Nanaerobic growth enables direct visualization of dynamic cellular processes in human gut symbionts

Metabolite dependence of antibiotic susceptibility in a gut microbe

Antibiotics save lives but can have unwanted effects on our gut microbes, thereby contributing to disease. A mechanistic understanding of how such microbes respond to antibiotics is hence critical. Recently in mSphere, Nilson et al. investigated the metabolite dependence of antibiotic susceptibility in Bacteroides thetaiotaomicron, an abundant and important member of our gut microbiota (R. Nilson, S. Penumutchu, F. S. Pagano, and P. Belenky, mSphere 9:e00103-24, 2024, https://doi.org/10.1128/msphere.00103-24). Their uncovered findings suggest the possibility of potentiating antibiotics with metabolites to reduce post-antibiotic "blooming" of B. thetaiotaomicron and the associated development of gut symbiosis.

A ubiquitous mobile genetic element changes the antagonistic weaponry of a human gut symbiont

DNA transfer is ubiquitous in the human gut microbiota, especially among species of the order Bacteroidales. In silico analyses have revealed hundreds of mobile genetic elements shared between these species, yet little is known about the phenotypes they encode, their effects on fitness, or pleiotropic consequences for the recipient's genome. In this work, we show that acquisition of a ubiquitous integrative conjugative element (ICE) encoding a type VI secretion system (T6SS) shuts down the native T6SS of Bacteroides fragilis. Despite inactivating this T6SS, ICE acquisition increases the fitness of the B. fragilis transconjugant over its progenitor by arming it with the new T6SS. DNA transfer causes the strain to change allegiances so that it no longer targets ecosystem members with the same element yet is armed for communal defense.

Oxygen tolerance in anaerobes as a virulence factor and a health-beneficial property

Oxygen tolerance of anaerobes is a virulence factor, but can also be a beneficial property. Many species have evolved to tolerate or take advantage of the presence of low, especially nanaerobic (≤0.14 %) oxygen concentrations. Oxygen tolerance is genus-, species- and strain-dependent according to their protective mechanisms. It was better expressed in some pathogenic species such as Bacteroides fragilis, Clostridioides difficile, and Clostridium perfringens, as well as in Akkermansia muciniphila than in other potential probiotics such as Alistipes, Blautia and Roseburia spp. Different degrees of oxygen sensitivity were found between the strains of Anaerostipes, Faecalibacterium, and Bifidobacterium spp. Importantly, clostridial spores and anaerobes in biofilms are protected from oxidation. Rubrerythrins and flavodiiron proteins and two regulators (sigma factor B and PerR) contribute to C. difficile protection from reactive oxygen species (ROS). The frequent pathogen, B. fragilis, has numerous protective factors such as enzymes (catalase, superoxide dismutase, alkyl hydroperoxidase, thioredoxin peroxidase, and aerobic-type NrdAB ribonucleotide reductase), and nanaerobic respiration. Seven proteins confer strain-specific oxygen adaptation of Faecalibacterium prausnitzii. Oxygen tolerance protects anaerobes from ROS, shields their DNA and modulates gene expression. Furthermore, oxygen can induce mutations leading to antibiotic resistance as shown in Prevotella melaninogenica. Some Faecalibacterium, Anaerostipes, Bifidobacterium, and Akkermansia strains from the intestinal microbiota exhibiting oxygen tolerance may become next-generation probiotic candidates. Further studies are needed to reveal oxygen effects on more anaerobic species and strains, and the influence of oxygen on antibiotic resistance. More studies on oxygen-tolerant probiotic strains can be useful to optimize biotechnological methods.

The type II secretion system as an underappreciated and understudied mediator of interbacterial antagonism

Interbacterial antagonism involves all major phyla, occurs across the full range of ecological niches, and has great significance for the environment, clinical arena, and agricultural and industrial sectors. Though the earliest insight into interbacterial antagonism traces back to the discovery of antibiotics, a paradigm shift happened when it was learned that protein secretion systems (e.g., types VI and IV secretion systems) deliver toxic "effectors" against competitors. However, a link between interbacterial antagonism and the Gram-negative type II secretion system (T2SS), which exists in many pathogens and environmental species, is not evident in prior reviews on bacterial competition or T2SS function. A current examination of the literature revealed four examples of a T2SS or one of its known substrates having a bactericidal activity against a Gram-positive target or another Gram-negative. When further studied, the T2SS effectors proved to be peptidases that target the peptidoglycan of the competitor. There are also reports of various bacteriolytic enzymes occurring in the culture supernatants of some other Gram-negative species, and a link between these bactericidal activities and T2SS is suggested. Thus, a T2SS can be a mediator of interbacterial antagonism, and it is possible that many T2SSs have antibacterial outputs. Yet, at present, the T2SS remains relatively understudied for its role in interbacterial competition. Arguably, there is a need to analyze the T2SSs of a broader range of species for their role in interbacterial antagonism. Such investigation offers, among other things, a possible pathway toward developing new antimicrobials for treating disease.

Far-Red Fluorescent Proteins: Tools for Advancing In Vivo Imaging

Far-red fluorescent proteins (FPs) have emerged as indispensable tools in in vivo imaging, playing a pivotal role in elucidating fundamental mechanisms and addressing application issues in biotechnology and biomedical fields. Their ability for deep penetration, coupled with reduced light scattering and absorption, robust resistance to autofluorescence, and diminished phototoxicity, has positioned far-red biosensors at the forefront of non-invasive visualization techniques for observing intracellular activities and intercellular behaviors. In this review, far-red FPs and their applications in living systems are mainly discussed. Firstly, various far-red FPs, characterized by emission peaks spanning from 600 nm to 650 nm, are introduced. This is followed by a detailed presentation of the fundamental principles enabling far-red biosensors to detect biomolecules and environmental changes. Furthermore, the review accentuates the superiority of far-red FPs in multi-color imaging. In addition, significant emphasis is placed on the value of far-red FPs in improving imaging resolution, highlighting their great contribution to the advancement of in vivo imaging.

Mechanisms of microbial co-aggregation in mixed anaerobic cultures

Co-aggregation of anaerobic microorganisms into suspended microbial biofilms (aggregates) serves ecological and biotechnological functions. Tightly packed aggregates of metabolically interdependent bacteria and archaea play key roles in cycling of carbon and nitrogen. Additionally, in biotechnological applications, such as wastewater treatment, microbial aggregates provide a complete metabolic network to convert complex organic material. Currently, experimental data explaining the mechanisms behind microbial co-aggregation in anoxic environments is scarce and scattered across the literature. To what extent does this process resemble co-aggregation in aerobic environments? Does the limited availability of terminal electron acceptors drive mutualistic microbial relationships, contrary to the commensal relationships observed in oxygen-rich environments? And do co-aggregating bacteria and archaea, which depend on each other to harvest the bare minimum Gibbs energy from energy-poor substrates, use similar cellular mechanisms as those used by pathogenic bacteria that form biofilms? Here, we provide an overview of the current understanding of why and how mixed anaerobic microbial communities co-aggregate and discuss potential future scientific advancements that could improve the study of anaerobic suspended aggregates. KEY POINTS: • Metabolic dependency promotes aggregation of anaerobic bacteria and archaea • Flagella, pili, and adhesins play a role in the formation of anaerobic aggregates • Cyclic di-GMP/AMP signaling may trigger the polysaccharides production in anaerobes.

Phosphorothioate DNA modification by BREX Type 4 systems in the human gut microbiome

Among dozens of microbial DNA modifications regulating gene expression and host defense, phosphorothioation (PT) is the only known backbone modification, with sulfur inserted at a non-bridging oxygen by dnd and ssp gene families. Here we explored the distribution of PT genes in 13,663 human gut microbiome genomes, finding that 6.3% possessed dnd or ssp genes predominantly in Bacillota, Bacteroidota, and Pseudomonadota. This analysis uncovered several putative new PT synthesis systems, including Type 4 Bacteriophage Exclusion (BREX) brx genes, which were genetically validated in Bacteroides salyersiae. Mass spectrometric analysis of DNA from 226 gut microbiome isolates possessing dnd, ssp, and brx genes revealed 8 PT dinucleotide settings confirmed in 6 consensus sequences by PT-specific DNA sequencing. Genomic analysis showed PT enrichment in rRNA genes and depletion at gene boundaries. These results illustrate the power of the microbiome for discovering prokaryotic epigenetics and the widespread distribution of oxidation-sensitive PTs in gut microbes.

The oral microbiome: diversity, biogeography and human health

The human oral microbiota is highly diverse and has a complex ecology, comprising bacteria, microeukaryotes, archaea and viruses. These communities have elaborate and highly structured biogeography that shapes metabolic exchange on a local scale and results from the diverse microenvironments present in the oral cavity. The oral microbiota also interfaces with the immune system of the human host and has an important role in not only the health of the oral cavity but also systemic health. In this Review, we highlight recent advances including novel insights into the biogeography of several oral niches at the species level, as well as the ecological role of candidate phyla radiation bacteria and non-bacterial members of the oral microbiome. In addition, we summarize the relationship between the oral microbiota and the pathology of oral diseases and systemic diseases. Together, these advances move the field towards a more holistic understanding of the oral microbiota and its role in health, which in turn opens the door to the study of novel preventive and therapeutic strategies.

Intestinal Translocation of Live Porphyromonas gingivalis Drives Insulin Resistance

Periodontitis has been emphasized as a risk factor of insulin resistance-related systemic diseases. Accumulating evidence has suggested a possible "oral-gut axis" linking oral infection and extraoral diseases, but it remains unclear whether periodontal pathogens can survive the barriers of the digestive tract and how they play their pathogenic roles. The present study established a periodontitis mouse model through oral ligature plus Porphyromonas gingivalis inoculation and demonstrated that periodontitis aggravated diet-induced obesity and insulin resistance, while also causing P. gingivalis enrichment in the intestine. Metabolic labeling strategy validated that P. gingivalis could translocate to the gastrointestinal tract in a viable state. Oral administration of living P. gingivalis elicited insulin resistance, while administration of pasteurized P. gingivalis had no such effect. Combination analysis of metagenome sequencing and nontargeted metabolomics suggested that the tryptophan metabolism pathway, specifically indole and its derivatives, was involved in the pathogenesis of insulin resistance caused by oral administration of living P. gingivalis. Moreover, liquid chromatography-high-resolution mass spectrometry analysis confirmed that the aryl hydrocarbon receptor (AhR) ligands, mainly indole acetic acid, tryptamine, and indole-3-aldehyde, were reduced in diet-induced obese mice with periodontitis, leading to inactivation of AhR signaling. Supplementation with Ficz (6-formylindolo (3,2-b) carbazole), an AhR agonist, alleviated periodontitis-associated insulin resistance, in which the restoration of gut barrier function might play an important role. Collectively, these findings reveal that the oral-gut translocation of viable P. gingivalis works as a fuel linking periodontitis and insulin resistance, in which reduction of AhR ligands and inactivation of AhR signaling are involved. This study provides novel insight into the role of the oral-gut axis in the pathogenesis of periodontitis-associated comorbidities.

A multilayer microfluidic system for studies of the dynamic responses of cellular proteins to oxygen switches at the single-cell level

Oxygen levels vary in the environment. Oxygen availability has a major effect on almost all organisms, and oxygen is far more than a substrate for energy production. However, less is known about related biological processes under hypoxic conditions and about the adaptations to changing oxygen concentrations. The yeast Saccharomyces cerevisiae can adapt its metabolism for growth under different oxygen concentrations and can grow even under anaerobic conditions. Therefore, we developed a microfluidic device that can generate serial, accurately controlled oxygen concentrations for single-cell studies of multiple yeast strains. This device can construct a broad range of oxygen concentrations, [O2] through on-chip gas-mixing channels from two gases fed to the inlets. Gas diffusion through thin polydimethylsiloxane (PDMS) can lead to the equilibration of [O2] in the medium in the cell culture layer under gas cover regions within 2 min. Here, we established six different and stable [O2] varying between ~0.1 and 20.9% in the corresponding layers of the device designed for multiple parallel single-cell culture of four different yeast strains. Using this device, the dynamic responses of different yeast transcription factors and metabolism-related proteins were studied when the [O2] decreased from 20.9% to serial hypoxic concentrations. We showed that different hypoxic conditions induced varying degrees of transcription factor responses and changes in respiratory metabolism levels. This device can also be used in studies of the aging and physiology of yeast under different oxygen conditions and can provide new insights into the relationship between oxygen and organisms. Integration, innovation and insight: Most living cells are sensitive to the oxygen concentration because they depend on oxygen for survival and proper cellular functions. Here, a composite microfluidic device was designed for yeast single-cell studies at a series of accurately controlled oxygen concentrations. Using this device, we studied the dynamic responses of various transcription factors and proteins to changes in the oxygen concentration. This study is the first to examine protein dynamics and temporal behaviors under different hypoxic conditions at the single yeast cell level, which may provide insights into the processes involved in yeast and even mammalian cells. This device also provides a base model that can be extended to oxygen-related biology and can acquire more information about the complex networks of organisms.

Assembly of a unique membrane complex in type VI secretion systems of Bacteroidota

The type VI secretion system (T6SS) of Gram-negative bacteria inhibits competitor cells through contact-dependent translocation of toxic effector proteins. In Proteobacteria, the T6SS is anchored to the cell envelope through a megadalton-sized membrane complex (MC). However, the genomes of Bacteroidota with T6SSs appear to lack genes encoding homologs of canonical MC components. Here, we identify five genes in Bacteroides fragilis (tssNQOPR) that are essential for T6SS function and encode a Bacteroidota-specific MC. We purify this complex, reveal its dimensions using electron microscopy, and identify a protein-protein interaction network underlying the assembly of the MC including the stoichiometry of the five TssNQOPR components. Protein TssN mediates the connection between the Bacteroidota MC and the conserved baseplate. Although MC gene content and organization varies across the phylum Bacteroidota, no MC homologs are detected outside of T6SS loci, suggesting ancient co-option and functional convergence with the non-homologous MC of Pseudomonadota.

Combinatorial fluorescent labeling of live anaerobic bacteria via the incorporation of azide-modified sugars into newly synthesized macromolecules

The human gut microbiome modulates physiological functions and pathologies; however, a mechanistic understanding of microbe-host and microbe-microbe interactions remains elusive owing to a lack of suitable approaches to monitor obligate anaerobic bacterial populations. Common genetically encoded fluorescent protein reporters, derived from the green fluorescent protein, require an oxidation step for fluorescent light emission and therefore are not suitable for use in anaerobic microbes residing in the intestine. Fluorescence in situ hybridization is a useful alternative to visualize bacterial communities in their natural niche; however, it requires tissue fixation. We therefore developed an approach for the real-time detection and monitoring of live communities of anaerobic gut commensals in their natural environment. We leverage the bacterial cells' reliance on sugars for macromolecule synthesis in combinatorial click chemistry labeling, where the addition of azide-modified sugars to the culturing media enables the fluorescence labeling of newly synthesized molecules via the addition of combinations of exogenous fluorophores conjugated to cyclooctynes. This process is suitable for labeling communities of live anaerobic gut bacteria with combinations of fluorophores that do not require oxygen to mature and fluoresce, and that can be detected over time in their natural environments. The labeling procedure requires 4-9 d, depending on the varying growth rates of different bacterial strains, and an additional 1-2 d for the detection and monitoring steps. The protocol can be completed by users with basic expertise in bacterial culturing.

Evaluation of oxidative stress in an experimental model of Crohn's disease treated with hyperbaric oxygen therapy

INTRODUCTION

Treatments of Inflammatory Bowel Disease (IBD) are able to control symptoms in most cases, however, a fraction of patients do not improve or have a loss of response to treatments, making it important to explore new therapeutic strategies. Hyperbaric oxygen therapy (HBO) may represent one of them. The aim of this study was to evaluate the effects of HBO therapy in an experimental model of IBD.

METHODS

Sixty male BALBc mice were divided into six groups. Group 1 was colitis-induced with trinitrobenzene sulfonic acid (TNBS) + ethanol, group 2 received TNBS + ethanol plus HBO, group 3 received only ethanol, group 4 received ethanol plus HBO, group 5 received saline solution, and group 6 received saline solution plus HBO. HBO was performed for four days, subsequently, the mice were evaluated daily. At the end of the study, samples from the intestine were collected for histological analysis as well as for measurement of antioxidant enzymes and cytokine levels.

RESULTS

HBO significantly improved the clinical and histological status of the animals. Treatment with HBO increased the activity of the antioxidant enzymes superoxide dismutase (SOD) and glutathione peroxidase (GPx) in all of the groups; moreover, the difference was only significant between the TNBS and TNBS + HBO groups and treatments promoted a reduction in the proinflammatory cytokines IFN-γ, IL-12, IL-17 and TNF-α and increased the anti-inflammatory cytokines IL-4 and IL-10, with no changes in IL-13.

CONCLUSION

HBO effectively treats TNBS-induced colitis by increasing the activity of antioxidant enzymes and modulating cytokine profiles.

Salmonella adhesion is decreased by hypoxia due to adhesion and motility structure crosstalk

Initial stages of Salmonella Typhimurium infection involve a series of coordinated events aimed at reaching, attaching to, and invading host cells. Virulence factors such as flagella, fimbriae, and secretion systems play crucial roles in these events and are regulated in response to the host environment. The first point of contact between the pathogen and host is the intestinal epithelial layer, which normally serves as a barrier against invading pathogens, but can also be an entry site for pathogens. The integrity of this barrier can be modulated by the hypoxic environment of the intestines, created by the presence of trillions of microbes. Variable oxygen concentrations can strongly affect many functions of the gut, including secretion of cytokines and growth factors from the host site and affect the ability of Salmonella to persist, invade, and replicate. In this study, we investigated the first stages of Salmonella Typhimurium infection under hypoxic conditions in vitro and found that low oxygen levels significantly decreased bacterial adhesion. Using adhesion and motility assays, biofilm formation tests, as well as gene expression and cytokine secretion analysis, we identified a hypoxia-specific cross-talk between the expression of type 1 fimbriae and flagella, suggesting that altered flagellin expression levels affect the motility of bacteria and further impact their adhesion level, biofilm formation ability, and innate immune response. Overall, understanding how Salmonella interacts with its variable host environment provides insights into the virulence mechanisms of the bacterium and information regarding strategies for preventing or treating infections. Further research is required to fully understand the complex interplay between Salmonella and its host environment.

A ubiquitous mobile genetic element disarms a bacterial antagonist of the gut microbiota

DNA transfer is ubiquitous in the gut microbiota, especially among species of Bacteroidales. In silico analyses have revealed hundreds of mobile genetic elements shared between these species, yet little is known about the phenotypes they encode, their effects on fitness, or pleiotropic consequences for the recipient's genome. Here, we show that acquisition of a ubiquitous integrative and conjugative element encoding an antagonistic system shuts down the native contact-dependent antagonistic system of Bacteroides fragilis . Despite inactivating the native antagonism system, mobile element acquisition increases fitness of the B. fragilis transconjugant over its progenitor by arming it with a new weapon. This DNA transfer causes the strain to change allegiances so that it no longer targets ecosystem members containing the same element yet is armed for communal defense.

Evaluation of Nanaerobic Digestion as a Mechanism to Explain Surplus Methane Production in Animal Rumina and Engineered Digesters

Nanaerobes are a newly described class of microorganisms that use a unique cytochrome bd oxidase to achieve nanaerobic respiration at <2 μM dissolved oxygen (∼1% of atmospheric oxygen) but are not viable above this value due to the lack of other terminal oxidases. Although sharing an overlapping ecological niche with methanogenic archaea, the role of nanaerobes in methanogenic systems has not been studied so far. To explore their occurrence and significance, we re-analyzed published meta-omic datasets from animal rumina and waste-to-energy digesters, including conventional anaerobic digesters and anaerobic digesters with ultra-low oxygenation. Results show that animal rumina share broad similarities in the microbial community and system performance with oxygenated digesters, rather than with conventional anaerobic digesters, implying that trace levels of oxygen drive the efficient digestion in ruminants. The rumen system serves as an ideal model for the newly named nanaerobic digestion, as it relies on the synergistic co-occurrence of nanaerobes and methanogens for methane yield enhancement. The most abundant ruminal bacterial family Prevotellaceae contains many nanaerobes, which perform not only anaerobic fermentation but also nanaerobic respiration using cytochrome bd oxidase. These nanaerobes generally accompany hydrogenotrophic methanogens to constitute a thermodynamically and physiologically consistent framework for efficient methane generation. Our findings provide new insights into ruminal methane emissions and strategies to enhance methane generation from biomass.

Human gut bacteria tailor extracellular vesicle cargo for the breakdown of diet- and host-derived glycans

Extracellular vesicles are produced in all three domains of life, and their biogenesis has common ancient origins in eukaryotes and archaea. Although bacterial vesicles were discovered several decades ago and multiple roles have been attributed to them, no mechanism has been established for vesicles biogenesis in bacteria. For this reason, there is a significant level of skepticism about the biological relevance of bacterial vesicles. Bacteroides thetaiotaomicron (Bt), a prominent member of the human intestinal microbiota, produces significant amounts of outer membrane vesicles (OMVs) which have been proposed to play key physiological roles. Here, we employed a dual marker system, consisting of outer membrane- and OMV-specific markers fused to fluorescent proteins to visualize OMV biogenesis by time-lapse microscopy. Furthermore, we performed comparative proteomic analyses to show that, in Bt, the OMV cargo is adapted for the optimal utilization of different polysaccharides. We also show that a negatively charged N-terminal motif acts as a signal for protein sorting into OMVs irrespective of the nutrient availability. Our results demonstrate that OMV production is the result of a highly regulated process in Bt.

Human gut bacteria tailor extracellular vesicle cargo for the breakdown of diet- and host-derived glycans

Extracellular vesicles (EV) are produced in all three domains of life, and their biogenesis have common ancient origins in eukaryotes and archaea. Although bacterial vesicles were discovered several decades ago and multiple roles have been attributed to them, no mechanism has been established for vesicles biogenesis in bacteria. For this reason, there is a significant level of skepticism about the biological relevance of bacterial vesicles. In Bacteroides thetaiotaomicron ( Bt ), a prominent member of the human intestinal microbiota, outer membrane vesicles (OMVs) have been proposed to play key physiological roles. By employing outer membrane- and OMV-specific markers fused to fluorescent proteins we visualized OMV biogenesis in live-cells. We performed comparative proteomic analyses to demonstrate that Bt actively tailors its vesicle cargo to optimize the breakdown of diet- and host-derived complex glycans. Surprisingly, our data suggests that OMV are not employed for mucin degradation. We also show that, in Bt , a negatively-charged N-terminal motif acts as a signal for protein sorting into OMVs irrespective of the nutrient availability. We conclude that OMVs are the result of an exquisitely orchestrated mechanism. This work lays the foundation for further investigations into the physiological relevance of OMVs and their roles in gut homeostasis. Furthermore, our work constitutes a roadmap to guide EV biogenesis research in other bacteria.

Bacteroides fragilis Maintains Concurrent Capability for Anaerobic and Nanaerobic Respiration

Bacteroides species can use fumarate and oxygen as terminal electron acceptors during cellular respiration. In the human gut, oxygen diffuses from intestinal epithelial cells supplying "nanaerobic" oxygen levels. Many components of the anaerobic respiratory pathway have been determined, but such analyses have not been performed for nanaerobic respiration. Here, we present genetic, biochemical, enzymatic, and mass spectrometry analyses to elucidate the nanaerobic respiratory pathway in Bacteroides fragilis. Under anaerobic conditions, the transfer of electrons from NADH to the quinone pool has been shown to be contributed by two enzymes, NQR and NDH2. We find that the activity contributed by each under nanaerobic conditions is 77 and 23%, respectively, similar to the activity levels under anaerobic conditions. Using mass spectrometry, we show that the quinone pool also does not differ under these two conditions and consists of a mixture of menaquinone-8 to menaquinone-11, with menaquinone-10 predominant under both conditions. Analysis of fumarate reductase showed that it is synthesized and active under anaerobic and nanaerobic conditions. Previous RNA sequencing data and new transcription reporter assays show that expression of the cytochrome bd oxidase gene does not change under these conditions. Under nanaerobic conditions, we find both increased CydA protein and increased cytochrome bd activity. Reduced-minus-oxidized spectra of membranes showed the presence of heme d when the bacteria were grown in the presence of protoporphyrin IX and iron under both anaerobic and nanaerobic conditions, suggesting that the active oxidase can be assembled with or without oxygen. IMPORTANCE By performing a comprehensive analysis of nanaerobic respiration in Bacteroides fragilis, we show that this organism maintains capabilities for anaerobic respiration on fumarate and nanaerobic respiration on oxygen simultaneously. The contribution of the two NADH:quinone oxidoreductases and the composition of the quinone pool are the same under both conditions. Fumarate reductase and cytochrome bd are both present, and which of these terminal enzymes is active in electron transfer depends on the availability of the final electron acceptor: fumarate or oxygen. The synthesis of cytochrome bd and fumarate reductase under both conditions serves as an adaptation to an environment with low oxygen concentrations so that the bacteria can maximize energy conservation during fluctuating environmental conditions or occupation of different spatial niches.

Harnessing gut microbes for glycan detection and quantification

Glycans facilitate critical biological functions and control the mammalian gut microbiota composition by supplying differentially accessible nutrients to distinct microbial subsets. Therefore, identifying unique glycan substrates that support defined microbial populations could inform therapeutic avenues to treat diseases via modulation of the gut microbiota composition and metabolism. However, examining heterogeneous glycan mixtures for individual microbial substrates is hindered by glycan structural complexity and diversity, which presents substantial challenges to glycomics approaches. Fortuitously, gut microbes encode specialized sensor proteins that recognize unique glycan structures and in-turn activate predictable, specific, and dynamic transcriptional responses. Here, we harness this microbial machinery to indicate the presence and abundance of compositionally similar, yet structurally distinct glycans, using a transcriptional reporter we develop. We implement these tools to examine glycan mixtures, isolate target molecules for downstream characterization, and quantify the recovered products. We assert that this toolkit could dramatically enhance our understanding of the mammalian intestinal environment and identify host-microbial interactions critical for human health.

Social complexity as a driving force of gut microbiota exchange among conspecific hosts in non-human primates

The emergent concept of the social microbiome implies a view of a highly connected biological world, in which microbial interchange across organisms may be influenced by social and ecological connections occurring at different levels of biological organization. We explore this idea reviewing evidence of whether increasing social complexity in primate societies is associated with both higher diversity and greater similarity in the composition of the gut microbiota. By proposing a series of predictions regarding such relationship, we evaluate the existence of a link between gut microbiota and primate social behavior. Overall, we find that enough empirical evidence already supports these predictions. Nonetheless, we conclude that studies with the necessary, sufficient, explicit, and available evidence are still scarce. Therefore, we reflect on the benefit of founding future analyses on the utility of social complexity as a theoretical framework.

Recent Advances in Structural Studies of Cytochrome bd and Its Potential Application as a Drug Target

Cytochrome bd is a triheme copper-free terminal oxidase in membrane respiratory chains of prokaryotes. This unique molecular machine couples electron transfer from quinol to O₂ with the generation of a proton motive force without proton pumping. Apart from energy conservation, the bd enzyme plays an additional key role in the microbial cell, being involved in the response to different environmental stressors. Cytochrome bd promotes virulence in a number of pathogenic species that makes it a suitable molecular drug target candidate. This review focuses on recent advances in understanding the structure of cytochrome bd and the development of its selective inhibitors.

Photomedicine based on heme-derived compounds

Photoimaging and phototherapy have become major platforms for the diagnosis and treatment of various health complications. These applications require a photosensitizer (PS) that is capable of absorbing light from a source and converting it into other energy forms for detection and therapy. While synthetic inorganic materials such as quantum dots and gold nanorods have been widely explored for their medical diagnosis and photodynamic (PDT) and photothermal (PTT) therapy capabilities, translation of these technologies has lagged, primarily owing to potential cytotoxicity and immunogenicity issues. Of the various photoreactive molecules, the naturally occurring endogenous compound heme, a constituent of red blood cells, and its derivatives, porphyrin, biliverdin and bilirubin, have shown immense potential as noteworthy candidates for clinically translatable photoreactive agents, as evidenced by previous reports. While porphyrin-based photomedicines have attracted significant attention and are well documented, research on photomedicines based on two other heme-derived compounds, biliverdin and bilirubin, has been relatively lacking. In this review, we summarize the unique photoproperties of heme-derived compounds and outline recent efforts to use them in biomedical imaging and phototherapy applications.

CRISPR/Cas-Based Genome Editing for Human Gut Commensal Bacteroides Species

Bacteroides is the most abundant genus in the human gut microbiome and has been increasingly used as model organisms for studying the function and ecology of the gut microbiome. However, genome editing tools for such commensal gut microbes are still lacking. Here we developed a versatile, highly efficient CRISPR/Cas-based genome editing tool that allows markerless gene deletion and insertion in human gut Bacteroides species. We constructed multiple CRISPR/Cas systems in all-in-one Bacteroides-E. coli shuttle plasmids and systematically evaluated the genome editing efficiency in Bacteroides thetaiotaomicron, including the mode of Cas protein expression (constitutive, inducible), different Cas proteins (FnCas12a, SpRY, SpCas9), and sgRNAs. Using the anhydrotetracycline (aTc)-inducible CRISPR/FnCas12a system, we successfully deleted large genomic fragments up to 50 kb to study the function of metabolic gene clusters. Furthermore, we demonstrated that CRISPR/FnCas12a can be broadly applied to engineer multiple human gut Bacteroides species, including Bacteroides fragilis, Bacteroides ovatus, Bacteroides uniformis, and Bacteroides vulgatus. We envision that CRISPR/Cas-based genome editing tools for Bacteroides will greatly facilitate mechanistic studies of the gut commensal and the development of engineered live biotherapeutics.

Dynamic regulation of gut Clostridium-derived short-chain fatty acids

Short-chain fatty acids (SCFAs) are major products of intestinal microbial fermentation with beneficial effects for human health. The dynamic balance and real-time monitoring of endogenous SCFA biosynthesis are important for understanding their physiological functions. We discuss the promising future of applying CRISPRi genetic systems and biosensors for targeted SCFA improvement.

Integrating Systems and Synthetic Biology to Understand and Engineer Microbiomes

Microbiomes are complex and ubiquitous networks of microorganisms whose seemingly limitless chemical transformations could be harnessed to benefit agriculture, medicine, and biotechnology. The spatial and temporal changes in microbiome composition and function are influenced by a multitude of molecular and ecological factors. This complexity yields both versatility and challenges in designing synthetic microbiomes and perturbing natural microbiomes in controlled, predictable ways. In this review, we describe factors that give rise to emergent spatial and temporal microbiome properties and the meta-omics and computational modeling tools that can be used to understand microbiomes at the cellular and system levels. We also describe strategies for designing and engineering microbiomes to enhance or build novel functions. Throughout the review, we discuss key knowledge and technology gaps for elucidating the networks and deciphering key control points for microbiome engineering, and highlight examples where multiple omics and modeling approaches can be integrated to address these gaps.

Cause or effect? The spatial organization of pathogens and the gut microbiota in disease

The human gut hosts a dense and diverse microbial community, spatially organized in multiple scales of structure. Here, we review how microbial organization differs between health and disease. We describe how changes in spatial organization may induce alterations in gut homeostasis, concluding with a future outlook to reveal causality.