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The mycobiome, mycobiota, or fungal microbiome, is the fungal community in and on an organism. [1] [2] [3]
The word “mycobiome” comes from the ancient Greek μύκης (mukēs), meaning "fungus" with the suffix “biome” derived from the Greek βίος (bíos), meaning “life.” The term was first coined in the 2009 paper by Gillevet et al. [4] [5]
Most species of fungi are decomposers with the ability to break down complex polymers. Fungi are commonly found within plant cells in an endophytic relationship or as a pathogen. Most plants also form mutualistic relationships with fungi that accelerate nutrient uptake among their root structures. The most common phyla present in the fungal communities that live alongside animals and in aquatic environments are Ascomycota and Basidiomycota. Animals will typically form a commensal relationship with fungi with the occasional occurrence of a pathogenic interaction.
Fungal microbes are amongst a wide variety of other microbes involved in a symbiotic relationship involving multicellular organisms. In mammals, the gut flora is usually met with vastly diverse populations of microbes from many kingdoms, where fungal populations make up less than 1% of the entire gut biome. [6] Due to the coexistence of fungal populations with other microbes in most cases of host-symbiont associations, it's important to assess common dynamics that may occur.
Most interactions between microbes in the gut are either competitive or cooperative. [7] This can be seen with multiple fungal microbes as well by observing populations through the treatment of antibiotics and antifungals. Research on microbial populations in animal models has resulted in noticeable fluctuations in microbe populations. [8] Antibiotic treatment has mostly shown increases in parasitic fungal presence, suggesting competitive behaviors between microbes against fungi. [9] Additionally, application of antifungal molecules have resulted in colitis in mice, suggesting that commensal fungi are responsible for balancing bacterial populations that are involved in dysbiosis. [10] Despite limited knowledge on the gut mycobiome, this research suggests that interactions between fungal and bacterial microbes in the mammalian gut are largely competitive.
Lichens are the symbiotic relationship between a wide range of fungi and photosynthesizing microbes that are either algal chlorophytes or cyanobacteria. The primary symbiosis involves fungi gaining energy from their photosynthesizing symbiotic microbe, and in return, provides a structural scaffold from the hyphae. [11] These interactions are classically seen as mutualistic, but have been questioned if fungi might also be capable of playing a controlled parasitic role. [12] [13]
Plants also have mutualistic symbiotic relationships with fungal communities that are found in a microbe abundant layer of the soil called the rhizosphere. [14] Fungi can be vertically transmitted to progeny plants, or horizontally through fungal diffusion in the soil. [15] Regardless of transmission, the most common cases of fungal plant symbiosis happens when fungal communities colonize plant root structure. There are some cases of symbiosis that Begin before maturity such as the Orchidaceae family, in which symbiosis begins at the seed germination phase. [15] Arbuscular mycorrhizal fungi supply the plant essential inorganic nutrients (in the form of minerals) for 80% of terrestrial plant species. In return the plant will provide fungi with plant assimilated carbon that can easily be metabolized and used for energy. [16]
DNA sequencing and fossil records have derived evidence that this mutualistic relationship between fungi and plants is extremely ancient. It is even suggested that different subspecies of fungi have played major roles in shaping the plants immune system in the early evolutionary moments of plants colonizing land. [17] Specific gene clusters in the plant's genome used to code for fungicidal properties like the production of isoflavonoids in the Fabaceae plant family are toxic and used to regulate the growth of expected fungal presence. [18] In a coevolutionary response, arbuscular fungal communities breed quickly and evolutionarily selected for the development of specific gene clusters to detoxify and metabolize a wide variety of organic antifungal chemicals. [19] This exchange of evolutionary pressures has given a variety of plant species better immune systems, while simultaneously giving mutualist fungal communities new sources of plant energy at the genomic level. [18]
Interactions between fungi and insects are incredibly common and most of these relationships are either commensal or pathogenic. [20]
The commensal relationships typically benefit the insect by allowing them to digest tough materials such as wood. For example, wood-ingesting passalid beetle guts are populated with bacteria, yeasts, and other eukaryotic organisms that aid in digestion and nutrient absorption. Without the help of the yeasts to break down xylose from plant cells, these beetles would be unable to efficiently digest this tough material. [21]
Insect pathogenic fungi slowly kill their hosts while replicating rapidly inside the infected insect. They typically attach to the external surface of the insect as spores before germinating, perforating the cuticle, and colonizing the internal cavity. The insect is typically killed and new spores are dispersed from the reproductive structures in the body of the insect. [22]
A prominent example of an entomopathogenic fungus is Ophiocordyceps unilateralis, nicknamed the “zombie-ant” fungus. The fungus infects ants and alters their natural behavioral patterns causing the ant to leave their usual environment in the trees in favor of the forest floor – a more suitable environment for fungal growth. The insects will then attach themselves to the underside of a leaf until it dies. Fungal spores then sprout out of the dead ant's head and are dispersed. [23] [24]
One of the first descriptions of insect pathogenic fungi was published in 1835 during an infestation of Beauveria bassiana in a silkworm population that upset the silk industry. [25]
There is a low abundance of fungi associated with most human body sites, such as the gastrointestinal tract, where fungi typically compose just 0.001 - 0.1% of the microbial community. [26] [27] However, fungi compose a significant fraction of the microbiome at some locations, such as the ear canal. [28]
The mycobiome is relevant to human physiology as fungi may be important in maintaining microbial community structure, metabolic function, and immune-priming. [29] [30] Mutualism between humans and fungi is not yet well understood, and there is much to be learned about how fungi interact with the nonfungal constituents of the microbiome.
Many human diseases, including hepatitis, cystic fibrosis, obesity, and inflammatory bowel disease have been associated with changes in the human mycobiome, [31] [32] and it has been proposed that any fungal colonization of the GI tract is a sign of disease. [26] Moreover, detecting tumor-associated mycobiomes may be a novel way of cancer screening – it could be used in synergy with other biomarkers such as of bacteriomes. [33] [34] [35]
Fungal interactions in the human host can be opportunistic, mutualistic, parasitic, commensalistic, and amensalistic. [36] Though there is a small amount of fungi considered to be true pathogens due to causing disease in healthy individuals, the majority of fungi are considered opportunistic, only affecting an immunosuppressed host. [36] In these cases, pathogen-mediated damage is dependent upon the host; disease could only occur if the host has either a hyperactive or weakened immune system. Commensal relationships between human host and pathogen are possible when the fungi are present in a host for long periods of time without causing disease. One example of a fungus that forms a commensal relationship in a human host is Cryptococcus. [36] Candida albicans can also form a long-time commensal relationship with its host.
Some fungi are niche specific symbionts in the human host. Some fungi can be introduced to the host from the environment by skin contact, oral or respiratory routes, while others are acquired vertically through birth. [36]
In the human GI tract, there are about 50 observed genera of fungi. Some are acquired through what the host eats; as such showing that the stability of the fungal population is dependent on the diet of the host and host immune status. [36]
About 20 genera of fungi have been observed in the vaginal niche. Most fungi that colonize the vagina show a commensalistic relationship with the host, but factors like antibiotic uses and pregnancy could influence the vaginal mycobiome. [37]
An endosymbiont or endobiont is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship. This phenomenon is known as endosymbiosis. Examples are nitrogen-fixing bacteria, which live in the root nodules of legumes, single-cell algae inside reef-building corals and bacterial endosymbionts that provide essential nutrients to insects.
Symbiosis is any type of a close and long-term biological interaction between two biological organisms of different species, termed symbionts, be it mutualistic, commensalistic, or parasitic. In 1879, Heinrich Anton de Bary defined it as "the living together of unlike organisms". The term is sometimes used in the more restricted sense of a mutually beneficial interaction in which both symbionts contribute to each other's support.
A mycorrhiza is a symbiotic association between a fungus and a plant. The term mycorrhiza refers to the role of the fungus in the plant's rhizosphere, its root system. Mycorrhizae play important roles in plant nutrition, soil biology, and soil chemistry.
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 skin, mammary glands, seminal fluid, uterus, ovarian follicles, lung, saliva, oral mucosa, conjunctiva, biliary tract, and gastrointestinal 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.
In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist guest (symbiont). The guest is typically provided with nourishment and shelter. Examples include animals playing host to parasitic worms, cells harbouring pathogenic (disease-causing) viruses, or a bean plant hosting mutualistic (helpful) nitrogen-fixing bacteria. More specifically in botany, a host plant supplies food resources to micropredators, which have an evolutionarily stable relationship with their hosts similar to ectoparasitism. The host range is the collection of hosts that an organism can use as a partner.
An endophyte is an endosymbiont, often a bacterium or fungus, that lives within a plant for at least part of its life cycle without causing apparent disease. Endophytes are ubiquitous and have been found in all species of plants studied to date; however, most of the endophyte/plant relationships are not well understood. Some endophytes may enhance host growth and nutrient acquisition and improve the plant's ability to tolerate abiotic stresses, such as drought, and decrease biotic stresses by enhancing plant resistance to insects, pathogens and herbivores. Although endophytic bacteria and fungi are frequently studied, endophytic archaea are increasingly being considered for their role in plant growth promotion as part of the core microbiome of a plant.
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.
Symbiotic bacteria are bacteria living in symbiosis with another organism or each other. For example, rhizobia living in root nodules of legumes provide nitrogen fixing activity for these plants.
Skin flora, also called skin microbiota, refers to microbiota that reside on the skin, typically human skin.
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.
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, and have been found to be crucial for immunologic, hormonal, and metabolic homeostasis of their host.
The hologenome theory of evolution recasts the individual animal or plant as a community or a "holobiont" – the host plus all of its symbiotic microbes. Consequently, the collective genomes of the holobiont form a "hologenome". Holobionts and hologenomes are structural entities that replace misnomers in the context of host-microbiota symbioses such as superorganism, organ, and metagenome. Variation in the hologenome may encode phenotypic plasticity of the holobiont and can be subject to evolutionary changes caused by selection and drift, if portions of the hologenome are transmitted between generations with reasonable fidelity. One of the important outcomes of recasting the individual as a holobiont subject to evolutionary forces is that genetic variation in the hologenome can be brought about by changes in the host genome and also by changes in the microbiome, including new acquisitions of microbes, horizontal gene transfers, and changes in microbial abundance within hosts. Although there is a rich literature on binary host–microbe symbioses, the hologenome concept distinguishes itself by including the vast symbiotic complexity inherent in many multicellular hosts. For recent literature on holobionts and hologenomes published in an open access platform, see the following reference.
The root microbiome is the dynamic community of microorganisms associated with plant roots. Because they are rich in a variety of carbon compounds, plant roots provide unique environments for a diverse assemblage of soil microorganisms, including bacteria, fungi, and archaea. The microbial communities inside the root and in the rhizosphere are distinct from each other, and from the microbial communities of bulk soil, although there is some overlap in species composition.
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 explanatory paragraph pronounces the dynamic character of the microbiome, and the second explanatory paragraph clearly separates the term microbiota from the term microbiome.
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.
A holobiont is an assemblage of a host and the many other species living in or around it, which together form a discrete ecological unit through symbiosis, though there is controversy over this discreteness. The components of a holobiont are individual species or bionts, while the combined genome of all bionts is the hologenome. The holobiont concept was initially introduced by the German theoretical biologist Adolf Meyer-Abich in 1943, and then apparently independently by Dr. Lynn Margulis in her 1991 book Symbiosis as a Source of Evolutionary Innovation. The concept has evolved since the original formulations. Holobionts include the host, virome, microbiome, and any other organisms which contribute in some way to the functioning of the whole. Well-studied holobionts include reef-building corals and humans.
A phytobiome consists of a plant (phyto) situated in its specific ecological area (biome), including its environment and the associated communities of organisms which inhabit it. These organisms include all macro- and micro-organisms living in, on, or around the plant including bacteria, archaea, fungi, protists, insects, animals, and other plants. The environment includes the soil, air, and climate. Examples of ecological areas are fields, rangelands, forests. Knowledge of the interactions within a phytobiome can be used to create tools for agriculture, crop management, increased health, preservation, productivity, and sustainability of cropping and forest systems.
Bacteroides thetaiotaomicron is a gram-negative, 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 members, 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.
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.
The plant microbiome, also known as the phytomicrobiome, plays roles in plant health and productivity and has received significant attention in recent years. The microbiome has been defined 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".
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