Microbiota


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 term microbiome describes either the collective genomes of the microbes that reside in an ecological niche or else the microbes themselves.
The microbiome and host emerged during evolution as a synergistic unit from epigenetics and genetic characteristics, sometimes collectively referred to as a holobiont. The presence of microbiota in human and other metazoan guts has been critical for understanding the co-evolution between metazoans and bacteria. Microbiota play key roles in the intestinal immune and metabolic responses via their fermentation product, acetate.

Introduction

All plants and animals, from simple life forms to humans, live in close association with microbial organisms. Several advances have driven the perception of microbiomes, including:
Biologists discovered that microbes make up an important part of an organism's phenotype, far beyond the occasional symbiotic case study.

Types of microbe-host relationships

, a concept developed by Pierre-Joseph van Beneden, a Belgian professor at the University of Louvain during the nineteenth century 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, 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. A nutrient exchange may be bidirectional or unidirectional, may be context dependent and may occur in diverse ways. Microbiota that are expected to be present, and that under normal circumstances do not cause disease, are deemed normal flora or normal microbiota; normal flora can not only be harmless, but can be protective of the host.

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. 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. Clinically, new microbiota can be acquired through fecal microbiota transplant to treat infections such as chronic C. difficile infection.

Microbiota by host

Humans

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.
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.
In fact, these are so small that there are around 100 trillion microbiota on the human body, around 39 trillion by revised estimates, with only 0.2 kg of total mass in a "reference" 70 kg human body.
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. It reached a milestone in 2012 when it published initial results.

Non-human animals

  • Amphibians have microbiota on their skin. 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.
  • Newborn marsupials are born with histologically immature immune tissues and unable to mount their own specific immune defence. They are therefore heavily reliant on their mother's immune system and the milk for their protection. Most marsupials have pouches, and their own microbiota changes throughout the reproductive stages: oestrus, birth/oestrus, and post-oestrus. Some pouch and skin secretions have had antimicrobial peptides identified, that presumably support the young at this vulnerable time.
  • 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, 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. 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. This combinatorial approach enables a systems-level understanding of microbial contributions to human biology. But also other mucoide tissues as lung and vagina have been studied in relation to diseases such as asthma, allergy and vaginosis.
  • 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. 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. When the moth Spodoptera exigua is infected with baculovirus immune-related genes are downregulated and the amount of its gut microbiota increases. In the dipteran intestine, enteroendocrine cells sense the gut microbiota-derived metabolites and coordinate antibacterial, mechanical, and metabolic branches of the host intestinal innate immune response to the commensal microbiota.
  • Fish have their own microbiomes, including the short-lived species Nothobranchius furzeri. Transferring the gut microbiota from young killfish into middle-aged killifish significantly extends the lifespans of the middle-aged killfish.

    Plants

The plant microbiome was recently discovered to originate from the seed. 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. 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.
  • The soil is the main reservoir for bacteria that colonize potato tubers
  • Bacteria are recruited from the soil more or less independent of the potato variety
  • Bacteria might colonize the tubers predominantly from the inside of plants via the stolon
  • The bacterial microbiota of potato tubers consists of bacteria transmitted from one tuber generation to the next and bacteria recruited from the soil colonize potato plants via the root.
Plants are attractive hosts for microorganisms since they provide a variety of nutrients. Microorganisms on plants can be epiphytes or endophytes. 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. Plant-growth promoting bacteria 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. 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