EssE Promotes Staphylococcus aureus ESS-Dependent Protein Secretion To Modify Host Immune Responses during Infection

Enterococcal-host interactions in the gastrointestinal tract and beyond

The gastrointestinal tract (GIT) is typically considered the natural niche of enterococci. However, these bacteria also inhabit extraintestinal tissues, where they can disrupt organ physiology and cause life-threatening infections. Here, we discuss how enterococci, primarily Enterococcus faecalis, interact with the intestine and other host anatomical locations such as the oral cavity, heart, liver, kidney, and vaginal tract. The metabolic flexibility of these bacteria allows them to quickly adapt to new environments, promoting their persistence in diverse tissues. In transitioning from commensals to pathogens, enterococci must overcome harsh conditions such as nutrient competition, exposure to antimicrobials, and immune pressure. Therefore, enterococci have evolved multiple mechanisms to adhere, colonize, persist, and endure these challenges in the host. This review provides a comprehensive overview of how enterococci interact with diverse host cells and tissues across multiple organ systems, highlighting the key molecular pathways that mediate enterococcal adaptation, persistence, and pathogenic behavior.

Teicoplanin associated gene tcaA inactivation increases persister cell formation in Staphylococcus aureus

Staphylococcus aureus is part of normal human flora and is widely associated with hospital-acquired bacteremia. S. aureus has shown a diverse array of resistance to environmental stresses and antibiotics. Methicillin-resistant S. aureus (MRSA) is on the high priority list of new antibiotics discovery and glycopeptides are considered the last drug of choice against MRSA. S. aureus has developed resistance against glycopeptides and the emergence of vancomycin-intermediate-resistant, vancomycin-resistant, and teicoplanin-resistant strains is globally reported. Teicoplanin-associated genes tcaR-tcaA-tcaB (tcaRAB) is known as the S. aureus glycopeptide resistance operon that is associated with glycopeptide resistance. Here, for the first time, the role of tcaRAB in S. aureus persister cells formation, and ΔtcaA dependent persisters' ability to resuscitate the bacterial population was explored. We recovered a clinical strain of MRSA from a COVID-19 patient which showed a high level of resistance to teicoplanin, vancomycin, and methicillin. Whole genome RNA sequencing revealed that the tcaRAB operon expression was altered followed by high expression of glyS and sgtB. The RNA-seq data revealed a significant decrease in tcaA (p = 0.008) and tcaB (p = 0.04) expression while tcaR was not significantly altered. We knocked down tcaA, tcaB, and tcaR using CRISPR-dCas9 and the results showed that when tcaA was suppressed by dCas9, a significant increase was witnessed in persister cells while tcaB suppression did not induce persistence. The results were further evaluated by creating a tcaA mutant that showed ΔtcaA formed a significant increase in persisters in comparison to the wild type. Based on our findings, we concluded that tcaA is the gene that increases persister cells and glycopeptide resistance and could be a potential therapeutic target in S. aureus.

Characterization of TelE, a T7SS LXG Effector Exhibiting a Conserved C-Terminal Glycine Zipper Motif Required for Toxicity

Streptococcus gallolyticus subsp. gallolyticus (SGG) is an opportunistic bacterial pathogen strongly associated with colorectal cancer. Here, through comparative genomics analysis, we demonstrated that the genetic locus encoding the type VIIb secretion system (T7SSb) machinery is uniquely present in SGG in two different arrangements. SGG UCN34 carrying the most prevalent T7SSb genetic arrangement was chosen as the reference strain. To identify the effectors secreted by this secretion system, we inactivated the essC gene encoding the motor of this machinery. A comparison of the proteins secreted by UCN34 wild type and its isogenic ΔessC mutant revealed six T7SSb effector proteins, including the expected WXG effector EsxA and three LXG-containing proteins. In this work, we characterized an LXG-family toxin named herein TelE promoting the loss of membrane integrity. Seven homologs of TelE harboring a conserved glycine zipper motif at the C terminus were identified in different SGG isolates. Scanning mutagenesis of this motif showed that the glycine residue at position 470 was crucial for TelE membrane destabilization activity. TelE activity was antagonized by a small protein TipE belonging to the DUF5085 family. Overall, we report herein a unique SGG T7SSb effector exhibiting a toxic activity against nonimmune bacteria. IMPORTANCE In this study, 38 clinical isolates of Streptococcus gallolyticus subsp. gallolyticus (SGG) were sequenced and a genetic locus encoding the type VIIb secretion system (T7SSb) was found conserved and absent from 16 genomes of the closely related S. gallolyticus subsp. pasteurianus (SGP). The T7SSb is a bona fide pathogenicity island. Here, we report that the model organism SGG strain UCN34 secretes six T7SSb effectors. One of the six effectors named TelE displayed a strong toxicity when overexpressed in Escherichia coli. Our results indicate that TelE is probably a pore-forming toxin whose activity can be antagonized by a specific immunity protein named TipE. Overall, we report a unique toxin-immunity protein pair and our data expand the range of effectors secreted through T7SSb.

The Type 7b Secretion System of S. aureus and Its Role in Colonization and Systemic Infection

Staphylococcus aureus bears a type 7b secretion system (T7bSS) that assembles in the bacterial envelope to promote the secretion of WXG-like proteins and toxic effectors bearing LXG domains. Cognate immunity proteins bind cytosolic effectors to mute their toxicity prior to secretion. T7b-secreted factors have been associated with the pathogenesis of staphylococcal disease and intraspecies competition. We identified earlier strain WU1, an S. aureus ST88 isolate that caused outbreaks of skin and soft tissue infections in mouse breeding facilities. WU1 was also found to persistently colonize the nasopharynx of animals, suggesting a strong host adaptation. In this manner, WU1 colonization and infectivity in mice resembles that of methicillin-sensitive and -resistant S. aureus strains in humans, where nasal carriage is a major risk factor for invasive infections. Here, animals were colonized with wild-type or T7-deficient WU1 strains or combinations thereof. Absence of the T7bSS did not affect colonization in the nasopharynx of animals, and although fluctuations were observed in weekly samplings, the wild-type strain did not replace the T7-deficient strain in cocolonization experiments. Bloodstream infection with a T7b-deficient strain resulted in enhanced survival and reduced bacterial loads and abscesses in soft tissues compared to infection with wild-type WU1. Together, experiments using a mouse-adapted strain suggest that the T7bSS of S. aureus is an important contributor to the pathogenesis of invasive disease.

Immunoglobulin G subclasses confer protection against Staphylococcus aureus bloodstream dissemination through distinct mechanisms in mouse models

Antibodies bind target molecules with exquisite specificity. The removal of these targets is mediated by the effector functions of antibodies. We reported earlier that the monoclonal antibody (mAb) 3F6 promotes opsonophagocytic killing of Staphylococcus aureus in blood and reduces bacterial replication in animals. Here, we generated mouse immunoglobulin G (mIgG) subclass variants and observed a hierarchy in protective efficacy 3F6-mIgG2a > 3F6-mIgG1 ≥ 3F6-mIgG2b >> 3F6-mIgG3 following bloodstream challenge of C57BL/6J mice. This hierarchy was not observed in BALB/cJ mice: All IgG subclasses conferred similar protection. IgG subclasses differ in their ability to activate complement and interact with Fcγ receptors (FcγR) on immune cells. 3F6-mIgG2a-dependent protection was lost in FcγR-deficient, but not in complement-deficient C57BL/6J animals. Measurements of the relative ratio of FcγRIV over complement receptor 3 (CR3) on neutrophils suggest the preferential expression of FcγRIV in C57BL/6 mice and of CR3 in BALB/cJ mice. To determine the physiological significance of these differing ratios, blocking antibodies against FcγRIV or CR3 were administered to animals before challenge. Correlating with the relative abundance of each receptor, 3F6-mIgG2a-dependent protection in C57BL/6J mice showed a greater reliance for FcγRIV while protection in BALB/cJ mice was only impaired upon neutralization of CR3. Thus, 3F6-based clearance of S. aureus in mice relies on a strain-specific contribution of variable FcγR- and complement-dependent pathways. We surmise that these variabilities are the result of genetic polymorphism(s) that may be encountered in other mammals including humans and may have clinical implications in predicting the efficacy of mAb-based therapies.

Contribution of the EssC ATPase to the assembly of the type 7b secretion system in Staphylococcus aureus

Secretion systems utilize ATPase activity to facilitate the translocation of proteins into and across membranes. In bacteria, the universally conserved SecA ATPase binds a large repertoire of preproteins and interacts with the SecYEG translocon. In contrast, the type 7b secretion system (T7bSS) of Staphylococcus aureus supports the secretion of a restricted subset of proteins. T7bSSs are found in several Firmicutes as gene clusters encoding secreted WXG100 proteins and FtsK/SpoIIIE-like ATPase. In S. aureus, this ATPase is called EssC and comprises two cytosolic forkhead-associated domains (FHA_1–2), two membrane-spanning segments (TM_1–2), and four cytosolic modules named DUF (domain of unknown function) and ATPases_1-3 (D1D2D3). However, a detailed understanding of the interactions of EssC in the T7bSS is not clear. Here, we tagged EssC and performed affinity chromatography of detergent-solubilized extracts of wild type and isogenic mutants of S. aureus. We found that EssC recruits EsaA, EssA, and EssB in a complex referred to as the ESS (ESAT-6 like secretion system) translocon, and secreted substrates were not required for translocon assembly. Furthermore, deletions of FHA₁ and DUF rendered EssC unstable, whereas FHA₂ was required for association with EssB. This interaction was independent of EsaA, but EsaA was required to recruit EssA to the EssC–EssB complex. Finally, we show that assembly of the ESS translocon was impaired upon mutation of D2 structural motifs. Together, our data indicate that the ESS translocon is maintained fully assembled at the plasma membrane and that D2 is fundamental in sustaining the integrity of this complex.

The Type VII Secretion System of Staphylococcus

The type VII protein secretion system (T7SS) of Staphylococcus aureus is encoded at the ess locus. T7 substrate recognition and protein transport are mediated by EssC, a membrane-bound multidomain ATPase. Four EssC sequence variants have been identified across S. aureus strains, each accompanied by a specific suite of substrate proteins. The ess genes are upregulated during persistent infection, and the secretion system contributes to virulence in disease models. It also plays a key role in intraspecies competition, secreting nuclease and membrane-depolarizing toxins that inhibit the growth of strains lacking neutralizing immunity proteins. A genomic survey indicates that the T7SS is widely conserved across staphylococci and is encoded in clusters that contain diverse arrays of toxin and immunity genes. The presence of genomic islands encoding multiple immunity proteins in species such as Staphylococcus warneri that lack the T7SS points to a major role for the secretion system in bacterial antagonism. Expected final online publication date for the Annual Review of Microbiology, Volume 75 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Extreme genetic diversity in the type VII secretion system of Listeria monocytogenes suggests a role in bacterial antagonism

The type VII protein secretion system (T7SS) has been characterized in members of the phyla Actinobacteria and Firmicutes. In mycobacteria the T7SS is intimately linked with pathogenesis and intracellular survival, while in Firmicutes there is mounting evidence that the system plays a key role in interbacterial competition. A conserved membrane-bound ATPase protein, termed EssC in Staphylococcus aureus, is a critical component of the T7SS and is the primary receptor for substrate proteins. Genetic diversity in the essC gene of S. aureus has previously been reported, resulting in four protein variants that are linked to specific subsets of substrates. Here we have analysed the genetic diversity of the T7SS-encoding genes and substrate proteins across Listeria monocytogenes genome sequences. We find that there are seven EssC variants across the species that differ in their C-terminal region; each variant is correlated with a distinct subset of genes for likely substrate and accessory proteins. EssC1 is most common and is exclusively linked with polymorphic toxins harbouring a YeeF domain, whereas EssC5, EssC6 and EssC7 variants all code for an LXG domain protein adjacent to essC. Some essC1 variant strains encode an additional, truncated essC at their T7 gene cluster. The truncated EssC, comprising only the C-terminal half of the protein, matches the sequence of either EssC2, EssC3 or EssC4. In each case the truncated gene directly precedes a cluster of substrate/accessory protein genes acquired from the corresponding strain. Across L. monocytogenes strains we identified 40 LXG domain proteins, most of which are encoded at conserved genomic loci. These loci also harbour genes encoding immunity proteins and sometimes additional toxin fragments. Collectively our findings strongly suggest that the T7SS plays an important role in bacterial antagonism in this species.

A type VII secretion system of Streptococcus gallolyticus subsp. gallolyticus contributes to gut colonization and the development of colon tumors

Streptococcus gallolyticus subspecies gallolyticus (Sgg) has a strong clinical association with colorectal cancer (CRC) and actively promotes the development of colon tumors. However, the molecular determinants involved in Sgg pathogenicity in the gut are unknown. Bacterial type VII secretion systems (T7SS) mediate pathogen interactions with their host and are important for virulence in pathogenic mycobacteria and Staphylococcus aureus. Through genome analysis, we identified a locus in Sgg strain TX20005 that encodes a putative type VII secretion system (designated as SggT7SS^T05). We showed that core genes within the SggT7SS^T05 locus are expressed in vitro and in the colon of mice. Western blot analysis showed that SggEsxA, a protein predicted to be a T7SS secretion substrate, is detected in the bacterial culture supernatant, indicating that this SggT7SS^T05 is functional. Deletion of SggT7SS^T05 (TX20005Δesx) resulted in impaired bacterial adherence to HT29 cells and abolished the ability of Sgg to stimulate HT29 cell proliferation. Analysis of bacterial culture supernatants suggest that SggT7SS^T05-secreted factors are responsible for the pro-proliferative activity of Sgg, whereas Sgg adherence to host cells requires both SggT7SS^T05-secreted and bacterial surface-associated factors. In a murine gut colonization model, TX20005Δesx showed significantly reduced colonization compared to the parent strain. Furthermore, in a mouse model of CRC, mice exposed to TX20005 had a significantly higher tumor burden compared to saline-treated mice, whereas those exposed to TX20005Δesx did not. Examination of the Sgg load in the colon in the CRC model suggests that SggT7SS^T05-mediated activities are directly involved in the promotion of colon tumors. Taken together, these results reveal SggT7SS^T05 as a previously unrecognized pathogenicity determinant for Sgg colonization of the colon and promotion of colon tumors.

Colorectal cancer (CRC) is a leading cause of cancer-related death. The development of CRC can be strongly influenced by specific gut microbes. Understanding how gut microbes modulate CRC is critical to developing novel strategies to improve clinical diagnosis and treatment of this disease. S. gallolyticus subsp. gallolyticus (Sgg) has a strong clinical association with CRC and actively promotes the development of colon tumors. However, the specific Sgg molecules that mediate its pro-tumor activity are unknown. Here we report the first characterization of a type VII secretion system (T7SS) in Sgg, designated as SggT7SS^T05. We further demonstrate that SggT7SS^T05-mediated activities are important for Sgg to colonize the colon and to promote the development of colon tumors. These findings reveal SggT7SS^T05 as a novel pathogenicity determinant of Sgg and provide a critical breakthrough in our efforts to understand how Sgg influences the development of CRC. Future investigations of the biological activities of specific effectors of SggT7SS^T05 will likely lead to the discovery of Sgg molecules that can be used as diagnostic markers and intervention targets aimed at mitigating the harmful effect of Sgg.

Staphylococcus lugdunensis: a Skin Commensal with Invasive Pathogenic Potential

Staphylococcus lugdunensis is a species of coagulase-negative staphylococcus (CoNS) that causes serious infections in humans akin to those of S. aureus It was often misidentified as S. aureus, but this has been rectified by recent routine use of matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) in diagnostic laboratories. It encodes a diverse array of virulence factors for adhesion, cytotoxicity, and innate immune evasion, but these are less diverse than those encoded by S. aureus It expresses an iron-regulated surface determinant (Isd) system combined with a novel energy-coupling factor (ECF) mechanism for extracting heme from hemoproteins. Small cytolytic S. lugdunensis synergistic hemolysins (SLUSH), peptides related to phenol-soluble modulins of S. aureus, act synergistically with β-toxin to lyse erythrocytes. S. lugdunensis expresses a novel peptide antibiotic, lugdunin, that can influence the nasal and skin microbiota. Endovascular infections are initiated by bacterial adherence to fibrinogen promoted by a homologue of Staphylococcus aureus clumping factor A and to von Willebrand factor on damaged endothelium by an uncharacterized mechanism. S. lugdunensis survives within mature phagolysosomes of macrophages without growing and is released only following apoptosis. This differs fundamentally from S. aureus, which actively grows and expresses bicomponent leukotoxins that cause membrane damage and could contribute to survival in the infected host. S. lugdunensis is being investigated as a probiotic to eradicate S. aureus from the nares of carriers. However, this is contraindicated by its innate virulence. Studies to obtain a deeper understanding of S. lugdunensis colonization, virulence, and microbiome interactions are therefore warranted.

The type VII secretion system protects Staphylococcus aureus against antimicrobial host fatty acids

The Staphylococcus aureus type VII secretion system (T7SS) exports several proteins that are pivotal for bacterial virulence. The mechanisms underlying T7SS-mediated staphylococcal survival during infection nevertheless remain unclear. Here we report that S. aureus lacking T7SS components are more susceptible to host-derived antimicrobial fatty acids. Unsaturated fatty acids such as linoleic acid (LA) elicited an increased inhibition of S. aureus mutants lacking T7SS effectors EsxC, EsxA and EsxB, or the membrane-bound ATPase EssC, compared to the wild-type (WT). T7SS mutants generated in different S. aureus strain backgrounds also displayed an increased sensitivity to LA. Analysis of bacterial membrane lipid profiles revealed that the esxC mutant was less able to incorporate LA into its membrane phospholipids. Although the ability to bind labelled LA did not differ between the WT and mutant strains, LA induced more cell membrane damage in the T7SS mutants compared to the WT. Furthermore, proteomic analyses of WT and mutant cell fractions revealed that, in addition to compromising membranes, T7SS defects induce oxidative stress and hamper their response to LA challenge. Thus, our findings indicate that T7SS contribute to maintaining S. aureus membrane integrity and homeostasis when bacteria encounter antimicrobial fatty acids.

A membrane-depolarizing toxin substrate of the Staphylococcus aureus type VII secretion system mediates intraspecies competition

The type VII protein secretion system (T7SS) is conserved across Staphylococcus aureus strains and plays important roles in virulence and interbacterial competition. To date, only one T7SS substrate protein, encoded in a subset of S. aureus genomes, has been functionally characterized. Here, using an unbiased proteomic approach, we identify TspA as a further T7SS substrate. TspA is encoded distantly from the T7SS gene cluster and is found across all S. aureus strains as well as in Listeria and Enterococci. Heterologous expression of TspA from S. aureus strain RN6390 indicates its C-terminal domain is toxic when targeted to the Escherichia coli periplasm and that it depolarizes the cytoplasmic membrane. The membrane-depolarizing activity is alleviated by coproduction of the membrane-bound TsaI immunity protein, which is encoded adjacent to tspA on the S. aureus chromosome. Using a zebrafish hindbrain ventricle infection model, we demonstrate that the T7SS of strain RN6390 promotes bacterial replication in vivo, and deletion of tspA leads to increased bacterial clearance. The toxin domain of TspA is highly polymorphic and S. aureus strains encode multiple tsaI homologs at the tspA locus, suggestive of additional roles in intraspecies competition. In agreement, we demonstrate TspA-dependent growth inhibition of RN6390 by strain COL in the zebrafish infection model that is alleviated by the presence of TsaI homologs.

Substrate Interaction with the EssC Coupling Protein of the Type VIIb Secretion System

Staphylococcus aureus employs the type VIIb secretion system (T7SSb) to secrete effector proteins that either have antibacterial activities or promote bacterial persistence in mouse infection models. Here, we present the crystal structure of the ATPase domain D3 of the EssC coupling protein from S. aureus USA300_FPR3757, an integral component of the T7SSb complex, resolved at a 1.7-Å resolution. EssC-D3 shares structural homology with FtsK/SpoIII-like ATPase domains of T7SSa and T7SSb and exhibits a conserved pocket on the surface with differential amino acid composition. In T7SSa, substrate EsxB interacts with the D3 domain through this pocket. Here, we identify amino acids in this pocket that are essential for effector protein secretion in the T7SSb. Our results reveal that the adjacent ATPase domain D2 is a substrate binding site on EssC and that substrates bound to D2 require domain D3 for further transport. Point mutations in the Walker B motif of domain D3 have diametric effects on secretion activity, either abolishing or boosting it, pointing to a critical role of domain D3 in the substrate transport. Finally, we identify ATPase domain D3 as a virulence determinant of S. aureus USA300_FPR3757 using an invertebrate in vivo infection model.IMPORTANCE The emergence of antibiotic-resistant bacteria poses a rising problem in antibiotic treatment (S. Boyle-Vavra and R. S. Daum, Lab Invest 87:3-9, 2007, https://doi.org/10.1038/labinvest.3700501). We have used the multidrug-resistant S. aureus USA300_FPR3757 as a model organism to study the T7SSb. Effector proteins of this system have been associated with abscess formation and bacterial persistence in mouse models (M. L. Burts, A. C. DeDent, and D. M. Missiakas, Mol Microbiol 69:736-746, 2008, https://doi.org/10.1111/j.1365-2958.2008.06324.x; M. L. Burts, W. A. Williams, K. DeBord, and D. M. Missiakas, Proc Natl Acad Sci U S A 102:1169-1174, 2005, https://doi.org/10.1073/pnas.0405620102). We determined the structure of the essential ATPase domain D3 of the T7SSb at atomic resolution and validated a surface-exposed pocket as a potential drug target to block secretion. Furthermore, our study provides new mechanistic insights into the T7SSb substrate transport.

Genome Plasticity of agr-Defective Staphylococcus aureus during Clinical Infection

Therapy for bacteremia caused by Staphylococcus aureus is often ineffective, even when treatment conditions are optimal according to experimental protocols. Adapted subclones, such as those bearing mutations that attenuate agr-mediated virulence activation, are associated with persistent infection and patient mortality.

Therapy for bacteremia caused by Staphylococcus aureus is often ineffective, even when treatment conditions are optimal according to experimental protocols. Adapted subclones, such as those bearing mutations that attenuate agr-mediated virulence activation, are associated with persistent infection and patient mortality. To identify additional alterations in agr-defective mutants, we sequenced and assembled the complete genomes of clone pairs from colonizing and infected sites of several patients in whom S. aureus demonstrated a within-host loss of agr function. We report that events associated with agr inactivation result in agr-defective blood and nares strain pairs that are enriched in mutations compared to pairs from wild-type controls. The random distribution of mutations between colonizing and infecting strains from the same patient, and between strains from different patients, suggests that much of the genetic complexity of agr-defective strains results from prolonged infection or therapy-induced stress. However, in one of the agr-defective infecting strains, multiple genetic changes resulted in increased virulence in a murine model of bloodstream infection, bypassing the mutation of agr and raising the possibility that some changes were selected. Expression profiling correlated the elevated virulence of this agr-defective mutant to restored expression of the agr-regulated ESAT6-like type VII secretion system, a known virulence factor. Thus, additional mutations outside the agr locus can contribute to diversification and adaptation during infection by S. aureus agr mutants associated with poor patient outcomes.

The C-terminus of the ESAT6-like secretion system virulence factor EsxC mediates divalent cation-dependent homodimerization

The human pathogen Staphylococcus aureus encodes the ESAT6-like Secretion System (ESS). The ESS pathway secretes pathogenic substrates such as EsxA, EsxB, EsxC, EsxD and EssD that mediate staphylococcal establishment in persistent abscess lesions. The biochemical behavior of these substrates is not fully understood. EsxC is species-specific lysine-rich homodimer that lacks recognizable topogenic sequence. Studies have shown that EsxC is required for the secretion of other substrates, thereby revealing its biomedical importance. Here, EsxC self-association was investigated in the presence of several metal ion chelators. Results show that EsxC homodimerization is abolished in the presence of EDTA and EGTA, suggesting a role for calcium in mediating EsxC self-association. Complementation experiments confirm that EsxC homodimerization is calcium-dependent. N- and C-terminal truncations of EsxC were constructed, followed by bacterial two-hybrid screening. Results show that EsxC self-association is mediated by its C-terminal domain. Affinity purification of recombinant EsxC to apparent homogeneity, followed by chemical crosslinking and SDS-PAGE led to the detection of the monomeric and dimeric forms of the protein. In contrast and when a purified EsxC variant lacking the C-terminus was subjected to similar conditions, only the monomeric form was observed. These in vivo and in vitro data highlight the contribution of the C-terminus of the virulence factor EsxC to self-association, and document a previously unreported role for calcium in mediating protein-protein interactions in this pathogenic secretion system.

EssC is a specificity determinant for Staphylococcus aureus type VII secretion

The type VII protein secretion system (T7SS) is found in actinobacteria and firmicutes, and plays important roles in virulence and interbacterial competition. A membrane-bound ATPase protein, EssC in Staphylococcus aureus, lies at the heart of the secretion machinery. The EssC protein from S. aureus strains can be grouped into four variants (EssC1-EssC4) that display sequence variability in the C-terminal region. Here we show that the EssC2, EssC3 and EssC4 variants can be produced in a strain deleted for essC1, and that they are able to mediate secretion of EsxA, an essential component of the secretion apparatus. They are, however, unable to support secretion of the substrate protein EsxC, which is only encoded in essC1-specific strains. This finding indicates that EssC is a specificity determinant for T7 protein secretion. Our results support a model in which the C-terminal domain of EssC interacts with substrate proteins, whereas EsxA interacts elsewhere.

Flotillin scaffold activity contributes to type VII secretion system assembly in Staphylococcus aureus

Scaffold proteins are ubiquitous chaperones that promote efficient interactions between partners of multi-enzymatic protein complexes; although they are well studied in eukaryotes, their role in prokaryotic systems is poorly understood. Bacterial membranes have functional membrane microdomains (FMM), a structure homologous to eukaryotic lipid rafts. Similar to their eukaryotic counterparts, bacterial FMM harbor a scaffold protein termed flotillin that is thought to promote interactions between proteins spatially confined to the FMM. Here we used biochemical approaches to define the scaffold activity of the flotillin homolog FloA of the human pathogen Staphylococcus aureus, using assembly of interacting protein partners of the type VII secretion system (T7SS) as a case study. Staphylococcus aureus cells that lacked FloA showed reduced T7SS function, and thus reduced secretion of T7SS-related effectors, probably due to the supporting scaffold activity of flotillin. We found that the presence of flotillin mediates intermolecular interactions of T7SS proteins. We tested several small molecules that interfere with flotillin scaffold activity, which perturbed T7SS activity in vitro and in vivo. Our results suggest that flotillin assists in the assembly of S. aureus membrane components that participate in infection and influences the infective potential of this pathogen.

The recently discovered functional membrane microdomains (FMM) of prokaryotic cells contain a protein homologous to the scaffold protein flotillin found in eukaryotic lipid rafts. It remains to be elucidated whether, like their eukaryotic counterparts, flotillin homolog proteins have a scaffold function in bacteria. Here we show that the Staphylococcus aureus flotillin FloA acts as a scaffold protein, to promote more efficient assembly of membrane-associated protein interacting partners of multi-enzyme complexes. In a case study, we provide biochemical evidence that FloA participates in assembly of the Type VII secretion system and thus contributes to S. aureus infective potential. Targeted dispersion of FMM-related processes using anti-FMM molecules opens up new perspectives for microbial therapies to treat persistent S. aureus infections.

Isolation of a Membrane Protein Complex for Type VII Secretion in Staphylococcus aureus

The ESAT6-like secretion system (ESS) of Staphylococcus aureus promotes effector protein transport across the bacterial envelope. Genes in the ESS cluster are required for S. aureus establishment of persistent abscess lesions and the modulation of immune responses during bloodstream infections. However, the biochemical functions of most of the ESS gene products, specifically the identity of secretion machine components, are unknown. Earlier work demonstrated that deletion of essB, which encodes a membrane protein, abolishes S. aureus ESS secretion. Loss-of-function mutations truncating the essB gene product cause dominant-negative phenotypes on ESS secretion, suggesting that EssB is a central component of the secretion machinery. To test this prediction, we purified native and affinity-tagged EssB from staphylococcal membranes via dodecyl-maltoside extraction, identifying a complex assembled from five proteins, EsaA, EssA, EssB, EssD, and EsxA. All five proteins are essential for secretion, as knockout mutations in the corresponding genes abolish ESS transport. Biochemical and bacterial two-hybrid analyses revealed a direct interaction between EssB and EsaA that, by engaging a mobile machine component, EsxA, may also recruit EssA and EssD.IMPORTANCE Type VII secretion systems support the lifestyle of Gram-positive bacteria, including important human pathogens such as Bacillus anthracis, Mycobacterium tuberculosis, and Staphylococcus aureus Genes encoding SpoIIIE-FtsK-like ATPases and WXG100-secreted products are conserved features of type VII secretion pathways; however, most of the genes in T7SS clusters are not conserved between different bacterial species. Here, we isolate a complex of proteins from the membranes of S. aureus that appears to represent the core secretion machinery, designated ESS. These results suggest that three membrane proteins, EsaA, EssB, and EssA, form a secretion complex that associates with EssC, the SpoIIIE-FtsK-like ATPase, and with EsxA, a mobile machine component and member of the WXG100 protein family.

Staphylococcus aureus Esx Factors Control Human Dendritic Cell Functions Conditioning Th1/Th17 Response

The opportunistic pathogen Staphylococcus aureus (S. aureus) is a major cause of nosocomial- and community-acquired infections. In addition, many antibiotic-resistant strains are emerging worldwide, thus, there is an urgent unmet need to pinpoint novel therapeutic and prophylactic strategies. In the present study, we characterized the impact of infection with the pandemic methicillin-resistant USA300 S. aureus strain on human primary dendritic cells (DC), key initiators and regulators of immune responses. In particular, among staphylococcal virulence factors, the function of EsxA and EsxB, two small acidic dimeric proteins secreted by the type VII-like secretion system Ess (ESAT-6-like secretion system), was investigated in human DC setting. A comparative analysis of bacterial entry, replication rate as well as DC maturation, apoptosis, signaling pathway activation and cytokine production was performed by using wild type (wt) USA300 and three isogenic mutants carrying the deletion of esxA (ΔesxA), esxB (ΔesxB), or both genes (ΔesxAB). The S. aureus mutant lacking only the EsxA protein (ΔesxA) stimulated a stronger pro-apoptotic phenotype in infected DC as compared to wt USA300, ΔesxAB, and ΔesxB strains. When the mutant carrying the esxB deletion (ΔesxB) was analyzed, a higher production of both regulatory and pro-inflammatory mediators was found in the infected DC with respect to those challenged with the wt counterpart and the other esx mutants. In accordance with these data, supernatant derived from ΔesxB-infected DC promoted a stronger release of both IFN-γ and IL-17 from CD4⁺ T cells as compared with those conditioned with supernatants derived from wild type USA300-, ΔesxAB-, and ΔesxA-infected cultures. Although, the interaction of S. aureus with human DC is not yet fully understood, our data suggest that both cytokine production and apoptotic process are modulated by Esx factors, thus indicating a possible role of these proteins in the modulation of DC-mediated immunity to S. aureus.

A Novel ESAT-6 Secretion System-Secreted Protein EsxX of Community-Associated Staphylococcus aureus Lineage ST398 Contributes to Immune Evasion and Virulence

The ESAT-6 secretion system (ESS) has been reported to contribute to the virulence and pathogenicity of several Staphylococcus aureus strains such as USA300 and Newman. However, the role of the ESS in community-associated S. aureus (CA-SA) lineage ST398 in China is not well understood. By comparing the ess locus of ST398 with the published S. aureus sequence in the NCBI database, we found one gene in the ess locus encoding a novel WXG superfamily protein that is highly conserved only in ST398. LC-MS/MS and Western blot analysis revealed that this protein is a novel secreted protein controlled by the ST398 ESS, and we named the protein EsxX. Although EsxX was not under the control of the accessory gene regulator like many other virulence factors and had no influence on several phenotypes of ST398, such as growth, hemolysis, and biofilm formation, it showed important impacts on immune evasion and virulence in ST398. An esxX deletion mutant led to significantly reduced resistance to neutrophil killing and decreased virulence in murine skin and blood infection models, indicating its essential contribution to the evasion of innate host defense and virulence to support the pathogenesis of ST398 infections. The function of this novel secreted protein EsxX might help us better understand the role of the ESS in the virulence and epidemic success of the CA-SA lineage ST398.

EssD, a Nuclease Effector of the Staphylococcus aureus ESS Pathway

Specialized secretion systems of bacteria evolved for selective advantage, either killing microbial competitors or implementing effector functions during parasitism. Earlier work characterized the ESAT-6 secretion system (ESS) of Staphylococcus aureus and demonstrated its contribution to persistent staphylococcal infection of vertebrate hosts. Here, we identify a novel secreted effector of the ESS pathway, EssD, that functions as a nuclease and cleaves DNA but not RNA. EssI, a protein of the DUF600 family, binds EssD to block its nuclease activity in the staphylococcal cytoplasm. An essD knockout mutant or a variant lacking nuclease activity, essD^L546P, elicited a diminished interleukin-12 (IL-12) cytokine response following bloodstream infection of mice, suggesting that the effector function of EssD stimulates immune signaling to support the pathogenesis of S. aureus infections.

IMPORTANCE

Bacterial type VII or ESAT-6-like secretion systems (ESS) may have evolved to modulate host immune responses during infection, thereby contributing to the pathogenesis of important diseases such as tuberculosis and methicillin-resistant S. aureus (MRSA) infection. The molecular mechanisms whereby type VII secretion systems achieve their goals are not fully elucidated as secreted effectors with biochemical functions have heretofore not been identified. We show here that MRSA infection relies on the secretion of a nuclease effector that cleaves DNA and contributes to the stimulation of IL-12 signaling during infection. These results identify a biological mechanism for the contribution of the ESS pathway toward the establishment of MRSA disease.