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

Mechanistic insights into the multitarget synergistic efficacy of farrerol and β-lactam antibiotics in combating methicillin-resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA), a principal causative agent of infections worldwide, urgently requires innovative interventions to counter its increasing risk. The present study revealed the profound impact of farrerol (FA), a robust bioactive agent, on the virulence and resistance mechanisms of MRSA. Our in-depth investigation revealed that FA significantly mitigated the β-lactam resistance of MRSA USA300, an achievement attributed to its precise interference with the BlaZ and Pbp2a protein. Additionally, FA indirectly diminishes the oligomerization of PBP2a by disrupting pigment synthesis, further contributing to its efficacy. In addition, FA extends its functional footprint beyond resistance modulation, exhibiting substantial antivirulence efficacy through selective inhibition of the accessory gene regulator (Agr) system, thereby significantly curbing MRSA pathogenicity in A549 cell and murine models. This study comprehensively explored the multiple impacts of FA on MRSA, shedding light on its versatile role as a BlaZ suppressor, pigment synthesis regulator, and AgrA activity modulator. These intricate findings firmly position FA as a compelling therapeutic candidate for addressing MRSA infections in the clinic.

The SPFH complex HflK-HflC regulates aerobic respiration in bacteria

The bacterial HflK-HflC membrane complex is a member of the highly conserved family of SPFH proteins, which are present in all domains of life and include eukaryotic stomatins, flotillins, and prohibitins. These proteins organize cell membranes and are involved in various processes. However, the exact physiological functions of most bacterial SPFH proteins remain unclear. Here, we report that the HflK-HflC complex in Escherichia coli is required for growth under high aeration. The absence of this complex causes a growth defect at high oxygen levels due to a reduced abundance of IspG, an essential iron-sulfur cluster enzyme in the isoprenoid biosynthetic pathway. This reduction might be related to lower stability of IspG and several other proteins, including the iron siderophore transporter TonB, in the absence of the HflK-HflC complex. Our results suggest that decreased IspG activity leads to lower levels of ubiquinone and misregulated expression of multiple respiratory enzymes, including cytochrome oxidases, and consequently reduced respiration and lower ATP levels. This impact of the hflK hflC deletion on aerobic respiration resembles the mitochondrial respiratory defects caused by the inactivation of prohibitins in mammalian and yeast cells, indicating functional parallels between these bacterial and eukaryotic SPFH proteins.

Comparative genome analyses of Staphylococcus aureus from platelet concentrates reveal rearrangements involving loss of type VII secretion genes

Staphylococcus aureus has been involved in transfusion-transmitted fatalities associated with platelet concentrates (PCs) due to its heightened pathogenicity enhanced by genome-encoded virulence and antibiotic resistance genes. This may be facilitated by mobile genetic elements (MGEs) that can cause rearrangements. Several factors contribute to S. aureus virulence, including the type VII secretion system (T7SS), composed of six core genes conserved across S. aureus strains. In this study, we conducted comparative genome analyses of five S. aureus isolates from PCs (CI/BAC/25/13 /W, PS/BAC/169/17 /W and PS/BAC/317/16 /W were detected during PCs screening with the BACT/ALERT automated culture system, and ATR-20003 and CBS2016-05 were missed during screening and caused septic transfusion reactions). Multiple alignments of the genomes revealed evidence of rearrangements involving phage Sa3int in PS/BAC/169/17 /W and PS/BAC/317/16 /W. While the former had undergone translocation of its immune evasion cluster (IEC), the latter had lost part of the phage, leaving behind the IEC. This observation highlights S. aureus genome plasticity. Unexpectedly, strain CBS2016-05 was found to encode a pseudo-type VII secretion system (T7SS) that had lost five of the conserved core genes (esxA, esaA, essA, esaB and essB) and contained a 5' truncated essC. Since these genes are essential for the function of the T7SS protein transport machinery, which plays a key role in S. aureus virulence, CBS2016-05 probably compensates by recruiting other export mechanisms and/or alternative virulence factors, such as neu-tralizing immunity proteins. This study unravels genome rearrangements in S. aureus isolated from PCs and reports the first S. aureus isolate lacking conserved T7SS core genes.

Flotillin-mediated stabilization of unfolded proteins in bacterial membrane microdomains

The function of many bacterial processes depends on the formation of functional membrane microdomains (FMMs), which resemble the lipid rafts of eukaryotic cells. However, the mechanism and the biological function of these membrane microdomains remain unclear. Here, we show that FMMs in the pathogen methicillin-resistant Staphylococcus aureus (MRSA) are dedicated to confining and stabilizing proteins unfolded due to cellular stress. The FMM scaffold protein flotillin forms a clamp-shaped oligomer that holds unfolded proteins, stabilizing them and favoring their correct folding. This process does not impose a direct energy cost on the cell and is crucial to survival of ATP-depleted bacteria, and thus to pathogenesis. Consequently, FMM disassembling causes the accumulation of unfolded proteins, which compromise MRSA viability during infection and cause penicillin re-sensitization due to PBP2a unfolding. Thus, our results indicate that FMMs mediate ATP-independent stabilization of unfolded proteins, which is essential for bacterial viability during infection.

Interbacterial competition mediated by the type VIIb secretion system

Successful occupancy of a given niche requires the colonising bacteria to interact extensively with the biotic and abiotic environment, including other resident microbes. Bacteria have evolved a range of protein secretion machines for this purpose with eleven such systems identified to date. The type VIIb secretion system (T7SSb) is utilised by Bacillota to secrete a range of protein substrates, including antibacterial toxins targeting closely related strains, and the system as a whole has been implicated in a range of activities such as iron acquisition, intercellular signalling, host colonisation and virulence. This review covers the components and secretion mechanism of the T7SSb, the substrates of these systems and their roles in Gram-positive bacteria, with a focus on interbacterial competition.

Modulation of bacterial membrane proteins activity by clustering into plasma membrane nanodomains

Recent research has demonstrated specific protein clustering within membrane subdomains in bacteria, challenging the long-held belief that prokaryotes lack these subdomains. This mini review provides examples of bacterial membrane protein clustering, discussing the benefits of protein assembly in membranes and highlighting how clustering regulates protein activity.

Bacterial lipid biophysics and membrane organization

The formation of lateral microdomains is emerging as a central organizing principle in bacterial membranes. These microdomains are targets of antibiotic development and have the potential to enhance natural product synthesis, but the rules governing their assembly are unclear. Previous studies have suggested that microdomain formation is promoted by lipid phase separation, particularly by cardiolipin (CL) and isoprenoid lipids, and there is strong evidence that CL biosynthesis is required for recruitment of membrane proteins to cell poles and division sites. New work demonstrates that additional bacterial lipids may mediate membrane protein localization and function, opening the field for mechanistic evaluation of lipid-driven membrane organization in vivo.

Skin-to-blood pH shift triggers metabolome and proteome global remodelling in Staphylococcus epidermidis

Staphylococcus epidermidis is one of the most common bacteria of the human skin microbiota. Despite its role as a commensal, S. epidermidis has emerged as an opportunistic pathogen, associated with 80% of medical devices related infections. Moreover, these bacteria are extremely difficult to treat due to their ability to form biofilms and accumulate resistance to almost all classes of antimicrobials. Thus new preventive and therapeutic strategies are urgently needed. However, the molecular mechanisms associated with S. epidermidis colonisation and disease are still poorly understood. A deeper understanding of the metabolic and cellular processes associated with response to environmental factors characteristic of SE ecological niches in health and disease might provide new clues on colonisation and disease processes. Here we studied the impact of pH conditions, mimicking the skin pH (5.5) and blood pH (7.4), in a S. epidermidis commensal strain by means of next-generation proteomics and 1H NMR-based metabolomics. Moreover, we evaluated the metabolic changes occurring during a sudden pH change, simulating the skin barrier break produced by a catheter. We found that exposure of S. epidermidis to skin pH induced oxidative phosphorylation and biosynthesis of peptidoglycan, lipoteichoic acids and betaine. In contrast, at blood pH, the bacterial assimilation of monosaccharides and its oxidation by glycolysis and fermentation was promoted. Additionally, several proteins related to virulence and immune evasion, namely extracellular proteases and membrane iron transporters were more abundant at blood pH. In the situation of an abrupt skin-to-blood pH shift we observed the decrease in the osmolyte betaine and changes in the levels of several metabolites and proteins involved in cellular redoxl homeostasis. Our results suggest that at the skin pH S. epidermidis cells are metabolically more active and adhesion is promoted, while at blood pH, metabolism is tuned down and cells have a more virulent profile. pH increase during commensal-to-pathogen conversion appears to be a critical environmental signal to the remodelling of the S. epidermidis metabolism toward a more pathogenic state. Targeting S. epidermidis proteins induced by pH 7.4 and promoting the acidification of the medical device surface or surrounding environment might be new strategies to treat and prevent S. epidermidis infections.

The Antibacterial Type VII Secretion System of Bacillus subtilis: Structure and Interactions of the Pseudokinase YukC/EssB

Type VIIb secretion systems (T7SSb) were recently proposed to mediate different aspects of Firmicutes physiology, including bacterial pathogenicity and competition. However, their architecture and mechanism of action remain largely obscure. Here, we present a detailed analysis of the T7SSb-mediated bacterial competition in Bacillus subtilis, using the effector YxiD as a model for the LXG secreted toxins. By systematically investigating protein-protein interactions, we reveal that the membrane subunit YukC contacts all T7SSb components, including the WXG100 substrate YukE and the LXG effector YxiD. YukC’s crystal structure shows unique features, suggesting an intrinsic flexibility that is required for T7SSb antibacterial activity. Overall, our results shed light on the role and molecular organization of the T7SSb and demonstrate the potential of B. subtilis as a model system for extensive structure-function studies of these secretion machineries.

Agrobacterium tumefaciens Type IV and Type VI Secretion Systems Reside in Detergent-Resistant Membranes

Cell membranes are not homogenous but compartmentalized into lateral microdomains, which are considered as biochemical reaction centers for various physiological processes in eukaryotes and prokaryotes. Due to their special lipid and protein composition, some of these microdomains are resistant to treatment with non-ionic detergents and can be purified as detergent-resistant membranes (DRMs). Here we report the proteome of DRMs from the Gram-negative phytopathogen Agrobacterium tumefaciens. Using label-free liquid chromatography-tandem mass spectrometry, we identified proteins enriched in DRMs isolated under normal and virulence-mimicking growth conditions. Prominent microdomain marker proteins such as the SPFH (stomatin/prohibitin/flotillin/HflKC) proteins HflK, HflC and Atu3772, along with the protease FtsH were highly enriched in DRMs isolated under any given condition. Moreover, proteins involved in cell envelope biogenesis, transport and secretion, as well as motility- and chemotaxis-associated proteins were overrepresented in DRMs. Most strikingly, we found virulence-associated proteins such as the VirA/VirG two-component system, and the membrane-spanning type IV and type VI secretion systems enriched in DRMs. Fluorescence microscopy of the cellular localization of both secretion systems and of marker proteins was in agreement with the results from the proteomics approach. These findings suggest that virulence traits are micro-compartmentalized into functional microdomains in A. tumefaciens.

The cell envelope of Staphylococcus aureus selectively controls the sorting of virulence factors

Staphylococcus aureus bi-component pore-forming leukocidins are secreted toxins that directly target and lyse immune cells. Intriguingly, one of the leukocidins, Leukocidin AB (LukAB), is found associated with the bacterial cell envelope in addition to secreted into the extracellular milieu. Here, we report that retention of LukAB on the bacterial cells provides S. aureus with a pre-synthesized active toxin that kills immune cells. On the bacteria, LukAB is distributed as discrete foci in two distinct compartments: membrane-proximal and surface-exposed. Through genetic screens, we show that a membrane lipid, lysyl-phosphatidylglycerol (LPG), and lipoteichoic acid (LTA) contribute to LukAB deposition and release. Furthermore, by studying non-covalently surface-bound proteins we discovered that the sorting of additional exoproteins, such as IsaB, Hel, ScaH, and Geh, are also controlled by LPG and LTA. Collectively, our study reveals a multistep secretion system that controls exoprotein storage and protein translocation across the S. aureus cell wall.

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.

Visualization and Analysis of the Dynamic Assembly of a Heterologous Lantibiotic Biosynthesis Complex in Bacillus subtilis

A membrane-associated lanthipeptide synthetase complex, consisting of the dehydratase NisB, the cyclase NisC, and the ABC transporter NisT, has been described for nisin biosynthesis in the coccoid bacterium Lactococcus lactis. Here, we used advanced fluorescence microscopy to visualize the functional nisin biosynthesis machinery in rod-shaped cells and analyzed its spatial distribution and dynamics employing a platform we developed for heterologous production of nisin in Bacillus subtilis. We observed that NisT, as well as NisB and NisC, were all distributed in a punctate pattern along the cell periphery, opposed to the situation in coccoid cells. NisBTC proteins were found to be highly colocalized, being visualized at the same spots by dual fluorescence microscopy. In conjunction with the successful isolation of the biosynthetic complex NisBTC from the cell membrane, this corroborated that the visual bright foci were the sites for nisin maturation and transportation. A strategy of differential timing of expression was employed to demonstrate the in vivo dynamic assembly of NisBTC, revealing the recruitment by NisT of NisBC to the membrane. Additionally, by use of mutated proteins, the nucleotide binding domain (NBD) of NisT was found to function as a membrane anchor for NisB and/or NisC. We also show that the nisin biosynthesis sites are static and likely associated with proteins residing in lipid rafts. Based on these data, we propose a model for a three-phase production of modified precursor nisin in rod-shaped bacteria, presenting the assembly dynamics of NisBTC and emphasizing the crucial role of NisBC, next to NisT, in the process of precursor nisin translocation.

Responses to chemical cross-talk between the Mycobacterium ulcerans toxin, mycolactone, and Staphylococcus aureus

Buruli ulcer is a neglected tropical disease caused by the environmental pathogen, Mycobacterium ulcerans whose major virulence factor is mycolactone, a lipid cytotoxic molecule. Buruli ulcer has high morbidity, particularly in rural West Africa where the disease is endemic. Data have shown that infected lesions of Buruli ulcer patients can be colonized by quorum sensing bacteria such as Staphylococcus aureus, S. epidermidis, and Pseudomonas aeruginosa, but without typical pathology associated with those pathogens' colonization. M. ulcerans pathogenesis may not only be an individual act but may also be dependent on synergistic or antagonistic mechanisms within a polymicrobial network. Furthermore, co-colonization by these pathogens may promote delayed wound healing, especially after the initiation of antibiotic therapy. Hence, it is important to understand the interaction of M. ulcerans with other bacteria encountered during skin infection. We added mycolactone to S. aureus and incubated for 3, 6 and 24 h. At each timepoint, S. aureus growth and hemolytic activity was measured, and RNA was isolated to measure virulence gene expression through qPCR and RNASeq analyses. Results showed that mycolactone reduced S. aureus hemolytic activity, suppressed hla promoter activity, and attenuated virulence genes, but did not affect S. aureus growth. RNASeq data showed mycolactone greatly impacted S. aureus metabolism. These data are relevant and significant as mycolactone and S. aureus sensing and response at the transcriptional, translational and regulation levels will provide insight into biological mechanisms of interspecific interactions that may play a role in regulation of responses such as effects between M. ulcerans, mycolactone, and S. aureus virulence that will be useful for treatment and prevention.

Mini Review: Bacterial Membrane Composition and Its Modulation in Response to Stress

Antibiotics and other agents that perturb the synthesis or integrity of the bacterial cell envelope trigger compensatory stress responses. Focusing on Bacillus subtilis as a model system, this mini-review summarizes current views of membrane structure and insights into how cell envelope stress responses remodel and protect the membrane. Altering the composition and properties of the membrane and its associated proteome can protect cells against detergents, antimicrobial peptides, and pore-forming compounds while also, indirectly, contributing to resistance against compounds that affect cell wall synthesis. Many of these regulatory responses are broadly conserved, even where the details of regulation may differ, and can be important in the emergence of antibiotic resistance in clinical settings.

Impact of Bicarbonate on PBP2a Production, Maturation, and Functionality in Methicillin-Resistant Staphylococcus aureus (MRSA)

Certain methicillin-resistant Staphylococcus aureus (MRSA) strains exhibit β-lactam-susceptibility in vitro, ex vivo and in vivo in the presence of NaHCO₃ (NaHCO₃-responsive MRSA). Herein, we investigate the impact of NaHCO₃ on factors required for PBP2a functionality. Prototype NaHCO₃-responsive and -nonresponsive MRSA strains (as defined in vitro) were assessed for the impact of NaHCO₃ on: expression of genes involved in PBP2a production-maturation pathways (mecA, blaZ, pbp4, vraSR, prsA, sigB, and floA); membrane PBP2a and PrsA protein content; and membrane carotenoid content. Following NaHCO₃ exposure in NaHCO₃-responsive (vs - nonresponsive) MRSA, there was significantly reduced expression of: i) mecA and blaZ; ii) the vraSR-prsA gene axis; and iii) pbp4 Carotenoid production was reduced, while floA expression was increased by NaHCO₃ exposure in all MRSA strains. This work underscores the distinct regulatory impact of NaHCO₃ on a cadre of genes encoding factors required for maintenance of the MRSA phenotype through PBP2a functionality and maturation.

Bacterial type VII secretion: An important player in host-microbe and microbe-microbe interactions

Type VII secretion systems (T7SSs) are poorly understood protein export apparatuses found in mycobacteria and many species of Gram-positive bacteria. To date, this pathway has predominantly been studied in Mycobacterium tuberculosis, where it has been shown to play an essential role in virulence; however, much less studied is an evolutionarily divergent subfamily of T7SSs referred to as the T7SSb. The T7SSb is found in the major Gram-positive phylum Firmicutes where it was recently shown to target both eukaryotic and prokaryotic cells, suggesting a dual role for this pathway in host-microbe and microbe-microbe interactions. In this review, we compare the current understanding of the molecular architectures and substrate repertoires of the well-studied mycobacterial T7SSa systems to that of recently characterized T7SSb pathways and highlight how these differences may explain the observed biological functions of this understudied protein export machine.

Mini Review: Bacterial Membrane Composition and Its Modulation in Response to Stress

Antibiotics and other agents that perturb the synthesis or integrity of the bacterial cell envelope trigger compensatory stress responses. Focusing on Bacillus subtilis as a model system, this mini-review summarizes current views of membrane structure and insights into how cell envelope stress responses remodel and protect the membrane. Altering the composition and properties of the membrane and its associated proteome can protect cells against detergents, antimicrobial peptides, and pore-forming compounds while also, indirectly, contributing to resistance against compounds that affect cell wall synthesis. Many of these regulatory responses are broadly conserved, even where the details of regulation may differ, and can be important in the emergence of antibiotic resistance in clinical settings.

Extracellular Vesicle Biogenesis and Functions in Gram-Positive Bacteria

Extracellular vesicles (EVs) are membrane-derived lipid bilayers secreted by bacteria and eukaryotic cells. Bacterial membrane vesicles were discovered over 60 years ago and have been extensively studied in Gram-negative bacteria. During their production, EVs are loaded with proteins, nucleic acids, and various compounds that are subsequently released into the environment. Depending on the packaged cargo, EVs have a broad spectrum of action and are involved in pathogenesis, antibiotic resistance, nutrient uptake, and nucleic acid transfer. Due to differences in cell wall structure, EVs in Gram-positive bacteria have been disregarded for decades, and our understanding of their biogenesis and host cell interaction is incomplete. Recently, studies on bacteria such as Staphylococcus aureus, Streptococcus spp., Bacillus subtilis, and Mycobacterium spp. have demonstrated EV production in Gram-positive bacteria and shown the great importance EVs have in Gram-positive bacterial physiology and disease progression. Here, we review the latest findings on the biogenesis and functions of EVs from Gram-positive bacteria and identify key areas for future research.

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.

Flotillin-mediated membrane fluidity controls peptidoglycan synthesis and MreB movement

The bacterial plasma membrane is an important cellular compartment. In recent years it has become obvious that protein complexes and lipids are not uniformly distributed within membranes. Current hypotheses suggest that flotillin proteins are required for the formation of complexes of membrane proteins including cell-wall synthetic proteins. We show here that bacterial flotillins are important factors for membrane fluidity homeostasis. Loss of flotillins leads to a decrease in membrane fluidity that in turn leads to alterations in MreB dynamics and, as a consequence, in peptidoglycan synthesis. These alterations are reverted when membrane fluidity is restored by a chemical fluidizer. In vitro, the addition of a flotillin increases membrane fluidity of liposomes. Our data support a model in which flotillins are required for direct control of membrane fluidity rather than for the formation of protein complexes via direct protein-protein interactions.

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.

An extracellular domain of the EsaA membrane component of the type VIIb secretion system: expression, purification and crystallization

The biophysical characterization and crystallization of an extracellular domain of the EsaA membrane component of the type VIIb secretion system is described.

The membrane protein EsaA is a conserved component of the type VIIb secretion system. Limited proteolysis of purified EsaA from Staphylococcus aureus USA300 identified a stable 48 kDa fragment, which was mapped by fingerprint mass spectrometry to an uncharacterized extracellular segment of EsaA. Analysis by circular dichroism spectroscopy showed that this fragment folds into a single stable domain made of mostly α-helices with a melting point of 34.5°C. Size-exclusion chromatography combined with multi-angle light scattering indicated the formation of a dimer of the purified extracellular domain. Octahedral crystals were grown in 0.2 M ammonium citrate tribasic pH 7.0, 16% PEG 3350 using the hanging-drop vapor-diffusion method. Diffraction data were analyzed to 4.0 Å resolution, showing that the crystals belonged to the enantiomorphic tetragonal space groups P4₁2₁2 or P4₃2₁2, with unit-cell parameters a = 197.5, b = 197.5, c = 368.3 Å, α = β = γ = 90°.

Proteomic analysis of Escherichia coli detergent-resistant membranes (DRM)

Membrane microdomains or lipid rafts compartmentalize cellular processes by laterally organizing membrane components. Such sub-membrane structures were mainly described in eukaryotic cells, but, recently, also in bacteria. Here, the protein content of lipid rafts in Escherichia coli was explored by mass spectrometry analyses of Detergent Resistant Membranes (DRM). We report that at least three of the four E. coli flotillin homologous proteins were found to reside in DRM, along with 77 more proteins. Moreover, the proteomic data were validated by subcellular localization, using immunoblot assays and fluorescence microscopy of selected proteins. Our results confirm the existence of lipid raft-like microdomains in the inner membrane of E. coli and represent the first comprehensive profiling of proteins in these bacterial membrane platforms.

Protein Export into and across the Atypical Diderm Cell Envelope of Mycobacteria

Mycobacteria, including the infamous pathogen Mycobacterium tuberculosis, are high-GC Gram-positive bacteria with a distinctive cell envelope. Although there is a typical inner membrane, the mycobacterial cell envelope is unusual in having its peptidoglycan layer connected to a polymer of arabinogalactan, which in turn is covalently attached to long-chain mycolic acids that help form a highly impermeable mycobacterial outer membrane. This complex double-membrane, or diderm, cell envelope imparts mycobacteria with unique requirements for protein export into and across the cell envelope for secretion into the extracellular environment. In this article, we review the four protein export pathways known to exist in mycobacteria: two conserved systems that exist in all types of bacteria (the Sec and Tat pathways) and two specialized systems that exist in mycobacteria, corynebacteria, and a subset of low-GC Gram-positive bacteria (the SecA2 and type VII secretion pathways). We describe the progress made over the past 15 years in understanding each of these mycobacterial export pathways, and we highlight the need for research to understand the specific steps of protein export across the mycobacterial outer membrane.

Recent Advances in Anti-virulence Therapeutic Strategies With a Focus on Dismantling Bacterial Membrane Microdomains, Toxin Neutralization, Quorum-Sensing Interference and Biofilm Inhibition

Antimicrobial resistance constitutes one of the major challenges facing humanity in the Twenty-First century. The spread of resistant pathogens has been such that the possibility of returning to a pre-antibiotic era is real. In this scenario, innovative therapeutic strategies must be employed to restrict resistance. Among the innovative proposed strategies, anti-virulence therapy has been envisioned as a promising alternative for effective control of the emergence and spread of resistant pathogens. This review presents some of the anti-virulence strategies that are currently being developed, it will cover strategies focused on quench pathogen quorum sensing (QS) systems, disassemble of bacterial functional membrane microdomains (FMMs), disruption of biofilm formation and bacterial toxin neutralization.

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.