Microorganisms or microbes are microscopic organisms which exist as unicellular, multicellular, or cell clusters. The possible existence of unseen microbial life was suspected from past. Antonie van Leeuwenhoek was the primary to review microorganisms, using simple microscopes of his own design. Microorganisms can be found almost anywhere on Earth.
Microorganims are widespread in nature and are beneficial to life, but some can cause serious harm. They can be divided into six major types: bacteria, archaea, fungi, protozoa, algae, and viruses. Bacteria and archaea are nearly always microscopic, while variety of eukaryotes also are microscopic,including most protozoa, some fungi, also as some micro-animals and plants. Viruses are generally considered not living and thus not considered as microorganisms, although a subfield of microbiology is virology, the study of viruses.
Archaea
Archaea are prokaryotic unicellular organisms, and form the primary domain of life, in Carl Woese's three-domain system. A prokaryote is defined as having no nucleus or other membrane bound-organelle. Archaea share this defining feature with the bacteria with which they were once grouped.
Archaea differ from bacteria in both their genetics and biochemistry. For example, while bacterial cell membranes are made up of phosphoglycerides with ester bonds, archaean membranes are made from ether lipids. The combined domains of archaea and bacteria structure the foremost diverse and abundant group of organisms.Archael cells of some species aggregate and transfer DNA from one cell to another through direct contact, particularly under stressful environmental conditions that cause DNA damage.
Bacteria
Bacteria are microscopic, with a couple of extremely rare exceptions, like Thiomargarita namibiensis. Bacteria function and reproduce as individual cells, but they will often aggregate in multicellular colonies. Some species like myxobacteria can aggregate into complex swarming structures, operating as multicellular groups as an area of their life cycle, or form clusters in bacterial colonies like E.coli.
Their genome is typically a circular bacterial chromosome – one loop of DNA, although they will also harbor small pieces of DNA called plasmids. These plasmids are often transferred between cells through bacterial conjugation. Bacteria have an enclosing cell membrane , which provides strength and rigidity to their cells. Under optimal conditions bacteria can grow extremely rapidly and their numbers can double as quickly as every 20 minutes.
Protozoa
Most living things that are visible to the eye in their adult form are eukaryotes, including humans. However, many eukaryotes are also microorganisms. Eukaryotes contain organelles like the nucleus , the Golgi body and mitochondria in their cells. The nucleus is an organelle that houses the DNA that creates up a cell's genome. Unicellular eukaryotes contains one cell throughout their life cycle. This qualification is critical since most multicellular eukaryotes contains one cell called a zygote only at the start of their life cycles. Microbial eukaryotes are often either haploid or diploid, and a few organisms have multiple cell nuclei.
Fungi
The fungi have several unicellular species, like brewer's yeast (Saccharomyces cerevisiae) and fission yeast (Schizosaccharomyces pombe). Some fungi, a bit like the pathogenic yeast Candida albicans , can undergo phenotypic switching and grow as single cells in some environments, and filamentous hyphae in others.
Algae
The chlorophyte are an outsized group of photosynthetic eukaryotes that include many microscopic organisms. Although some chlorophyte are classified as protists, others like charophyta are classified with embryophyte plants, which are the foremost familiar group of land plants. Algae can grow as single cells, or in long chains of cells. The chlorophyte include unicellular and colonial flagellates, usually but not always with two flagella per cell, also as various colonial, coccoid, and filamentous forms. In the Charales, which are the algae most closely associated with higher plants, cells differentiate into several distinct tissues within the organism. There are about 6000 species of green algae.
Viruses
Viruses are noncellular entities that contains a macromolecule core (DNA or RNA) surrounded by a protein coat. Although viruses are classified as microorganisms, they're not considered living organisms. Viruses cannot reproduce outside a number cell and can't metabolize on their own. Viruses often infest prokaryotic and eukaryotic cells causing diseases.
Humans depend on microorganisms for many things varying from health, welfare, food, and industrial products. A variety of microbes exists throughout the physical body and have fundamental role in human health. In the body, these microbes are present in several parts like skin, cavity , mouth , gut and therefore stay in different habitats.
Many microorganisms (generally bacteria and archaea) live in the digestive tracts of humans they are called as human gastrointestinal microbiota, also known as gut flora or gut microbiota. In humans, the gut microbiota has the most important numbers of bacteria and therefore the greatest number of species compared to other areas of the body. In humans, the gut flora is established at one to 2 years after birth, by which era the intestinal epithelium and therefore the intestinal mucosal barrier that it secretes have co-developed during a way that's tolerant to, and even supportive of, the gut flora.
The microbial composition of the gut microbiota varies across the alimentary canal . In the stomach and little intestine, relatively few species of bacteria are generally present. The colon, in contrast, contains the very best microbial density recorded in any habitat on Earth with up to 1012 cells per gram of intestinal content. These bacteria represent between 300 and 1000 different species. Bacteria also structure to 60% of the dry mass of feces. Fungi, protists, archaea, and viruses also are present within the gut flora, but less is understood about their activities. It is estimated that these gut flora have around 100 times as many genes in total as there are within the human genome.
Bacteria within the alimentary canal can contribute to and be suffering from disease in various ways.
Ulcers
Helicobacter pylori can cause stomach ulcers by crossing the epithelial lining of the stomach. Here the body produces an immune response. During this response, parietal cells are stimulated and release extra acid (HCl+) into the stomach. However, the response doesn't stimulate the mucus-secreting cells that protect and line the epithelium of the stomach. The extra acid sears holes into the epithelial lining of the stomach, leading to stomach ulcers.
Inflammatory bowel diseases
The two main types of inflammatory bowel diseases, Crohn's disease and ulcerative colitis, are chronic inflammatory disorders of the gut; the causes of these diseases are unknown and issues with the gut flora and its relationship with the host are implicated in these conditions. Additionally, it appears that interactions of gut flora with the gut-brain axis have a task in IBD, with physiological stress mediated through the hypothalamic–pituitary–adrenal axis driving changes to intestinal epithelium and therefore the refore the gut flora successively releasing factors and metabolites that trigger signaling within the enteric systema nervosum and the vagus . The diversity of gut flora appears to be significantly diminished in people with inflammatory bowel diseases compared to healthy people.
Asthma
With asthma, two hypotheses have been posed to explain its rising prevalence in the developed world. The hygiene hypothesis posits that children in the developed world are not exposed to enough microbes and thus may contain lower prevalence of specific bacterial taxa that play protective roles. The second hypothesis focuses on the Western pattern diet, which lacks whole grains and fiber and has an overabundance of straightforward sugars. Both hypotheses converge on the role of short-chain fatty acids (SCFAs) in immunomodulation. These bacterial fermentation metabolites are involved in immune signalling that forestalls the triggering of asthma and lower SCFA levels are related to the disease.
Diabetes mellitus type 1
The connection between the gut microbiota and diabetes mellitus type 1 has also been linked to SCFAs, such as butyrate and acetate. Diets yielding butyrate and acetate from bacterial fermentation show increased Treg expression.
Liver disease
As the liver is fed directly by the hepatic portal vein , whatever crosses the intestinal epithelium and therefore the intestinal mucosal barrier enters the liver, as do cytokines generated there. Dysbiosis within the gut flora has been linked with the event of cirrhosis and non-alcoholic liver disease disease.
Cancer
Some genera of bacteria, like Bacteroides and Clostridium, are related to a rise in tumor rate of growth , while other genera, like Lactobacillus and Bifidobacteria, are known to prevent tumor formation.
Microorganisms are also important in human body. Each physical body hosts 10 microorganisms for each human cell. These microbes contribute to digestion by producing vitamin K, promoting development of the immune system, and detoxify harmful chemicals.
References
https://en.m.wikipedia.org/wiki/Microorganism
https://en.m.wikipedia.org/wiki/Human_gastrointestinal_microbiota