Humans studies have revealed consistent alterations in the gut microbiomes of patients with cardiometabolic and aging-associated diseases. A major focus of my group is to understand how variations in the gut microbiome modulate the effects of diet and host’s susceptibility to cardiometabolic disease. To address these questions we use a combination of hypothesis-generating, sequencing-centered analyses of microbiomes from humans and mice, followed by proof-of-principle/proof-of-mechanism studies in gnotobiotic mouse models of disease and classic bacteriology experiments.
Dissecting the impact of microbe-diet interactions on host health:
1. Bacterial choline metabolism: Choline is a water-soluble nutrient essential for human life. Gut microbial metabolism of choline results in the production of trimethylamine (TMA), which upon absorption by the host is converted in the liver to trimethylamine N-oxide (TMAO). TMAO exacerbates thrombosis and atherosclerosis in mice, and its abundance correlates with the severity of these diseases in humans. We have recently discovered that subjects with Alzheimer’s Disease (AD) have alterations in their microbiome and higher levels of TMAO in cerebrospinal fluid (CSF), and we have identified associations between this molecule and markers of AD (Vogt et al., 2017, Vogt et al., 2018). Given the importance of choline and vascular disease to AD development we are exploring the impact of choline-consuming/TMA(O)-producing bacteria on AD development using a gnotobiotic mouse model of AD (collaboration with Dr. Barbara Bendlin and Dr. Luigi Puglielli). Additionally, we are working with Dr. John Denu to explore the role of bacterial choline depletion, TMAO and bacterial metabolism on epigenetic regulation.
2. Diet-microbiota interactions that impact atherosclerosis: (a) variation in butyrate-producing bacteria: Consumption of dietary fiber decreases the risk of cardiometabolic disease. Recent studies suggest that some of the benefits associated with consumption of dietary fiber are mediated in part by gut microbes, with increased representation of butyrate-producing microbes proposed as one of the potential mediators. Moreover, gut butyrate-producing bacteria have been found at higher levels in healthy populations relative to patients suffering various inflammatory diseases such as atherosclerosis. We recently showed that the butyrate-producing bacterium Roseburia intestinalis interacts with dietary plant polysaccharides to generate large amounts of butyrate, modify epigenetic programming, impact gene expression in the intestine, and improve intestinal barrier function. These changes were associated with lower levels of pro-inflammatory molecules in systemic circulation, reduced inflammation in the aorta and smaller atherosclerotic lesions. By administering tributyrin in the diet, which delivers butyrate in the distal gut, we demonstrated that butyrate mimics the effects of R. intestinalis (Kasahara et al. 2018). We plan to identify fiber types and dietary sources that promote the growth of R. intestinalis and test whether these results can be extended to other butyrate-producing bacteria. Additionally, we are testing causal relationships of other associations that we have identified between gut microbes and atherosclerosis. (b) Interactions between gut bacteria and flavonoids: some of the health benefits associated with the consumption of fruits and vegetables are derived from flavonoids. However, our bodies do not absorb most flavonoids as they exist in food, and there is a large degree of interpersonal variation in the reported benefits. These observations in part reflect individual differences in gut microbiota, which metabolize ingested flavonoids to a variety of products that are more readily absorbed by the host, although the nature and extent of these transformations remain poorly characterized. We recently found that colonized mice fed a diet supplemented with quercetin accumulate high levels of biologically active (e.g., anti-thrombotic) metabolites and develop smaller atherosclerotic lesions relative to mice consuming a control diet, whereas age-matched germ-free Apolipoprotein E-deficient mice do not benefit from flavonoid supplementation. These preliminary results suggest that bacterial metabolism of quercetin may be necessary for some of the beneficial effects associated with flavonoid consumption. We are currently examining whether specific routes of flavonoid metabolism are required for its beneficial effects.
Deciphering the impact of host genetics on gut microbiome composition: Host genetic variation modulates the composition of the gut microbiota. We found significant variation in diabetes-related phenotypes, atherosclerosis and gut microbiota composition among different mouse strains, and identified microbes associated with these traits. Microbiota transplant experiments showed that altering the composition of the gut microbiota modifies strain-specific susceptibility to diet-induced metabolic disease (Kreznar et al., 2017) and atherosclerosis (Kasahara and Rey, unpublished). By using a genetically-diverse mouse cohort and quantitative trait loci (QTL) mapping, we are identifying gut microbes, microbial functions, microbial-derived metabolites and clinical phenotypes that are modulated by host genetics (collaboration with Dr. Alan Attie).
Using untargeted metabolomics (n = 1,162 subjects), the plasma metabolite (m/z = 265.1188) phenylacetylglutamine (PAGln) was discovered and then shown in an independent cohort (n = 4,000 subjects) to be associated with cardiovascular disease (CVD) and incident major adverse cardiovascular events (myocardial infarction, stroke, or death). A gut microbiota-derived metabolite, PAGln, was shown to enhance platelet activation-related phenotypes and thrombosis potential in whole blood, isolated platelets, and animal models of arterial injury. Functional and genetic engineering studies with human commensals, coupled with microbial colonization of germ-free mice, showed the microbial porA gene facilitates dietary phenylalanine conversion into phenylacetic acid, with subsequent host generation of PAGln and phenylacetylglycine (PAGly) fostering platelet responsiveness and thrombosis potential. Both gain- and loss-of-function studies employing genetic and pharmacological tools reveal PAGln mediates cellular events through G-protein coupled receptors, including α2A, α2B, and β2-adrenergic receptors. PAGln thus represents a new CVD-promoting gut microbiota-dependent metabolite that signals via adrenergic receptors.
The gut microbiome has been implicated in the progression of cardiovascular diseases (CVD) including hypertension, dyslipidemia, atherosclerosis, thrombosis, heart failure, and ischemic stroke. Metabolomics studies in humans and diverse mouse populations have revealed associations between diet-derived gut bacterial metabolites, including trimethylamine-N-oxide, short-chain fatty acids, and intermediates of aromatic amino acid breakdown, with progression of CVD. Functional studies in animals fed diets of defined composition have been instrumental for establishing causal links between these metabolites, the microbes that produce them, dietary substrates and disease. The purpose of this review is to discuss recent progress in our understanding of how gut microbial metabolism of food influences the development of CVD and to outline experimental approaches that can be useful for addressing crucial knowledge gaps in the field. Together, this body of work supports the notion that the gut microbiomes mediate many of the effects of diet.
The gut-microbe-derived metabolite trimethylamine N-oxide (TMAO) is increased by insulin resistance and associated with several sequelae of metabolic syndrome in humans, including cardiovascular, renal, and neurodegenerative disease. The mechanism by which TMAO promotes disease is unclear. We now reveal the endoplasmic reticulum stress kinase PERK (EIF2AK3) as a receptor for TMAO: TMAO binds to PERK at physiologically relevant concentrations; selectively activates the PERK branch of the unfolded protein response; and induces the transcription factor FoxO1, a key driver of metabolic disease, in a PERK-dependent manner. Furthermore, interventions to reduce TMAO, either by manipulation of the gut microbiota or by inhibition of the TMAO synthesizing enzyme, flavin-containing monooxygenase 3, can reduce PERK activation and FoxO1 levels in the liver. Taken together, these data suggest TMAO and PERK may be central to the pathogenesis of the metabolic syndrome.
The microbial communities that inhabit the distal gut of humans and other mammals exhibit large inter-individual variation. While host genetics is a known factor that influences gut microbiota composition, the mechanisms underlying this variation remain largely unknown. Bile acids (BAs) are hormones that are produced by the host and chemically modified by gut bacteria. BAs serve as environmental cues and nutrients to microbes, but they can also have antibacterial effects. We hypothesized that host genetic variation in BA metabolism and homeostasis influence gut microbiota composition. To address this, we used the Diversity Outbred (DO) stock, a population of genetically distinct mice derived from eight founder strains. We characterized the fecal microbiota composition and plasma and cecal BA profiles from 400 DO mice maintained on a high-fat high-sucrose diet for ~22 weeks. Using quantitative trait locus (QTL) analysis, we identified several genomic regions associated with variations in both bacterial and BA profiles. Notably, we found overlapping QTL for Turicibacter sp. and plasma cholic acid, which mapped to a locus containing the gene for the ileal bile acid transporter, Slc10a2. Mediation analysis and subsequent follow-up validation experiments suggest that differences in Slc10a2 gene expression associated with the different strains influences levels of both traits and revealed novel interactions between Turicibacter and BAs. This work illustrates how systems genetics can be utilized to generate testable hypotheses and provide insight into host-microbe interactions.
Consumption of flavonoids has been associated with protection against cardiovascular and neurodegenerative diseases. Most dietary flavonoids are subjected to bacterial transformations in the gut where they are converted into biologically active metabolites that are more bioavailable and have distinct effects relative to the parent compounds. While some of the pathways involved in the breakdown of flavonoids are emerging, little it is known about the impact of carbon source availability and community dynamics on flavonoid metabolism. This is relevant in the gut where there is a fierce competition for nutrients. In this study, we show that metabolism of one of the most commonly consumed flavonoids, quercetin, by the gut-associated bacterium is dependent on interspecies cross-feeding interactions when starch is the only energy source available. can degrade quercetin in the presence of glucose but is unable to use starch for growth or quercetin degradation. However, the starch-metabolizing bacterium , which does not metabolize quercetin, stimulates degradation of quercetin and butyrate production by via cross-feeding of glucose and maltose molecules released from starch. These results suggest that dietary substrates and interactions between species modulate the degradation of flavonoids and production of butyrate, thus shaping their bioavailability and bioactivity, and likely impacting their health-promoting effects in humans.
Elderberries are good sources of anthocyanins, which are poorly absorbed in the upper gastrointestinal tract but extensively transformed into phenolic metabolites at the colonic level. Because different gut microbiota strains have different metabolism, the catabolism of anthocyanins may lead to interindividual differences in metabolite production. In this work, an anthocyanin-rich elderberry extract was incubated with three single gut microbial strains (, , and ) up to 4 days, to assess differences in their phenolic metabolism. All of the strains degraded the elderberry anthocyanins, but the metabolic pathways followed were different. Although some metabolites were common for all of the strains, a wide disparity was observed in the kind and amount of several phenolic metabolites produced by each species. These preliminary results may be of help in the interpretation of the bioavailability of anthocyanins and give a clue to understand interindividual variability in metabolite production.
Shotgun metagenomics is a powerful, high-resolution technique enabling the study of microbial communities in situ. However, species-level resolution is only achieved after a process of 'binning' where contigs predicted to originate from the same genome are clustered. Such culture-independent sequencing frequently unearths novel microbes, and so various methods have been devised for reference-free binning. As novel microbiomes of increasing complexity are explored, sometimes associated with non-model hosts, robust automated binning methods are required. Existing methods struggle with eukaryotic contamination and cannot handle highly complex single metagenomes. We therefore developed an automated binning pipeline, termed 'Autometa', to address these issues. This command-line application integrates sequence homology, nucleotide composition, coverage and the presence of single-copy marker genes to separate microbial genomes from non-model host genomes and other eukaryotic contaminants, before deconvoluting individual genomes from single metagenomes. The method is able to effectively separate over 1000 genomes from a metagenome, allowing the study of previously intractably complex environments at the level of single species. Autometa is freely available at https://bitbucket.org/jason_c_kwan/autometa and as a docker image at https://hub.docker.com/r/jasonkwan/autometa under the GNU Affero General Public License 3 (AGPL 3).
The colonization of an animal's tissues by its microbial partners creates networks of communication across the host's body. We used the natural binary light-organ symbiosis between the squid and its luminous bacterial partner, , to define the impact of colonization on transcriptomic networks in the host. A night-active predator, coordinates the bioluminescence of its symbiont with visual cues from the environment to camouflage against moon and starlight. Like mammals, this symbiosis has a complex developmental program and a strong day/night rhythm. We determined how symbiont colonization impacted gene expression in the light organ itself, as well as in two anatomically remote organs: the eye and gill. While the overall transcriptional signature of light organ and gill were more alike, the impact of symbiosis was most pronounced and similar in light organ and eye, both in juvenile and adult animals. Furthermore, the presence of a symbiosis drove daily rhythms of transcription within all three organs. Finally, a single mutation in -specifically, deletion of the operon, which abrogates symbiont luminescence-reduced the symbiosis-dependent transcriptome of the light organ by two-thirds. In addition, while the gills responded similarly to light-organ colonization by either the wild-type or mutant, luminescence was required for all of the colonization-associated transcriptional responses in the juvenile eye. This study defines not only the impact of symbiont colonization on the coordination of animal transcriptomes, but also provides insight into how such changes might impact the behavior and ecology of the host.
Social relationships shape human health and mortality via behavioral, psychosocial, and physiological mechanisms, including inflammatory and immune responses. Though not tested in human studies, recent primate studies indicate that the gut microbiome may also be a biological mechanism linking relationships to health. Integrating microbiota data into the 60-year-old Wisconsin Longitudinal Study, we found that socialness with family and friends is associated with differences in the human fecal microbiota. Analysis of spouse (N = 94) and sibling pairs (N = 83) further revealed that spouses have more similar microbiota and more bacterial taxa in common than siblings, with no observed differences between sibling and unrelated pairs. These differences held even after accounting for dietary factors. The differences between unrelated individuals and married couples was driven entirely by couples who reported close relationships; there were no differences in similarity between couples reporting somewhat close relationships and unrelated individuals. Moreover, married individuals harbor microbial communities of greater diversity and richness relative to those living alone, with the greatest diversity among couples reporting close relationships, which is notable given decades of research documenting the health benefits of marriage. These results suggest that human interactions, especially sustained, close marital relationships, influence the gut microbiota.
The impact of gut microbiota on the regulation of host physiology has recently garnered considerable attention, particularly in key areas such as the immune system and metabolism. These areas are also crucial for the pathophysiology of and repair after myocardial infarction (MI). However, the role of the gut microbiota in the context of MI remains to be fully elucidated. To investigate the effects of gut microbiota on cardiac repair after MI, C57BL/6J mice were treated with antibiotics 7 days before MI to deplete mouse gut microbiota. Flow cytometry was applied to examine the changes in immune cell composition in the heart. 16S rDNA sequencing was conducted as a readout for changes in gut microbial composition. Short-chain fatty acid (SCFA) species altered after antibiotic treatment were identified by high-performance liquid chromatography. Fecal reconstitution, transplantation of monocytes, or dietary SCFA or Lactobacillus probiotic supplementation was conducted to evaluate the cardioprotective effects of microbiota on the mice after MI. Antibiotic-treated mice displayed drastic, dose-dependent mortality after MI. We observed an association between the gut microbiota depletion and significant reductions in the proportion of myeloid cells and SCFAs, more specifically acetate, butyrate, and propionate. Infiltration of CX3CR1+ monocytes to the peri-infarct zone after MI was also reduced, suggesting impairment of repair after MI. Accordingly, the physiological status and survival of mice were significantly improved after fecal reconstitution, transplantation of monocytes, or dietary SCFA supplementation. MI was associated with a reorganization of the gut microbial community such as a reduction in Lactobacillus. Supplementing antibiotic-treated mice with a Lactobacillus probiotic before MI restored myeloid cell proportions, yielded cardioprotective effects, and shifted the balance of SCFAs toward propionate. Gut microbiota-derived SCFAs play an important role in maintaining host immune composition and repair capacity after MI. This suggests that manipulation of these elements may provide opportunities to modulate pathological outcome after MI and indeed human health and disease as a whole.
Trimethylamine N-oxide (TMAO), a small molecule produced by the metaorganismal metabolism of dietary choline, has been implicated in human disease pathogenesis, including known risk factors for Alzheimer's disease (AD), such as metabolic, cardiovascular, and cerebrovascular disease. In this study, we tested whether TMAO is linked to AD by examining TMAO levels in cerebrospinal fluid (CSF) collected from a large sample (n = 410) of individuals with Alzheimer's clinical syndrome (n = 40), individuals with mild cognitive impairment (MCI) (n = 35), and cognitively-unimpaired individuals (n = 335). Linear regression analyses were used to determine differences in CSF TMAO between groups (controlling for age, sex, and APOE ε4 genotype), as well as to determine relationships between CSF TMAO and CSF biomarkers of AD (phosphorylated tau and beta-amyloid) and neuronal degeneration (total tau, neurogranin, and neurofilament light chain protein). CSF TMAO is higher in individuals with MCI and AD dementia compared to cognitively-unimpaired individuals, and elevated CSF TMAO is associated with biomarkers of AD pathology (phosphorylated tau and phosphorylated tau/Aβ) and neuronal degeneration (total tau and neurofilament light chain protein). These findings provide additional insight into gut microbial involvement in AD and add to the growing understanding of the gut-brain axis.
Increased fruit consumption is associated with reduced risk of colitis. It has been investigated whether the anti-colitic effects of the polyphenol-rich aronia berry (Aronia mitschurinii 'Viking') are mediated through Th17 and Treg. Colitis is induced in recombinase activating gene-1 deficient mice injected with syngeneic CD4 CD62L naïve T cells. Mice consume either 4.5% w/w aronia-berry-supplemented or a control diet concurrent with T cell transfer. The extent of colitis and immunocyte populations are evaluated at weeks 3 to 7 after transfer. Aronia consumption prevents colitic wasting and reduces colon weight/length ratios relative to the control diet at weeks 5 and 7. Compared to the control diet, aronia feeding increases Treg in mesenteric lymph node at all colitis stages. Treg and regulatory Th17 subpopulations (IL-17A IL-10 and IL-17A IL-22 ) are increased in lamina propria and spleen at week 5 in aronia-fed mice. Aronia feeding also decreases total CD4 cells but increases colonic Tregs. The ability of aronia to modulate colonic cytokines is associated with functional T cell IL-10 and increased diversity of microbiota. Aronia berry consumption inhibits adoptive transfer colitis by increasing Treg and regulatory Th17 cells. Dietary modulation of T cells is dynamic and precedes colitic wasting.
The gut microbiota confers resistance to pathogens of the intestinal ecosystem, yet the dynamics of pathogen-microbiome interactions and the metabolites involved in this process remain largely unknown. Here, we use gnotobiotic mice infected with the virulent pathogen serovar Typhimurium or the opportunistic pathogen in combination with metagenomics and discovery metabolomics to identify changes in the community and metabolome during infection. To isolate the role of the microbiota in response to pathogens, we compared mice monocolonized with the pathogen, uninfected mice "humanized" with a synthetic human microbiome, or infected humanized mice. In -infected mice, by 3 days into infection, microbiome community structure and function changed substantially, with a rise in strains and a reduction in biosynthetic gene cluster potential. In contrast, -infected mice had few microbiome changes. The LC-MS metabolomic fingerprint of the cecum differed between mice monocolonized with either pathogen and humanized infected mice. Specifically, we identified an increase in glutathione disulfide, glutathione cysteine disulfide, inosine 5'-monophosphate, and hydroxybutyrylcarnitine in mice infected with in contrast to uninfected mice and mice monocolonized with These metabolites potentially play a role in pathogen-induced oxidative stress. These results provide insight into how the microbiota community members interact with each other and with pathogens on a metabolic level. The gut microbiota is increasingly recognized for playing a critical role in human health and disease, especially in conferring resistance to both virulent pathogens such as , which infects 1.2 million people in the United States every year (E. Scallan, R. M. Hoekstra, F. J. Angulo, R. V. Tauxe, et al., Emerg Infect Dis 17:7-15, 2011, https://doi.org/10.3201/eid1701.P11101), and opportunistic pathogens like , which causes an estimated 46,000 cases of invasive candidiasis each year in the United States (Centers for Disease Control and Prevention, s, , 2013). Using a gnotobiotic mouse model, we investigate potential changes in gut microbial community structure and function during infection using metagenomics and metabolomics. We observe that changes in the community and in biosynthetic gene cluster potential occur within 3 days for the virulent serovar Typhimurium, but there are minimal changes with a poorly colonizing In addition, the metabolome shifts depending on infection status, including changes in glutathione metabolites in response to a infection, potentially in response to host oxidative stress.
Humans with metabolic and inflammatory diseases frequently harbour lower levels of butyrate-producing bacteria in their gut. However, it is not known whether variation in the levels of these organisms is causally linked with disease development and whether diet modifies the impact of these bacteria on health. Here we show that a prominent gut-associated butyrate-producing bacterial genus (Roseburia) is inversely correlated with atherosclerotic lesion development in a genetically diverse mouse population. We use germ-free apolipoprotein E-deficient mice colonized with synthetic microbial communities that differ in their capacity to generate butyrate to demonstrate that Roseburia intestinalis interacts with dietary plant polysaccharides to: impact gene expression in the intestine, directing metabolism away from glycolysis and toward fatty acid utilization; lower systemic inflammation; and ameliorate atherosclerosis. Furthermore, intestinal administration of butyrate reduces endotoxaemia and atherosclerosis development. Together, our results illustrate how modifiable diet-by-microbiota interactions impact cardiovascular disease, and suggest that interventions aimed at increasing the representation of butyrate-producing bacteria may provide protection against atherosclerosis.
Gut microbes influence cardiovascular disease and thrombosis risks through the production of trimethylamine N-oxide (TMAO). Microbiota-dependent generation of trimethylamine (TMA)-the precursor to TMAO-is rate limiting in the metaorganismal TMAO pathway in most humans and is catalyzed by several distinct microbial choline TMA-lyases, including the proteins encoded by the cutC/D (choline utilization C/D) genes in multiple human commensals. Direct demonstration that the gut microbial cutC gene is sufficient to transmit enhanced platelet reactivity and thrombosis potential in a host via TMA/TMAO generation has not yet been reported. Herein, we use gnotobiotic mice and a series of microbial colonization studies to show that microbial cutC-dependent TMA/TMAO production is sufficient to transmit heightened platelet reactivity and thrombosis potential in a host. Specifically, we examine in vivo thrombosis potential employing germ-free mice colonized with either high TMA-producing stable human fecal polymcrobial communities or a defined CutC-deficient background microbial community coupled with a CutC-expressing human commensal±genetic disruption of its cutC gene (ie, Clostridium sporogenes Δ cutC). Collectively, these studies point to the microbial choline TMA-lyase pathway as a rational molecular target for the treatment of atherothrombotic heart disease.
Mounting evidence suggests that environmental stress experienced in utero (for example, maternal nutritional deficits) establishes a predisposition in the newborn to the development of chronic diseases later in life. This concept is often referred to as the "fetal origins hypothesis" or "developmental origins of health and disease". Since its first proposal, epigenetics has emerged as an underlying mechanism explaining how environmental cues become gestationally "encoded". Many of the enzymes that impart and maintain epigenetic modifications are highly sensitive to nutrient availability, which can be influenced by the metabolic activities of the intestinal microbiota. Therefore, the maternal microbiome has the potential to influence epigenetics in utero and modulate offspring's long-term health trajectories. Here we summarize the current understanding of the interactions that occur between the maternal gut microbiome and the essential nutrient choline, that is not only required for fetal development and epigenetic regulation but is also a growth substrate for some microbes. Bacteria able to metabolize choline benefit from the presence of this nutrient and compete with the host for its access, which under extreme conditions may elicit signatures of choline deficiency. Another consequence of bacterial choline metabolism is the accumulation of the pro-inflammatory, pro-thrombotic metabolite trimethylamine-N-oxide (TMAO). Finally, we discuss how these different facets of microbial choline metabolism may influence infant development and health trajectories via epigenetic mechanisms and more broadly place a call to action to better understand how maternal microbial metabolism can shape their offspring's propensity to chronic disease development later in life.
Sex is one of the most powerful modifiers of disease development. Clear sexual dimorphism exists in cardiometabolic health susceptibility, likely due to differences in sex steroid hormones. Changes in the gut microbiome have been linked with the development of obesity, type 2 diabetes, and atherosclerosis; however, the impact of microbes in sex-biased cardiometabolic disorders remains unclear. The gut microbiome is critical for maintaining a normal estrous cycle, testosterone levels, and reproductive function. Gut microbes modulate the enterohepatic recirculation of estrogens and androgens, affecting local and systemic levels of sex steroid hormones. Gut bacteria can also generate androgens from glucocorticoids. This review summarizes current knowledge of the complex interplay between sexual dimorphism in cardiometabolic disease and the gut microbiome. Emerging evidence suggests the role of gut microbiome as a modifier of disease susceptibility due to sex; however, the impact on cardiometabolic disease in this complex interplay is lacking. Elucidating the role of gut microbiome on sex-biased susceptibility in cardiometabolic disease is of high relevance to public health given its high prevalence and significant financial burden.
Triclosan (TCS) is a high-volume chemical used as an antimicrobial ingredient in more than 2000 consumer products, such as toothpaste, cosmetics, kitchenware, and toys. We report that brief exposure to TCS, at relatively low doses, causes low-grade colonic inflammation, increases colitis, and exacerbates colitis-associated colon cancer in mice. Exposure to TCS alters gut microbiota in mice, and its proinflammatory effect is attenuated in germ-free mice. In addition, TCS treatment increases activation of Toll-like receptor 4 (TLR4) signaling in vivo and fails to promote colitis in mice. Together, our results demonstrate that this widely used antimicrobial ingredient could have adverse effects on colonic inflammation and associated colon tumorigenesis through modulation of the gut microbiota and TLR4 signaling. Together, these results highlight the need to reassess the effects of TCS on human health and potentially update policies regulating the use of this widely used antimicrobial.
Convenient, reproducible, and rapid preservation of unique biological specimens is pivotal to their use in microbiome analyses. As an increasing number of human studies incorporate the gut microbiome in their design, there is a high demand for streamlined sample collection and storage methods that are amenable to different settings and experimental needs. While several commercial kits address collection/shipping needs for sequence-based studies, these methods do not preserve samples properly for studies that require viable microbes. We describe the Fecal Aliquot Straw Technique (FAST) of fecal sample processing for storage and subsampling. This method uses a straw to collect fecal material from samples recently voided or preserved at low temperature but not frozen (i.e., 4 °C). Different straw aliquots collected from the same sample yielded highly reproducible communities as disclosed by 16S rRNA gene sequencing; operational taxonomic units that were lost, or gained, between the two aliquots represented very low-abundance taxa (i.e., < 0.3% of the community). FAST-processed samples inoculated into germ-free animals resulted in gut communities that retained on average ~ 80% of the donor's bacterial community. Assessment of choline metabolism and trimethylamine-N-oxide accumulation in transplanted mice suggests large interpersonal variation. Overall, FAST allows for repetitive subsampling without thawing of the specimens and requires minimal supplies and storage space, making it convenient to utilize both in the lab and in the field. FAST has the potential to advance microbiome research through easy, reproducible sample processing.
Using an untargeted metabolomics approach in initial (N = 99 subjects) and replication cohorts (N = 1,162), we discovered and structurally identified a plasma metabolite associated with cardiovascular disease (CVD) risks, N6,N6,N6-trimethyl-L-lysine (trimethyllysine, TML). Stable-isotope-dilution tandem mass spectrometry analyses of an independent validation cohort (N = 2,140) confirmed TML levels are independently associated with incident (3-year) major adverse cardiovascular event risks (hazards ratio [HR], 2.4; 95% CI, 1.7-3.4) and incident (5-year) mortality risk (HR, 2.9; 95% CI, 2.0-4.2). Genome-wide association studies identified several suggestive loci for TML levels, but none reached genome-wide significance; and d9(trimethyl)-TML isotope tracer studies confirmed TML can serve as a nutrient precursor for gut microbiota-dependent generation of trimethylamine (TMA) and the atherogenic metabolite trimethylamine N-oxide (TMAO). Although TML was shown to be abundant in both plant- and animal-derived foods, mouse and human fecal cultures (omnivores and vegans) showed slow conversion of TML to TMA. Furthermore, unlike chronic dietary choline, TML supplementation in mice failed to elevate plasma TMAO or heighten thrombosis potential in vivo. Thus, TML is identified as a strong predictor of incident CVD risks in subjects and to serve as a dietary precursor for gut microbiota-dependent generation of TMAO; however, TML does not appear to be a major microbial source for TMAO generation in vivo.
Alzheimer's disease (AD) is the most common form of dementia. However, the etiopathogenesis of this devastating disease is not fully understood. Recent studies in rodents suggest that alterations in the gut microbiome may contribute to amyloid deposition, yet the microbial communities associated with AD have not been characterized in humans. Towards this end, we characterized the bacterial taxonomic composition of fecal samples from participants with and without a diagnosis of dementia due to AD. Our analyses revealed that the gut microbiome of AD participants has decreased microbial diversity and is compositionally distinct from control age- and sex-matched individuals. We identified phylum- through genus-wide differences in bacterial abundance including decreased Firmicutes, increased Bacteroidetes, and decreased Bifidobacterium in the microbiome of AD participants. Furthermore, we observed correlations between levels of differentially abundant genera and cerebrospinal fluid (CSF) biomarkers of AD. These findings add AD to the growing list of diseases associated with gut microbial alterations, as well as suggest that gut bacterial communities may be a target for therapeutic intervention.
Choline is an essential nutrient and methyl donor required for epigenetic regulation. Here, we assessed the impact of gut microbial choline metabolism on bacterial fitness and host biology by engineering a microbial community that lacks a single choline-utilizing enzyme. Our results indicate that choline-utilizing bacteria compete with the host for this nutrient, significantly impacting plasma and hepatic levels of methyl-donor metabolites and recapitulating biochemical signatures of choline deficiency. Mice harboring high levels of choline-consuming bacteria showed increased susceptibility to metabolic disease in the context of a high-fat diet. Furthermore, bacterially induced reduction of methyl-donor availability influenced global DNA methylation patterns in both adult mice and their offspring and engendered behavioral alterations. Our results reveal an underappreciated effect of bacterial choline metabolism on host metabolism, epigenetics, and behavior. This work suggests that interpersonal differences in microbial metabolism should be considered when determining optimal nutrient intake requirements.
Mammals and their gut microbial communities share extensive and tightly coordinated co-metabolism of dietary substrates. A large number of microbial metabolites have been detected in host circulation and tissues and, in many cases, are linked to host metabolic, developmental, and immunological states. The presence of these metabolites in host tissues intersects with regulation of the host's epigenetic machinery. Although it is established that the host's epigenetic machinery is sensitive to levels of endogenous metabolites, the roles for microbial metabolites in epigenetic regulation are just beginning to be elucidated. This review focuses on eukaryotic chromatin regulation by endogenous and gut microbial metabolites and how these regulatory events may impact host developmental and metabolic phenotypes.
Genetic variation drives phenotypic diversity and influences the predisposition to metabolic disease. Here, we characterize the metabolic phenotypes of eight genetically distinct inbred mouse strains in response to a high-fat/high-sucrose diet. We found significant variation in diabetes-related phenotypes and gut microbiota composition among the different mouse strains in response to the dietary challenge and identified taxa associated with these traits. Follow-up microbiota transplant experiments showed that altering the composition of the gut microbiota modifies strain-specific susceptibility to diet-induced metabolic disease. Animals harboring microbial communities with enhanced capacity for processing dietary sugars and for generating hydrophobic bile acids showed increased susceptibility to metabolic disease. Notably, differences in glucose-stimulated insulin secretion between different mouse strains were partially recapitulated via gut microbiota transfer. Our results suggest that the gut microbiome contributes to the genetic and phenotypic diversity observed among mouse strains and provide a link between the gut microbiome and insulin secretion.
Histone-modifying enzymes regulate transcription and are sensitive to availability of endogenous small-molecule metabolites, allowing chromatin to respond to changes in environment. The gut microbiota produces a myriad of metabolites that affect host physiology and susceptibility to disease; however, the underlying molecular events remain largely unknown. Here we demonstrate that microbial colonization regulates global histone acetylation and methylation in multiple host tissues in a diet-dependent manner: consumption of a "Western-type" diet prevents many of the microbiota-dependent chromatin changes that occur in a polysaccharide-rich diet. Finally, we demonstrate that supplementation of germ-free mice with short-chain fatty acids, major products of gut bacterial fermentation, is sufficient to recapitulate chromatin modification states and transcriptional responses associated with colonization. These findings have profound implications for understanding the complex functional interactions between diet, gut microbiota, and host health.
Thrombosis plays an important role in cardiovascular disease (CVD). Platelet activation is an essential step in the genesis and propagation of atherothrombotic complications. In a recent publication, Zhu and colleagues report that gut microbe-derived TMAO enhances platelet responsiveness and thrombosis, providing a novel mechanistic connection between microbes and CVD (Zhu et al., 2016).
Choline is a water-soluble nutrient essential for human life. Gut microbial metabolism of choline results in the production of trimethylamine (TMA), which upon absorption by the host is converted in the liver to trimethylamine-N-oxide (TMAO). Recent studies revealed that TMAO exacerbates atherosclerosis in mice and positively correlates with the severity of this disease in humans. However, which microbes contribute to TMA production in the human gut, the extent to which host factors (e.g., genotype) and diet affect TMA production and colonization of these microbes, and the effects TMA-producing microbes have on the bioavailability of dietary choline remain largely unknown. We screened a collection of 79 sequenced human intestinal isolates encompassing the major phyla found in the human gut and identified nine strains capable of producing TMA from choline in vitro. Gnotobiotic mouse studies showed that TMAO accumulates in the serum of animals colonized with TMA-producing species, but not in the serum of animals colonized with intestinal isolates that do not generate TMA from choline in vitro. Remarkably, low levels of colonization by TMA-producing bacteria significantly reduced choline levels available to the host. This effect was more pronounced as the abundance of TMA-producing bacteria increased. Our findings provide a framework for designing strategies aimed at changing the representation or activity of TMA-producing bacteria in the human gut and suggest that the TMA-producing status of the gut microbiota should be considered when making recommendations about choline intake requirements for humans. Cardiovascular disease (CVD) is the leading cause of death and disability worldwide, and increased trimethylamine N-oxide (TMAO) levels have been causally linked with CVD development. This work identifies members of the human gut microbiota responsible for both the accumulation of trimethylamine (TMA), the precursor of the proatherogenic compound TMAO, and subsequent decreased choline bioavailability to the host. Understanding how to manipulate the representation and function of choline-consuming, TMA-producing species in the intestinal microbiota could potentially lead to novel means for preventing or treating atherosclerosis and choline deficiency-associated diseases.
To study how microbes establish themselves in a mammalian gut environment, we colonized germ-free mice with microbial communities from human, zebrafish, and termite guts, human skin and tongue, soil, and estuarine microbial mats. Bacteria from these foreign environments colonized and persisted in the mouse gut; their capacity to metabolize dietary and host carbohydrates and bile acids correlated with colonization success. Cohousing mice harboring these xenomicrobiota or a mouse cecal microbiota, along with germ-free "bystanders," revealed the success of particular bacterial taxa in invading guts with established communities and empty gut habitats. Unanticipated patterns of ecological succession were observed; for example, a soil-derived bacterium dominated even in the presence of bacteria from other gut communities (zebrafish and termite), and human-derived bacteria colonized germ-free bystander mice before mouse-derived organisms. This approach can be generalized to address a variety of mechanistic questions about succession, including succession in the context of microbiota-directed therapeutics.
The role of specific gut microbes in shaping body composition remains unclear. We transplanted fecal microbiota from adult female twin pairs discordant for obesity into germ-free mice fed low-fat mouse chow, as well as diets representing different levels of saturated fat and fruit and vegetable consumption typical of the U.S. diet. Increased total body and fat mass, as well as obesity-associated metabolic phenotypes, were transmissible with uncultured fecal communities and with their corresponding fecal bacterial culture collections. Cohousing mice harboring an obese twin's microbiota (Ob) with mice containing the lean co-twin's microbiota (Ln) prevented the development of increased body mass and obesity-associated metabolic phenotypes in Ob cage mates. Rescue correlated with invasion of specific members of Bacteroidetes from the Ln microbiota into Ob microbiota and was diet-dependent. These findings reveal transmissible, rapid, and modifiable effects of diet-by-microbiota interactions.
Sulfate-reducing bacteria (SRB) colonize the guts of ∼50% of humans. We used genome-wide transposon mutagenesis and insertion-site sequencing, RNA-Seq, plus mass spectrometry to characterize genetic and environmental factors that impact the niche of Desulfovibrio piger, the most common SRB in a surveyed cohort of healthy US adults. Gnotobiotic mice were colonized with an assemblage of sequenced human gut bacterial species with or without D. piger and fed diets with different levels and types of carbohydrates and sulfur sources. Diet was a major determinant of functions expressed by this artificial nine-member community and of the genes that impact D. piger fitness; the latter includes high- and low-affinity systems for using ammonia, a limiting resource for D. piger in mice consuming a polysaccharide-rich diet. Although genes involved in hydrogen consumption and sulfate reduction are necessary for its colonization, varying dietary-free sulfate levels did not significantly alter levels of D. piger, which can obtain sulfate from the host in part via cross-feeding mediated by Bacteroides-encoded sulfatases. Chondroitin sulfate, a common dietary supplement, increased D. piger and H2S levels without compromising gut barrier integrity. A chondroitin sulfate-supplemented diet together with D. piger impacted the assemblage's substrate utilization preferences, allowing consumption of more reduced carbon sources and increasing the abundance of the H2-producing Actinobacterium, Collinsella aerofaciens. Our findings provide genetic and metabolic details of how this H2-consuming SRB shapes the responses of a microbiota to diet ingredients and a framework for examining how individuals lacking D. piger differ from those who harbor it.
Gut microbial communities represent one source of human genetic and metabolic diversity. To examine how gut microbiomes differ among human populations, here we characterize bacterial species in fecal samples from 531 individuals, plus the gene content of 110 of them. The cohort encompassed healthy children and adults from the Amazonas of Venezuela, rural Malawi and US metropolitan areas and included mono- and dizygotic twins. Shared features of the functional maturation of the gut microbiome were identified during the first three years of life in all three populations, including age-associated changes in the genes involved in vitamin biosynthesis and metabolism. Pronounced differences in bacterial assemblages and functional gene repertoires were noted between US residents and those in the other two countries. These distinctive features are evident in early infancy as well as adulthood. Our findings underscore the need to consider the microbiome when evaluating human development, nutritional needs, physiological variations and the impact of westernization.
No abstract available.
The interrelationships between our diets and the structure and operations of our gut microbial communities are poorly understood. A model community of 10 sequenced human gut bacteria was introduced into gnotobiotic mice, and changes in species abundance and microbial gene expression were measured in response to randomized perturbations of four defined ingredients in the host diet. From the responses, we developed a statistical model that predicted over 60% of the variation in species abundance evoked by diet perturbations, and we were able to identify which factors in the diet best explained changes seen for each community member. The approach is generally applicable, as shown by a follow-up study involving diets containing various mixtures of pureed human baby foods.
The human gut microbiota harbors three main groups of H(2)-consuming microbes: methanogens including the dominant archaeon, Methanobrevibacter smithii, a polyphyletic group of acetogens, and sulfate-reducing bacteria. Defining their roles in the gut is important for understanding how hydrogen metabolism affects the efficiency of fermentation of dietary components. We quantified methanogens in fecal samples from 40 healthy adult female monozygotic (MZ) and 28 dizygotic (DZ) twin pairs, analyzed bacterial 16S rRNA datasets generated from their fecal samples to identify taxa that co-occur with methanogens, sequenced the genomes of 20 M. smithii strains isolated from families of MZ and DZ twins, and performed RNA-Seq of a subset of strains to identify their responses to varied formate concentrations. The concordance rate for methanogen carriage was significantly higher for MZ versus DZ twin pairs. Co-occurrence analysis revealed 22 bacterial species-level taxa positively correlated with methanogens: all but two were members of the Clostridiales, with several being, or related to, known hydrogen-producing and -consuming bacteria. The M. smithii pan-genome contains 987 genes conserved in all strains, and 1,860 variably represented genes. Strains from MZ and DZ twin pairs had a similar degree of shared genes and SNPs, and were significantly more similar than strains isolated from mothers or members of other families. The 101 adhesin-like proteins (ALPs) in the pan-genome (45 ± 6 per strain) exhibit strain-specific differences in expression and responsiveness to formate. We hypothesize that M. smithii strains use their different repertoires of ALPs to create diversity in their metabolic niches, by allowing them to establish syntrophic relationships with bacterial partners with differing metabolic capabilities and patterns of co-occurrence.
No abstract available.
Fermenting microbial communities generate hydrogen; its removal through the production of acetate, methane, or hydrogen sulfide modulates the efficiency of energy extraction from available nutrients in many ecosystems. We noted that pathway components for acetogenesis are more abundantly and consistently represented in the gut microbiomes of monozygotic twins and their mothers than components for methanogenesis or sulfate reduction and subsequently analyzed the metabolic potential of two sequenced human gut acetogens, Blautia hydrogenotrophica and Marvinbryantia formatexigens in vitro and in the intestines of gnotobiotic mice harboring a prominent saccharolytic bacterium. To do so, we developed a generally applicable method for multiplex sequencing of expressed microbial mRNAs (microbial RNA-Seq) and, together with mass spectrometry of metabolites, showed that these organisms have distinct patterns of substrate utilization. B. hydrogenotrophica targets aliphatic and aromatic amino acids. It increases the efficiency of fermentation by consuming reducing equivalents, thereby maintaining a high NAD(+)/NADH ratio and boosting acetate production. In contrast, M. formatexigens consumes oligosaccharides, does not impact the redox state of the gut, and boosts the yield of succinate. These findings have strategic implications for those who wish to manipulate the hydrogen economy of gut microbial communities in ways that modulate energy harvest.
Purple non-sulphur phototrophic bacteria (PNSB) are excellent models for analysing the co-ordination of major metabolisms, including oxidative phosphorylation, photophosphorylation, carbon dioxide fixation and nitrogen fixation. In species studied to date, a two-component system called RegBA controls these functions and it has been thought that this redox sensing regulatory system is essential for co-ordinating electron flow and cannot be easily replaced. Here we show that this is not the case for all PNSB and that the oxygen-sensing FixLJ-K system, initially described in rhizobia, controls microaerobic respiration, photophosphorylation and other major metabolic traits in Rhodopseudomonas palustris. A R. palustris fixK mutant grew normally aerobically but was impaired in microaerobic growth. It was also severely impaired in photosynthetic growth. Transcriptome analyses indicated that FixK positively regulates haem and bacteriochlorophyll biosynthesis, cbb3 oxidase and NADH dehydrogenase genes, as well as genes for autotrophy and aromatic compound degradation. Purified FixK interacted with the promoters of a bacteriochlorophyll biosynthesis operon, a bacteriophytochrome-histidine kinase gene and the fnr-type regulatory gene, aadR. A FixK-AadR hierarchy mediates the transition from microaerobic to anaerobic growth. These results underscore that physiologically similar bacteria can use very different regulatory strategies to control common major metabolisms.
No abstract available.
Diet and nutritional status are among the most important modifiable determinants of human health. The nutritional value of food is influenced in part by a person's gut microbial community (microbiota) and its component genes (microbiome). Unraveling the interrelations among diet, the structure and operations of the gut microbiota, and nutrient and energy harvest is confounded by variations in human environmental exposures, microbial ecology, and genotype. To help overcome these problems, we created a well-defined, representative animal model of the human gut ecosystem by transplanting fresh or frozen adult human fecal microbial communities into germ-free C57BL/6J mice. Culture-independent metagenomic analysis of the temporal, spatial, and intergenerational patterns of bacterial colonization showed that these humanized mice were stably and heritably colonized and reproduced much of the bacterial diversity of the donor's microbiota. Switching from a low-fat, plant polysaccharide-rich diet to a high-fat, high-sugar "Western" diet shifted the structure of the microbiota within a single day, changed the representation of metabolic pathways in the microbiome, and altered microbiome gene expression. Reciprocal transplants involving various combinations of donor and recipient diets revealed that colonization history influences the initial structure of the microbial community but that these effects can be rapidly altered by diet. Humanized mice fed the Western diet have increased adiposity; this trait is transmissible via microbiota transplantation. Humanized gnotobiotic mice will be useful for conducting proof-of-principle "clinical trials" that test the effects of environmental and genetic factors on the gut microbiota and host physiology. Nearly full-length 16S rRNA gene sequences are deposited in GenBank under the accession numbers GQ491120 to GQ493997.
The adult human distal gut microbial community is typically dominated by 2 bacterial phyla (divisions), the Firmicutes and the Bacteroidetes. Little is known about the factors that govern the interactions between their members. Here, we examine the niches of representatives of both phyla in vivo. Finished genome sequences were generated from Eubacterium rectale and E. eligens, which belong to Clostridium Cluster XIVa, one of the most common gut Firmicute clades. Comparison of these and 25 other gut Firmicutes and Bacteroidetes indicated that the Firmicutes possess smaller genomes and a disproportionately smaller number of glycan-degrading enzymes. Germ-free mice were then colonized with E. rectale and/or a prominent human gut Bacteroidetes, Bacteroides thetaiotaomicron, followed by whole-genome transcriptional profiling, high-resolution proteomic analysis, and biochemical assays of microbial-microbial and microbial-host interactions. B. thetaiotaomicron adapts to E. rectale by up-regulating expression of a variety of polysaccharide utilization loci encoding numerous glycoside hydrolases, and by signaling the host to produce mucosal glycans that it, but not E. rectale, can access. E. rectale adapts to B. thetaiotaomicron by decreasing production of its glycan-degrading enzymes, increasing expression of selected amino acid and sugar transporters, and facilitating glycolysis by reducing levels of NADH, in part via generation of butyrate from acetate, which in turn is used by the gut epithelium. This simplified model of the human gut microbiota illustrates niche specialization and functional redundancy within members of its major bacterial phyla, and the importance of host glycans as a nutrient foundation that ensures ecosystem stability.
The distal human intestine harbors trillions of microbes that allow us to extract calories from otherwise indigestible dietary polysaccharides. The products of polysaccharide fermentation include short-chain fatty acids that are ligands for Gpr41, a G protein-coupled receptor expressed by a subset of enteroendocrine cells in the gut epithelium. To examine the contribution of Gpr41 to energy balance, we compared Gpr41-/- and Gpr41+/+ mice that were either conventionally-raised with a complete gut microbiota or were reared germ-free and then cocolonized as young adults with two prominent members of the human distal gut microbial community: the saccharolytic bacterium, Bacteroides thetaiotaomicron and the methanogenic archaeon, Methanobrevibacter smithii. Both conventionally-raised and gnotobiotic Gpr41-/- mice colonized with the model fermentative community are significantly leaner and weigh less than their WT (+/+) littermates, despite similar levels of chow consumption. These differences are not evident when germ-free WT and germ-free Gpr41 knockout animals are compared. Functional genomic, biochemical, and physiologic studies of germ-free and cocolonized Gpr41-/- and +/+ littermates disclosed that Gpr41-deficiency is associated with reduced expression of PYY, an enteroendocrine cell-derived hormone that normally inhibits gut motility, increased intestinal transit rate, and reduced harvest of energy (short-chain fatty acids) from the diet. These results reveal that Gpr41 is a regulator of host energy balance through effects that are dependent upon the gut microbiota.
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Rhodopseudomonas palustris is a purple, facultatively phototrophic bacterium that uses hydrogen gas as an electron donor for carbon dioxide fixation during photoautotrophic growth or for ammonia synthesis during nitrogen fixation. It also uses hydrogen as an electron supplement to enable the complete assimilation of oxidized carbon compounds, such as malate, into cell material during photoheterotrophic growth. The R. palustris genome predicts a membrane-bound nickel-iron uptake hydrogenase and several regulatory proteins to control hydrogenase synthesis. There is also a novel sensor kinase gene (RPA0981) directly adjacent to the hydrogenase gene cluster. Here we show that the R. palustris regulatory sensor hydrogenase HupUV acts in conjunction with the sensor kinase-response regulator protein pair HoxJ-HoxA to activate hydrogenase expression in response to hydrogen gas. Transcriptome analysis indicated that the HupUV-HoxJA regulatory system also controls the expression of genes encoding a predicted dicarboxylic acid transport system, a putative formate transporter, and a glutamine synthetase. RPA0981 had a small effect in repressing hydrogenase synthesis. We also determined that the two-component system RegS-RegR repressed expression of the uptake hydrogenase, probably in response to changes in intracellular redox status. Transcriptome analysis indicated that about 30 genes were differentially expressed in R. palustris cells that utilized hydrogen when growing photoheterotrophically on malate under nitrogen-fixing conditions compared to a mutant strain that lacked uptake hydrogenase. From this it appears that the recycling of reductant in the form of hydrogen does not have extensive nonspecific effects on gene expression in R. palustris.
The photosynthetic bacterium Rhodopseudomonas palustris is one of just a few prokaryotes described so far that has vnf and anf genes for alternative vanadium cofactor (V) and iron cofactor (Fe) nitrogenases in addition to nif genes for a molybdenum cofactor (Mo) nitrogenase. Transcriptome data indicated that the 32 genes in the nif gene cluster, but not the anf or vnf genes, were induced in wild-type and Mo nitrogenase-expressing strains grown under nitrogen-fixing conditions in Mo-containing medium. Strains that were unable to express a functional Mo nitrogenase due to mutations in Mo nitrogenase structural genes synthesized functional V and Fe nitrogenases and expressed vnf and anf genes in nitrogen-fixing growth media that contained Mo and V at concentrations far in excess of those that repress alternative nitrogenase gene expression in other bacteria. Thus, not only does R. palustris have multiple enzymatic options for nitrogen fixation, but in contrast to reports on other nitrogen-fixing bacteria, the expression of its alternative nitrogenases is not repressed by transition metals. Between 95 and 295 genes that are not directly associated with nitrogenase synthesis and assembly were induced under nitrogen-fixing conditions, depending on which nitrogenase was being used by R. palustris. Genes for nitrogen acquisition were expressed at particularly high levels during alternative nitrogenase-dependent growth. This suggests that alternative nitrogenase-expressing cells are relatively starved for nitrogen and raises the possibility that fixed nitrogen availability may be the primary signal that controls the synthesis of the V and Fe nitrogenases.
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