A microbiota is an "ecological community of commensal, symbiotic and pathogenic microorganisms"[1][2] found in and on all multicellular organisms studied to date from plants to animals. A microbiota includes bacteria, archaea, protists, fungi and viruses. Microbiota have been found to be crucial for immunologic, hormonal and metabolic homeostasis of their host. The synonymous term microbiome describes either the collective genomes of the microorganisms that reside in an environmental niche or the microorganisms themselves.[3][4][5]

The microbiome and host emerged during evolution as a synergistic unit from epigenetics and genetic characteristics, sometimes collectively referred to as a holobiont.[6][7]

Skin Microbiome20169-300
The predominant bacteria on human skin


All plants and animals, from simple life forms to humans, live in close association with microbial organisms.[8] Several advances have driven the perception of microbiomes, including:

  • the ability to perform genomic and gene expression analyses of single cells and of entire microbial communities in the disciplines of metagenomics and metatranscriptomics[9]
  • databases accessible to researchers across multiple disciplines[9]
  • methods of mathematical analysis suitable for complex data sets[9]

Biologists have come to appreciate that microbes make up an important part of an organism's phenotype, far beyond the occasional symbiotic case study.[9]

Types of host relationships

Commensalism, a concept developed by Pierre-Joseph van Beneden (1809-1894), a Belgian professor at the University of Louvain during the nineteenth century [10] 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,[11]:700[12] 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.[13] A nutrient exchange may be bidirectional or unidirectional, may be context dependent and may occur in diverse ways.[13] Microbiota that are expected to be present, and that under normal circumstances do not cause disease, are deemed normal flora or normal microbiota.[11]

Acquisition and change

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.[14] 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.[15]

Microbiota by host

Consensus exists among evolutionary biologists that one should not separate an organism's genes from the context of its resident microbes.


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 genomes.[11]

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.[16][17][18][19]

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.[11] It reached a milestone in 2012 when it published initial results.[20]

Non-human animals

  • Amphibians have microbiota on their skin.[21] Some species are able to carry a fungus named Batrachochytrium dendrobatidis, which in others can cause a deadly infection Chytridiomycosis depending on their microbiome, resisting pathogen colonization or inhibiting their growth with antimicrobial skin peptides.[22]
  • In mammals, herbivores such as cattle depend on their rumen microbiome to convert cellulose into proteins, short chain fatty acids, and gases. Culture methods cannot provide information on all microorganisms present. Comparative metagenomic studies yielded the surprising result that individual cattle possess markedly different community structures, predicted phenotype, and metabolic potentials,[23] even though they were fed identical diets, were housed together, and were apparently functionally identical in their utilization of plant cell wall resources.
  • Mice have become the most studied mammalian regarding their microbiomes. The gut microbiota have been studied in relation to allergic airway disease, obesity, gastrointestinal diseases and diabetes. Perinatal shifting of microbiota through low dose antibiotics can have long-lasting effects on future susceptibility to allergic airway disease. The frequency of certain subsets of microbes has been linked to disease severity. The presence of specific microbes early in postnatal life, instruct future immune responses.[24][25] In gnotobiotic mice certain gut bacteria were found to transmit a particular phenotype to recipient germ-free mice, that promoted accumulation of colonic regulatory T cells, and strains that modulated mouse adiposity and cecal metabolite concentrations.[26] This combinatorial approach enables a systems-level understanding of microbial contributions to human biology.[27] But also other mucoide tissues as lung and vagina have been studied in relation to diseases such as asthma, allergy and vaginosis.[28]
  • Insects have their own microbiomes. For example, leaf-cutter ants form huge underground colonies harvesting hundreds of kilograms of leaves each year and are unable to digest the cellulose in the leaves directly. They maintain fungus gardens as the colony's primary food source. While the fungus itself does not digest cellulose, a microbial community containing a diversity of bacteria is doing so. Analysis of the microbial population's genome revealed many genes with a role in cellulose digestion. This microbiome's predicted carbohydrate-degrading enzyme profile is similar to that of the bovine rumen, but the species composition is almost entirely different.[29] Gut microbiota of the fruit fly can affect the way its gut looks, by impacting epithelial renewal rate, cellular spacing, and the composition of different cell types in the epithelium.[30] When the moth Spodoptera exigua is infected with baculovirus immune-related genes are downregulated and the amount of its gut microbiota increases.[31]


Cycas coralloid root XS high
Light micrograph of a cross section of a coralloid root of a cycad, showing the layer that hosts symbiotic cyanobacteria
  • 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).[32][33] 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.[13][34] 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.[35][36] 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[14]


The symbiotic relationship between a host and its microbiota is under laboratory research for how it may shape the immune system of mammals.[37][38] 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.[39] Bacteria can be transferred from mother to child through direct contact and after birth.[40] 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.[39] 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.[39]

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.[41][42]

Co-evolution of microbiota

Bleached branching coral (foreground) and normal branching coral (background). Keppel Islands, Great Barrier Reef

Organisms evolve within eco-systems so that the change of one organism affects the change of others. Co-evolution (also called "hologenome theory") 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. 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,[43] and do not have adaptive immune systems. 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.

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".[44]

As of 2007 the hologenome theory was still being debated.[45] 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.[46] 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.[6]

Research methods

Targeted amplicon sequencing

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).[47] 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 rDNA gene contains both slowly evolving regions and fast evolving regions; 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. Different sets of primers have been shown to amplify different taxonomic groups due to sequence variation.

Targeted studies of eukaryotic and viral communities are limited[48] and subject to the challenge of excluding host DNA from amplification and the reduced eukaryotic and viral biomass in the human microbiome.[49]

After the amplicons are sequenced, molecular phylogenetic methods are used to infer the composition of the microbial community. This is done by clustering the amplicons into operational taxonomic units (OTUs) and inferring phylogenetic relationships 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,[50] QIIME[51] and mothur.[52]

Metagenomic sequencing

Metagenomics is also used extensively for studying microbial communities.[53][54][55] 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.[56] 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.[47] 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.[57]

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.[58][59] 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,[60] CAMERA[61] and IMG/M.[62]

RNA and protein-based approaches

Metatranscriptomics studies have been performed to study the gene expression of microbial communities through methods such as the pyrosequencing of extracted RNA.[63] Structure based studies have also identified non-coding RNAs (ncRNAs) such as ribozymes from microbiota.[64] Metaproteomics is an approach that studies the proteins expressed by microbiota, giving insight into its functional potential.[65]


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.[66] 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.[66]

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, 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,[67] 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.[68]

The Brazilian Microbiome Project aims to assemble a Brazilian Microbiome Consortium/Database. This is the first attempt to collect and collate information about Brazilian microbial genetic and functional diversity in a systematic and holistic manner. New sequence data have been generated from samples collected in all Brazilian regions.[69]

Privacy issues

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.[70][71][72]

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Bifidobacterium is a genus of gram-positive, nonmotile, often branched anaerobic bacteria. They are ubiquitous inhabitants of the gastrointestinal tract, vagina and mouth (B. dentium) of mammals, including humans. Bifidobacteria are one of the major genera of bacteria that make up the gastrointestinal tract microbiota in mammals. Some bifidobacteria are used as probiotics.

Before the 1960s, Bifidobacterium species were collectively referred to as "Lactobacillus bifidus".


Cypress is a common name for various coniferous trees or shrubs of northern temperate regions that belong to the family Cupressaceae. The word cypress is derived from Old French cipres, which was imported from Latin cypressus, the latinisation of the Greek κυπάρισσος (kyparissos).Species that are commonly known as cypresses include:

The family Cupressaceae also contains 13–16 other genera (not listed above) that do not bear cypress in their common names.

Plants named cypress


Dysbiosis (also called dysbacteriosis) is a term for a microbial imbalance or maladaptation on or inside the body, such as an impaired microbiota. For example, a part of the human microbiota, such as the skin flora, gut flora, or vaginal flora, can become deranged, with normally dominating species underrepresented and normally outcompeted or contained species increasing to fill the void. Dysbiosis is most commonly reported as a condition in the gastrointestinal tract, particularly during small intestinal bacterial overgrowth (SIBO) or small intestinal fungal overgrowth (SIFO).Typical microbial colonies found on or in the body are normally benign or beneficial. These beneficial and appropriately sized microbial colonies carry out a series of helpful and necessary functions, such as aiding in digestion. They also help protect the body from the penetration of pathogenic microbes. These beneficial microbial colonies compete with each other for space and resources.

Fecal microbiota transplant

Fecal microbiota transplant (FMT), also known as a stool transplant, is the process of transplantation of fecal bacteria from a healthy individual into a recipient. FMT involves restoration of the colonic microflora by introducing healthy bacterial flora through infusion of stool, e.g. by colonoscopy, enema, orogastric tube or by mouth in the form of a capsule containing freeze-dried material, obtained from a healthy donor. The effectiveness of FMT has been established in clinical trials for the treatment of Clostridioides difficile infection (CDI), whose effects can range from diarrhea to pseudomembranous colitis.

Due to an epidemic of CDI in North America and Europe, FMT has gained increasing prominence, with some experts calling for it to become first-line therapy for CDI. In 2013 a randomized, controlled trial of FMT from healthy donors showed it to be highly effective in treating recurrent C. difficile in adults, and more effective than vancomycin alone. FMT has been used experimentally to treat other gastrointestinal diseases, including colitis, constipation, irritable bowel syndrome, and neurological conditions such as multiple sclerosis and Parkinson's. In the United States, human feces has been regulated as an experimental drug since 2013. In the United Kingdom, FMT regulation is under the remit of the Medicines and Healthcare products Regulatory Agency.

Gut flora

Gut flora, or gut microbiota, or gastrointestinal microbiota, is the complex community of microorganisms that live in the digestive tracts of humans and non-human animals, including insects. The gut metagenome is the aggregate of all the genomes of gut microbiota. The gut is one niche that human microbiota inhabit.

Human microbiota

The human microbiota is the aggregate of microorganisms that resides on or within any of a number of human tissues and biofluids, including the skin, mammary glands, placenta, seminal fluid, uterus, ovarian follicles, lung, saliva, oral mucosa, conjunctiva, biliary and gastrointestinal tracts. They include bacteria, archaea, fungi, protists and viruses. Though micro-animals can also live on the human body, they are typically excluded from this definition. The human microbiome refers specifically to the collective genomes of resident microorganisms.Humans are colonized by many microorganisms; the traditional estimate is that the average human body is inhabited by ten times as many non-human cells as human cells, but more recent estimates have lowered that ratio to 3:1 or even to approximately the same number. Some microorganisms that colonize humans are commensal, meaning they co-exist without harming humans; others have a mutualistic relationship with their human hosts. Conversely, some non-pathogenic microorganisms can harm human hosts via the metabolites they produce, like trimethylamine, which the human body converts to trimethylamine N-oxide via FMO3-mediated oxidation. Certain microorganisms perform tasks that are known to be useful to the human host but the role of most of them is not well understood. Those that are expected to be present, and that under normal circumstances do not cause disease, are sometimes deemed normal flora or normal microbiota.The Human Microbiome Project took on the project of sequencing the genome of the human microbiota, focusing particularly on the microbiota that normally inhabit the skin, mouth, nose, digestive tract, and vagina. It reached a milestone in 2012 when it published its initial results.


Lactobacillus is a genus of Gram-positive, facultative anaerobic or microaerophilic, rod-shaped, non-spore-forming bacteria. They are a major part of the lactic acid bacteria group (i.e., they convert sugars to lactic acid). In humans, they constitute a significant component of the microbiota at a number of body sites, such as the digestive system, urinary system, and genital system. In women of European ancestry, Lactobacillus species are normally a major part of the vaginal microbiota. Lactobacillus forms biofilms in the vaginal and gut microbiota, allowing them to persist during harsh environmental conditions and maintain ample populations. Lactobacillus exhibits a mutualistic relationship with the human body, as it protects the host against potential invasions by pathogens, and in turn, the host provides a source of nutrients. Lactobacillus is the most common probiotic found in food such as yogurt, and it is diverse in its application to maintain human well-being, as it can help treat diarrhea, vaginal infections, and skin disorders such as eczema.

List of bacterial vaginosis microbiota

Bacterial vaginosis is caused by an imbalance of the naturally occurring bacteria in the vagina. The normally predominant species of Lactobacilli are markedly reduced. This is the list of organisms that are found in the vagina that are associated with bacterial vaginosis, an infectious disease of the vagina caused by excessive growth of specific bacteria. The census and relationships among the microbiota are altered in BV resulting in a complex bacterial milieu. Some species have been identified relatively recently. Having infections with the listed pathogens increases the risk of acquiring other sexually transmitted infections including HIV/AIDS.

List of human microbiota

This article lists some of the species recognized as belonging to the human microbiota.

List of microbiota species of the lower reproductive tract of women

This is the list of healthy vaginal microbiota (VMB), which is defined as the group of species and genera that generally are found to have lack of symptoms, absence of various infections, and result in good pregnancy outcomes. VMB is dominated mainly by Lactobacillus species. This is the list of organisms that are found in the lower reproductive tract of sexually mature women who are not immunocompromised. A partial description of pathogens that can be found in the lower and upper reproductive tract of women can be found in the article sexually transmitted disease. The organisms listed below are capable of causing illness if for some reason there is a change in vaginal pH or a change in the ratio of one organism to another. For example, Candida is a normal inhabitant of a healthy reproductive tract but an overgrowth of this organism can cause candidiasis.

Lung microbiota

The lung microbiota, is the pulmonary microbial community consisting of a complex variety of microorganisms found in the lower respiratory tract particularly on the mucous layer and the epithelial surfaces. These microorganisms include bacteria, fungi, viruses and bacteriophages. The bacterial part of the microbiota has been more closely studied. It consists of a core of nine genera: Prevotella, Sphingomonas, Pseudomonas, Acinetobacter, Fusobacterium, Megasphaera, Veillonella, Staphylococcus, and Streptococcus. They are aerobes as well as anaerobes and aerotolerant bacteria. The microbial communities are highly variable in particular individuals and compose of about 140 distinct families. The bronchial tree for instance contains a mean of 2000 bacterial genomes per cm2 surface. The harmful or potentially harmful bacteria are also detected routinely in respiratory specimens. The most significant are Moraxella catarrhalis, Haemophilus influenzae, and Streptococcus pneumoniae. They are known to cause respiratory disorders under particular conditions namely if the human immune system is impaired. The mechanism by which they persist in the lower airways in healthy individuals is unknown.

Fungal genera that are commonly found in the lung microbiota include Candida, Malassezia, Neosartorya, Saccharomyces, and Aspergillus, among others.

Metabolic disorder

A metabolic disorder can happen when abnormal chemical reactions in the body alter the normal metabolic process. It can also be defined as inherited single gene anomaly, most of which are autosomal recessive.

Microbiota decussata

Microbiota is a monotypic genus of evergreen coniferous shrub in the cypress family Cupressaceae, containing only one species, Microbiota decussata (Siberian carpet cypress, Russian arbor-vitae). The plant is native and endemic to a limited area of the Sikhote-Alin mountains in Primorskiy Krai in the Russian Far East. The name causes much confusion because of other meanings for the word "microbiota," but the genus name was derived from micro-, meaning "small," + Biota, the genus name for a closely related conifer, a species formerly called Biota orientalis, now renamed Platycladus orientalis.

Opportunistic infection

An opportunistic infection is an infection caused by pathogens (bacteria, viruses, fungi, or protozoa) that take advantage of an opportunity not normally available, such as a host with a weakened immune system, an altered microbiota (such as a disrupted gut microbiota), or breached integumentary barriers. Many of these pathogens do not cause disease in a healthy host that has a normal immune system. However, a compromised immune system, which is seriously debilitated and has lowered resistance to infection, a penetrating injury, or a lack of competition from normal commensals presents an opportunity for the pathogen to infect.

Oral microbiology

See also: Oral Ecology

Oral microbiology is the study of the microorganisms (microbiota) of the oral cavity and their interactions between oral microorganisms or with the host. The environment present in the human mouth is suited to the growth of characteristic microorganisms found there. It provides a source of water and nutrients, as well as a moderate temperature. Resident microbes of the mouth adhere to the teeth and gums to resist mechanical flushing from the mouth to stomach where acid-sensitive microbes are destroyed by hydrochloric acid.Anaerobic bacteria in the oral cavity include: Actinomyces, Arachnia, Bacteroides, Bifidobacterium, Eubacterium, Fusobacterium, Lactobacillus, Leptotrichia, Peptococcus, Peptostreptococcus, Propionibacterium, Selenomonas, Treponema, and Veillonella. Genera of fungi that are frequently found in the mouth include Candida, Cladosporium, Aspergillus, Fusarium, Glomus, Alternaria, Penicillium, and Cryptococcus, among others. Bacteria accumulate on both the hard and soft oral tissues in biofilms. Bacterial adhesion is particularly important for oral bacteria.

Oral bacteria have evolved mechanisms to sense their environment and evade or modify the host. Bacteria occupy the ecological niche provided by both the tooth surface and gingival epithelium. However, a highly efficient innate host defense system constantly monitors the bacterial colonization and prevents bacterial invasion of local tissues. A dynamic equilibrium exists between dental plaque bacteria and the innate host defense system. Of particular interest is the role of oral microorganisms in the two major dental diseases: dental caries and periodontal disease. Additionally, research has correlated poor oral heath and the resulting ability of the oral microbiota to invade the body to affect cardiac health as well as cognitive function.

Placental microbiome

The placental microbiome is the nonpathogenic, commensal bacteria claimed to be present in a healthy human placenta and is distinct from bacteria that cause infection and preterm birth in chorioamnionitis. Until recently, the healthy placenta was considered to be a sterile organ but now genera and species have been identified that reside in the basal layer.It should be stressed that the evidence for a placental microbiome is controversial. Most studies supporting the existence of a placental microbiome lack the appropriate experimental controls, and it has been found that contamination is most likely responsible for reports of a placental microbiome.The placental microbiome more closely resembles that of the oral microbiome than either the vaginal or rectal microbiome. Changes in the microbiome can result is disease.

Skin flora

The term skin flora (also commonly referred to as skin microbiota) refers to the microorganisms which reside on the skin, typically human skin.

Many of them are bacteria of which there are around 1000 species upon human skin from nineteen phyla. Most are found in the superficial layers of the epidermis and the upper parts of hair follicles.

Skin flora is usually non-pathogenic, and either commensal (are not harmful to their host) or mutualistic (offer a benefit). The benefits bacteria can offer include preventing transient pathogenic organisms from colonizing the skin surface, either by competing for nutrients, secreting chemicals against them, or stimulating the skin's immune system. However, resident microbes can cause skin diseases and enter the blood system, creating life-threatening diseases, particularly in immunosuppressed people.A major non-human skin flora is Batrachochytrium dendrobatidis, a chytrid and non-hyphal zoosporic fungus that causes chytridiomycosis, an infectious disease thought to be responsible for the decline in amphibian populations.

Vaginal flora

Vaginal flora or vaginal microbiota are the microorganisms that colonize the vagina. They were discovered by the German gynecologist Albert Döderlein in 1892 and are part of the overall human flora.

The amount and type of bacteria present have significant implications for a woman's overall health. The primary colonizing bacteria of a healthy individual are of the genus Lactobacillus, such as L. crispatus, and the lactic acid they produce is thought to protect against infection by pathogenic species.

Vaginal flora in pregnancy

The vaginal flora in pregnancy, or vaginal microbiota in pregnancy, is different from the vaginal flora (the population of microorganisms that resides in the vagina) before sexual maturity, during reproductive years, and after menopause. A description of the vaginal flora of pregnant women who are immunocompromised is not covered in this article. The composition of the vaginal flora significantly differs in pregnancy. Bacteria or viruses that are infectious most often have no symptoms.

Human flora
Disorders and therapies

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