Microbiota are the range of microorganisms that may be commensal, mutualistic, or pathogenic found in and on all multicellular organisms, including plants. Microbiota include bacteria, archaea, protists, fungi, and viruses, [2] [3] and have been found to be crucial for immunologic, hormonal, and metabolic homeostasis of their host.
The term microbiome describes either the collective genomes of the microbes that reside in an ecological niche or else the microbes themselves. [4] [5] [6]
The microbiome and host emerged during evolution as a synergistic unit from epigenetics and genetic characteristics, sometimes collectively referred to as a holobiont. [7] [8] The presence of microbiota in human and other metazoan guts has been critical for understanding the co-evolution between metazoans and bacteria. [9] [10] Microbiota play key roles in the intestinal immune and metabolic responses via their fermentation product (short-chain fatty acid), acetate. [11]
All plants and animals, from simple life forms to humans, live in close association with microbial organisms. [12] Several advances have driven the perception of microbiomes, including:
Biologists have come to appreciate that microbes make up an important part of an organism's phenotype, far beyond the occasional symbiotic case study. [13]
Commensalism, a concept developed by Pierre-Joseph van Beneden (1809–1894), a Belgian professor at the University of Louvain during the nineteenth century [14] is central to the microbiome, where microbiota colonize a host in a non-harmful coexistence. The relationship with their host is called mutualistic when organisms perform tasks that are known to be useful for the host, [15] : 700 [16] parasitic, when disadvantageous to the host. Other authors define a situation as mutualistic where both benefit, and commensal, where the unaffected host benefits the symbiont. [17] A nutrient exchange may be bidirectional or unidirectional, may be context dependent and may occur in diverse ways. [17] Microbiota that are expected to be present, and that under normal circumstances do not cause disease, are deemed normal flora or normal microbiota; [15] normal flora can not only be harmless, but can be protective of the host. [18]
The initial acquisition of microbiota in animals from mammalians to marine sponges is at birth, and may even occur through the germ cell line. In plants, the colonizing process can be initiated below ground in the root zone, around the germinating seed, the spermosphere, or originate from the above ground parts, the phyllosphere and the flower zone or anthosphere. [19] The stability of the rhizosphere microbiota over generations depends upon the plant type but even more on the soil composition, i.e. living and non living environment. [20] Clinically, new microbiota can be acquired through fecal microbiota transplant to treat infections such as chronic C. difficile infection. [21]
The human microbiota includes bacteria, fungi, archaea and viruses. Micro-animals which live on the human body are excluded. The human microbiome refers to their collective genomes. [15]
Humans are colonized by many microorganisms; the traditional estimate was that humans live with ten times more non-human cells than human cells; more recent estimates have lowered this to 3:1 and even to about 1:1 by number (1:350 by mass). [22] [23] [24] [25] [26]
In fact, these are so small that there are around 100 trillion microbiota on the human body, [27] around 39 trillion by revised estimates, with only 0.2 kg of total mass in a "reference" 70 kg human body. [26]
The Human Microbiome Project sequenced the genome of the human microbiota, focusing particularly on the microbiota that normally inhabit the skin, mouth, nose, digestive tract, and vagina. [15] It reached a milestone in 2012 when it published initial results. [28]
The plant microbiome was recently discovered to originate from the seed. [46] Microorganism which are transmitted via seed migrate into the developing seedling in a specific route in which certain community move to the leaves and others to the roots. [46] In the diagram on the right, microbiota colonizing the rhizosphere, entering the roots and colonizing the next tuber generation via the stolons, are visualized with a red color. Bacteria present in the mother tuber, passing through the stolons and migrating into the plant as well as into the next generation of tubers are shown in blue. [45]
Plants are attractive hosts for microorganisms since they provide a variety of nutrients. Microorganisms on plants can be epiphytes (found on the plants) or endophytes (found inside plant tissue). [47] [48] Oomycetes and fungi have, through convergent evolution, developed similar morphology and occupy similar ecological niches. They develop hyphae, threadlike structures that penetrate the host cell. In mutualistic situations the plant often exchanges hexose sugars for inorganic phosphate from the fungal symbiont. It is speculated that such very ancient associations have aided plants when they first colonized land. [17] [49] Plant-growth promoting bacteria (PGPB) provide the plant with essential services such as nitrogen fixation, solubilization of minerals such as phosphorus, synthesis of plant hormones, direct enhancement of mineral uptake, and protection from pathogens. [50] [51] PGPBs may protect plants from pathogens by competing with the pathogen for an ecological niche or a substrate, producing inhibitory allelochemicals, or inducing systemic resistance in host plants to the pathogen [19]
The symbiotic relationship between a host and its microbiota is under laboratory research for how it may shape the immune system of mammals. [52] [53] In many animals, the immune system and microbiota may engage in "cross-talk" by exchanging chemical signals, which may enable the microbiota to influence immune reactivity and targeting. [54] Bacteria can be transferred from mother to child through direct contact and after birth. [55] As the infant microbiome is established, commensal bacteria quickly populate the gut, prompting a range of immune responses and "programming" the immune system with long-lasting effects. [54] The bacteria are able to stimulate lymphoid tissue associated with the gut mucosa, which enables the tissue to produce antibodies for pathogens that may enter the gut. [54]
The human microbiome may play a role in the activation of toll-like receptors in the intestines, a type of pattern recognition receptor host cells use to recognize dangers and repair damage. Pathogens can influence this coexistence leading to immune dysregulation including and susceptibility to diseases, mechanisms of inflammation, immune tolerance, and autoimmune diseases. [56] [57]
Organisms evolve within ecosystems so that the change of one organism affects the change of others. The hologenome theory of evolution proposes that an object of natural selection is not the individual organism, but the organism together with its associated organisms, including its microbial communities.
Coral reefs. The hologenome theory originated in studies on coral reefs. [58] Coral reefs are the largest structures created by living organisms, and contain abundant and highly complex microbial communities. Over the past several decades, major declines in coral populations have occurred. Climate change, water pollution and over-fishing are three stress factors that have been described as leading to disease susceptibility. Over twenty different coral diseases have been described, but of these, only a handful have had their causative agents isolated and characterized. Coral bleaching is the most serious of these diseases. In the Mediterranean Sea, the bleaching of Oculina patagonica was first described in 1994 and shortly determined to be due to infection by Vibrio shiloi. From 1994 to 2002, bacterial bleaching of O. patagonica occurred every summer in the eastern Mediterranean. Surprisingly, however, after 2003, O. patagonica in the eastern Mediterranean has been resistant to V. shiloi infection, although other diseases still cause bleaching. The surprise stems from the knowledge that corals are long lived, with lifespans on the order of decades, [59] and do not have adaptive immune systems.[ citation needed ] Their innate immune systems do not produce antibodies, and they should seemingly not be able to respond to new challenges except over evolutionary time scales.[ citation needed ]
The puzzle of how corals managed to acquire resistance to a specific pathogen led to a 2007 proposal, that a dynamic relationship exists between corals and their symbiotic microbial communities. It is thought that by altering its composition, the holobiont can adapt to changing environmental conditions far more rapidly than by genetic mutation and selection alone. Extrapolating this hypothesis to other organisms, including higher plants and animals, led to the proposal of the hologenome theory of evolution. [58]
As of 2007 [update] the hologenome theory was still being debated. [60] A major criticism has been the claim that V. shiloi was misidentified as the causative agent of coral bleaching, and that its presence in bleached O. patagonica was simply that of opportunistic colonization. [61] If this is true, the basic observation leading to the theory would be invalid. The theory has gained significant popularity as a way of explaining rapid changes in adaptation that cannot otherwise be explained by traditional mechanisms of natural selection. Within the hologenome theory, the holobiont has not only become the principal unit of natural selection but also the result of other step of integration that it is also observed at the cell (symbiogenesis, endosymbiosis) and genomic levels. [7]
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Targeted amplicon sequencing relies on having some expectations about the composition of the community that is being studied. In target amplicon sequencing a phylogenetically informative marker is targeted for sequencing. Such a marker should be present in ideally all the expected organisms. It should also evolve in such a way that it is conserved enough that primers can target genes from a wide range of organisms while evolving quickly enough to allow for finer resolution at the taxonomic level. A common marker for human microbiome studies is the gene for bacterial 16S rRNA (i.e. "16S rDNA", the sequence of DNA which encodes the ribosomal RNA molecule). [62] Since ribosomes are present in all living organisms, using 16S rDNA allows for DNA to be amplified from many more organisms than if another marker were used. The 16S rRNA gene contains both slowly evolving regions and 9 fast evolving regions, also known as hypervariable regions (HVRs); [63] the former can be used to design broad primers while the latter allow for finer taxonomic distinction. However, species-level resolution is not typically possible using the 16S rDNA. Primer selection is an important step, as anything that cannot be targeted by the primer will not be amplified and thus will not be detected, moreover different sets of primers can be selected to amplify different HVRs in the gene, or pairs of them. The appropriate choice of which HVRs to amplify has to be made according to the taxonomic groups of interest, as different target regions has been shown to influence taxonomical classification. [64]
Targeted studies of eukaryotic and viral communities are limited [65] and subject to the challenge of excluding host DNA from amplification and the reduced eukaryotic and viral biomass in the human microbiome. [66]
After the amplicons are sequenced, molecular phylogenetic methods are used to infer the composition of the microbial community. This can be done through clustering methodologies, by clustering the amplicons into operational taxonomic units (OTUs); or alternatively with denoising methodologies, identifying amplicon sequence variants (ASVs).
Phylogenetic relationships are then inferred between the sequences. Due to the complexity of the data, distance measures such as UniFrac distances are usually defined between microbiome samples, and downstream multivariate methods are carried out on the distance matrices. An important point is that the scale of data is extensive, and further approaches must be taken to identify patterns from the available information. Tools used to analyze the data include VAMPS, [67] QIIME, [68] mothur [69] and DADA2 [70] or UNOISE3 [71] for denoising.
Metagenomics is also used extensively for studying microbial communities. [72] [73] [74] In metagenomic sequencing, DNA is recovered directly from environmental samples in an untargeted manner with the goal of obtaining an unbiased sample from all genes of all members of the community. Recent studies use shotgun Sanger sequencing or pyrosequencing to recover the sequences of the reads. [75] The reads can then be assembled into contigs. To determine the phylogenetic identity of a sequence, it is compared to available full genome sequences using methods such as BLAST. One drawback of this approach is that many members of microbial communities do not have a representative sequenced genome, but this applies to 16S rRNA amplicon sequencing as well and is a fundamental problem. [62] With shotgun sequencing, it can be resolved by having a high coverage (50-100x) of the unknown genome, effectively doing a de novo genome assembly. As soon as there is a complete genome of an unknown organism available it can be compared phylogenetically and the organism put into its place in the tree of life, by creating new taxa. An emerging approach is to combine shotgun sequencing with proximity-ligation data (Hi-C) to assemble complete microbial genomes without culturing. [76]
Despite the fact that metagenomics is limited by the availability of reference sequences, one significant advantage of metagenomics over targeted amplicon sequencing is that metagenomics data can elucidate the functional potential of the community DNA. [77] [78] Targeted gene surveys cannot do this as they only reveal the phylogenetic relationship between the same gene from different organisms. Functional analysis is done by comparing the recovered sequences to databases of metagenomic annotations such as KEGG. The metabolic pathways that these genes are involved in can then be predicted with tools such as MG-RAST, [79] CAMERA [80] and IMG/M. [81]
Metatranscriptomics studies have been performed to study the gene expression of microbial communities through methods such as the pyrosequencing of extracted RNA. [82] Structure based studies have also identified non-coding RNAs (ncRNAs) such as ribozymes from microbiota. [83] Metaproteomics is an approach that studies the proteins expressed by microbiota, giving insight into its functional potential. [84]
The Human Microbiome Project launched in 2008 was a United States National Institutes of Health initiative to identify and characterize microorganisms found in both healthy and diseased humans. [85] The five-year project, best characterized as a feasibility study with a budget of $115 million, tested how changes in the human microbiome are associated with human health or disease. [85]
The Earth Microbiome Project (EMP) is an initiative to collect natural samples and analyze the microbial community around the globe. Microbes are highly abundant, diverse and have an important role in the ecological system. Yet as of 2010 [update] , it was estimated that the total global environmental DNA sequencing effort had produced less than 1 percent of the total DNA found in a liter of seawater or a gram of soil, [86] and the specific interactions between microbes are largely unknown. The EMP aims to process as many as 200,000 samples in different biomes, generating a complete database of microbes on earth to characterize environments and ecosystems by microbial composition and interaction. Using these data, new ecological and evolutionary theories can be proposed and tested. [87]
The gut microbiota are very important for the host health because they play role in degradation of non-digestible polysaccharides (fermentation of resistant starch, oligosaccharides, inulin) strengthening gut integrity or shaping the intestinal epithelium, harvesting energy, protecting against pathogens, and regulating host immunity. [88] [89]
Several studies showed that the gut bacterial composition in diabetic patients became altered with increased levels of Lactobacillus gasseri, Streptococcus mutans and Clostridiales members, with decrease in butyrate-producing bacteria such as Roseburia intestinalis and Faecalibacterium prausnitzii. [90] [91] This alteration is due to many factors such as antibiotic abuse, diet, and age.
The decrease in butyrate production is associated with defects in intestinal permeability, which could lead to endotoxemia, which is the increased level of circulating Lipopolysaccharides from gram negative bacterial cells wall. It is found that endotoxemia has association with development of insulin resistance. [90]
In addition that butyrate production affects serotonin level. [90] Elevated serotonin level has contribution in obesity, which is known to be a risk factor for development of diabetes.
The colonization of the human gut microbiota may start already before birth. [92] There are multiple factors in the environment that affects the development of the microbiota with birthmode being one of the most impactful. [93]
Another factor that has been observed to cause huge changes in the gut microbiota, particularly in children, is the use of antibiotics, associating with health issues such as higher BMI, [94] [95] and further an increased risk towards metabolic diseases such as obesity. [96] In infants it was observed that amoxicillin and macrolides cause significant shifts in the gut microbiota characterized by a change in the bacterial classes Bifidobacteria, Enterobacteria and Clostridia. [97] A single course of antibiotics in adults causes changes in both the bacterial and fungal microbiota, with even more persistent changes in the fungal communities. [98] The bacteria and fungi live together in the gut and there is most likely a competition for nutrient sources present. [99] [100] Seelbinder et al. found that commensal bacteria in the gut regulate the growth and pathogenicity of Candida albicans by their metabolites, particularly by propionate, acetic acid and 5-dodecenoate. [98] Candida has previously been associated with IBD [101] and further it has been observed to be increased in non-responders to a biological drug, infliximab, given to IBD patients with severe IBD. [102] Propionate and acetic acid are both short-chain fatty acids (SCFAs) that have been observed to be beneficial to gut microbiota health. [103] [104] [105] When antibiotics affect the growth of bacteria in the gut, there might be an overgrowth of certain fungi, which might be pathogenic when not regulated. [98]
Microbial DNA inhabiting a person's human body can uniquely identify the person. A person's privacy may be compromised if the person anonymously donated microbe DNA data. Their medical condition and identity could be revealed. [106] [107] [108]
The human microbiome is the aggregate of all microbiota that reside on or within human tissues and biofluids along with the corresponding anatomical sites in which they reside, including the gastrointestinal tract, skin, mammary glands, seminal fluid, uterus, ovarian follicles, lung, saliva, oral mucosa, conjunctiva, and the biliary tract. Types of human microbiota include bacteria, archaea, fungi, protists, and viruses. Though micro-animals can also live on the human body, they are typically excluded from this definition. In the context of genomics, the term human microbiome is sometimes used to refer to the collective genomes of resident microorganisms; however, the term human metagenome has the same meaning.
Metagenomics is the study of genetic material recovered directly from environmental or clinical samples by a method called sequencing. The broad field may also be referred to as environmental genomics, ecogenomics, community genomics or microbiomics.
Gut microbiota, gut microbiome, or gut flora are the microorganisms, including bacteria, archaea, fungi, and viruses, that live in the digestive tracts of animals. The gastrointestinal metagenome is the aggregate of all the genomes of the gut microbiota. The gut is the main location of the human microbiome. The gut microbiota has broad impacts, including effects on colonization, resistance to pathogens, maintaining the intestinal epithelium, metabolizing dietary and pharmaceutical compounds, controlling immune function, and even behavior through the gut–brain axis.
Dysbiosis is characterized by a disruption to the microbiome resulting in an imbalance in the microbiota, changes in their functional composition and metabolic activities, or a shift in their local distribution. For example, a part of the human microbiota such as the skin flora, gut flora, or vaginal flora, can become deranged (unbalanced), when normally dominating species become underrepresented and species that normally are outcompeted or contained increase to fill the void. Similar to the human gut microbiome, diverse microbes colonize the plant rhizosphere, and dysbiosis in the rhizosphere, can negatively impact plant health. Dysbiosis is most commonly reported as a condition in the gastrointestinal tract or plant rhizosphere.
Skin flora, also called skin microbiota, refers to microbiota that reside on the skin, typically human skin.
Jeffrey Ivan Gordon is a biologist and the Dr. Robert J. Glaser Distinguished University Professor and Director of the Center for Genome Sciences and Systems Biology at Washington University in St. Louis. He is internationally known for his research on gastrointestinal development and how gut microbial communities affect normal intestinal function, shape various aspects of human physiology including our nutritional status, and affect predisposition to diseases. He is a member of the National Academy of Sciences, the American Academy of Arts and Sciences, the Institute of Medicine of the National Academies, and the American Philosophical Society.
The Human Microbiome Project (HMP) was a United States National Institutes of Health (NIH) research initiative to improve understanding of the microbiota involved in human health and disease. Launched in 2007, the first phase (HMP1) focused on identifying and characterizing human microbiota. The second phase, known as the Integrative Human Microbiome Project (iHMP) launched in 2014 with the aim of generating resources to characterize the microbiome and elucidating the roles of microbes in health and disease states. The program received $170 million in funding by the NIH Common Fund from 2007 to 2016.
Long-term close-knit interactions between symbiotic microbes and their host can alter host immune system responses to other microorganisms, including pathogens, and are required to maintain proper homeostasis. The immune system is a host defense system consisting of anatomical physical barriers as well as physiological and cellular responses, which protect the host against harmful microorganisms while limiting host responses to harmless symbionts. Humans are home to 1013 to 1014 bacteria, roughly equivalent to the number of human cells, and while these bacteria can be pathogenic to their host most of them are mutually beneficial to both the host and bacteria.
Prevotella is a genus of Gram-negative bacteria.
Mark J. Pallen is a research leader at the Quadram Institute and Professor of Microbial Genomics at the University of East Anglia. In recent years, he has been at the forefront of efforts to apply next-generation sequencing to problems in microbiology and ancient DNA research.
A microbiome is the community of microorganisms that can usually be found living together in any given habitat. It was defined more precisely in 1988 by Whipps et al. as "a characteristic microbial community occupying a reasonably well-defined habitat which has distinct physio-chemical properties. The term thus not only refers to the microorganisms involved but also encompasses their theatre of activity". In 2020, an international panel of experts published the outcome of their discussions on the definition of the microbiome. They proposed a definition of the microbiome based on a revival of the "compact, clear, and comprehensive description of the term" as originally provided by Whipps et al., but supplemented with two explanatory paragraphs, the first pronouncing the dynamic character of the microbiome, and the second clearly separating the term microbiota from the term microbiome.
Metatranscriptomics is the set of techniques used to study gene expression of microbes within natural environments, i.e., the metatranscriptome.
The initial acquisition of microbiota is the formation of an organism's microbiota immediately before and after birth. The microbiota are all the microorganisms including bacteria, archaea and fungi that colonize the organism. The microbiome is another term for microbiota or can refer to the collected genomes.
The microbiota are the sum of all symbiotic microorganisms living on or in an organism. The fruit fly Drosophila melanogaster is a model organism and known as one of the most investigated organisms worldwide. The microbiota in flies is less complex than that found in humans. It still has an influence on the fitness of the fly, and it affects different life-history characteristics such as lifespan, resistance against pathogens (immunity) and metabolic processes (digestion). Considering the comprehensive toolkit available for research in Drosophila, analysis of its microbiome could enhance our understanding of similar processes in other types of host-microbiota interactions, including those involving humans. Microbiota plays key roles in the intestinal immune and metabolic responses via their fermentation product, acetate.
Hologenomics is the omics study of hologenomes. A hologenome is the whole set of genomes of a holobiont, an organism together with all co-habitating microbes, other life forms, and viruses. While the term hologenome originated from the hologenome theory of evolution, which postulates that natural selection occurs on the holobiont level, hologenomics uses an integrative framework to investigate interactions between the host and its associated species. Examples include gut microbe or viral genomes linked to human or animal genomes for host-microbe interaction research. Hologenomics approaches have also been used to explain genetic diversity in the microbial communities of marine sponges.
Pharmacomicrobiomics, proposed by Prof. Marco Candela for the ERC-2009-StG project call, and publicly coined for the first time in 2010 by Rizkallah et al., is defined as the effect of microbiome variations on drug disposition, action, and toxicity. Pharmacomicrobiomics is concerned with the interaction between xenobiotics, or foreign compounds, and the gut microbiome. It is estimated that over 100 trillion prokaryotes representing more than 1000 species reside in the gut. Within the gut, microbes help modulate developmental, immunological and nutrition host functions. The aggregate genome of microbes extends the metabolic capabilities of humans, allowing them to capture nutrients from diverse sources. Namely, through the secretion of enzymes that assist in the metabolism of chemicals foreign to the body, modification of liver and intestinal enzymes, and modulation of the expression of human metabolic genes, microbes can significantly impact the ingestion of xenobiotics.
The human milk microbiota, also known as human milk probiotics (HMP), encompasses the microbiota–the community of microorganisms–present within the human mammary glands and breast milk. Contrary to the traditional belief that human breast milk is sterile, advancements in both microbial culture and culture-independent methods have confirmed that human milk harbors diverse communities of bacteria. These communities are distinct in composition from other microbial populations found within the human body which constitute the human microbiome.
Bacteroides thetaiotaomicron is a gram-negative, non-motile, rod shaped obligate anaerobic bacterium that is a prominent member of the normal gut microbiome in the distal intestines. Its proteome, consisting of 4,779 proteins, includes a system for obtaining and breaking down dietary polysaccharides that would otherwise be difficult to digest. B. thetaiotaomicron is also an opportunistic pathogen, meaning it may become virulent in immunocompromised individuals. It is often used in research as a model organism for functional studies of the human microbiota in the gut.
All animals on Earth form associations with microorganisms, including protists, bacteria, archaea, fungi, and viruses. In the ocean, animal–microbial relationships were historically explored in single host–symbiont systems. However, new explorations into the diversity of marine microorganisms associating with diverse marine animal hosts is moving the field into studies that address interactions between the animal host and a more multi-member microbiome. The potential for microbiomes to influence the health, physiology, behavior, and ecology of marine animals could alter current understandings of how marine animals adapt to change, and especially the growing climate-related and anthropogenic-induced changes already impacting the ocean environment.
Culturomics is the high-throughput cell culture of bacteria that aims to comprehensively identify strains or species in samples obtained from tissues such as the human gut or from the environment. This approach was conceived as an alternative, complementary method to metagenomics, which relies on the presence of homologous sequences to identify new bacteria. Due to the limited phylogenetic information available on bacteria, metagenomic data generally contains large amounts of "microbial dark matter", sequences of unknown origin. Culturomics provides some of the missing gaps with the added advantage of enabling the functional study of the generated cultures. Its main drawback is that many bacterial species remain effectively uncultivable until their growth conditions are better understood. Therefore, optimization of the culturomics approach has been done by improving culture conditions.
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