Codiversification of gut microbiota with humans

Integrating multi-omics data to reveal the host-microbiota interactome in inflammatory bowel disease

Numerous studies have accelerated the knowledge expansion on the role of gut microbiota in inflammatory bowel disease (IBD). However, the precise mechanisms behind host-microbe cross-talk remain largely undefined, due to the complexity of the human intestinal ecosystem and multiple external factors. In this review, we introduce the interactome concept to systematically summarize how intestinal dysbiosis is involved in IBD pathogenesis in terms of microbial composition, functionality, genomic structure, transcriptional activity, and downstream proteins and metabolites. Meanwhile, this review also aims to present an updated overview of the relevant mechanisms, high-throughput multi-omics methodologies, different types of multi-omics cohort resources, and computational methods used to understand host-microbiota interactions in the context of IBD. Finally, we discuss the challenges pertaining to the integration of multi-omics data in order to reveal host-microbiota cross-talk and offer insights into relevant future research directions.

Oat β-Glucan and Lacticaseibacillus paracasei K56 synergistically ameliorate hypercholesterolemia in mice by modulating gut microbiota and bile acid metabolism

Hypercholesterolemia is one of the major risk factors for cardiovascular diseases. While probiotics and prebiotics show cholesterol-lowering potential, current evidence suggests limited efficacy when used alone. This study investigated the synergistic effects of oat β-glucan (OBG) and Lacticaseibacillus paracasei K56 (L. paracasei K56) against diet-induced hypercholesterolemia in mice. Hypercholesterolemic mice were treated for 12 weeks with atorvastatin (5 mg/kg/day), L. paracasei K56 (5 × 109 CFU/kg), OBG (1 g/kg/day) or their combination. The combination treatment demonstrated superior efficacy, reducing total cholesterol (TC) by 41.19 ± 7.13 % (vs model group) and low-density lipoprotein cholesterol (LDL-C) by 37.47 ± 6.55 % (vs model group), with 24.37 ± 9.18 % (vs OBG group) and 15.10 ± 10.30 % (vs K56 group) greater TC reduction. Liver histopathology showed 3.7-fold reduction in steatosis scores versus model group. Mechanistically, compared with the model group, the combination upregulated hepatic CYP7A1/CYP27A1 expression by 5.75 ± 0.51-fold and 2.34 ± 0.21-fold respectively, increased fecal bile acids and enriched bile acid producing bacteria. In summary, L. paracasei K56 and OBG synergistically regulate gut microbiota and bile acid metabolism, promoting the conversion of cholesterol into bile acids, thereby alleviating hypercholesterolemia in mice.

Strain tracking in complex microbiomes using synteny analysis reveals per-species modes of evolution

Microbial species diversify into strains through single-nucleotide mutations and structural changes, such as recombination, insertions and deletions. Most strain-comparison methods quantify differences in single-nucleotide polymorphisms (SNPs) and are insensitive to structural changes. However, recombination is an important driver of phenotypic diversification in many species, including human pathogens. We introduce SynTracker, a tool that compares microbial strains using genome synteny-the order of sequence blocks in homologous genomic regions-in pairs of metagenomic assemblies or genomes. Genome synteny is a rich source of genomic information untapped by current strain-comparison tools. SynTracker has low sensitivity to SNPs, has no database requirement and is robust to sequencing errors. It outperforms existing tools when tracking strains in metagenomic data and is particularly suited for phages, plasmids and other low-data contexts. Applied to single-species datasets and human gut metagenomes, SynTracker, combined with an SNP-based tool, detects strains enriched in either point mutations or structural changes, providing insights into microbial evolution in situ.

Gut microbiota reshapes the TNBC immune microenvironment: Emerging immunotherapeutic strategies

Triple-negative breast cancer (TNBC) is an aggressive subtype of breast cancer with limited treatment options and poor prognosis. The gut microbiota, a diverse community of microorganisms in the gastrointestinal tract, plays a crucial role in regulating immune responses through the gut-immune axis. Recent studies have highlighted its significant impact on TNBC progression and the efficacy of immunotherapies. This review examines the interactions between gut microbiota and the immune system in TNBC, focusing on key immune cells and pathways involved in tumor immunity. It also explores microbiota modulation strategies, including probiotics, prebiotics, dietary interventions, and fecal microbiota transplantation, as potential methods to enhance immunotherapeutic outcomes. Understanding these mechanisms offers promising avenues for improving treatment efficacy and patient prognosis in TNBC.

The Respiratory Tract Microbiome and Human Health

The respiratory tract microbiome (RTM) is a multi-kingdom microbial ecosystem that inhabits various niches of the respiratory system. While previously overlooked, there is now sufficient evidence that the RTM plays a crucial role in human health related to immune system training and protection against pathogens. Accordingly, dysbiosis or disequilibrium of the RTM has been linked to several communicable and non-communicable respiratory diseases, highlighting the need to unveil its role in health and disease. Here, we define the RTM and its place in microbiome medicine. Moreover, we outline the challenges of RTM research, emphasising the need for combining methodologies, including multi-omics and computational tools. We also discuss the RTM's potential for diagnosing, preventing and treating respiratory diseases and developing novel microbiome-based therapies to improve pulmonary health.

Integrating natural commensals and pathogens into preclinical mouse models

Fundamental discoveries in many aspects of mammalian physiology have been made using laboratory mice as research models. These studies have been facilitated by the genetic tractability and inbreeding of such mice, the large set of immunological reagents that are available, and the establishment of environmentally controlled, high-throughput facilities. Such facilities typically include barriers to keep the mouse colonies free of pathogens and the frequent re-derivation of the mice severely limits their commensal flora. Because humans have co-evolved with microorganisms and are exposed to a variety of pathogens, a growing community of researchers posits that preclinical disease research can be improved by studying mice in the context of the microbiota and pathogens that they would encounter in the natural world. Here, we provide a perspective of how these different approaches can be combined and integrated to improve existing mouse models to enhance our understanding of disease mechanisms and develop new therapies for humans. We also propose that the term 'mice with natural microbiota' is more appropriate for describing these models than existing terms such as 'dirty mice'.

Global genetic diversity of human gut microbiome species is related to geographic location and host health

The human gut harbors thousands of microbial species, each exhibiting significant inter-individual genetic variability. Although many studies have associated microbial relative abundances with human-health-related phenotypes, the substantial intraspecies genetic variability of gut microbes has not yet been comprehensively considered, limiting the potential of linking such genetic traits with host conditions. Here, we analyzed 32,152 metagenomes from 94 microbiome studies across the globe to investigate the human microbiome intraspecies genetic diversity. We reconstructed 583 species-specific phylogenies and linked them to geographic information and species' horizontal transmissibility. We identified 484 microbial-strain-level associations with 241 host phenotypes, encompassing human anthropometric factors, biochemical measurements, diseases, and lifestyle. We observed a higher prevalence of a Ruminococcus gnavus clade in nonagenarians correlated with distinct plasma bile acid profiles and a melanoma and prostate-cancer-associated Collinsella clade. Our large-scale intraspecies genetic analysis highlights the relevance of strain diversity as it relates to human health.

Expansion of a bacterial operon during cancer treatment ameliorates fluoropyrimidine toxicity

Dose-limiting toxicities remain a major barrier to drug development and therapy, revealing the limited predictive power of human genetics. Here, we demonstrate the utility of a more comprehensive approach to studying drug toxicity through longitudinal profiling of the human gut microbiome during colorectal cancer (CRC) treatment (NCT04054908) coupled to cell culture and mouse experiments. Substantial shifts in gut microbial community structure during oral fluoropyrimidine treatment across multiple patient cohorts, in mouse small and large intestinal contents, and in patient-derived ex vivo communities were revealed by 16S rRNA gene sequencing. Metagenomic sequencing revealed marked shifts in pyrimidine-related gene abundance during oral fluoropyrimidine treatment, including enrichment of the preTA operon, which was sufficient for the inactivation of active metabolite 5-fluorouracil (5-FU). preTA+ bacteria depleted 5-FU in gut microbiota grown ex vivo and in the mouse distal gut. Germ-free and antibiotic-treated mice experienced increased fluoropyrimidine toxicity, which was rescued by colonization with the mouse gut microbiota, preTA+ Escherichia coli, or preTA-high stool from patients with CRC. Last, preTA abundance was negatively associated with fluoropyrimidine toxicity in patients. Together, these data support a causal, clinically relevant interaction between a human gut bacterial operon and the dose-limiting side effects of cancer treatment. Our approach may be generalizable to other drugs, including cancer immunotherapies, and provides valuable insights into host-microbiome interactions in the context of disease.

Human and bacterial genetic variation shape oral microbiomes and health

It is largely unknown which human genetic variants shape a person's oral microbiome and potentially promote its dysbiosis. We characterized the oral microbiomes of 12,519 people by analyzing whole-genome sequencing reads from previously sequenced saliva-derived DNA. Human genetic variation at 11 loci (10 novel) associated with differences in oral microbiome composition. Nearly all of these associations implicated candidate genes with readily interpretable functions, several related to carbohydrate availability. The strongest association ( p =3.0x10 -188 ) involved the common FUT2 W154X loss-of-function variant, which associated with the abundances of 32 bacterial species. Human host genetics also appeared to powerfully shape within-species genetic variation in oral bacteria. Variation at the 11 human loci associated with variation in gene dosages in 68 regions of bacterial genomes. Several such associations implicated interactions of bacterial proteins with histo-blood group antigens presented on host mucosal cell surfaces and salivary proteins. Common, multi-allelic copy-number variation of AMY1 , which encodes salivary amylase, associated with oral microbiome composition ( p =1.5x10 -53 ) and with dentures use in UK Biobank ( p =5.9x10 -35 , n=418k), suggesting that amylase abundance impacts oral health by influencing the oral microbiome. Two other microbiome composition-associated loci, FUT2 and PITX1 , also significantly associated with dentures risk, collectively nominating numerous microbial taxa that might contribute to tooth decay.

Strain diversity and host specificity of the gut symbiont Gilliamella in Apis mellifera, Apis cerana and Bombus terrestris

Social bees, with their specialized gut microbiota and societal transmission between individuals, provide an ideal model for studying host-gut microbiota interactions. While the functional disparities arising from strain-level diversity of gut symbionts and their effects on host health have been studied in Apis mellifera and bumblebees, studies focusing on host-specific investigations of individual strains across different honeybee hosts remain relatively unexplored. In this study, the complete genomic sequences of 17 strains of Gilliamella from A. mellifera, Apis cerana and Bombus terrestris were analyzed. The analysis revealed that the strains of A. mellifera display a more expansive genomic and functional content compared to the strains of A. cerana and B. terrestris. Phylogenetic analysis showed a deep divergence among the Gilliamella strains from different hosts. Additionally, biochemistry tests and antibiotic susceptibility tests revealed that gut strains from A. mellifera exhibited a more extensive pathway for carbohydrate metabolism and a greater resistance to antibiotics than gut strains from A. cerana and B. terrestris. Strains from A. mellifera and A. cerana showed higher colonization efficiency and competitive ability whithin their respective host species, indicating a higher degree of host-specific adaptation of local gut microbiota. In addition, colonization by A. mellifera-derived strain triggers a stronger transcriptional response in the host than A. cerana-derived strain. The variation in the number of differentially expressed genes and the involvement of distinct signaling pathways across these two host species suggest species-specific adaptations to Gilliamella strains. These findings suggest that despite occupying similar niches in the bee gut, strain-level variations can influence microbial functions, and their impact on host physiological functions may vary across different strains.

Human microbiome acquisition and transmission

As humans, we host personal microbiomes intricately connected to our biology and health. Far from being isolated entities, our microbiomes are dynamically shaped by microbial exchange with the surroundings, in lifelong microbiome acquisition and transmission processes. In this Review, we explore recent studies on how our microbiomes are transmitted, beginning at birth and during interactions with other humans and the environment. We also describe the key methodological aspects of transmission inference, based on the uniqueness of the building blocks of the microbiome - single microbial strains. A better understanding of human microbiome transmission will have implications for studies of microbial host regulation, of microbiome-associated diseases, and for effective microbiome-targeting strategies. Besides exchanging strains with other humans, there is also preliminary evidence we acquire microorganisms from animals and food, and thus a complete understanding of microbiome acquisition and transmission can only be attained by adopting a One Health perspective.

Host specificity of gut microbiota associated with social bees: patterns and processes

SUMMARYGut microbes provide benefits to some animals, but their distribution and effects across diverse hosts are still poorly described. There is accumulating evidence for host specificity (i.e., a pattern where different microbes tend to associate with distinct host lineages), but the causes and consequences of this pattern are unclear. Combining experimental tests in the laboratory with broad surveys in the wild is a promising approach to gaining a comprehensive and mechanistic understanding of host specificity prevalence, origin, and importance. Social bees represent an ideal testbed for this endeavor because they are phylogenetically and functionally diverse, with host-specific, stable, and tractable gut microbiota. Furthermore, the western honeybee (Apis mellifera) is an emerging experimental model system for studying microbiota-host interactions. In this review, we summarize data on the prevalence and strength of host specificity of the social bee gut microbiota (bumblebees, stingless bees, and honeybees), as well as the potential and proven ecological and molecular mechanisms that maintain host specificity. Overall, we found that host specificity in bees is relatively strong and likely results from several processes, including host filtering mediated by the immune system and priority effects. However, more research is needed across multiple social bee species to confirm these findings. To help future research, we summarize emerging hypotheses in the field and propose several experimental and comparative tests. Finally, we conclude this review by highlighting the need to understand how host specificity can influence host health.

Recent genetic drift in the co-diversified gut bacterial symbionts of laboratory mice

Laboratory mice (Mus musculus domesticus) harbor gut bacterial strains that are distinct from those of wild mice but whose evolutionary histories are unclear. Here, we show that laboratory mice have retained gut bacterial lineages that diversified in parallel (co-diversified) with rodent species for > 25 million years, but that laboratory-mouse gut microbiota (LGM) strains of these ancestral symbionts have experienced accelerated accumulation of genetic load during the past ~ 120 years of captivity. Compared to closely related wild-mouse gut microbiota (WGM) strains, co-diversified LGM strains displayed significantly faster genome-wide rates of nonsynonymous substitutions, indicating elevated genetic drift-a difference that was absent in non-co-diversified symbiont clades. Competition experiments in germ-free mice further indicated that LGM strains within co-diversified clades displayed significantly reduced fitness in vivo compared to WGM relatives to an extent not observed within non-co-diversified clades. Thus, stochastic processes (e.g., bottlenecks), not natural selection in the laboratory, have been the predominant evolutionary forces underlying divergence of co-diversified symbiont strains between laboratory and wild house mice. Our results show that gut bacterial lineages conserved in diverse rodent species have acquired novel mutational burdens in laboratory mice, providing an evolutionary rationale for restoring laboratory mice with wild gut bacterial strain diversity.

Study on the difference of gut microbiota in DLY and Diqing Tibetan pigs induce by high fiber diet

In order to investigate the regularity of fecal microorganisms changes in Landrace × Large White × Duroc (DLY) and Diqing Tibetan pigs (TP) induced by dietary fiber, and further explore the buffering effect of different intestinal flora structures on dietary stress. DLY (n = 15) and TP (n = 15) were divided into two treatments. Then, diet with 20% neutral detergent fiber (NDF) was supplemented for 9 days. Our results showed that the feed conversion efficiency of TP was significantly higher (p < 0.05) than that of DLY. The fecal microorganisms shared by the two groups gradually increased with the feeding cycle. In addition, the dispersion of Shannon, Simpson, ACE and Chao of TP decreased. Also, we found that the fecal microorganisms of TP (R2 = 0.2089, p < 0.01) and DLY (R2 = 0.3982, p < 0.01) showed significant differences in different feeding cycles. With the prolongation of feeding cycle, the similarity of fecal microbial composition between DLY and TP increased. Our study strongly suggests that the complex environment and diet structure have shaped the unique gut microbiota of TP, which plays a vital role in the buffering effect of high-fiber diets.

Crosstalk between gut microbiotas and fatty acid metabolism in colorectal cancer

Colorectal cancer (CRC) is the third most common malignancy globally and the second leading cause of cancer-related mortality. Its development is a multifactorial and multistage process influenced by a dynamic interplay between gut microbiota, environmental factors, and fatty acid metabolism. Dysbiosis of intestinal microbiota and abnormalities in microbiota-associated metabolites have been implicated in colorectal carcinogenesis, highlighting the pivotal role of microbial and metabolic interactions. Fatty acid metabolism serves as a critical nexus linking dietary patterns with gut microbial activity, significantly impacting intestinal health. In CRC patients, reduced levels of short-chain fatty acids (SCFAs) and SCFA-producing bacteria have been consistently observed. Supplementation with SCFA-producing probiotics has demonstrated tumor-suppressive effects, while therapeutic strategies aimed at modulating SCFA levels have shown potential in enhancing the efficacy of radiation therapy and immunotherapy in both preclinical and clinical settings. This review explores the intricate relationship between gut microbiota, fatty acid metabolism, and CRC, offering insights into the underlying mechanisms and their potential translational applications. Understanding this interplay could pave the way for novel diagnostic, therapeutic, and preventive strategies in the management of CRC.

Association of Circulating Phenylacetylglutamine With Multi-Vessel Coronary Disease Severity and Outcomes in ST-Segment-Elevation Myocardial Infarction

BACKGROUND

There is a lack of evidence regarding the association between plasma phenylacetylglutamine levels and lesion severity and clinical prognosis in patients with ST-segment elevation myocardial infarction (STEMI) with multivessel coronary disease (MVCD). This study aims to investigate the potential of phenylacetylglutamine as a biomarker for major adverse cardiovascular events (MACEs) of patients with STEMI and MVCD.

METHODS AND RESULTS

Clinical data and blood samples were collected from 631 patients with STEMI and MVCD, who underwent primary percutaneous coronary intervention. Quantitative coronary angiography analysis was performed using the QAngio XA 7.3 system. Plasma phenylacetylglutamine concentrations were measured by rapid resolution liquid chromatography quadrupole time-of-flight mass spectrometry. Among a total of 631 patients, median plasma phenylacetylglutamine level was 3.8 (2.1-6.8) μmol/L and the cumulative MACE rate at follow-up was 12%. Plasma phenylacetylglutamine levels of patients with MACE were significantly higher than patients without MACE. We employed restricted cubic spline, Kaplan-Meier curves, and Cox proportional hazard models to explore the association between plasma phenylacetylglutamine and prognosis of patients with STEMI and MVCD. Per SD, an increment in phenylacetylglutamine was associated with a 24% higher risk of complexity lesion. Higher phenylacetylglutamine level was an independent predictor of MACEs (hazard ratio [HR], 2.76 [95% CI, 1.62-4.72]). A novel prognostic scoring system was established by combining phenylacetylglutamine levels with the synergy between percutaneous coronary intervention with Taxus and cardiac surgery score, with higher scores significantly increasing the risk of MACEs (HR, 4.01 [95% CI, 2.04-7.89]).

CONCLUSIONS

Phenylacetylglutamine levels were associated with lesion complexity and prognosis, may serve as a novel biomarker in patients with STEMI and MVCD.

Intraspecies associations from strain-rich metagenome samples

Genetically distinct strains of a species can vary widely in phenotype, reducing the utility of species-resolved microbiome measurements for detecting associations with health or disease. While metagenomics theoretically provides information on all strains in a sample, current strain-resolved analysis methods face a tradeoff: de novo genotyping approaches can detect novel strains but struggle when applied to strain-rich or low-coverage samples, while reference database methods work robustly across sample types but are insensitive to novel diversity. We present PHLAME, a method that bridges this divide by combining the advantages of reference-based approaches with novelty awareness. PHLAME explicitly defines clades at multiple phylogenetic levels and introduces a probabilistic, mutation-based, framework to accurately quantify novelty from the nearest reference. By applying PHLAME to publicly available human skin and vaginal metagenomes, we uncover previously undetected clade associations with coexisting species, geography, and host age. The ability to characterize intraspecies associations and dynamics in previously inaccessible environments will propel new mechanistic insights from accumulating metagenomic data.

Longitudinal phage-bacteria dynamics in the early life gut microbiome

Microbial colonization of the human gut occurs soon after birth, proceeds through well-studied phases and is affected by lifestyle and other factors. Less is known about phage community dynamics during infant gut colonization due to small study sizes, an inability to leverage large databases and a lack of appropriate bioinformatics tools. Here we reanalysed whole microbial community shotgun sequencing data of 12,262 longitudinal samples from 887 children from four countries across four years of life as part of the The Environmental Determinants of Diabetes in the Young (TEDDY) study. We developed an extensive metagenome-assembled genome catalogue using the Marker-MAGu pipeline, which comprised 49,111 phage taxa from existing human microbiome datasets. This was used to identify phage marker genes and their integration into the MetaPhlAn 4 bacterial marker gene database enabled simultaneous assessment of phage and bacterial dynamics. We found that individual children are colonized by hundreds of different phages, which are more transitory than bacteria, accumulating a more diverse phage community over time. Type 1 diabetes correlated with a decreased rate of change in bacterial and viral communities in children aged one and two. The addition of phage data improved the ability of machine learning models to discriminate samples by country. Finally, although phage populations were specific to individuals, we observed trends of phage ecological succession that correlated well with putative host bacteria. This resource improves our understanding of phage-bacteria interactions in the developing early life microbiome.

Harnessing gut microbial communities to unravel microbiome functions

The gut microbiome impacts human health in direct and indirect ways. While many associations have been discovered between specific microbiome compositions and diseases, establishing causality, understanding the underlying mechanisms, and developing successful microbiome-based therapies require novel experimental approaches. In this opinion, we discuss how in vitro cultivation of diverse communities enables systematic investigation of the individual and collective functions of gut microbes. Up to now, the field has relied mostly on simple, bottom-up assembled synthetic communities or more complex, undefined stool-derived communities. Although powerful for dissecting interactions and mapping causal effects, these communities suffer either from ignoring the complexity, diversity, coevolution, and dynamics of natural communities or from lack of control of community composition. These limitations can be overcome in the future by establishing personalized culture collections from stool samples of different donors and assembling personalized communities to investigate native interactions and ecological relationships in a controlled manner.

Clade-specific long-read sequencing increases the accuracy and specificity of the gyrB phylogenetic marker gene

Phylogenetic marker gene sequencing is often used as a quick and cost-effective way of evaluating microbial composition within a community. While 16S rRNA gene sequencing (16S) is commonly used for bacteria and archaea, other marker genes are preferable in certain situations, such as when 16S sequences cannot distinguish between taxa within a group. Another situation is when researchers want to study cospeciation of host taxa that diverged much more recently than the slowly evolving 16S rRNA gene. For example, the bacterial gyrase subunit B (gyrB) gene has been used to investigate cospeciation between the microbiome and various hominid species. However, to date, only primers that generate short-read Illumina MiSeq-length amplicons exist to investigate gyrB of the Bacteroidaceae, Bifidobacteriaceae, and Lachnospiraceae families. Here, we update this methodology by creating gyrB primers for the Bacteroidaceae, Bifidobacteriaceae, and Lachnospiraceae families for long-read PacBio sequencing and characterize them against established short-read gyrB primer sets. We demonstrate both bioinformatically and analytically that these longer amplicons offer more sequence space for greater taxonomic resolution, lower off-target amplification rates, and lower error rates with PacBio CCS sequencing versus established short-read sequencing. The availability of these long-read gyrB primers will prove to be integral to the continued analysis of cospeciation between bacterial members of the gut microbiome and recently diverging host species.

IMPORTANCE

Previous studies have shown that the marker gene gyrase subunit B (gyrB) can be used to study codiversification between the gut microbiome and hominids. However, only primers for short-read sequencing have been developed which have limited resolution for subspecies assignment. In the present study, we create new gyrB primer sets for long-read sequencing approaches and compare them to the existing short-read gyrB primers. We show that using longer reads leads to better taxonomic resolution, lower off-target amplification, and lower error rates, which are vital for accurate estimates of codiversification.

MAIT cells: Conserved watchers on the wall

MAIT cells are innate-like T cells residing in barrier tissues such as the lung, skin, and intestine. Both the semi-invariant T cell receptor of MAIT cells and the restricting element MR1 are deeply conserved across mammals, indicating non-redundant functions linked to antigenic specificity. MAIT cells across species concomitantly express cytotoxicity and tissue-repair genes, suggesting versatile functions. Accordingly, MAIT cells contribute to antibacterial responses as well as to the repair of damaged barrier tissues. MAIT cells recognize riboflavin biosynthetic pathway-derived metabolites, which rapidly cross epithelial barriers to be presented by antigen-presenting cells. Changes in gut ecology during intestinal inflammation drive the expansion of strong riboflavin and MAIT ligand producers. Thus, MAIT cells may enable real-time surveillance of microbiota dysbiosis across intact epithelia and provide rapid and context-dependent responses. Here, we discuss recent findings regarding the origin and regulation of MAIT ligands and the role of MAIT cells in barrier tissues. We speculate on the potential reasons for MAIT cell conservation during evolution.

Partner fidelity, not geography, drives co-diversification of gut microbiota with hominids

Bacterial strains that inhabit the gastrointestinal tracts of hominids have diversified in parallel (co-diversified) with their host species. The extent to which co-diversification has been mediated by partner fidelity between strains and hosts or by geographical distance between hosts is not clear due to a lack of strain-level data from clades of hosts with unconfounded phylogenetic relationships and geographical distributions. Here, I tested these competing hypotheses through meta-analyses of 7121 gut bacterial genomes assembled from wild-living ape species and subspecies sampled throughout their ranges in equatorial Africa. Across the gut bacterial phylogeny, strain diversification was more strongly associated with host phylogeny than with geography. In total, approximately 14% of the branch length of the gut bacterial phylogeny showed significant evidence of co-diversification independent of geography, whereas only approximately 4% showed significant evidence of diversification associated with geography independent of host phylogeny. Geographically co-occurring heterospecific hosts (Pan and Gorilla) universally maintained distinct co-diversified bacterial strains. Strains whose diversification was associated with geography independent of host phylogeny included clades of Proteobacteria known to adopt free-living lifestyles (e.g. Escherichia). These results show that co-diversification of gut bacterial strains with hominids has been driven primarily by fidelity of strains to host lineages rather than geography.

[Did humans co-evolve with the gut microbiota?]

The gut microbiome plays an important role in animal physiology and development. While the molecular, cellular and ecological mechanisms that determine its diversity and impact on animal health are beginning to unfold, we still know relatively little about its evolutionary history. Fundamental questions such as "Is the microbiota evolving and at what race?", "What are its origins?", "What are the consequences of microbiota evolution for human health?" or "Did we co-evolve with our gut bacteria?" are only beginning to be explored. In the short term (from a few days to a few years, or microevolution), gut microbes can evolve and adapt very rapidly within an individual in responses to environmental changes, such as diet shifts, which can affect human health. On the longer term (ten to millions of years, or macroevolution), evolution within individuals is counterbalanced by the transfer of microbes from other people, so that human evolution is decoupled from the evolution of most gut microbes over many generations. This suggests that, while gut microbes have probably evolved rapidly within humans, most of them have a history of exchange between host populations over millennia. Whether the evolution of the microbiota over the last hundreds of thousands of years has facilitated human adaptations remains an open question and an exciting avenue for future research.

A metagenome-assembled genome inventory for children reveals early-life gut bacteriome and virome dynamics

Existing microbiota databases are biased toward adult samples, hampering accurate profiling of the infant gut microbiome. Here, we generated a metagenome-assembled genome inventory for children (MAGIC) from a large collection of bulk and viral-like particle-enriched metagenomes from 0 to 7 years of age, encompassing 3,299 prokaryotic and 139,624 viral species-level genomes, 8.5% and 63.9% of which are unique to MAGIC. MAGIC improves early-life microbiome profiling, with the greatest improvement in read mapping observed in Africans. We then identified 54 candidate keystone species, including several Bifidobacterium spp. and four phages, forming guilds that fluctuated in abundance with time. Their abundances were reduced in preterm infants and were associated with childhood allergies. By analyzing the B. longum pangenome, we found evidence of phage-mediated evolution and quorum sensing-related ecological adaptation. Together, the MAGIC database recovers genomes that enable characterization of the dynamics of early-life microbiomes, identification of candidate keystone species, and strain-level study of target species.

A conserved bacterial genetic basis for commensal-host specificity

Animals selectively acquire specific symbiotic gut bacteria from their environments that aid host fitness. To colonize, a symbiont must locate its niche and sustain growth within the gut. Adhesins are bacterial cell surface proteins that facilitate attachment to host tissues and are often virulence factors for opportunistic pathogens. However, the attachments are often transient and nonspecific, and additional mechanisms are required to sustain infection. In this work, we use live imaging of individual symbiotic bacterial cells colonizing the gut of living Drosophila melanogaster to show that Lactiplantibacillus plantarum specifically recognizes the fruit fly foregut as a distinct physical niche. L. plantarum establishes stably within its niche through host-specific adhesins encoded by genes carried on a colonization island. The adhesin binding domains are conserved throughout the Lactobacillales, and the island also encodes a secretion system widely conserved among commensal and pathogenic bacteria.

Linkage equilibrium between rare mutations

Recombination breaks down genetic linkage by reshuffling existing variants onto new genetic backgrounds. These dynamics are traditionally quantified by examining the correlations between alleles, and how they decay as a function of the recombination rate. However, the magnitudes of these correlations are strongly influenced by other evolutionary forces like natural selection and genetic drift, making it difficult to tease out the effects of recombination. Here, we introduce a theoretical framework for analyzing an alternative family of statistics that measure the homoplasy produced by recombination. We derive analytical expressions that predict how these statistics depend on the rates of recombination and recurrent mutation, the strength of negative selection and genetic drift, and the present-day frequencies of the mutant alleles. We find that the degree of homoplasy can strongly depend on this frequency scale, which reflects the underlying timescales over which these mutations occurred. We show how these scaling properties can be used to isolate the effects of recombination and discuss their implications for the rates of horizontal gene transfer in bacteria.

Expansion of a bacterial operon during cancer treatment ameliorates drug toxicity

Dose-limiting toxicities remain a major barrier to drug development and therapy, revealing the limited predictive power of human genetics. Herein, we demonstrate the utility of a more comprehensive approach to studying drug toxicity through longitudinal study of the human gut microbiome during colorectal cancer (CRC) treatment (NCT04054908) coupled to cell culture and mouse experiments. 16S rRNA gene sequencing revealed significant shifts in gut microbial community structure during oral fluoropyrimidine treatment across multiple patient cohorts, in mouse small and large intestinal contents, and in patient-derived ex vivo communities. Metagenomic sequencing revealed marked shifts in pyrimidine-related gene abundance during oral fluoropyrimidine treatment, including enrichment of the preTA operon, which is sufficient for the inactivation of active metabolite 5-fluorouracil (5-FU). preTA + bacteria depleted 5-FU in gut microbiota grown ex vivo and the mouse distal gut. Germ-free and antibiotic-treated mice experienced increased fluoropyrimidine toxicity, which was rescued by colonization with the mouse gut microbiota, preTA + E. coli, or preTA-high CRC patient stool. Finally, preTA abundance was negatively associated with fluoropyrimidine toxicity in patients. Together, these data support a causal, clinically relevant interaction between a human gut bacterial operon and the dose-limiting side effects of cancer treatment. Our approach is generalizable to other drugs, including cancer immunotherapies, and provides valuable insights into host-microbiome interactions in the context of disease.

LPA3 agonist-producing Bacillus velezensis ADS024 is efficacious in multiple neuroinflammatory disease models

Neuroinflammation is a common component of neurological disorders. In the gut-brain-immune axis, bacteria and their metabolites are now thought to play a role in the modulation of the nervous and immune systems which may impact neuroinflammation. In this respect, commensal bacteria of humans have recently been shown to produce metabolites that mimic endogenous G-protein coupled receptor (GPCR) ligands. To date, it has not been established whether plant commensal bacteria, which may be ingested by animals including humans, can impact the gut-brain-immune axis via GPCR agonism. We screened an isopropanol (IPA) extract of the plant commensal Bacillus velezensis ADS024, a non-engrafting live biotherapeutic product (LBP) with anti-inflammatory properties isolated from human feces, against a panel of 168 GPCRs and identified strong agonism of the lysophosphatidic acid (LPA) receptor LPA3. The ADS024 IPA extracted material (ADS024-IPA) did not agonize LPA2, and only very weakly agonized LPA1. The agonism of LPA3 was inhibited by the reversible LPA1/3 antagonist Ki16425. ADS024-IPA signaled downstream of LPA3 through G-protein-induced calcium release, recruitment of β-arrestin, and recruitment of the neurodegeneration-associated proteins 14-3-3γ, ε and ζ but did not recruit the β isoform. Since LPA3 agonism was previously indirectly implicated in the reduction of pathology in models of Parkinson's disease (PD) and multiple sclerosis (MS) by use of the nonselective antagonist Ki16425, and since we identified an LPA3-specific agonist within ADS024, we sought to examine whether LPA3 might indeed be part of a broad underlying mechanism to control neuroinflammation. We tested oral treatment of ADS024 in multiple models of neuroinflammatory diseases using three models of PD, two models of MS, and a model each of amyotrophic lateral sclerosis (ALS), Huntington's disease (HD), and chemo-induced peripheral neuropathy (CIPN). ADS024 treatment improved model-specific functional effects including improvements in motor movement, breathing and swallowing, and allodynia suggesting that ADS024 treatment impacted a universal underlying neuroinflammatory mechanism regardless of the initiating cause of disease. We used the MOG-EAE mouse model to examine early events after disease initiation and found that ADS024 attenuated the increase in circulating lymphocytes and changes in neutrophil subtypes, and ADS024 attenuated the early loss of cell-surface LPA3 receptor expression on circulating white blood cells. ADS024 efficacy was partially inhibited by Ki16425 in vivo suggesting LPA3 may be part of its mechanism. Altogether, these data suggest that ADS024 and its LPA3 agonism activity should be investigated further as a possible treatment for diseases with a neuroinflammatory component.

A systematic framework for understanding the microbiome in human health and disease: from basic principles to clinical translation

The human microbiome is a complex and dynamic system that plays important roles in human health and disease. However, there remain limitations and theoretical gaps in our current understanding of the intricate relationship between microbes and humans. In this narrative review, we integrate the knowledge and insights from various fields, including anatomy, physiology, immunology, histology, genetics, and evolution, to propose a systematic framework. It introduces key concepts such as the 'innate and adaptive genomes', which enhance genetic and evolutionary comprehension of the human genome. The 'germ-free syndrome' challenges the traditional 'microbes as pathogens' view, advocating for the necessity of microbes for health. The 'slave tissue' concept underscores the symbiotic intricacies between human tissues and their microbial counterparts, highlighting the dynamic health implications of microbial interactions. 'Acquired microbial immunity' positions the microbiome as an adjunct to human immune systems, providing a rationale for probiotic therapies and prudent antibiotic use. The 'homeostatic reprogramming hypothesis' integrates the microbiome into the internal environment theory, potentially explaining the change in homeostatic indicators post-industrialization. The 'cell-microbe co-ecology model' elucidates the symbiotic regulation affecting cellular balance, while the 'meta-host model' broadens the host definition to include symbiotic microbes. The 'health-illness conversion model' encapsulates the innate and adaptive genomes' interplay and dysbiosis patterns. The aim here is to provide a more focused and coherent understanding of microbiome and highlight future research avenues that could lead to a more effective and efficient healthcare system.

Distinguishing Cophylogenetic Signal from Phylogenetic Congruence Clarifies the Interplay Between Evolutionary History and Species Interactions

Interspecific interactions, including host-symbiont associations, can profoundly affect the evolution of the interacting species. Given the phylogenies of host and symbiont clades and knowledge of which host species interact with which symbiont, two questions are often asked: "Do closely related hosts interact with closely related symbionts?" and "Do host and symbiont phylogenies mirror one another?." These questions are intertwined and can even collapse under specific situations, such that they are often confused one with the other. However, in most situations, a positive answer to the first question, hereafter referred to as "cophylogenetic signal," does not imply a close match between the host and symbiont phylogenies. It suggests only that past evolutionary history has contributed to shaping present-day interactions, which can arise, for example, through present-day trait matching, or from a single ancient vicariance event that increases the probability that closely related species overlap geographically. A positive answer to the second, referred to as "phylogenetic congruence," is more restrictive as it suggests a close match between the two phylogenies, which may happen, for example, if symbiont diversification tracks host diversification or if the diversifications of the two clades were subject to the same succession of vicariance events. Here we apply a set of methods (ParaFit, PACo, and eMPRess), whose significance is often interpreted as evidence for phylogenetic congruence, to simulations under 3 biologically realistic scenarios of trait matching, a single ancient vicariance event, and phylogenetic tracking with frequent cospeciation events. The latter is the only scenario that generates phylogenetic congruence, whereas the first 2 generate a cophylogenetic signal in the absence of phylogenetic congruence. We find that tests of global-fit methods (ParaFit and PACo) are significant under the 3 scenarios, whereas tests of event-based methods (eMPRess) are only significant under the scenario of phylogenetic tracking. Therefore, significant results from global-fit methods should be interpreted in terms of cophylogenetic signal and not phylogenetic congruence; such significant results can arise under scenarios when hosts and symbionts had independent evolutionary histories. Conversely, significant results from event-based methods suggest a strong form of dependency between hosts and symbionts evolutionary histories. Clarifying the patterns detected by different cophylogenetic methods is key to understanding how interspecific interactions shape and are shaped by evolution.

Mind over matter: the microbial mindscapes of psychedelics and the gut-brain axis

Psychedelics have emerged as promising therapeutics for several psychiatric disorders. Hypotheses around their mechanisms have revolved around their partial agonism at the serotonin 2 A receptor, leading to enhanced neuroplasticity and brain connectivity changes that underlie positive mindset shifts. However, these accounts fail to recognise that the gut microbiota, acting via the gut-brain axis, may also have a role in mediating the positive effects of psychedelics on behaviour. In this review, we present existing evidence that the composition of the gut microbiota may be responsive to psychedelic drugs, and in turn, that the effect of psychedelics could be modulated by microbial metabolism. We discuss various alternative mechanistic models and emphasize the importance of incorporating hypotheses that address the contributions of the microbiome in future research. Awareness of the microbial contribution to psychedelic action has the potential to significantly shape clinical practice, for example, by allowing personalised psychedelic therapies based on the heterogeneity of the gut microbiota.

Recent genetic drift in the co-diversified gut bacterial symbionts of laboratory mice

Laboratory mice (Mus musculus domesticus) harbor gut bacterial strains that are distinct from those of wild mice1 but whose evolutionary histories are poorly understood. Understanding the divergence of laboratory-mouse gut microbiota (LGM) from wild-mouse gut microbiota (WGM) is critical, because LGM and WGM have been previously shown to differentially affect mouse immune-cell proliferation2,3, infection resistance4, cancer progression2, and ability to model drug outcomes for humans5. Here, we show that laboratory mice have retained gut bacterial symbiont lineages that diversified in parallel (co-diversified) with rodent species for > 25 million years, but that LGM strains of these ancestral symbionts have experienced accelerated accumulation of genetic load during the past ~ 120 years of captivity. Compared to closely related WGM strains, co-diversified LGM strains displayed significantly faster genome-wide rates of fixation of nonsynonymous mutations, indicating elevated genetic drift, a difference that was absent in non-co-diversified symbiont clades. Competition experiments in germ-free mice further indicated that LGM strains within co-diversified clades displayed significantly reduced fitness in vivo compared to WGM relatives to an extent not observed within non-co-diversified clades. Thus, stochastic processes (e.g., bottlenecks), not natural selection in the laboratory, have been the predominant evolutionary forces underlying divergence of co-diversified symbiont strains between laboratory and wild house mice. Our results show that gut bacterial lineages conserved in diverse rodent species have acquired novel mutational burdens in laboratory mice, providing an evolutionary rationale for restoring laboratory mice with wild gut bacterial strain diversity.

Integrating research on bacterial pathogens and commensals to fight infections-an ecological perspective

The incidence of antibiotic-resistant bacterial infections is increasing, and development of new antibiotics has been deprioritised by the pharmaceutical industry. Interdisciplinary research approaches, based on the ecological principles of bacterial fitness, competition, and transmission, could open new avenues to combat antibiotic-resistant infections. Many facultative bacterial pathogens use human mucosal surfaces as their major reservoirs and induce infectious diseases to aid their lateral transmission to new host organisms under some pathological states of the microbiome and host. Beneficial bacterial commensals can outcompete specific pathogens, thereby lowering the capacity of the pathogens to spread and cause serious infections. Despite the clinical relevance, however, the understanding of commensal-pathogen interactions in their natural habitats remains poor. In this Personal View, we highlight directions to intensify research on the interactions between bacterial pathogens and commensals in the context of human microbiomes and host biology that can lead to the development of innovative and sustainable ways of preventing and treating infectious diseases.

Western diets and chronic diseases

'Westernization', which incorporates industrial, cultural and dietary trends, has paralleled the rise of noncommunicable diseases across the globe. Today, the Western-style diet emerges as a key stimulus for gut microbial vulnerability, chronic inflammation and chronic diseases, affecting mainly the cardiovascular system, systemic metabolism and the gut. Here we review the diet of modern times and evaluate the threat it poses for human health by summarizing recent epidemiological, translational and clinical studies. We discuss the links between diet and disease in the context of obesity and type 2 diabetes, cardiovascular diseases, gut and liver diseases and solid malignancies. We collectively interpret the evidence and its limitations and discuss future challenges and strategies to overcome these. We argue that healthcare professionals and societies must react today to the detrimental effects of the Western diet to bring about sustainable change and improved outcomes in the future.

Domestication and evolutionary histories of specialized gut symbionts across cephalotine ants

The evolution of animals and their gut symbionts is a complex phenomenon, obscured by lability and diversity. In social organisms, transmission of symbionts among relatives may yield systems with more stable associations. Here, we study the history of a social insect symbiosis involving cephalotine ants and their extracellular gut bacteria, which come predominantly from host-specialized lineages. We perform multi-locus phylogenetics for symbionts from nine bacterial orders, and map prior amplicon sequence data to lineage-assigned symbiont genomes, studying distributions of rigorously defined symbionts across 20 host species. Based on monophyly and additional hypothesis testing, we estimate that these specialized gut bacteria belong to 18 distinct lineages, of which 15 have been successfully isolated and cultured. Several symbiont lineages showed evidence for domestication events that occurred later in cephalotine evolutionary history, and only one lineage was ubiquitously detected in all 20 host species and 48 colonies sampled with amplicon 16S rRNA sequencing. We found evidence for phylogenetically constrained distributions in four symbionts, suggesting historical or genetic impacts on community composition. Two lineages showed evidence for frequent intra-lineage co-infections, highlighting the potential for niche divergence after initial domestication. Nearly all symbionts showed evidence for occasional host switching, but four may, more often, co-diversify with their hosts. Through our further assessment of symbiont localization and genomic functional profiles, we demonstrate distinct niches for symbionts with shared evolutionary histories, prompting further questions on the forces underlying the evolution of hosts and their gut microbiomes.

Host control of the microbiome: Mechanisms, evolution, and disease

Many species, including humans, host communities of symbiotic microbes. There is a vast literature on the ways these microbiomes affect hosts, but here we argue for an increased focus on how hosts affect their microbiomes. Hosts exert control over their symbionts through diverse mechanisms, including immunity, barrier function, physiological homeostasis, and transit. These mechanisms enable hosts to shape the ecology and evolution of microbiomes and generate natural selection for microbial traits that benefit the host. Our microbiomes result from a perpetual tension between host control and symbiont evolution, and we can leverage the host's evolved abilities to regulate the microbiota to prevent and treat disease. The study of host control will be central to our ability to both understand and manipulate microbiotas for better health.

The gut microbiota-constipation connection: Insights from a two sample bidirectional Mendelian randomization study

OBJECTIVE

The dysbiosis of the gut microbiota has been implicated in various maladies. Research has identified an association between the dysbiosis of the gut microbiota and the risk of constipation, prompting this study to elucidate the potential causal relationship between gut microbiota imbalance with constipation through a two sample bidirectional Mendelian randomization (MR) study, shedding light on the genetic mechanisms underlying the connection between gut microbiota and constipation.

METHODS

The forward MR analysis aimed to scrutinize whether alterations in the composition and abundance of gut microbiota impact the risk of constipation, while the reverse MR analysis explored whether the genetic predisposition to constipation influences the abundance of gut microbiota. Genomic correlation data for the gut microbiota were sourced from the comprehensive statistics of the MiBioGen consortium. Genomic correlation data for constipation were obtained from the IEU database, encoded as the dataset ebi-a-GCST90018829. The correlation was assessed using various analytical techniques, including inverse variance weighting (IVW), Mendelian randomization-Egger regression (MR-Egger), and weighted median and mode methodologies. To ensure the robustness of the results, a meticulous sensitivity analysis was conducted, incorporating Cochran's Q test, MR-Egger intercept test, Mendelian Randomization Pleiotropy RESidual Sum and Outlier (MR-PRESSO), and a Leave-one-out analysis.

RESULTS

In the forward Mendelian randomization analyses, a negative correlation was discerned between the abundance of Coprococcus in the gut microbiota and the occurrence of constipation (IVW: OR = 0.74, 95 % CI = 0.64-0.86, p = 0.0001), whereas a positive correlation was observed between the abundance of Bacteroidetes in the gut microbiota and constipation (IVW: OR = 1.22, 95 % CI = 1.00-1.50, p = 0.04). In the forward Mendelian randomization analyses, we were unsuccessful in obtaining valid instrumental variables for scrutiny, and we deemed that constipation exerts no influence on the composition of the gut microbiota.

CONCLUSION

Genetic predisposition towards increased abundance of Coprococcus and decreased abundance of Bacteroidetes is correlated with a diminished susceptibility to constipation. This investigation showed that alterations in the gut microbiota precipitated the onset of constipation, rather than constipation inducing modifications in the microbial flora.

Exploring the Impact of Short Term Travel on Gut Microbiota and Probiotic Bacteria Mediated Stability

Although travelers are frequently accompanied by abdominal discomfort and even diarrhea, not every trip can cause this issue. Many studies have reported that intestinal microbes play an important role in it. However, little is known about the reason for the dynamics of these intestinal microbes. Here, we delved into the effects of short-term travel on the gut microbiota of 12 healthy individuals. A total of 72 fecal samples collected before and after one-week travel, alongside non-traveling controls, underwent amplicon sequencing and a series of bioinformatic analyses. We found that travel significantly increased intra-individual gut microbiota fluctuations without diarrhea symptoms. In addition, the initial composition of the gut microbiota before travel emerged as a crucial factor in understanding these fluctuations. Travelers with stable microbiota exhibited an enrichment of specific probiotic bacteria (Agathobaculum, Faecalibacterium, Bifidobacterium, Roseburia, Lactobacillus) before travel. Another batch of data validated their predictive role in distinguishing travelers with and without the gut microbial disorder. This work provided valuable insights into understanding the relationship between gut microbiota and travel. It offered a microbiota-centric perspective and a potential avenue for interventions to preserve gut health during travel.

Gut microbial genomes with paired isolates from China illustrate probiotic and cardiometabolic effects

The gut microbiome displays genetic differences among populations, and characterization of the genomic landscape of the gut microbiome in China remains limited. Here, we present the Chinese Gut Microbial Reference (CGMR) set, comprising 101,060 high-quality metagenomic assembled genomes (MAGs) of 3,707 nonredundant species from 3,234 fecal samples across primarily rural Chinese locations, 1,376 live isolates mainly from lactic acid bacteria, and 987 novel species relative to worldwide databases. We observed region-specific coexisting MAGs and MAGs with probiotic and cardiometabolic functionalities. Preliminary mouse experiments suggest a probiotic effect of two Faecalibacillus intestinalis isolates in alleviating constipation, cardiometabolic influences of three Bacteroides fragilis_A isolates in obesity, and isolates from the genera Parabacteroides and Lactobacillus in host lipid metabolism. Our study expands the current microbial genomes with paired isolates and demonstrates potential host effects, contributing to the mechanistic understanding of host-microbe interactions.

Long-term evolution of Streptococcus mitis and Streptococcus pneumoniae leads to higher genetic diversity within rather than between human populations

Evaluation of the apportionment of genetic diversity of human bacterial commensals within and between human populations is an important step in the characterization of their evolutionary potential. Recent studies showed a correlation between the genomic diversity of human commensal strains and that of their host, but the strength of this correlation and of the geographic structure among human populations is a matter of debate. Here, we studied the genomic diversity and evolution of the phylogenetically related oro-nasopharyngeal healthy-carriage Streptococcus mitis and Streptococcus pneumoniae, whose lifestyles range from stricter commensalism to high pathogenic potential. A total of 119 S. mitis genomes showed higher within- and among-host variation than 810 S. pneumoniae genomes in European, East Asian and African populations. Summary statistics of the site-frequency spectrum for synonymous and non-synonymous variation and ABC modelling showed this difference to be due to higher ancestral bacterial population effective size (Ne) in S. mitis, whose genomic variation has been maintained close to mutation-drift equilibrium across (at least many) generations, whereas S. pneumoniae has been expanding from a smaller ancestral bacterial population. Strikingly, both species show limited differentiation among human populations. As genetic differentiation is inversely proportional to the product of effective population size and migration rate (Nem), we argue that large Ne have led to similar differentiation patterns, even if m is very low for S. mitis. We conclude that more diversity within than among human populations and limited population differentiation must be common features of the human microbiome due to large Ne.

Comparative analysis of gut microbiome of mangrove brachyuran crabs revealed patterns of phylosymbiosis and codiversification

The acquisition of microbial symbionts enables animals to rapidly adapt to and exploit novel ecological niches, thus significantly enhancing the evolutionary fitness and success of their hosts. However, the dynamics of host-microbe interactions and their evolutionary implications remain largely underexplored in marine invertebrates. Crabs of the family Sesarmidae (Crustacea: Brachyura) are dominant inhabitants of mangrove forests and are considered keystone species there. Their rapid diversification, particularly after adopting a plant-feeding lifestyle, is believed to have been facilitated by symbiotic gut microbes, enabling successful colonization of intertidal and terrestrial environments. To investigate the patterns and mechanisms shaping the microbial communities and the role of microbes in the evolution of Sesarmidae, we characterized and compared the gut microbiome compositions across 43 crab species from Sesarmidae and other mangrove-associated families using 16S metabarcoding. We found that the gut microbiome assemblages in crabs are primarily determined by host identity, with a secondary influence from environmental factors such as microhabitat and sampling location, and to a lesser extent influenced by biological factors such as sex and gut region. While patterns of phylosymbiosis (i.e. when microbial community relationships recapitulate the phylogeny of their hosts) were consistently observed in all beta-diversity metrics analysed, the strength of phylosymbiosis varied across crab families. This suggests that the bacterial assemblages in each family were differentially shaped by different degrees of host filtering and/or other evolutionary processes. Notably, Sesarmidae displayed signals of cophylogeny with its core gut bacterial genera, which likely play crucial functional roles in their hosts by providing lignocellulolytic enzymes, essential amino acids, and fatty acids supplementation. Our results support the hypothesis of microbial contribution to herbivory and terrestrialization in mangrove crabs, highlighting the tight association and codiversification of the crab holobiont.

Evolution and the critical role of the microbiota in the reduced mental and physical health associated with low socioeconomic status (SES)

The evolution of the gut-microbiota-brain axis in animals reveals that microbial inputs influence metabolism, the regulation of inflammation and the development of organs, including the brain. Inflammatory, neurodegenerative and psychiatric disorders are more prevalent in people of low socioeconomic status (SES). Many aspects of low SES reduce exposure to the microbial inputs on which we are in a state of evolved dependence, whereas the lifestyle of wealthy citizens maintains these exposures. This partially explains the health deficit of low SES, so focussing on our evolutionary history and on environmental and lifestyle factors that distort microbial exposures might help to mitigate that deficit. But the human microbiota is complex and we have poor understanding of its functions at the microbial and mechanistic levels, and in the brain. Perhaps its composition is more flexible than the microbiota of animals that have restricted habitats and less diverse diets? These uncertainties are discussed in relation to the encouraging but frustrating results of attempts to treat psychiatric disorders by modulating the microbiota.

Exposure to environmental pollutants selects for xenobiotic-degrading functions in the human gut microbiome

Environmental pollutants from different chemical families may reach the gut microbiome, where they can be metabolized and transformed. However, how our gut symbionts respond to the exposure to environmental pollution is still underexplored. In this observational, cohort study, we aim to investigate the influence of environmental pollution on the gut microbiome composition and potential activity by shotgun metagenomics. We select as a case study a population living in a highly polluted area in Campania region (Southern Italy), proposed as an ideal field for exposomic studies and we compare the fecal microbiome of 359 subjects living in areas with high, medium and low environmental pollution. We highlight changes in gut microbiome composition and functionality that were driven by pollution exposure. Subjects from highly polluted areas show higher blood concentrations of dioxin and heavy metals, as well as an increase in microbial genes related to degradation and/or resistance to these molecules. Here we demonstrate the dramatic effect that environmental xenobiotics have on gut microbial communities, shaping their composition and boosting the selection of strains with degrading capacity. The gut microbiome can be considered as a pivotal player in the environment-health interaction that may contribute to detoxifying toxic compounds and should be taken into account when developing risk assessment models. The study was registered at ClinicalTrials.gov with the identifier NCT05976126.

The strength of gut microbiota transfer along social networks and genealogical lineages in the house mouse

The gut microbiota of vertebrates is acquired from the environment and other individuals, including parents and unrelated conspecifics. In the laboratory mouse, a key animal model, inter-individual interactions are severely limited and its gut microbiota is abnormal. Surprisingly, our understanding of how inter-individual transmission impacts house mouse gut microbiota is solely derived from laboratory experiments. We investigated the effects of inter-individual transmission on gut microbiota in two subspecies of house mice (Mus musculus musculus and M. m. domesticus) raised in a semi-natural environment without social or mating restrictions. We assessed the correlation between microbiota composition (16S rRNA profiles), social contact intensity (microtransponder-based social networks), and mouse relatedness (microsatellite-based pedigrees). Inter-individual transmission had a greater impact on the lower gut (colon and cecum) than on the small intestine (ileum). In the lower gut, relatedness and social contact independently influenced microbiota similarity. Despite female-biased parental care, both parents exerted a similar influence on their offspring's microbiota, diminishing with the offspring's age in adulthood. Inter-individual transmission was more pronounced in M. m. domesticus, a subspecies, with a social and reproductive network divided into more closed modules. This suggests that the transmission magnitude depends on the social and genetic structure of the studied population.

Critical role of the gut microbiota in immune responses and cancer immunotherapy

The gut microbiota plays a critical role in the progression of human diseases, especially cancer. In recent decades, there has been accumulating evidence of the connections between the gut microbiota and cancer immunotherapy. Therefore, understanding the functional role of the gut microbiota in regulating immune responses to cancer immunotherapy is crucial for developing precision medicine. In this review, we extract insights from state-of-the-art research to decipher the complicated crosstalk among the gut microbiota, the systemic immune system, and immunotherapy in the context of cancer. Additionally, as the gut microbiota can account for immune-related adverse events, we discuss potential interventions to minimize these adverse effects and discuss the clinical application of five microbiota-targeted strategies that precisely increase the efficacy of cancer immunotherapy. Finally, as the gut microbiota holds promising potential as a target for precision cancer immunotherapeutics, we summarize current challenges and provide a general outlook on future directions in this field.

The importance of host physical niches for the stability of gut microbiome composition

Gut bacteria are prevalent throughout the Metazoa and form complex microbial communities associated with food breakdown, nutrient provision and disease prevention. How hosts acquire and maintain a consistent bacterial flora remains mysterious even in the best-studied animals, including humans, mice, fishes, squid, bugs, worms and flies. This essay visits the evidence that hosts have co-evolved relationships with specific bacteria and that some of these relationships are supported by specialized physical niches that select, sequester and maintain microbial symbionts. Genetics approaches could uncover the mechanisms for recruiting and maintaining the stable and consistent members of the microbiome. This article is part of the theme issue 'Sculpting the microbiome: how host factors determine and respond to microbial colonization'.

The microbiome: an integral player in immune homeostasis and inflammation in the respiratory tract

The last decade of microbiome research has highlighted its fundamental role in systemic immune and metabolic homeostasis. The microbiome plays a prominent role during gestation and into early life, when maternal lifestyle factors shape immune development of the newborn. Breast milk further shapes gut colonization, supporting the development of tolerance to commensal bacteria and harmless antigens while preventing outgrowth of pathogens. Environmental microbial and lifestyle factors that disrupt this process can dysregulate immune homeostasis, predisposing infants to atopic disease and childhood asthma. In health, the low-biomass lung microbiome, together with inhaled environmental microbial constituents, establishes the immunological set point that is necessary to maintain pulmonary immune defense. However, in disease perturbations to immunological and physiological processes allow the upper respiratory tract to act as a reservoir of pathogenic bacteria, which can colonize the diseased lung and cause severe inflammation. Studying these host-microbe interactions in respiratory diseases holds great promise to stratify patients for suitable treatment regimens and biomarker discovery to predict disease progression. Preclinical studies show that commensal gut microbes are in a constant flux of cell division and death, releasing microbial constituents, metabolic by-products, and vesicles that shape the immune system and can protect against respiratory diseases. The next major advances may come from testing and utilizing these microbial factors for clinical benefit and exploiting the predictive power of the microbiome by employing multiomics analysis approaches.

Linkage equilibrium between rare mutations

Recombination breaks down genetic linkage by reshuffling existing variants onto new genetic backgrounds. These dynamics are traditionally quantified by examining the correlations between alleles, and how they decay as a function of the recombination rate. However, the magnitudes of these correlations are strongly influenced by other evolutionary forces like natural selection and genetic drift, making it difficult to tease out the effects of recombination. Here we introduce a theoretical framework for analyzing an alternative family of statistics that measure the homoplasy produced by recombination. We derive analytical expressions that predict how these statistics depend on the rates of recombination and recurrent mutation, the strength of negative selection and genetic drift, and the present-day frequencies of the mutant alleles. We find that the degree of homoplasy can strongly depend on this frequency scale, which reflects the underlying timescales over which these mutations occurred. We show how these scaling properties can be used to isolate the effects of recombination, and discuss their implications for the rates of horizontal gene transfer in bacteria.