The Microbiota-Gut-Brain Axis.

The importance of the gut-brain axis in maintaining homeostasis has long been appreciated. However, the past 15 yr have seen the emergence of the microbiota (the trillions of microorganisms within and on our bodies) as one of the key regulators of gut-brain function and has led to the appreciation of the importance of a distinct microbiota-gut-brain axis. This axis is gaining ever more traction in fields investigating the biological and physiological basis of psychiatric, neurodevelopmental, age-related, and neurodegenerative disorders. The microbiota and the brain communicate with each other via various routes including the immune system, tryptophan metabolism, the vagus nerve and the enteric nervous system, involving microbial metabolites such as short-chain fatty acids, branched chain amino acids, and peptidoglycans. Many factors can influence microbiota composition in early life, including infection, mode of birth delivery, use of antibiotic medications, the nature of nutritional provision, environmental stressors, and host genetics. At the other extreme of life, microbial diversity diminishes with aging. Stress, in particular, can significantly impact the microbiota-gut-brain axis at all stages of life. Much recent work has implicated the gut microbiota in many conditions including autism, anxiety, obesity, schizophrenia, Parkinson's disease, and Alzheimer's disease. Animal models have been paramount in linking the regulation of fundamental neural processes, such as neurogenesis and myelination, to microbiome activation of microglia. Moreover, translational human studies are ongoing and will greatly enhance the field. Future studies will focus on understanding the mechanisms underlying the microbiota-gut-brain axis and attempt to elucidate microbial-based intervention and therapeutic strategies for neuropsychiatric disorders.

Kefir ameliorates specific microbiota-gut-brain axis impairments in a mouse model relevant to autism spectrum disorder.

Autism spectrum disorder (ASD) is one of the most severe developmental disorders, affecting on average 1 in 150 children worldwide. There is a great need for more effective strategies to improve quality of life in ASD subjects. The gut microbiome has emerged as a potential therapeutic target in ASD. A novel modulator of the gut microbiome, the traditionally fermented milk drink kefir, has recently been shown to modulate the microbiota and decrease repetitive behaviour, one of the hallmarks of ASD, in mice. As such, we hypothesized that kefir could ameliorate behavioural deficits in a mouse model relevant to ASD; the BTBR T+ Itpr3tf/J mouse strain. To this end, adult mice were administered either kefir (UK4) or a milk control for three weeks as treatment lead-in, after which they were assessed for their behavioural phenotype using a battery of tests. In addition, we assessed systemic immunity by flow cytometry and the gut microbiome using shotgun metagenomic sequencing. We found that indeed kefir decreased repetitive behaviour in this mouse model. Furthermore, kefir prolonged stress-induced increases in corticosterone 60 min post-stress, which was accompanied by an ameliorated innate immune response as measured by LY6Chi monocyte levels. In addition, kefir increased the levels of anti-inflammatory Treg cells in mesenteric lymph nodes (MLNs). Kefir also increased the relative abundance of Lachnospiraceae bacterium A2, which correlated with reduced repetitive behaviour and increased Treg cells in MLNs. Functionally, kefir modulated various predicted gut microbial pathways, including the gut-brain module S-Adenosylmethionine (SAM) synthesis, as well as L-valine biosynthesis and pyruvate fermentation to isobutanol, which all correlated with repetitive behaviour. Taken together our data show that kefir modulates peripheral immunoregulation, can ameliorate specific ASD behavioural dysfunctions and modulates selective aspects of the composition and function of the gut microbiome, indicating that kefir supplementation might prove a viable strategy in improving quality of life in ASD subjects.

Exploring the Impact of the Microbiome on Neuroactive Steroid Levels in Germ-Free Animals.

Steroid hormones are essential biomolecules for human physiology as they modulate the endocrine system, nervous function and behaviour. Recent studies have shown that the gut microbiota is directly involved in the production and metabolism of steroid hormones in the periphery. However, the influence of the gut microbiota on levels of steroids acting and present in the brain (i.e., neuroactive steroids) is not fully understood. Therefore, using liquid chromatography-tandem mass spectrometry, we assessed the levels of several neuroactive steroids in various brain areas and the plasma of germ-free (GF) male mice and conventionally colonized controls. The data obtained indicate an increase in allopregnanolone levels associated with a decrease in those of 5α-androstane-3α, 17ß-diol (3α-diol) in the plasma of GF mice. Moreover, an increase of dihydroprogesterone and isoallopregnanolone in the hippocampus, cerebellum, and cerebral cortex was also reported. Changes in dihydrotestosterone and 3α-diol levels were also observed in the hippocampus of GF mice. In addition, an increase in dehydroepiandrosterone was associated with a decrease in testosterone levels in the hypothalamus of GF mice. Our findings suggest that the absence of microbes affects the neuroactive steroids in the periphery and the brain, supporting the evidence of a microbiota-mediated modulation of neuroendocrine pathways involved in preserving host brain functioning.

The role of the microbiota in acute stress-induced myeloid immune cell trafficking.

There has been a growing recognition of the involvement of the gastrointestinal microbiota in the development of stress-related disorders. Acute stress leads to activation of neuroendocrine systems, which in turn orchestrate a large-scale redistribution of innate immune cells. Both these response systems are independently known to be primed by the microbiota, even though much is still unclear about the role of the gastrointestinal microbiota in acute stress-induced immune activation. In this study, we investigated whether the microbiota influences acute stress-induced changes in innate immunity using conventionally colonised mice, mice devoid of any microbiota (i.e. germ-free, GF), and colonised GF mice (CGF). We also explored the kinetics of stress-induced immune cell mobilisation in the blood, the spleen and mesenteric lymph nodes (MLNs). Mice were either euthanised prior to stress or underwent restraint stress and were then euthanised at various time points (i.e. 0, 45- and 240-minutes) post-stress. Plasma adrenaline and noradrenaline levels were analysed using ELISA and immune cell levels were quantified using flow cytometry. GF mice had increased baseline levels of adrenaline and noradrenaline, of which adrenaline was normalised in CGF mice. In tandem, GF mice had decreased circulating levels of LY6Chi and LY6Cmid, CCR2+ monocytes, and granulocytes, but not LY6C-, CX3CR1+ monocytes. These deficits were normalised in CGF mice. Acute stress decreased blood LY6Chi and LY6Cmid, CCR2+ monocytes while increasing granulocyte levels in all groups 45 min post-stress. However, only GF mice showed stress-induced changes in LY6Chi monocytes and granulocytes 240 min post-stress, indicating impairments in the recovery from acute stress-induced changes in levels of specific innate immune cell types. LY6C-, CX3CR1+ monocytes remained unaffected by stress, indicating that acute stress impacts systemic innate immunity in a cell-type-specific manner. Overall, these data reveal novel cell-type-specific changes in the innate immune system in response to acute stress, which in turn are impacted by the microbiota. In conclusion, the microbiota influences the priming and recovery of the innate immune system to an acute stressor and may inform future microbiota-targeted therapeutics aimed at modulating stress-induced immune activation in stress-related disorders.

Resilience to chronic stress is associated with specific neurobiological, neuroendocrine and immune responses.

Research into the molecular basis of stress resilience is a novel strategy to identify potential therapeutic strategies to treat stress-induced psychopathologies such as anxiety and depression. Stress resilience is a phenomenon which is not solely driven by effects within the central nervous system (CNS) but involves multiple systems, central and peripheral, which interact with and influence each other. Accordingly, we used the chronic social defeat stress paradigm and investigated specific CNS, endocrine and immune responses to identify signatures of stress-resilience and stress susceptibility in mice. Our results showed that mice behaviourally susceptible to stress (indexed by a reduction in social interaction behaviour) had higher plasma corticosterone levels and adrenal hypertrophy. An increase in inflammatory circulating monocytes was another hallmark of stress susceptibility. Furthermore, prefrontal cortex mRNA expression of corticotrophin-releasing factor (Crf) was increased in susceptible mice relative to resilient mice. We also report differences in hippocampal synaptic plasticity between resilient and susceptible mice. Ongoing studies will interpret the functional relevance of these signatures which could potentially inform the development of novel psychotherapeutic strategies.

Short-chain fatty acids: microbial metabolites that alleviate stress-induced brain-gut axis alterations.

KEY POINTS: Chronic (psychosocial) stress changes gut microbiota composition, as well as inducing behavioural and physiological deficits. The microbial metabolites short-chain fatty acids (SCFAs) have been implicated in gastrointestinal functional, (neuro)immune regulation and host metabolism, but their role in stress-induced behavioural and physiological alterations is poorly understood. Administration of SCFAs to mice undergoing psychosocial stress alleviates enduring alterations in anhedonia and heightened stress-responsiveness, as well as stress-induced increases in intestinal permeability. In contrast, chronic stress-induced alterations in body weight gain, faecal SCFAs and the gene expression of the SCFA receptors FFAR2 and FFAR3 remained unaffected by SCFA supplementation. These results present novel insights into mechanisms underpinning the influence of the gut microbiota on brain homeostasis, behaviour and host metabolism, informing the development of microbiota-targeted therapies for stress-related disorders. ABSTRACT: There is a growing recognition of the involvement of the gastrointestinal microbiota in the regulation of physiology and behaviour. Microbiota-derived metabolites play a central role in the communication between microbes and their host, with short-chain fatty acids (SCFAs) being perhaps the most studied. SCFAs are primarily derived from fermentation of dietary fibres and play a pivotal role in host gut, metabolic and immune function. All these factors have previously been demonstrated to be adversely affected by stress. Therefore, we sought to assess whether SCFA supplementation could counteract the enduring effects of chronic psychosocial stress. C57BL/6J male mice received oral supplementation of a mixture of the three principle SCFAs (acetate, propionate and butyrate). One week later, mice underwent 3 weeks of repeated psychosocial stress, followed by a comprehensive behavioural analysis. Finally, plasma corticosterone, faecal SCFAs and caecal microbiota composition were assessed. SCFA treatment alleviated psychosocial stress-induced alterations in reward-seeking behaviour, and increased responsiveness to an acute stressor and in vivo intestinal permeability. In addition, SCFAs exhibited behavioural test-specific antidepressant and anxiolytic effects, which were not present when mice had also undergone psychosocial stress. Stress-induced increases in body weight gain, faecal SCFAs and the colonic gene expression of the SCFA receptors free fatty acid receptors 2 and 3 remained unaffected by SCFA supplementation. Moreover, there were no collateral effects on caecal microbiota composition. Taken together, these data show that SCFA supplementation alleviates selective and enduring alterations induced by repeated psychosocial stress and these data may inform future research into microbiota-targeted therapies for stress-related disorders.

Postprandial serum endotoxin in healthy humans is modulated by dietary fat in a randomized, controlled, cross-over study.

BACKGROUND: High-fat diets may contribute to metabolic disease via postprandial changes in serum endotoxin and inflammation. It is unclear how dietary fat composition may alter these parameters. We hypothesized that a meal rich in n-3 (ω3) fatty acids would reduce endotoxemia and associated inflammation but a saturated or n-6 (ω6) fatty acid-rich meal would increase postprandial serum endotoxin concentrations and systemic inflammation in healthy adults. METHODS: Healthy adults (n = 20; mean age 25 ± 3.2 S.D. years) were enrolled in this single-blind, randomized, cross-over study. Participants were randomized to treatment and reported to the laboratory, after an overnight fast, on four occasions separated by at least one week. Participants were blinded to treatment meal and consumed one of four isoenergetic meals that provided: 1) 20 % fat (control; olive oil) or 35 % fat provided from 2) n-3 (ω3) (DHA = 500 mg; fish oil); 3) n-6 (ω6) (7.4 g; grapeseed oil) or 4) saturated fat (16 g; coconut oil). Baseline and postprandial blood samples were collected. Primary outcome was defined as the effect of treatment meal on postprandial endotoxemia. Serum was analyzed for metabolites, inflammatory markers, and endotoxin. Data from all 20 participants were analyzed using repeated-measures ANCOVA. RESULTS: Participant serum endotoxin concentration was increased during the postprandial period after the consumption of the saturated fat meal but decreased after the n-3 meal (p < 0.05). The n-6 meal did not effect a different outcome in participant postprandial serum endotoxin concentration from that of the control meal (p > 0.05). There was no treatment meal effect on participant postprandial serum biomarkers of inflammation. Postprandial serum triacylglycerols were significantly elevated following the n-6 meal compared to the n-3 meal. Non-esterified fatty acids were significantly increased after consumption of the saturated fat meal compared to other treatment meals. CONCLUSIONS: Meal fatty acid composition modulates postprandial serum endotoxin concentration in healthy adults. However, postprandial endotoxin was not associated with systemic inflammation in vivo. TRIAL REGISTRATION: This study was retrospectively registered at clinicaltrials.gov as NCT02521779 on July 28, 2015.

Leaching of antibiotic resistance genes and microbial assemblages following poultry litter applications in karst and non-karst landscapes.

Antibiotic resistance is increasingly recognized as a critical challenge affecting human, animal, and environmental health. Yet, environmental dynamics and transport of antibiotic resistance genes (ARGs) and microbial communities in karst and non-karst leachate following poultry litter land applications are not well understood. This study investigates impacts of broiler poultry litter application on the proliferation of ARGs (tetW, qnrS, ermB, sulI, and blaCTX-M-32), class 1 integron (intI1 i), and alterations in microbial communities (16S rRNA) within karst derived soils, which are crucial and under-researched systems in the global hydrological cycle, and non-karst landscapes. Using large, intact soil columns (45 cm diam. × 100 cm depth) from karst and non-karst landscapes, the role of preferential flow and ARG transport in leachate was enumerated following surface application of poultry litter and simulated rain events. This research demonstrated that in poultry litter amended karst soils, ARG (i.e., ermB and tetW) abundance in leachate increased 1.5 times compared to non-karst systems (p < 0.05), highlighting the influence of geological factors on ARG proliferation. Notably, microbial communities in karst soil leachate exhibited increased diversity and abundance, suggesting a potential linkage between microbial composition and ARG presence. Further, our correlation and network analyses identified relationships between leachate ARGs, microbial taxa, and physicochemical properties, underscoring the complex interplay in these environmentally sensitive areas. These findings illuminate the critical role of karst systems in shaping ARG abundance and pollutant dispersal and microbial community dynamics, thus emphasizing the need for landscape-specific approaches in managing ARG dissemination to the environment. This study provides a deeper understanding of hydrogeological ARG dynamics but also lays the groundwork for future research and strategies to mitigate ARG dissemination through targeted manure applications across agricultural landscapes.

Inclusion of trans-cinnamaldehyde and caprylic acid in feed results in detectable concentrations in the chicken gut and reduces foodborne pathogen carriage.

Poultry act as a major reservoir host for Salmonella and Campylobacter spp., the 2 leading causes of foodborne illnesses globally and in the United States. Preharvest stage interventions to reduce foodborne pathogen carriage in poultry are increasingly informed by consumer preference for antibiotic-free poultry production. The in-feed inclusion of plant-derived antimicrobial compounds is a promising antibiotic alternative strategy to reduce foodborne pathogen load in the broiler chicken gut. Yet, the fate of these phytochemicals through the broiler chicken gastrointestinal tract is unknown. Likewise, while in-feed phytochemicals have been widely demonstrated in challenge models to reduce foodborne pathogen carriage, little is known regarding efficacy to curb natural routes of infection. As such, the aim of the present study was 2-fold. We sought to determine the concentrations of 2 phytochemicals, trans-cinnamaldehyde and caprylic acid, in each region of the chicken gastrointestinal tract following their in-feed inclusion over a 6-wk production period. In addition, we investigated how the in-feed provision of these phytochemicals may protect against environmental acquisition of Campylobacter jejuni and Salmonella spp. Trans-cinnamaldehyde and caprylic acid were detected in crop, gizzard, duodenal, jejunal, and ileal contents. Crop and gizzard concentrations were not significantly (P > 0.05) different. A significant (P < 0.05) decrease in phytochemical concentration was observed in intestinal regions compared to crop and gizzard. Trans-cinnamaldehyde was consistently identified in cecal and colon contents, while caprylic acid was not detectable in these regions. Trans-cinnamaldehyde and caprylic acid were found to reduce (P < 0.05) Salmonella load. Together, our data establish that the in-feed addition of trans-cinnamaldehyde and caprylic acid, 2 phytochemicals that have previously been shown to exert antimicrobial activity against poultry-associated foodborne pathogens, results in detectable concentrations in the broiler chicken gastrointestinal tract. By providing researchers with a gastrointestinal region-by-region map of phytochemical concentrations, the present study is expected to inform the choice of in-feed phytochemicals targeting foodborne pathogen carriage in the broiler chicken gastrointestinal tract.

Cold stress initiates catecholaminergic and serotonergic responses in the chicken gut that are associated with functional shifts in the microbiome.

Climate change is one of the most significant challenges facing the sustainability of global poultry production. Stress resulting from extreme temperature swings, including cold snaps, is a major concern for food production birds. Despite being well-documented in mammals, the effect of environmental stress on enteric neurophysiology and concomitant impact on host-microbiome interactions remains poorly understood in birds. As early life stressors may imprint long-term adaptive changes in the host, the present study sought to determine whether cold temperature stress, a prominent form of early life stress in chickens, elicits changes in enteric stress-related neurochemical concentrations that coincide with compositional and functional changes in the microbiome that persist into the later life of the bird. Chicks were, or were not, subjected to cold ambient temperature stress during the first week post-hatch and then remained at normal temperature for the remainder of the study. 16S rRNA gene and shallow shotgun metagenomic analyses demonstrated taxonomic and functional divergence between the cecal microbiomes of control and cold stressed chickens that persisted for weeks following cessation of the stressor. Enteric concentrations of serotonin, norepinephrine, and other monoamine neurochemicals were elevated (P < 0.05) in both cecal tissue and luminal content of cold stressed chickens. Significant (P < 0.05) associations were identified between cecal neurochemical concentrations and microbial taxa, suggesting host enteric neurochemical responses to environmental stress may shape the cecal microbiome. These findings demonstrate for the first time that early life exposure to environmental temperature stress can change the developmental trajectory of both the chicken cecal microbiome and host neuroendocrine enteric physiology. As many neurochemicals serve as interkingdom signaling molecules, the relationships identified here could be exploited to control the impact of climate change-driven stress on avian enteric host-microbe interactions.

The microbiota drives diurnal rhythms in tryptophan metabolism in the stressed gut.

Chronic stress disrupts microbiota-gut-brain axis function and is associated with altered tryptophan metabolism, impaired gut barrier function, and disrupted diurnal rhythms. However, little is known about the effects of acute stress on the gut and how it is influenced by diurnal physiology. Here, we used germ-free and antibiotic-depleted mice to understand how microbiota-dependent oscillations in tryptophan metabolism would alter gut barrier function at baseline and in response to an acute stressor. Cecal metabolomics identified tryptophan metabolism as most responsive to a 15-min acute stressor, while shotgun metagenomics revealed that most bacterial species exhibiting rhythmicity metabolize tryptophan. Our findings highlight that the gastrointestinal response to acute stress is dependent on the time of day and the microbiome, with a signature of stress-induced functional alterations in the ileum and altered tryptophan metabolism in the colon.

Complete genome sequence of an avian pathogenic Escherichia coli strain isolated from poultry.

Avian pathogenic Escherichia coli found in the avian intestinal tract can cause systemic disease in birds and act as a foodborne zoonotic pathogen associated with human disease. Here, we report the complete genome sequence of E. coli strain H1998 isolated from a chicken with colisepticemia.

Catecholamine concentrations in duck eggs are impacted by hen exposure to heat stress.

Rapid "fight-or-flight" responses to stress are largely orchestrated by the catecholamines. Moreover, catecholamines and catecholamine precursors are widely recognized to act as interkingdom signaling molecules among host and microbiota, as well as to serve as chemotactic signals for bacterial foodborne pathogens. While albumen and yolk concentrations of glucocorticoids have received extensive attention as non-invasive indicators of hen response to stress, little is known regarding the impact of the hen's stress response on in ovo catecholamine and catecholamine precursor concentrations. The aim of the present study was to determine norepinephrine and L-dopa concentrations in albumen and yolk of eggs laid by hens maintained under normal or heat stress conditions. Norepinephrine and L-dopa concentrations were also measured in oviductal tissue. Breeder ducks (∼35 weeks/age) were kept under normal (22°C) conditions or subjected to cyclical heat stress (35°C day/29.5°C night) for 3 weeks. Eggs (n = 12 per timepoint/group) were collected on a weekly basis. Hens were sacrificed at baseline or after 3 weeks of heat stress for oviductal tissue collection. Albumen, yolk, and oviduct concentrations of norepinephrine and L-dopa were determined using ultra high-performance liquid chromatography with electrochemical detection. Norepinephrine and L-dopa were detected in oviductal tissue as well as egg albumen and yolk. Norepinephrine concentrations were elevated (p < 0.05) in the yolk of eggs laid by the heat stress group compared to those of the control group. Norepinephrine concentrations in albumen were elevated (p < 0.05) in the heat stress group compared to control group at week 2. L-dopa concentrations were not significantly affected (p > 0.05) by heat stress in albumen, yolk, or oviductal tissue. Together, the present study provides the first evidence of the stress neurohormone, norepinephrine, in duck eggs and identifies that hen exposure to heat stress can affect in ovo norepinephrine concentrations. These data highlight the potential utility of in ovo catecholamine concentrations as non-invasive measures of the hen's response to stress, as well as warrants future research into whether hen deposition of stress-related neurochemicals into the egg could serve as a chemotactic signal in the vertical transmission of foodborne pathogens.

Distinct Cecal and Fecal Microbiome Responses to Stress Are Accompanied by Sex- and Diet-Dependent Changes in Behavior and Gut Serotonin.

Although diet- and stress-induced perturbations in the microbiome (biotic and abiotic factors) associate with changes in host behavior via the microbiota-gut-brain axis, few mechanisms have been identified. The identification of causative pathways by which the microbiome influences host behavior therefore would benefit from the application of evidence-based conceptual frameworks. One such causal framework is microbial endocrinology which is the study of neuroendocrine axes as avenues of bi-directional neurochemical-based host-microbe crosstalk. As such, we investigated the relationship between diet- and stress-induced alterations in behavior, regional gut serotonergic response, and concomitant changes in the cecal and fecal bacterial populations of male and female mice. Our results demonstrate that sex is a dominant factor in determining compositional changes in the gut microbiome in response to stress and diet modifications. Intestinal serotonergic responses to stress were observed in both sexes but dietary modifications uniquely affected region-specific changes in males and females. Likewise, behavioral alterations diverged between male and female mice. Together, these results demonstrate distinct sex-dependent relationships between cecal and fecal bacterial taxa and behavioral- and serotonergic-responses to stress and diet. The present study demonstrates the importance of including both male and female sexes in the examination of the microbiota-gut-brain axis. As different microbial taxa were identified to associate with the behavioral and gut serotonergic responses of male and female mice, certain bacterial species may hold sex-dependent functional relevance for the host. Future investigations seeking to develop microbiome-based strategies to afford host stress resilience should include sex-based differences in the microbiome.

A neurochemical biogeography of the broiler chicken intestinal tract.

The study of neurochemical-based interkingdom signaling and its impact on host-microbe interaction is called microbial endocrinology. Neurochemicals play a recognized role in determining bacterial colonization and interaction with the gut epithelium. While much attention has been devoted to the determination of neurochemical concentrations in the mammalian gut to better understand tissue and region-specific microbial endocrinology-based mechanisms of host-microbe interaction, little is known regarding the biogeography of neurochemicals in the avian gut. Greater resolution of avian gut neurochemical concentrations is needed especially as recent microbial endocrinology-based investigations into bacterial foodborne pathogen colonization of the chicken gut have demonstrated neurochemicals to affect Campylobacter jejuni and Salmonella spp. in vivo and in vitro. The aim of the present study was to determine the concentrations of stress-related neurochemicals in the tissue and luminal content of the duodenum, jejunum, ileum, cecum, and colon of the broiler intestinal tract, and to investigate if this biogeography changes with age of the bird. While all neurochemicals measured were detected in the intestinal tract, many displayed differences in regional concentrations. Whereas the catecholamine norepinephrine was detected in each region of the intestinal tract, epinephrine was present only in the cecum and colon. Likewise, dopamine, and its metabolite 3,4-dihydroxyphenylacetic acid were found in the greatest quantities in the cecum and colon. Serotonin and histamine were identified in each gut region. Region-specific age-related changes were observed (P < 0.05) for serotonin, its metabolite 5-hydroxyindole acetic acid as well as for histamine. Several neurochemicals, including norepinephrine, were found in the contents of each gut region. Epinephrine was not detected in the gut content of any region. Salsolinol, a microbial-produced neuroactive compound was detected in the gut content but not in tissue. Together, our data establish a neurochemical biogeography of the broiler chicken intestinal tract. By providing researchers with a region-by-region map of in vivo gut neurochemical concentrations of a modern broiler chicken breed, this neurochemical map is expected to inform future investigations that seek to utilize avian enteric neurochemistry.

Informal nutrition symposium: leveraging the microbiome (and the metabolome) for poultry production.

Knowledge of gut microbiology of poultry has advanced from a limited ability to culture relatively few microbial species, to attempting to understand the complex interactions between the bird and its microbiome. The Informal Nutrition Symposium 2021 was intended to help poultry scientists to make sense of the implications of the vast amounts of information being generated by researchers. This paper represents a compilation of the talks given at the symposium by leading international researchers in this field. The symposium began with an overview of the historical developments in the field of intestinal microbiology and microbiome research in poultry. Next, the systemic effects of the microbiome on health in the context of the interplay between the intestinal microbiota and the immune system were presented. Because the microbiome and the host communicate and influence each other, the novel field of kinomics (the study of protein phosphorylation) as used in the study of the poultry microbiome was discussed. Protein phosphorylation is a rapid response to the complex of signals among the microbiome, intestinal lumen metabolites, and the host. Then, a description of why an understanding of the role of microbial endocrinology in poultry production can lead to new understanding of the mechanisms by which the gut microbiota and the host can interact in defined mechanisms that ultimately determine health, pathogenesis of infectious disease, and behavior was given. Finally, a view forward was presented underscoring the importance of understanding mechanisms in microbiomes in other organ systems and other species. Additionally, the importance of the development of new -omics platforms and data management tools to more completely understand host microbiomes was stressed.

Serotonin modulates Campylobacter jejuni physiology and invitro interaction with the gut epithelium.

Microbial endocrinology, which is the study of neurochemical-based host-microbe interaction, has demonstrated that neurochemicals affect bacterial pathogenicity. A variety of neurochemicals, including norepinephrine, were shown to enhance intestinal epithelial colonization by Campylobacter jejuni. Yet, little is known whether serotonin, an abundant neurochemical produced in the gut, affects the physiology of C. jejuni and its interaction with the host gut epithelium. Considering the avian gut produces serotonin and serves as a major reservoir of C. jejuni, we sought to investigate whether serotonin can affect C. jejuni physiology and gut epithelial colonization in vitro. We first determined the biogeographical distribution of serotonin concentrations in the serosa, mucosa, as well as the luminal contents of the broiler chicken ileum, cecum, and colon. Serotonin concentrations were greater (P < 0.05) in the mucosa and serosa compared to the luminal content in each gut region examined. Among the ileum, colon, and cecum, the colon was found to contain the greatest concentrations of serotonin. We then investigated whether serotonin may effect changes in C. jejuni growth and motility in vitro. The C. jejuni used in this study was previously isolated from the broiler chicken ceca. Serotonin at concentrations of 1mM or below did not elicit changes in growth (P > 0.05) or motility (P > 0.05) of C. jejuni. Next, we utilized liquid chromatography tandem mass spectrometry to investigate whether serotonin affected the proteome of C. jejuni. Serotonin caused (P < 0.05) the downregulation of a protein (CJJ81176_1037) previously identified to be essential in C. jejuni colonization. Based on our findings, we evaluated whether serotonin would cause a functional change in C. jejuni adhesion and invasion of the HT29MTX-E12 colonic epithelial cell line. Serotonin was found to cause a reduction in adhesion (P < 0.05) but not invasion (P > 0.05). Together, we have identified a potential role for serotonin in modulating C. jejuni colonization in the gut in vitro. Further studies are required to understand the practical implications of these findings for the control of C. jejuni enteric colonization in vivo.

Japanese quail (Coturnix japonica) as a novel model to study the relationship between the avian microbiome and microbial endocrinology-based host-microbe interactions.

BACKGROUND: Microbial endocrinology, which is the study of neuroendocrine-based interkingdom signaling, provides a causal mechanistic framework for understanding the bi-directional crosstalk between the host and microbiome, especially as regards the effect of stress on health and disease. The importance of the cecal microbiome in avian health is well-recognized, yet little is understood regarding the mechanisms underpinning the avian host-microbiome relationship. Neuroendocrine plasticity of avian tissues that are focal points of host-microbiome interaction, such as the gut and lung, has likewise received limited attention. Avian in vivo models that enable the study of the neuroendocrine dynamic between host and microbiome are needed. As such, we utilized Japanese quail (Coturnix japonica) that diverge in corticosterone response to stress to examine the relationship between stress-related neurochemical concentrations at sites of host-microbe interaction, such as the gut, and the cecal microbiome. RESULTS: Our results demonstrate that birds which contrast in corticosterone response to stress show profound separation in cecal microbial community structure as well as exhibit differences in tissue neurochemical concentrations and structural morphologies of the gut. Changes in neurochemicals known to be affected by the microbiome were also identified in tissues outside of the gut, suggesting a potential relationship in birds between the cecal microbiome and overall avian physiology. CONCLUSIONS: The present study provides the first evidence that the structure of the avian cecal microbial community is shaped by selection pressure on the bird for neuroendocrine response to stress. Identification of unique region-dependent neurochemical changes in the intestinal tract following stress highlights environmental stressors as potential drivers of microbial endocrinology-based mechanisms of avian host-microbiome dialogue. Together, these results demonstrate that tissue neurochemical concentrations in the avian gut may be related to the cecal microbiome and reveal the Japanese quail as a novel avian model in which to further examine the mechanisms underpinning these relationships. Video abstract.

Volatility as a Concept to Understand the Impact of Stress on the Microbiome.

The microbiome-gut-brain-axis is a complex phenomenon spanning several dynamic systems in the body which can be parsed at a molecular, cellular, physiological and ecological level. A growing body of evidence indicates that this axis is particularly sensitive to the effects of stress and that it may be relevant to stress resilience and susceptibility. Although stress-induced changes in the composition of the microbiome have been reported, the degree of compositional change over time, which we define as volatility, has not been the subject of in-depth scrutiny. Using a chronic psychosocial stress paradigm in male mice, we report that the volatility of the microbiome significantly correlated with several readouts of the stress response, including behaviour and corticosterone response. We then validated these findings in a second independent group of stressed mice. Additionally, we assessed the relationship between volatility and stress parameters in a cohort of health volunteers who were undergoing academic exams and report similar observations. Finally, we found inter-species similarities in the microbiome stress response on a functional level. Our research highlights the effects of stress on the dynamic microbiome and underscores the informative value of volatility as a parameter that should be considered in all future analyses of the microbiome.

Gut-brain axis serotonergic responses to acute stress exposure are microbiome-dependent.

BACKGROUND: Understanding the mechanisms underpinning the response to acute stress is critical for determining how this can be modulated in both health and disease and across sexes. Stress can markedly alter the microbiome and gut-brain axis signaling with the serotonergic system being particularly sensitive to acute stress. As the impact of acute stress on regional serotonergic dynamics in the gut-brain axis and the contribution of the microbiome to this are poorly appreciated, we used microbiota-deficient mice to assess whether the serotonergic response to acute stress exposure is microbiome dependent. METHODS: Adult male and female conventional, germ-free, and colonized germ-free mice underwent a single acute stressor and samples were harvested immediately or 45 minutes following stress. Serotonin and related metabolites and serotonergic gene expression were determined. KEY RESULTS: Our data clearly show the microbiota influenced gastrointestinal serotonergic response to acute stress in a sex- and region-dependent manner. Male-specific poststress increases in colonic serotonin were absent in germ-free mice but normalized following colonization. mRNA serotonergic gene expression was differentially expressed in colon and ileum of germ-free mice on a sex-dependent basis. Within the frontal cortex, absence of the microbiome altered basal serotonin, its main metabolite 5-hydroxyindoleacetic acid, and prevented stress-induced increases in serotonin turnover. CONCLUSIONS AND INFERENCES: The gut microbiome influences the set points of the brain and gastrointestinal serotonergic systems and affected their response to acute stress in a sex- and region-dependent manner.