Lipopolysaccharide Biosynthesis and Transport to the Outer Membrane of Gram-Negative Bacteria

A molecular overview of the polymyxin-LPS interaction in the context of its mode of action and resistance development

With the rising incidences of antimicrobial resistance (AMR) and the diminishing options of novel antimicrobial agents, it is paramount to decipher the molecular mechanisms of action and the emergence of resistance to the existing drugs. Polymyxin, a cationic antimicrobial lipopeptide, is used to treat infections by Gram-negative bacterial pathogens as a last option. Though polymyxins were identified almost seventy years back, their use has been restricted owing to toxicity issues in humans. However, their clinical use has been increasing in recent times resulting in the rise of polymyxin resistance. Moreover, the detection of "mobile colistin resistance (mcr)" genes in the environment and their spread across the globe have complicated the scenario. The mechanism of polymyxin action and the development of resistance is not thoroughly understood. Specifically, the polymyxin-bacterial lipopolysaccharide (LPS) interaction is a challenging area of investigation. The use of advanced biophysical techniques and improvement in molecular dynamics simulation approaches have furthered our understanding of this interaction, which will help develop polymyxin analogs with better bactericidal effects and lesser toxicity in the future. In this review, we have delved deeper into the mechanisms of polymyxin-LPS interactions, highlighting several models proposed, and the mechanisms of polymyxin resistance development in some of the most critical Gram-negative pathogens.

Microbiota substances modulate dendritic cells activity: A critical view

Contemporary research in the field of microbiota shows that commensal bacteria influence physiological activity of different organs and systems of a human organism, such as brain, lungs, immune and metabolic systems. This influence is realized by various processes. One of them is trough modulation of immune mechanisms. Interactions between microbiota and the human immune system are known to be complex and ambiguous. Dendritic cells (DCs) are unique cells, which initiate the development and polarization of adaptive immune response. These cells also interconnect native and specific immune reactivity. A large set of biochemical signals from microbiota in the form of different microbiota associated molecular patterns (MAMPs) and bacterial metabolites that act locally and distantly in the human organism. As a result, commensal bacteria influence the maturity and activity of dendritic cells and affect the overall immune reactivity of the human organism. It then determines the response to pathogenic microorganisms, inflammation, associated with different pathological conditions and even affects the effectiveness of vaccination.

Redundant essentiality of AsmA-like proteins in Pseudomonas aeruginosa

The outer membrane (OM) is an essential structure of Gram-negative bacteria that provides mechanical strength and protection from large and/or hydrophobic toxic molecules, including many antibiotics. The OM is composed of glycerophospholipids (GPLs) and lipopolysaccharide (LPS) in the inner and outer leaflets, respectively, and hosts integral β-barrel proteins and lipoproteins. While the systems responsible for translocation and insertion of LPS and OM proteins have been elucidated, the mechanism(s) mediating transport of GPLs from the inner membrane to the OM has remained elusive for decades. Very recently, studies performed in Escherichia coli proposed a role in this process for AsmA-like proteins that are predicted to share structural features with eukaryotic lipid transporters. In this study, we provide the first systematic investigation of AsmA-like proteins in a bacterium other than E. coli, the opportunistic human pathogen Pseudomonas aeruginosa. Bioinformatic analyses revealed that P. aeruginosa possesses seven AsmA-like proteins. Deletion of asmA-like genes in many different combinations, coupled with conditional mutagenesis, revealed that four AsmA-like proteins are redundantly essential for growth and OM integrity in P. aeruginosa, including a novel AsmA-like protein (PA4735) that is not present in E. coli. Cells depleted of AsmA-like proteins showed severe defects in the OM permeability barrier that were partially rescued by lowering the synthesis or transport of LPS. Since fine balancing of GPL and LPS levels is crucial for OM integrity, this evidence supports the role of AsmA-like proteins in GPL transport toward the OM.

IMPORTANCE

Given the importance of the outer membrane (OM) for viability and antibiotic resistance in Gram-negative bacteria, in the last decades, several studies have focused on the characterization of the systems involved in OM biogenesis, which have also been explored as targets for antibacterial drug development. However, the mechanism mediating translocation of glycerophospholipids (GPLs) to the OM remained unknown until recent studies provided evidence that AsmA-like proteins could be responsible for this process. Here, we demonstrate for the first time that AsmA-like proteins are essential and redundant for growth and OM integrity in a Gram-negative bacterium other than the model organism Escherichia coli and demonstrate that the human pathogen Pseudomonas aeruginosa has an additional essential AsmA-like protein that is not present in E. coli, thus expanding the range of AsmA-like proteins that play key functions in Gram-negative bacteria.

Xanthorrhizol inhibits mitochondrial damage, oxidative stress and inflammation in LPS-induced MLE-12 cells by regulating MAPK pathway

LPS-induced injury in lung epithelial cells is a crucial part of the process of acute lung injury (ALI). The aim of this study is to explore whether Xanthorrhizol, a medicine that has antioxidant and anti-inflammatory activity, could mitigate the injury of lung epithelial cells caused by LPS. Mouse lung epithelial cell line (MLE-12 cells) were treated with LPS in the absence and presence of Xanthorrhizol. As a results, we observed that LPS could induce MLE-12 cells death, mitochondrial dysfunction, oxidative stress and inflammation, and activate MAPK signaling pathways. However, Xanthorrhizol mitigated the injury in MEL-12 caused by LPS by promoting cell viability and MDA, GSH production as well as inhibiting LDH release, mitochondria damage, IL-1β, IL-6 and TNF-α production and the phosphorylation levels of ERK, P38 and JNK. Our results indicated that Xanthorrhizol could protect lung epithelial cells from LPS-induced injury, more likely by inhibiting the phosphorylation of MAPK pathway related proteins.

Glucose Transport in Escherichia coli: From Basics to Transport Engineering

Escherichia coli is the best-known model for the biotechnological production of many biotechnological products, including housekeeping and heterologous primary and secondary metabolites and recombinant proteins, and is an efficient biofactory model to produce biofuels to nanomaterials. Glucose is the primary substrate used as the carbon source for laboratory and industrial cultivation of E. coli for production purposes. Efficient growth and associated production and yield of desired products depend on the efficient sugar transport capabilities, sugar catabolism through the central carbon catabolism, and the efficient carbon flux through specific biosynthetic pathways. The genome of E. coli MG1655 is 4,641,642 bp, corresponding to 4702 genes encoding 4328 proteins. The EcoCyc database describes 532 transport reactions, 480 transporters, and 97 proteins involved in sugar transport. Nevertheless, due to the high number of sugar transporters, E. coli uses preferentially few systems to grow in glucose as the sole carbon source. E. coli nonspecifically transports glucose from the extracellular medium into the periplasmic space through the outer membrane porins. Once in periplasmic space, glucose is transported into the cytoplasm by several systems, including the phosphoenolpyruvate-dependent phosphotransferase system (PTS), the ATP-dependent cassette (ABC) transporters, and the major facilitator (MFS) superfamily proton symporters. In this contribution, we review the structures and mechanisms of the E. coli central glucose transport systems, including the regulatory circuits recruiting the specific use of these transport systems under specific growing conditions. Finally, we describe several successful examples of transport engineering, including introducing heterologous and non-sugar transport systems for producing several valuable metabolites.

Recombinant Cas9 protein production in an endotoxin-free system and evaluation with editing the BCL11A gene in human cells

Many therapeutic proteins are expressed in Escherichia coli bacteria for the low cost and high yield obtained. However, these gram-negative bacteria also generate undesirable endotoxin byproducts such as lipopolysaccharides (LPS). These endotoxins can induce a human immune response and cause severe inflammation. To mitigate this problem, we have employed the ClearColi BL21 (DE3) endotoxin-free cells as an expression host for Cas9 protein production. Cas9 is an endonuclease enzyme that plays a key role in the Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) and CRISPR associated protein 9 (CRISPR/Cas9) genome editing technique. This technology is very promising for use in diagnostics as well as treatment of diseases, especially for genetic diseases such as thalassemia. The potential uses for this technology thus generate a considerable interest for Cas9 utilization as a therapeutic protein in clinical treatment. Therefore, special care in protein production should be a major concern. Accordingly, we expressed the Cas9 protein in endotoxin-free bacterial cells achieving 99% purity with activity comparable to commercially available Cas9. Our protocol therefore yields a cost-effective product suitable for invitro experiments with stem cells.

Challenges for the development of a universal vaccine against leptospirosis revealed by the evaluation of 22 vaccine candidates

Leptospirosis is a neglected disease of man and animals that affects nearly half a million people annually and causes considerable economic losses. Current human vaccines are inactivated whole-cell preparations (bacterins) of Leptospira spp. that provide strong homologous protection yet fail to induce a cross-protective immune response. Yearly boosters are required, and serious side-effects are frequently reported so the vaccine is licensed for use in humans in only a handful of countries. Novel universal vaccines require identification of conserved surface-exposed epitopes of leptospiral antigens. Outer membrane β-barrel proteins (βb-OMPs) meet these requirements and have been successfully used as vaccines for other diseases. We report the evaluation of 22 constructs containing protein fragments from 33 leptospiral βb-OMPs, previously identified by reverse and structural vaccinology and cell-surface immunoprecipitation. Three-dimensional structures for each leptospiral βb-OMP were predicted by I-TASSER. The surface-exposed epitopes were predicted using NetMHCII 2.2 and BepiPred 2.0. Recombinant constructs containing regions from one or more βb-OMPs were cloned and expressed in Escherichia coli. IMAC-purified recombinant proteins were adsorbed to an aluminium hydroxide adjuvant to produce the vaccine formulations. Hamsters (4-6 weeks old) were vaccinated with 2 doses containing 50 – 125 μg of recombinant protein, with a 14-day interval between doses. Immunoprotection was evaluated in the hamster model of leptospirosis against a homologous challenge (10 – 20× ED₅₀) with L. interrogans serogroup Icterohaemorrhagiae serovar Copenhageni strain Fiocruz L1-130. Of the vaccine formulations, 20/22 were immunogenic and induced significant humoral immune responses (IgG) prior to challenge. Four constructs induced significant protection (100%, P < 0.001) and sterilizing immunity in two independent experiments, however, this was not reproducible in subsequent evaluations (0 – 33.3% protection, P > 0.05). The lack of reproducibility seen in these challenge experiments and in other reports in the literature, together with the lack of immune correlates and commercially available reagents to characterize the immune response, suggest that the hamster may not be the ideal model for evaluation of leptospirosis vaccines and highlight the need for evaluation of alternative models, such as the mouse.

Lipopolysaccharide detection by the innate immune system may be an uncommon defence strategy used in nature

Since the publication of the Janeway's Pattern Recognition hypothesis in 1989, study of pathogen-associated molecular patterns (PAMPs) and their immuno-stimulatory activities has accelerated. Most studies in this area have been conducted in model organisms, which leaves many open questions about the universality of PAMP biology across living systems. Mammals have evolved multiple proteins that operate as receptors for the PAMP lipopolysaccharide (LPS) from Gram-negative bacteria, but LPS is not immuno-stimulatory in all eukaryotes. In this review, we examine the history of LPS as a PAMP in mammals, recent data on LPS structure and its ability to activate mammalian innate immune receptors, and how these activities compare across commonly studied eukaryotes. We discuss why LPS may have evolved to be immuno-stimulatory in some eukaryotes but not others and propose two hypotheses about the evolution of PAMP structure based on the ecology and environmental context of the organism in question. Understanding PAMP structures and stimulatory mechanisms across multi-cellular life will provide insights into the evolutionary origins of innate immunity and may lead to the discovery of new PAMP variations of scientific and therapeutic interest.

Ginkgolide A Participates in LPS-Induced PMVEC Injury by Regulating miR-224 and Inhibiting p21 in a Targeted Manner

Most studies have focused on the protective effects of ginkgolide A against ischemia/reperfusion-induced cardiomyopathy and injury of the brain, liver, and other organs, but there are few reports about the protection of lung tissues. This study was designed to clarify the protection of ginkgolide A against lipopolysaccharide (LPS)-induced pulmonary microvascular endothelial cell (PMVEC) injury. PMVECs were extracted and fell into control, LPS, and ginkgolide A groups. Next, we delved into the growth activity and apoptosis rate of cells via the CCK-8 assay and Hoechst staining, independently. Beyond that, western blotting (WB) was implemented for measurement of the expressions of cyclin D1, cyclin-dependent kinase 4 (CDK4), and CDK inhibitor (p21) that pertained to the cell cycle. The target sites of ginkgolide A were confirmed by miRNA array and real-time quantitative PCR. The relationship between miR-224 and p21 was analyzed using dual-luciferase reporter gene assay. Compared with the control group, the LPS group and ginkgolide A group had significantly decreased cell growth activity and relative expressions of cyclin D1 and CDK4 and elevated apoptosis rate and p21 expression. Pronounced elevations were observable in the cell growth activity and expressions of cyclin D1, CDK4, and p21, while the ginkgolide A group presented with a reduced apoptosis rate in comparison with the LPS group (P < 0.05). MiR-224 was the target of ginkgolide A, which had targeted regulatory effects on p21. Ginkgolide A can modulate miR-224 expression and regulate p21 expression in a targeted manner to enhance the resistance of PMVECs to LPS-induced cell apoptosis.

Periodontitis and cardiometabolic disorders: The role of lipopolysaccharide and endotoxemia

Lipopolysaccharide is a virulence factor of gram-negative bacteria with a crucial importance to the bacterial surface integrity. From the host's perspective, lipopolysaccharide plays a role in both local and systemic inflammation, activates both innate and adaptive immunity, and can trigger inflammation either directly (as a microbe-associated molecular pattern) or indirectly (by inducing the generation of nonmicrobial, danger-associated molecular patterns). Translocation of lipopolysaccharide into the circulation causes endotoxemia, which is typically measured as the biological activity of lipopolysaccharide to induce coagulation of an aqueous extract of blood cells of the assay. Apparently healthy subjects have a low circulating lipopolysaccharide activity, since it is neutralized and cleared rapidly. However, chronic endotoxemia is involved in the pathogenesis of many inflammation-driven conditions, especially cardiometabolic disorders. These include atherosclerotic cardiovascular diseases, obesity, liver diseases, diabetes, and metabolic syndrome, where endotoxemia has been recognized as a risk factor. The main source of endotoxemia is thought to be the gut microbiota. However, the oral dysbiosis in periodontitis, which is typically enriched with gram-negative bacterial species, may also contribute to endotoxemia. As endotoxemia is associated with an increased risk of cardiometabolic disorders, lipopolysaccharide could be considered as a molecular link between periodontal microbiota and cardiometabolic diseases.

Degradation of Components of the Lpt Transenvelope Machinery Reveals LPS-Dependent Lpt Complex Stability in Escherichia coli

Lipopolysaccharide (LPS) is a peculiar component of the outer membrane (OM) of many Gram-negative bacteria that renders these bacteria highly impermeable to many toxic molecules, including antibiotics. LPS is assembled at the OM by a dedicated intermembrane transport system, the Lpt (LPS transport) machinery, composed of seven essential proteins located in the inner membrane (IM) (LptB₂CFG), periplasm (LptA), and OM (LptDE). Defects in LPS transport compromise LPS insertion and assembly at the OM and result in an overall modification of the cell envelope and its permeability barrier properties. LptA is a key component of the Lpt machine. It connects the IM and OM sub-complexes by interacting with the IM protein LptC and the OM protein LptD, thus enabling the LPS transport across the periplasm. Defects in Lpt system assembly result in LptA degradation whose stability can be considered a marker of an improperly assembled Lpt system. Indeed, LptA recruitment by its IM and OM docking sites requires correct maturation of the LptB₂CFG and LptDE sub-complexes, respectively. These quality control checkpoints are crucial to avoid LPS mistargeting. To further dissect the requirements for the complete Lpt transenvelope bridge assembly, we explored the importance of LPS presence by blocking its synthesis using an inhibitor compound. Here, we found that the interruption of LPS synthesis results in the degradation of both LptA and LptD, suggesting that, in the absence of the LPS substrate, the stability of the Lpt complex is compromised. Under these conditions, DegP, a major chaperone–protease in Escherichia coli, is responsible for LptD but not LptA degradation. Importantly, LptD and LptA stability is not affected by stressors disturbing the integrity of LPS or peptidoglycan layers, further supporting the notion that the LPS substrate is fundamental to keeping the Lpt transenvelope complex assembled and that LptA and LptD play a major role in the stability of the Lpt system.

Membrane Fractionation by Isopycnic Sucrose Density Gradient Centrifugation for Qualitative Analysis of LPS in Escherichia coli

Gram-negative diderm bacteria are characterized by a tripartite cell envelope, composed of an inner membrane (IM) and a lipopolysaccharide (LPS)-containing outer membrane (OM), separated by an aqueous space where the peptidoglycan is embedded. LPS is a peculiar glycolipid endowed with several biological activities. The biosynthesis and transport of LPS to its final location take place in every compartment of the cell envelope. Proteins and protein machineries with different subcellular localization are involved in this process to facilitate the trafficking of LPS across subcellular compartments that differ in their physicochemical proprieties. The fractionation of bacterial cell envelopes can give information on the status of the LPS biogenesis by allowing the analysis of LPS profiles and of the localization of proteins involved in the transport. Here, we describe a standardized protocol for membrane fractionation in Escherichia coli using sucrose density gradient centrifugation that separates the IM from the OM cellular fractions. Bacterial cells are first converted into spheroplasts and lysed; then the membrane fractions are collected by ultracentrifugation and separated at high speed by exploiting the differences in membrane density. The fractions obtained are analyzed for LPS total amount and electrophoretic profile.

Salidroside attenuates acute lung injury via inhibition of inflammatory cytokine production

Acute lung injury is a fatal condition characterized by excessive inflammation responses. Salidroside, the active constituent of Rhodiola rosea, possesses properties including anti-oxidation, anti-aging, anti-inflammatory, anti-hypoxia, and anti-cancer activities. In the present study, Salidroside attenuated acute lung injury via inhibition of inflammatory cytokine production. Rats pre-treated with Salidroside showed attenuated lipopolysaccharide (LPS)-induced pathological damage and suppressed tumor necrosis factor-alpha (TNFα) and interleukin 6 (IL-6) secretion in the lung. Furthermore, flow cytometry showed that Salidroside reduced the production of TNFα and IL-6 in NR8383 alveolar macrophages. These findings suggest that Salidroside may attenuate LPS-induced acute lung injury.

Synthesis of structurally-defined polymeric glycosylated phosphoprenols as potential lipopolysaccharide biosynthetic probes

The biosynthesis of lipopolysaccharide (LPS), a key immunomodulatory molecule produced by gram-negative bacteria, has been a topic of long-term interest. To date, the chemical probes used as tools to study LPS biosynthetic pathways have consisted primarily of small fragments of the larger structure (e.g., the O-chain repeating unit). While such compounds have helped to provide significant insight into many aspects of LPS assembly, understanding other aspects will require larger, more complex probes. For example, the molecular interactions between polymeric LPS biosynthetic intermediates and the proteins that transfer them across the inner and outer membrane remain largely unknown. We describe the synthesis of two lipid-linked polysaccharides, containing 11 and 27 monosaccharide residues, that are related to LPS O-chain biosynthesis in Escherichia coli O9a. This work has led not only to multi-milligram quantities of two biosynthetic probes, but also provided insights into challenges that must be overcome in the chemical synthesis of structurally-defined polysaccharides.

The synthesis of lipid-linked polysaccharides containing 11 and 27 monosaccharides via a ‘frame-shift’ strategy is described. The work provides biosynthetic probes and highlights challenges in synthesizing structurally-defined polymeric glycans.

Mice Plasmacytoid Dendritic Cells Were Activated by Lipopolysaccharides Through Toll-Like Receptor 4/Myeloid Differentiation Factor 2

Plasmacytoid dendritic cells (pDCs) are known to respond to viral infections. However, the activation of pDCs by bacterial components such as lipopolysaccharides (LPS) has not been well studied. Here, we found that pDCs, conventional dendritic cells (cDCs), and B cells express high levels of toll-like receptor 4 (TLR4), a receptor for LPS. Moreover, LPS could effectively bind to not only cDCs but also pDCs and B cells. Intraperitoneal administration of LPS promoted activation of splenic pDCs and cDCs. LPS treatment led to upregulation of interferon regulatory factor 7 (IRF7) and induced production of interferon-alpha (IFN-α) in splenic pDCs. Furthermore, LPS-dependent upregulation of co-stimulatory molecules in pDCs did not require the assistance of other immune cells, such as cDCs. However, the production levels of IFN-α were decreased in cDC-depleted splenocytes, indicating that cDCs may contribute to the enhancement of IFN-α production in pDCs. Finally, we showed that activation of pDCs by LPS requires the TLR4 and myeloid differentiation factor 2 (MD2) signaling pathways. Thus, these results demonstrate that the gram-negative component LPS can directly stimulate pDCs via TLR4/MD2 stimulation in mice.

Psychological and Gastrointestinal Symptoms of Patients with Irritable Bowel Syndrome Undergoing a Low-FODMAP Diet: The Role of the Intestinal Barrier

A diet low in fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (LFD) improves both gastrointestinal (GI) symptoms and the psychological profile of patients with irritable bowel syndrome with diarrhea (IBS-D). The effects of 12 weeks of LFD on GI symptom and psychological profiles in relation to inflammation and the involvement of the intestinal barrier were studied in twenty IBS-D patients. The IBS Severity Scoring System, the Symptom Checklist-90-Revised, the Italian version of the 36-Item Short-Form Health Survey, the IBS-Quality of Life (QoL) questionnaire, and the Psychophysiological questionnaire were administered. The GI barrier function was assessed by sugar absorption test, the serum and fecal zonulin levels, and the serum levels of intestinal fatty-acid binding protein and diamine oxidase. Interleukins (ILs) and lipopolysaccharide (LPS) serum levels were evaluated along with dysbiosis. At the end of LFD, GI symptoms, psychological state (mainly anxiety, somatization, psychoticism, and interpersonal sensitivity), and QoL significantly improved in these patients. Simultaneously, an improvement in small intestinal permeability and intestinal mucosal integrity occurred, while IL-6, Il-10, LPS, and fermentative dysbiosis significantly decreased. The LFD can modify the immune-inflammatory features and enhance intestinal permeability and mucosal integrity, thus determining a concurrent improvement in the clinical and psychological conditions.

ActS activates peptidoglycan amidases during outer membrane stress in Escherichia coli

The integrity of the cell envelope of E. coli relies on the concerted activity of multi-protein machineries that synthesize the peptidoglycan (PG) and the outer membrane (OM). Our previous work found that the depletion of lipopolysaccharide (LPS) export to the OM induces an essential PG remodeling process involving LD-transpeptidases (LDTs), the glycosyltransferase function of PBP1B and the carboxypeptidase PBP6a. Consequently, cells with defective OM biogenesis lyse if they lack any of these PG enzymes. Here we report that the morphological defects, and lysis associated with a ldtF mutant with impaired LPS transport, are alleviated by the loss of the predicted OM-anchored lipoprotein ActS (formerly YgeR). We show that ActS is an inactive member of LytM-type peptidoglycan endopeptidases due to a degenerated catalytic domain. ActS is capable of activating all three main periplasmic peptidoglycan amidases, AmiA, AmiB, and AmiC, which were previously reported to be activated only by EnvC and/or NlpD. Our data also suggest that in vivo ActS preferentially activates AmiC and that its function is linked to cell envelope stress.

Biosynthesis and function of cell-surface polysaccharides in the social bacterium Myxococcus xanthus

In bacteria, cell-surface polysaccharides fulfill important physiological functions, including interactions with the environment and other cells as well as protection from diverse stresses. The Gram-negative delta-proteobacterium Myxococcus xanthus is a model to study social behaviors in bacteria. M. xanthus synthesizes four cell-surface polysaccharides, i.e., exopolysaccharide (EPS), biosurfactant polysaccharide (BPS), spore coat polysaccharide, and O-antigen. Here, we describe recent progress in elucidating the three Wzx/Wzy-dependent pathways for EPS, BPS and spore coat polysaccharide biosynthesis and the ABC transporter-dependent pathway for O-antigen biosynthesis. Moreover, we describe the functions of these four cell-surface polysaccharides in the social life cycle of M. xanthus.

Integrated Proteomic and Transcriptomic Analyses Reveal the Roles of Brucella Homolog of BAX Inhibitor 1 in Cell Division and Membrane Homeostasis of Brucella suis S2

BAX inhibitor 1 (BI-1) is an evolutionarily conserved transmembrane protein first identified in a screening process for human proteins that suppress BAX-induced apoptosis in yeast cells. Eukaryotic BI-1 is a cytoprotective protein that suppresses cell death induced by multiple stimuli in eukaryotes. Brucella, the causative agent of brucellosis that threatens public health and animal husbandry, contains a conserved gene that encodes BI-1-like protein. To explore the role of the Brucella homolog of BI-1, BrBI, in Brucella suis S2, we constructed the brbI deletion mutant strain and its complemented strain. brbI deletion altered the membrane properties of Brucella suis S2 and decreased its resistance to acidic pH, H₂O₂, polymyxin B, and lincomycin. Additionally, deleting brbI led to defective growth, cell division, and viability in Brucella suis S2. We then revealed the effect of brbI deletion on the physiological characteristics of Brucella suis S2 via integrated transcriptomic and proteomic analyses. The integrated analysis showed that brbI deletion significantly affected the expression of multiple genes at the mRNA and/or protein levels. Specifically, the affected divisome proteins, FtsB, FtsI, FtsL, and FtsQ, may be the molecular basis of the impaired cell division of the brbI mutant strain, and the extensively affected membrane proteins and transporter-associated proteins were consistent with the phenotype of the membrane properties’ alterations of the brbI mutant strain. In conclusion, our results revealed that BrBI is a bacterial cytoprotective protein involved in membrane homeostasis, cell division, and stress resistance in Brucella suis S2.

Structural insights into outer membrane asymmetry maintenance in Gram-negative bacteria by MlaFEDB

The highly asymmetric outer membrane of Gram-negative bacteria functions in the defense against cytotoxic substances, such as antibiotics. The Mla pathway maintains outer membrane lipid asymmetry by transporting phospholipids between the inner and outer membranes. It comprises six Mla proteins, MlaFEDBCA, including the ABC transporter MlaFEDB, which functions via an unknown mechanism. Here we determine cryo-EM structures of Escherichia coli MlaFEDB in an apo state and bound to phospholipid, ADP or AMP-PNP to a resolution of 3.3-4.1 Å and establish a proteoliposome-based transport system that includes MlaFEDB, MlaC and MlaA-OmpF to monitor the transport direction of phospholipids. In vitro transport assays and in vivo membrane permeability assays combined with mutagenesis identify functional residues that not only recognize and transport phospholipids but also regulate the activity and structural stability of the MlaFEDB complex. Our results provide mechanistic insights into the Mla pathway, which could aid antimicrobial drug development.

Gut microbiome and cardiovascular disease

PURPOSE OF REVIEW

This review aims to highlight the association between gut microbiome and cardiovascular disease (CVD) with emphasis on the possible molecular mechanisms by which how gut microbiome contributes to CVD.

RECENT FINDINGS

Increasingly, the roles of gut microbiome in cardiovascular health and disease have gained much attention. Most of the investigations focus on how the gut dysbiosis contributes to CVD risk factors and which gut microbial-derived metabolites mediate such effects.

SUMMARY

In this review, we discuss the molecular mechanisms of gut microbiome contributing to CVD, which include gut microbes translocalization to aortic artery because of gut barrier defect to initiate inflammation and microbial-derived metabolites inducing inflammation-signaling pathway and renal insufficiency. Specifically, we categorize beneficial and deleterious microbial-derived metabolites in cardiovascular health. We also summarize recent findings in the gut microbiome modulation of drug efficacy in treatment of CVD and the microbiome mechanisms by which how physical exercise ameliorates cardiovascular health. Gut microbiome has become an essential component of cardiovascular research and a crucial consideration factor in cardiovascular health and disease.

Good Gone Bad: One Toxin Away From Disease for Bacteroides fragilis

The human gut is colonized by hundreds of trillions of microorganisms whose acquisition begins during early infancy. Species from the Bacteroides genus are ubiquitous commensals, comprising about thirty percent of the human gut microbiota. Bacteroides fragilis is one of the least abundant Bacteroides species, yet is the most common anaerobe isolated from extraintestinal infections in humans. A subset of B. fragilis strains carry a genetic element that encodes a metalloprotease enterotoxin named Bacteroides fragilis toxin, or BFT. Toxin-bearing strains, or Enterotoxigenic B. fragilis (ETBF) cause acute and chronic intestinal disease in children and adults. Despite this association with disease, around twenty percent of the human population appear to be asymptomatic carriers of ETBF. BFT damages the colonic epithelial barrier by inducing cleavage of the zonula adherens protein E-cadherin and initiating a cell signaling response characterized by inflammation and c-Myc-dependent pro-oncogenic hyperproliferation. As a consequence, mice harboring genetic mutations that predispose to colonic inflammation or tumor formation are uniquely susceptible to toxin-mediated injury. The recent observation of ETBF-bearing biofilms in colon biopsies from humans with colon cancer susceptibility loci strongly suggests that ETBF is a driver of colorectal cancer. This article will address ETBF biology from a host-pathobiont perspective, including clinical data, analysis of molecular mechanisms of disease, and the complex ecological context of the human gut.