Staphylococcus aureus infects hospitalized or healthy individuals and represents the most frequent cause of bacteremia, treatment of which is complicated by the emergence of methicillin-resistant S. aureus. We examined the ability of S. aureus to escape phagocytic clearance in blood and identified adenosine synthase A (AdsA), a cell wall–anchored enzyme that converts adenosine monophosphate to adenosine, as a critical virulence factor. Staphylococcal synthesis of adenosine in blood, escape from phagocytic clearance, and subsequent formation of organ abscesses were all dependent on adsA and could be rescued by an exogenous supply of adenosine. An AdsA homologue was identified in the anthrax pathogen, and adenosine synthesis also enabled escape of Bacillus anthracis from phagocytic clearance. Collectively, these results suggest that staphylococci and other bacterial pathogens exploit the immunomodulatory attributes of adenosine to escape host immune responses.
Bacillus anthracis, the causative agent of anthrax, requires surface (S)-layer proteins for the pathogenesis of infection. Previous work characterized S-layer protein binding via the surface layer homology domain to a pyruvylated carbohydrate in the envelope of vegetative forms. The molecular identity of this carbohydrate and the mechanism of its display in the bacterial envelope are still unknown. Analyzing acid-solubilized, purified carbohydrates by mass spectrometry and NMR spectroscopy, we identify secondary cell wall polysaccharide (SCWP) as the ligand of S-layer proteins. In agreement with the model that surface layer homology domains bind to pyruvylated carbohydrate, SCWP was observed to be linked to pyruvate in a manner requiring csaB, the only structural gene known to be required for S-layer assembly. B. anthracis does not elaborate wall teichoic acids; however, its genome harbors tagO and tagA, genes responsible for the synthesis of the linkage unit that tethers teichoic acids to the peptidoglycan layer. The tagO gene appears essential for B. anthracis growth and complements the tagO mutant phenotypes of staphylococci. Tunicamycin-mediated inhibition of TagO resulted in deformed, S-layer-deficient bacilli. Together, these results suggest that tagO-mediated assembly of linkage units tethers pyruvylated SCWP to the B. anthracis envelope, thereby enabling S-layer assembly and providing for the pathogenesis of anthrax infections.
SummaryThe Gram-positive pathogen Bacillus anthracis causes anthrax, a fulminant and lethal infection of mammals. Two large virulence plasmids, pXO1 and pXO2, harbour genes required for anthrax pathogenesis and encode secreted toxins or provide for the poly g-D-glutamic acid capsule. In addition to capsule, B. anthracis harbours additional cell wall envelope structures, including the surface layer (S-layer), which is composed of crystalline protein arrays. We sought to identify the B. anthracis envelope factor that mediates adherence of vegetative forms to human cells and isolated BslA (B. anthracis S-layer protein A). Its structural gene, bslA, is located on the pXO1 pathogenicity island (pXO1-90) and bslA expression is both necessary and sufficient for adherence of vegetative forms to host cells. BslA assembly into S-layers and surface exposure is presumably mediated by three N-terminal SLH domains. Twenty-three B. anthracis genes, whose products harbour similar SLH domains, may provide additional surface molecules that allow bacilli to engage cells or tissues of specific hosts during anthrax pathogenesis.
Sortases anchor surface proteins to the cell wall of Gram-positive pathogens through recognition of specific motif sequences. Loss of sortase leads to large reductions in virulence, which identifies sortase as a target for the development of antibacterials. By screening 135,625 small molecules for inhibition, we report here that aryl (-amino)ethyl ketones inhibit sortase enzymes from staphylococci and bacilli. Inhibition of sortases occurs through an irreversible, covalent modification of their active site cysteine. Sortases specifically activate this class of molecules via -elimination, generating a reactive olefin intermediate that covalently modifies the cysteine thiol. Analysis of the three-dimensional structure of Bacillus anthracis sortase B with and without inhibitor provides insights into the mechanism of inhibition and reveals binding pockets that can be exploited for drug discovery.The emergence of bacterial strains resistant to antibiotic therapy is a major public health threat (1). Of particular concern is Staphylococcus aureus, because this Gram-positive pathogen is the leading cause of infections in the bloodstream, lower respiratory tract, skin, and soft tissue in the United States (2). S. aureus strains exhibiting resistance against multiple antibiotics, such as methicillin-resistant S. aureus, are isolated in 30 -60% of community and Ͼ80% of hospital infections with this pathogen (3). Vancomycin or other glycopeptides are considered last-resort therapies for methicillin-resistant S. aureus; however, S. aureus strains with intermediate or full resistance to vancomycin can cause infections for which antimicrobial treatment may no longer be effective (4).Surface proteins of Gram-positive bacteria play important roles during pathogenesis (5). Sortases anchor these polypeptides to the bacterial cell wall envelope (6). For example, S. aureus sortase A recognizes proteins destined for the cell surface via an LPXTG motif in their C-terminal sorting signal (7). Following cleavage between the threonine and the glycine residues, an acyl-enzyme intermediate captures cleaved substrate at the active site thiol of sortase (8). Nucleophilic attack of the amino group of the peptidoglycan precursor lipid II (at the thioester intermediate resolves the acyl enzyme and forms an amide bond between the C-terminal threonine of surface protein and pentaglycine crossbridges (9). Lipid II-linked polypeptide is subsequently incorporated into the cell wall envelope of staphylococci (10). The final product of this pathway, protein linked to cell wall pentaglycine cross-bridges, is displayed on the bacterial surface and enables interactions between the pathogen and tissues of its host.Surface protein anchoring to the cell wall envelope is thought to be an essential strategy for bacterial survival during infection, because mutants lacking genes for one or more sortase enzymes are attenuated in virulence (11). Inhibition of sortases by small molecules may therefore function as a therapeutic strategy for bacterial infections. ...
Surface (S)-layers, para-crystalline arrays of protein, are deposited in the envelope of most bacterial species. These surface organelles are retained in the bacterial envelope through the non-covalent association of proteins with cell wall carbohydrates. Bacillus anthracis, a Gram-positive pathogen, produces S-layers of the protein Sap, which uses three consecutive repeats of the surface-layer homology (SLH) domain to engage secondary cell wall polysaccharides (SCWP). Using x-ray crystallography, we reveal here the structure of these SLH domains, which assume the shape of a three-prong spindle. Each SLH domain contributes to a three-helical bundle at the spindle base, whereas another ␣-helix and its connecting loops generate the three prongs. The inter-prong grooves contain conserved cationic and anionic residues, which are necessary for SLH domains to bind the B. anthracis SCWP. Modeling experiments suggest that the SLH domains of other S-layer proteins also fold into three-prong spindles and capture bacterial envelope carbohydrates by a similar mechanism.Surface layers (S-layers) 3 are para-crystalline sheets of protein, which self-assemble on the surface of microbial cells to form contiguous layers (1, 2). Most organisms that elaborate S-layers do so by abundantly producing and secreting a single protein species (3). Whether an organism produces an S-layer as a component of its envelope structure is assessed by electron microscopy of the cell surface (4). In this manner, species from nearly every branch of the Bacteria and Archaea have been discovered to produce S-layers (2). Proteins within S-layers fulfill variable functions in that they act either as a scaffold or enzyme in the bacterial envelope (5), promote nutrient diffusion or transport (6), or contribute to virulence by enabling microbial adhesion to infected host tissues (7).Most, but not all, S-layer proteins of bacteria share three tandem ϳ55 amino acid repeats of the Surface Layer Homology (SLH) domain (8 -10). Secreted proteins encoding three tandem SLH domains are tethered to the bacterial envelope by non-covalent interactions between the SLH domains and a secondary cell wall carbohydrate (11). SLH domains are remarkable for being both necessary and sufficient for the incorporation of chimeric proteins into S-layers (12, 13). The SbsC protein of Geobacillus stearothermophilus is an example for a class of protein that forms S-layers without SLH domains (14). SbsC binds to the secondary cell wall polysaccharide (SCWP) of G. stearothermophilus via its N-terminal domain, which consists of three triple-helical bundles that are connected by two contiguous helices (14). The N-terminal domain of SbsC has high similarity with S-layer proteins from G. stearothermophilus, Geobacillus kaustophilus, and Geobacillus tepidamans (14) and is not similar to proteins with SLH domains.The Gram-positive bacterium Bacillus anthracis is a rodshaped, spore-forming pathogen of mammalian hosts (15). The envelope of its vegetative forms is composed of a plasma membrane a...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.