths in developing countries—from detection by fruit flies.

The findings of Atilano, Pereira et al. highlight how bacteria employ a number of ways to evade detection. If similar behavior is observed when bacteria infect humans, autolysins could represent a new drug target for the treatment of bacterial diseases.

DOI:http://dx.doi.org/10.7554/eLife.02277.002

Human pathogenic bacteria, fungi, and viruses in Drosophila: disease modeling, lessons, and shortcomings

Drosophila has been the invertebrate model organism of choice for the study of innate immune responses during the past few decades. Many Drosophila-microbe interaction studies have helped to define innate immunity pathways, and significant effort has been made lately to decipher mechanisms of microbial pathogenesis. Here we catalog 68 bacterial, fungal, and viral species studied in flies, 43 of which are relevant to human health. We discuss studies of human pathogens in flies revealing not only the elicitation and avoidance of immune response but also mechanisms of tolerance, host tissue homeostasis, regeneration, and predisposition to cancer. Prominent among those is the emerging pattern of intestinal regeneration as a defense response induced by pathogenic and innocuous bacteria. Immunopathology mechanisms and many microbial virulence factors have been elucidated, but their relevance to human health conventionally necessitates validation in mammalian models of infection.

Teichoic acid biosynthesis as an antibiotic target

The relentless spread of antibiotic-resistant pathogens makes it imperative to develop new chemotherapeutic strategies to overcome infection. The bacterial cell wall has served as a rich source for both validated and unexploited pathways that are essential for virulence and survival. Lipoteichoic acids (LTAs) and wall teichoic acids (WTAs) are cell wall polymers that play fundamental roles in Gram-positive bacterial physiology and pathogenesis, and both have been proposed as novel antibacterial targets. Here we describe recent progress toward the discovery of teichoic acid biosynthesis inhibitors and their potential as antibiotics to combat Staphylococcus aureus infections.

Drosophila as a model system to unravel the layers of innate immunity to infection

Innate immunity relies entirely upon germ-line encoded receptors, signalling components and effector molecules for the recognition and elimination of invading pathogens. The fruit fly Drosophila melanogaster with its powerful collection of genetic and genomic tools has been the model of choice to develop ideas about innate immunity and host–pathogen interactions. Here, we review current research in the field, encompassing all layers of defence from the role of the microbiota to systemic immune activation, and attempt to speculate on future directions and open questions.

Dissecting the mechanisms of linezolid resistance in a Drosophila melanogaster infection model of Staphylococcus aureus

BACKGROUND

 Mini-host models are simple experimental systems to study host-pathogen interactions. We adapted a Drosophila melanogaster infection model to evaluate the in vivo effect of different mechanisms of linezolid (LNZ) resistance in Staphylococcus aureus.

METHODS

 Fly survival was evaluated after infection with LNZ-resistant S. aureus strains NRS119 (which has mutations in 23S ribosomal RNA [rRNA]), CM-05 and 004-737X (which carry cfr), LNZ-susceptible derivatives of CM-05 and 004-737X (which lack cfr), and ATCC 29213 (an LNZ-susceptible control). Flies were then fed food mixed with LNZ (concentration, 15-500 µg/mL). Results were compared to those in mouse peritonitis, using LNZ via oral gavage at 80 and 120 mg/kg every 12 hours.

RESULTS

 LNZ at 500 µg/mL in fly food protected against all strains, while concentrations of 15-250 µg/mL failed to protect against NRS119 (survival, 1.6%-20%). An in vivo effect of cfr was only detected at concentrations of 30 and 15 µg/mL. In the mouse peritonitis model, LNZ (at doses that mimic human pharmacokinetics) protected mice from challenge with the cfr+ 004-737X strain but was ineffective against the NRS119 strain, which carried 23S rRNA mutations.

CONCLUSIONS

 The fly model offers promising advantages to dissect the in vivo effect of LNZ resistance in S. aureus, and findings from this model appear to be concordant with those from the mouse peritonitis model.

Drosophila melanogaster as a model to explore the effects of methicillin-resistant Staphylococcus aureus strain type on virulence and response to linezolid treatment

BACKGROUND

USA300 is a uniquely successful methicillin-resistant Staphylococcus aureus (MRSA) clone that has been associated with Panton-Valentine leukocidin (PVL) production and severe infections. However, conflicting experimental and epidemiological data exist regarding the virulence of USA300 relative to other MRSA clones. We aimed to address this issue using Drosophila melanogaster as a model host to study strain and PVL-dependent variations in virulence among MRSA clinical isolates.

RESULTS

We studied the relative virulence of 39 MRSA isolates: 17 (43%) were PFGE type USA300. Lethal MRSA infection was reproducibly induced both in wild-type (WT) and Toll-deficient D. melanogaster. USA300 strains had significantly lower lethality than non-USA300 strains in a WT background but not in Toll-deficient flies. PFGE type (USA300 versus non-USA300) and PVL status did not affect the response to treatment with linezolid. Virulence was similar in strains with high vancomycin MIC (≥2 μg/mL) versus those with vancomycin MIC<2 μg/mL.

CONCLUSIONS

D. melanogaster is a potentially useful model host to study pathogenicity and response to antibiotic treatment in S. aureus. Our results imply that the attenuated virulence of PVL(+)/USA300 requires intact host innate immunity.