Peptidoglycan Fragments Research Articles

Detection of “Non-culturable” <i>Mycobacterium tuberculosis</i> Cells by Culture Methods

The specific features of the reactivation of “non-culturable” (NC) Mycobacterium tuberculosis (MTB) cells for the following propagation in liquid medium were defined, as well as the approach for the quantification of these cells by culture methods was suggested. When NC cells obtained in vitro were inoculated into standard Sauton’s liquid medium, a prolonged (up to 20 days) lag-phase is observed, in which no cell propagation is occurred. During the lag-phase, dormant cells secrete substances of unknown nature that inhibit or completely arrest the reactivation and growth of cultures when initial dormant cells concentration is above 107–108 cells/mL. Addition of meat-peptone broth (3.25 g/L) into a standard Sauton’s medium with a 10-fold reduced concentration of glycerol significantly stimulated the reactivation and propagation of the initially dormant cells inoculated at a concentration above the threshold. This modification of the medium composition made it possible to detect up to 103 times more cells in the MTB population by the method of the Most Probable Number (MPN) of cells. Increased number of the detected dormant cells in the population (up to an average of 2.5 orders of magnitude) was also achieved by adding to the reactivation medium recombinant protein RpfB (5 ng/mL), a protein of the Rpf family – resuscitation promoting factor of dormant bacteria. Perhaps the action of a Rpf enzyme is related to the products of its enzymatic activity, since an increased MPN value in the dormant cells population was also observed when the products of mycobacteria peptidoglycan hydrolysis obtained by the coaction of RpfB and endopeptidase RipA were added. The addition of sonicated peptidoglycan fragments at a concentration up to 1 μg/mL had a similar effect. The obtained results may be used as approaches to optimize liquid media composition and culture conditions aiming to identify in clinical samples the pathogen of tuberculosis remain in “non-culturable” state.

Methotrexate suppresses psoriatic skin inflammation by inhibiting muropeptide transporter SLC46A2 activity

Cytosolic innate immune sensing is critical for protecting barrier tissues. NOD1 and NOD2 are cytosolic sensors of small peptidoglycan fragments (muropeptides) derived from the bacterial cell wall. These muropeptides enter cells, especially epithelial cells, through unclear mechanisms. We previously implicated SLC46 transporters in muropeptide transport in Drosophila immunity. Here, we focused on Slc46a2, which was highly expressed in mammalian epidermal keratinocytes, and showed that it was critical for the delivery of diaminopimelic acid (DAP)-muropeptides and activation of NOD1 in keratinocytes, whereas the related transporter Slc46a3 was critical for delivering the NOD2 ligand MDP to keratinocytes. In a mouse model, Slc46a2 and Nod1 deficiency strongly suppressed psoriatic inflammation, whereas methotrexate, a commonly used psoriasis therapeutic, inhibited Slc46a2-dependent transport of DAP-muropeptides. Collectively, these studies define SLC46A2 as a transporter of NOD1-activating muropeptides, with critical roles in the skin barrier, and identify this transporter as an important target for anti-inflammatory intervention.

Peptidoglycan-induced modulation of metabolic and inflammatory responses.

Bacterial cell wall peptidoglycan is composed of innate immune ligands and, due to its important structural role, also regulates access to many other innate immune ligands contained within the bacteria. There is a growing body of literature demonstrating how innate immune recognition impacts the metabolic functions of immune cells and how metabolic changes are not only important to inflammatory responses but are often essential. Peptidoglycan is primarily sensed in the context of the whole bacteria during lysosomal degradation; consequently, the innate immune receptors for peptidoglycan are primarily intracellular cytosolic innate immune sensors. However, during bacterial growth, peptidoglycan fragments are shed and can be found in the bloodstream of humans and mice, not only during infection but also derived from the abundant bacterial component of the gut microbiota. These peptidoglycan fragments influence cells throughout the body and are important for regulating inflammation and whole-body metabolic function. Therefore, it is important to understand how peptidoglycan-induced signals in innate immune cells and cells throughout the body interact to regulate how the body responds to both pathogenic and nonpathogenic bacteria. This mini-review will highlight key research regarding how cellular metabolism shifts in response to peptidoglycan and how systemic peptidoglycan sensing impacts whole-body metabolic function.

Bacterial virulence regulation through soluble peptidoglycan fragments sensing and response: knowledge gaps and therapeutic potential.

Given the growing clinical-epidemiological threat posed by the phenomenon of antibiotic resistance, new therapeutic options are urgently needed, especially against top nosocomial pathogens such as those within the ESKAPE group. In this scenario, research is pushed to explore therapeutic alternatives and, among these, those oriented toward reducing bacterial pathogenic power could pose encouraging options. However, the first step in developing these antivirulence weapons is to find weak points in the bacterial biology to be attacked with the goal of dampening pathogenesis. In this regard, during the last decades some studies have directly/indirectly suggested that certain soluble peptidoglycan-derived fragments display virulence-regulatory capacities, likely through similar mechanisms to those followed to regulate the production of several β-lactamases: binding to specific transcriptional regulators and/or sensing/activation of two-component systems. These data suggest the existence of intra- and also intercellular peptidoglycan-derived signaling capable of impacting bacterial behavior, and hence likely exploitable from the therapeutic perspective. Using the well-known phenomenon of peptidoglycan metabolism-linked β-lactamase regulation as a starting point, we gather and integrate the studies connecting soluble peptidoglycan sensing with fitness/virulence regulation in Gram-negatives, dissecting the gaps in current knowledge that need filling to enable potential therapeutic strategy development, a topic which is also finally discussed.

Mechanistic Insights into the Activities of Major Families of Enzymes in Bacterial Peptidoglycan Assembly and Breakdown.

Serving as an exoskeletal scaffold, peptidoglycan is a polymeric macromolecule that is essential and conserved across all bacteria, yet is absent in mammalian cells; this has made bacterial peptidoglycan a well-established excellent antibiotic target. In addition, soluble peptidoglycan fragments derived from bacteria are increasingly recognised as key signalling molecules in mediating diverse intra- and inter-species communication in nature, including in gut microbiota-host crosstalk. Each bacterial species encodes multiple redundant enzymes for key enzymatic activities involved in peptidoglycan assembly and breakdown. In this review, we discuss recent findings on the biochemical activities of major peptidoglycan enzymes, including peptidoglycan glycosyltransferases (PGT) and transpeptidases (TPs) in the final stage of peptidoglycan assembly, as well as peptidoglycan glycosidases, lytic transglycosylase (LTs), amidases, endopeptidases (EPs) and carboxypeptidases (CPs) in peptidoglycan turnover and metabolism. Biochemical characterisation of these enzymes provides valuable insights into their substrate specificity, regulation mechanisms and potential modes of inhibition.

Soluble peptidoglycan fragments produced by Limosilactobacillus fermentum with antiproliferative activity are suitable for potential therapeutic development: A preliminary report.

Currently, the use of probiotic strains and their products represents a promising innovative approach as an antagonist treatment against many human diseases. Previous studies showed that a strain of Limosilactobacillus fermentum (LAC92), previously defined as Lactobacillus fermentum, exhibited a suitable amensalistic property. The present study aimed to purify the active components from LAC92 to evaluate the biological properties of soluble peptidoglycan fragments (SPFs). The cell-free supernatant (CFS) and bacterial cells were separated after 48h of growth in MRS medium broth and treated for isolation of SPFs. Antimicrobial activity and proliferation analysis on the human cell line HTC116 were performed using technologies such as xCELLigence, count and viability, and clonogenic analysis. MALDI-MS investigation and docking analysis were performed to determine the molecular structure and hypothetical mode of action, respectively. Our results showed that the antimicrobial activity was mainly due to SPFs. Moreover, the results obtained when investigating the SPF effect on the cell line HCT116 showed substantial preliminary evidence, suggesting their significant cytostatic and quite antiproliferative properties. Although MALDI was unable to identify the molecular structure, it was subsequently revealed by analysis of the bacterial genome. The amino acid structure is called peptide 92. Furthermore, we confirmed by molecular docking studies the interaction of peptide 92 with MDM2 protein, the negative regulator of p53. This study showed that SPFs from the LAC92 strain exerted anticancer effects on the human colon cancer HCT116 cell line via antiproliferation and inducing apoptosis. These findings indicated that this probiotic strain might be a potential candidate for applications in functional products in the future. Further examination is needed to understand the specific advantages of this probiotic strain and improve its functional features to confirm these data. Moreover, deeper research on peptide 92 could increase our knowledge and help us understand if it will be possible to apply to specific diseases such as CRC.

Masters of Misdirection: Peptidoglycan Glycosidases in Bacterial Growth.

The dynamic composition of the peptidoglycan cell wall has been the subject of intense research for decades, yet how bacteria coordinate the synthesis of new peptidoglycan with the turnover and remodeling of existing peptidoglycan remains elusive. Diversity and redundancy within peptidoglycan synthases and peptidoglycan autolysins, enzymes that degrade peptidoglycan, have often made it challenging to assign physiological roles to individual enzymes and determine how those activities are regulated. For these reasons, peptidoglycan glycosidases, which cleave within the glycan strands of peptidoglycan, have proven veritable masters of misdirection over the years. Unlike many of the broadly conserved peptidoglycan synthetic complexes, diverse bacteria can employ unrelated glycosidases to achieve the same physiological outcome. Additionally, although the mechanisms of action for many individual enzymes have been characterized, apparent conserved homologs in other organisms can exhibit an entirely different biochemistry. This flexibility has been recently demonstrated in the context of three functions critical to vegetative growth: (i) release of newly synthesized peptidoglycan strands from their membrane anchors, (ii) processing of peptidoglycan turned over during cell wall expansion, and (iii) removal of peptidoglycan fragments that interfere with daughter cell separation during cell division. Finally, the regulation of glycosidase activity during these cell processes may be a cumulation of many factors, including protein-protein interactions, intrinsic substrate preferences, substrate availability, and subcellular localization. Understanding the true scope of peptidoglycan glycosidase activity will require the exploration of enzymes from diverse organisms with equally diverse growth and division strategies.

An improved lysozyme substrate preparation method for the biochemistry kinetics laboratory

The mutation and growth of Bacteriophage T4 Lysozyme (T4L) is easily accomplished in an undergraduate biochemistry laboratory and allows students to visualize how mutations affect enzyme stability and kinetics. However, full Michaelis-Menten parameters have been difficult to observe due to lack of a readily available, small molecule substrate. A FITC-tagged peptidoglycan has been used previously, but it is not commercially available and its preparation method was not appropriate for the undergraduate laboratory. Here, we describe a cost-effective, non-toxic, and reproducible means of generating FITC-tagged peptidoglycan fragments. Fluorescence assays monitoring substrate digestion using HEWL and T4L produced signals sufficient for reliable analysis of lysozyme kinetics.

Hijacking the Peptidoglycan Recycling Pathway of Escherichia coli to Produce Muropeptides.

Soluble fragments of peptidoglycan called muropeptides are released from the cell wall of bacteria as part of their metabolism or as a result of biological stresses. These compounds trigger immune responses in mammals and plants. In bacteria, they play a major role in the induction of antibiotic resistance. The development of efficient methods to produce muropeptides is, therefore, desirable both to address their mechanism of action and to design new antibacterial and immunostimulant agents. Herein, we engineered the peptidoglycan recycling pathway of Escherichia coli to produce N-acetyl-β-D-glucosaminyl-(1→4)-1,6-anhydro-N-acetyl-β-D-muramic acid (GlcNAc-anhMurNAc), a common precursor of Gram-negative and Gram-positive muropeptides. Inactivation of the hexosaminidase nagZ gene allowed the efficient production of this key disaccharide, providing access to Gram-positive muropeptides through subsequent chemical peptide conjugation. E. coli strains deficient in both NagZ hexosaminidase and amidase activities further enabled the in vivo production of Gram-negative muropeptides containing meso-diaminopimelic acid, a rarely available amino acid.

Chemoenzymatic Probes Reveal Peptidoglycan Recognition and Uptake Mechanisms in Candida albicans.

Candida albicans, the major fungal pathogen in humans, is under the strong influence of bacterial peptidoglycan fragments to undergo the yeast-to-hyphae transition, a key virulent step in C. albicans pathogenesis and infections. However, due to the synthetic difficulties of obtaining peptidoglycan fragments for biological studies, mechanistic details of how C. albicans recognizes and uptakes these peptidoglycan fragments have not been well elucidated. Notably, previous works have solely focused on the synthetic peptidoglycan ligand, muramyl dipeptide (MDP), despite its poor hyphal-inducing activity in C. albicans. In this work, we isolated and purified natural peptidoglycan fragments via enzymatic degradation of bacteria cell wall sacculi and chemoenzymatically installed a series of functional d-amino acids into the natural muropeptide, creating peptidoglycan probes that bear photoaffinity, bio-orthogonal, or fluorescent functionality. Using these chemoenzymatic peptidoglycan probes, we established that natural peptidoglycan fragments, which are potent hyphal-inducers, interact with the C. albicans Cyr1 sensor protein in the in-gel fluorescence assay as well as in in vitro pulldown studies. Moreover, we established that bacterial peptidoglycan probes enter C. albicans cells via an energy-dependent endocytic process.

Synthesis and Immunological Evaluation of Mannosylated Desmuramyl Dipeptides Modified by Lipophilic Triazole Substituents.

Muramyl dipeptide (N-acetylmuramyl-L-alanyl-D-isoglutamine, MDP) is the smallest peptidoglycan fragment able to trigger an immune response by activating the NOD2 receptor. Structural modification of MDP can lead to analogues with improved immunostimulating properties. The aim of this work was to prepare mannosylated desmuramyl peptides (ManDMP) containing lipophilic triazole substituents to study their immunomodulating activities in vivo. The adjuvant activity of the prepared compounds was evaluated in the mouse model using ovalbumin as an antigen and compared to the MDP and referent adjuvant ManDMPTAd. The obtained results confirm that the α-position of D-isoGln is the best position for the attachment of lipophilic substituents, especially adamantylethyl triazole. Compound 6c exhibited the strongest adjuvant activity, comparable to the MDP and better than referent ManDMPTAd.

Frontispiece: Recent Advances in the Synthesis and Biological Applications of Peptidoglycan Fragments

Peptidoglycan (PG) biosynthesis and breakdown can play vital roles in both bacterial viability and in the response of human physiology to bacterial infection. Chemical synthesis is therefore an important means to obtain PG fragments that may serve as enzyme substrates and elicitors of the human immune response since the fact that PG is an inhomogeneous insoluble macromolecule can hamper studies on PG biochemistry. More information can be found in the Review article by C.M. Vacariu and M.E. Tanner. (DOI: 10.1002/chem.202200788).

Novel Scaffolds for Modulation of NOD2 Identified by Pharmacophore-Based Virtual Screening.

Nucleotide-binding oligomerization domain-containing protein 2 (NOD2) is an innate immune pattern recognition receptor responsible for the recognition of bacterial peptidoglycan fragments. Given its central role in the formation of innate and adaptive immune responses, NOD2 represents a valuable target for modulation with agonists and antagonists. A major challenge in the discovery of novel small-molecule NOD2 modulators is the lack of a co-crystallized complex with a ligand, which has limited previous progress to ligand-based design approaches and high-throughput screening campaigns. To that end, a hybrid docking and pharmacophore modeling approach was used to identify key interactions between NOD2 ligands and residues in the putative ligand-binding site. Following docking of previously reported NOD2 ligands to a homology model of human NOD2, a structure-based pharmacophore model was created and used to virtually screen a library of commercially available compounds. Two compounds, 1 and 3, identified as hits by the pharmacophore model, exhibited NOD2 antagonist activity and are the first small-molecule NOD2 modulators identified by virtual screening to date. The newly identified NOD2 antagonist scaffolds represent valuable starting points for further optimization.

Purification and Characterization of a Stable, Membrane-Associated Peptidoglycan Responsive Adenylate Cyclase LRR Domain from Human Commensal Candida albicans.

The evolutionarily conserved leucine rich repeat (LRR) protein domain is a unique structural motif found in many viral, bacterial, archaeal, and eukaryotic proteins. The LRR domain serves many roles, including being a signaling domain and a pathogen recognition receptor. In the human innate immune system, it serves an essential role by recognizing fragments of bacterial cell walls. Interestingly, the human fungal pathogen Candida albicans also uses an LRR domain-containing protein, Cyrp1, to sense bacterial cell wall fragments. However, the dynamics of signaling and detection of bacterial peptidoglycan fragments by the LRR of Cyr1p remains poorly characterized. Here we develop optimal recombinant expression workflows and provide characterization of the entire region of the LRR domain of Cyr1p as a peripheral membrane protein. Using a newly designed peptidoglycan enrichment bead assay, we demonstrate that this domain can bind bacterial peptidoglycan fragments under native conditions. The new membrane-associated Cyr1p-LRR construct sets the stage for the development of antifungal agents via high-throughput campaigns to inhibit cell wall-Cyr1p interactions.

Detection of peptidoglycan in yeast as a marker for the presence or abundance of intracellular Helicobacter pylori and Staphylococcus.

Peptidoglycan (PG) was targeted as the marker for bacterial occurrence inside yeast. Detection of only few bacteria in old and new generations of yeast raised the question of how yeast controls the abundance of its intracellular bacteria. One gastric C. tropicalis that showed concurrence of H. pylori and Staphylococcus 16S rDNA was stained for assessing the viability of intracellular bacteria. Fluorescein isothiocyanate (FITC)-labeled anti-PG monoclonal antibody (APGMAb) was used for detection of PG inside yeast by direct immunofluorescence. APGMAb-coated magnetic beads were used for separation of bacteria from disrupted yeasts. Bead-bound bacteria were separated, fixed, stained, and examined by scanning electron microscope (SEM). Bead-bound bacteria were cultured and identified by amplification and sequencing of 16S rDNA. Fluorescence microscopy demonstrated occurrence of few live bacteria inside yeast cells. FITC- APGMAb interacted with PG of intracellular bacteria, appearing as few green spots in mother and daughter yeast cells. Interestingly, PG fragments were also detected in the exterior of yeast cells. SEM observations showed separated bead-bound bacilli and cocci. Culture of Staphylococcus was positive. Sequencing results confirmed identity of separated bacteria as H. pylori and Staphylococcus. PG detected inside yeast may have belonged to H. pylori, Staphylococcus or any other intracellular bacteria that coexisted in yeast as its microbiome. Detection of only few intracellular bacteria in old and new generations of yeast as well as PG fragments in their exterior suggested that yeast controls the abundance of its intracellular bacteria at low rate by hydrolysis and exporting of PG.

Recent Advances in the Synthesis and Biological Applications of Peptidoglycan Fragments.

The biosynthesis, breakdown, and modification of peptidoglycan (PG) play vital roles in both bacterial viability and in the response of human physiology to bacterial infection. Studies on PG biochemistry are hampered by the fact that PG is an inhomogeneous insoluble macromolecule. Chemical synthesis is therefore an important means to obtain PG fragments that may serve as enzyme substrates and elicitors of the human immune response. This review outlines the recent advances in the synthesis and biochemical studies of PG fragments, PG biosynthetic intermediates (such as Park's nucleotides and PG lipids), and PG breakdown products (such as muramyl dipeptides and anhydro-muramic acid-containing fragments). A rich variety of synthetic approaches has been applied to preparing such compounds since carbohydrate, peptide, and phospholipid chemical methodologies must all be applied.

Elucidation of Bacterial Peptidoglycan Signaling and Recognition in the Human Commensal Candida. albicans through the LRR domain of the peripheral Membrane Protein Cyr1p

The human commensal Candida. albicans (C. albicans) is the most common fungal resident of the human microbiome. In healthy individuals this fungus occupies a budding morphology, but during immune incompetence morphologically transitions to a pathogenic hyphal form. The long filamentous form of hyphae readily invades human epithelial cells and if not successfully treated, cause lethal systemic infections. One of the key molecular integrators in this morphological transition is the binding of bacterial peptidoglycan (PG) fragments to the Leucine Rich Repeat (LRR) domain of the adenyl cyclase Cyr1p. PG is a highly complex polymer consisting of repeating units of the disaccharide N‐acetyl glucosamine‐N acetyl muramic acid, which are crosslinked by pentapeptide side chains. Considerable diversity exists in this polymer within and among Gram (‐) and Gram (+) species of bacteria, rendering a plethora of potential ligands for Cyr1p. Identification of specific PG fragments that are able to induce hyphal formation in C. albicans, and stabilize Cyr1p through the LRR domain can elude to small molecule inhibitors to combat these lethal fungal infections that are continuously becoming resistant to current antifungal therapies. To approach a congruency of PG fragments from both Gram (‐) and Gram (+) bacterium that can serve as ligands for the LRR domain, we began by characterizing recombinant LRR and in doing so have determined this protein is a peripheral membrane protein that adopts an alpha helical conformation. Furthermore, to assess Cyr1p's ability to interact with a repertoire of PG fragments, we conducted Cellular Thermal Shift Assays (CETSA) with various PG fragments and have observed a temperature induced stabilization in comparison to apo‐protein, suggesting that there is a positive interaction with the LRR domain and the fragments. We further investigated the ability of these PG fragments to induce hyphal formation in WT C. albicans cells via microscopy assays and evaluated upregulation of hyphae specific genes at various timepoints via RT‐PCR. We observed differential hyphal gene regulation at various timepoints, suggesting that some fragments although able to initiate hyphal growth, failed to maintain growth and subsequent elongation. In addition, we observed that despite a positive association of the LRR with some fragments through CETSA assays, these fragments failed to illicit hyphal formation in WT cells. We infer that stabilization of the LRR is not linearly attributed to hyphal elongation, although hyphal initiation may be present. Collectively, these data help to identify new mechanisms to treat pathogenic fungal infections through the elucidation of promiscuity of the Cyr1p LRR domain, and steers us towards the identification of PG decoys and/or small molecule inhibitors for the treatment of these infections.

Metabolic Regulation of Quiescence and Antibiotic Tolerance in Uropathogenic Escherichia coli

Urinary tract infections (UTIs) pose a global public health threat. Uropathogenic Escherichia coli (UPEC) cause the majority of UTIs and a quarter of infections are recurrent within six months. This may be attributed to UPEC entering a non‐proliferative, or quiescent state that allows some cell populations to evade the host immune response and survive exposure to antibiotics, thus causing reinfection upon cessation of antibiotic treatment. A significant proportion of UPEC isolates, including the classic endemic UPEC lineage CFT073, are unable to proliferate at low cell density in vitro with glucose as the sole carbon source, but proliferate robustly at high cell density. Quiescence at low cell density is inhibited by external cues, such as peptidoglycan fragments, L‐Lysine, L‐Methionine, and succinate, which induce proliferation of quiescent cells and lead to a rapid increase in cell number. Here, we show that quiescent CFT073 cells are tolerant to antibiotics in vitro; however, proliferative cells are susceptible to antibiotics. Furthermore, we show that succinate fails to induce proliferation of cells deleted for sucCD, which encode enzymes that interconvert succinate and succinyl‐CoA in the tricarboxylic acid cycle. This suggests that succinyl‐CoA is limiting in the cell during quiescence and that quiescent cells may have lower metabolic activity. We demonstrate that increased levels of SucAB, which synthesize succinyl‐CoA from alpha ketoglutarate, prevent quiescence and induce proliferation. These results are consistent with a model in which quiescent UPEC cells exhibit reduced metabolism with limited succinyl‐CoA, and this underlies the antibiotic tolerance exhibited during UPEC quiescence in vitro.

The unusual peptidoglycan cell wall of Borrelia burgdorferi and it’s role in Lyme disease pathogenesis

Abstract Borrelia burgdorferi — the Lyme disease agent — does not produce typical bacterial proinflammatory compounds, so the genesis of Lyme arthritis (LA) is largely unknown. To better understand this manifestation, we’ve begun investigating B. burgdorferi peptidoglycan (PG). PG is a large polymer that protects bacteria from various stressors. While most double-membraned bacteria recycle PG fragments during cell wall turnover, B. burgdorferi lack this pathway, instead releasing ~45% of their total PG into their host per generation. Released PG can interact with the immune system as a pathogen associated molecular pattern (PAMP). To that end, we discovered PG lingers in the synovial fluid of LA patients and PG, alone, can cause arthritis in a mouse model. Virtually all bacteria produce PG, but the chemical composition of B. burgdorferi PG is unlike any previously described. We hypothesize the unique chemical structure of B. burgdorferi PG, coupled with shedding, impact host immune responses, and contribute to cell wall half-life and LA pathogenesis. To study this relationship, we developed an in vivo imaging system to track the real-time biodistribution of B. burgdorferi PG in mice. Our results suggest certain organs act as reservoirs, retaining material well after arthritis resolution. Apart from half-life, transcriptomic studies reveal that B. burgdorferi PG induces unique immunological responses in human PBMCs, that appear to be dictated by PG chemistry. Our findings provide a fundamental basis for understanding the complex interplay between a unique PAMP and a host, in addition to deciphering the role of the B. burgdorferi cell wall in Lyme disease pathogenesis, results that may lead to new therapeutic approaches to alleviate LA symptoms. Supported by grants from NIH (R21 AI159800), USDA (VA-160113), the Steven & Alexandra Cohen Foundation, and the Bay Area Lyme Foundation

Messengers From the Gut: Gut Microbiota-Derived Metabolites on Host Regulation.

The human gut is the natural habitat for trillions of microorganisms, known as the gut microbiota, which play indispensable roles in maintaining host health. Defining the underlying mechanistic basis of the gut microbiota-host interactions has important implications for treating microbiota-associated diseases. At the fundamental level, the gut microbiota encodes a myriad of microbial enzymes that can modify various dietary precursors and host metabolites and synthesize, de novo, unique microbiota-derived metabolites that traverse from the host gut into the blood circulation. These gut microbiota-derived metabolites serve as key effector molecules to elicit host responses. In this review, we summarize recent studies in the understanding of the major classes of gut microbiota-derived metabolites, including short-chain fatty acids (SCFAs), bile acids (BAs) and peptidoglycan fragments (PGNs) on their regulatory effects on host functions. Elucidation of the structures and biological activities of such gut microbiota-derived metabolites in the host represents an exciting and critical area of research.