Ploem

                                           Ploem

                                            

Structure

Multiple cross-sections of a stem showing phloem and companion cells^[2]

Phloem tissue consists of: conducting cells, generally called sieve elements; parenchyma cells, including both specialized companion cells or albuminous cells and unspecialized cells; and supportive cells, such as fibres and sclereids.

simplified phloem and companion cells:
1. Xylem
2. Phloem
3. Cambium
4. Pith
5. Companion Cells

[edit]Parenchyma cells

[edit]Companion cells

The metabolic functioning of sieve-tube members depends on a close association with the companion cells, a specialized form ofparenchyma cell. All of the cellular functions of a sieve-tube element are carried out by the (much smaller) companion cell, a typical nucleate plant cell except the companion cell usually has a larger number of ribosomes and mitochondria. The cytoplasm of a companion cell is connected to the sieve-tube element by plasmodesmata.^[3]

There are three types of companion cell.

1. Ordinary companions cells, which have smooth walls and few or no plasmodesmata connections to cells other than the sieve tube.
2. Transfer cells, which have much-folded walls that are adjacent to non-sieve cells, allowing for larger areas of transfer. They are specialised in scavenging solutes from those in the cell walls that are actively pumped requiring energy.
3. Intermediary cells, which have smooth walls and numerous plasmodesmata connecting them to other cells.

The first two types of cell collect solutes through apoplastic (cell wall) transfers, whilst the third type can collect solutes via the symplastthrough the plasmodesmata connections.

[edit]Albuminous cells

Albuminous cells have a similar role to companion cells, but are associated with sieve cells only and are therefore found only in seedless vascular plants and gymnosperms.^[3]

[edit]Other parenchyma cells

Other parenchyma cells within the phloem are generally undifferentiated and used for food storage.^[3]

[edit]Supportive cells

Although its primary function is transport of sugars, phloem may also contain cells that have a mechanical support function. These generally fall into two categories: fibres andsclereids. Both cell types have a secondary cell wall and are therefore dead at maturity. The secondary cell wall increases their rigidity and tensile strength.

[edit]Fibers

Fibers are the long, narrow supportive cells that provide tension strength without limiting flexibility. They are also found in xylem, and are the main component of many textiles such as paper, linen, and cotton.^[3] [3]

[edit]Sclereids

Sclereids are irregularly shaped cells that add compression strength^[3] but may reduce flexibility to some extent. They also serve as anti-herbivory structures, as their irregular shape and hardness will increase wear on teeth as the herbivores chew. For example, they are responsible for the gritty texture in pears.

[edit]Function

Unlike xylem (which is composed primarily of dead cells), the phloem is composed of still-living cells that transport sap. The sap is a water-based solution, but rich in sugars made by the photosynthetic areas. These sugars are transported to non-photosynthetic parts of the plant, such as the roots, or into storage structures, such as tubers or bulbs.

The Pressure flow hypothesis was a hypothesis proposed by Ernst Münch in 1930 that explained the mechanism of phloem translocation.^[4]

During the plant's growth period, usually during the spring, storage organs such as the roots are sugar sources, and the plant's many growing areas are sugar sinks. The movement in phloem is multidirectional, whereas, in xylem cells, it is unidirectional (upward).

After the growth period, when the meristems are dormant, the leaves are sources, and storage organs are sinks. Developing seed-bearing organs (such as fruit) are always sinks. Because of this multi-directional flow, coupled with the fact that sap cannot move with ease between adjacent sieve-tubes, it is not unusual for sap in adjacent sieve-tubes to be flowing in opposite directions.

While movement of water and minerals through the xylem is driven by negative pressures (tension) most of the time, movement through the phloem is driven by positive hydrostatic pressures. This process is termedtranslocation, and is accomplished by a process called phloem loading and unloading. Cells in a sugar source "load" a sieve-tube element by actively transporting solute molecules into it. This causes water to move into the sieve-tube element by osmosis, creating pressure that pushes the sap down the tube. In sugar sinks, cells actively transport solutes out of the sieve-tube elements, producing the exactly opposite effect.

Some plants, however, appear not to load phloem by active transport. In these cases a mechanism known as thepolymer trap mechanism was proposed by Robert Turgeon.^[5] In this case small sugars such as sucrose move into intermediary cells through narrow plasmodesmata, where they are polymerised to raffinose and other largeroligosaccharides. Now they are unable to move back, but can proceed through wider plasmodesmata into the sieve tube element.

The symplastic phloem loading (polymer trap mechanism above) is confined mostly to plants in tropical rain forests and is seen as more primitive. The actively transported apoplastic phloem loading is viewed as more advanced, as it is found in the later-evolved plants, and particularly in those in temperate and arid conditions. This mechanism may, therefore, have allowed plants to colonise the cooler locations.

Organic molecules such as sugars, amino acids, certain hormones, and even messenger RNAs are transported in the phloem through sieve tube elements.

[edit]Girdling

Main article: Girdling

Because phloem tubes sit on the outside of the xylem in most plants, a tree or other plant can be effectively killed by stripping away the bark in a ring on the trunk or stem. With the phloem destroyed, nutrients cannot reach the roots, and the tree/plant will die. Trees located in areas with animals such as beavers are vulnerable since beavers chew off the bark at a fairly precise height. This process is known as girdling, and can be used for agricultural purposes. For example, enormous fruits and vegetables seen at fairs and carnivals are produced via girdling. A farmer would place a girdle at the base of a large branch, and remove all but one fruit/vegetable from that branch. Thus, all the sugars manufactured by leaves on that branch have no sinks to go to but the one fruit/vegetable, which thus expands to many times normal size.

[edit]Origin

The phloem originates, and grows outwards from, meristematic cells in the vascular cambium. Phloem is produced in phases. Primary phloem is laid down by the apical meristem and develops from the procambium.Secondary phloem is laid down by the vascular cambium to the inside of the established layer(s) of phloem.

In some eudicot families (Apocynaceae, Convolvulaceae, Cucurbitaceae, Solanaceae, Myrtaceae, Asteraceae), phloem also develops on the inner side of the vascular cambium; in this case, a distinction between external phloem and internal phloem or intraxylary phloem is made. Internal phloem is mostly primary, and begins differentiation later than the external phloem and protoxylem, though it is not without exceptions. In some other families (Amaranthaceae, Nyctaginaceae, Salvadoraceae), the cambium also periodically forms inward strands or layers of phloem, embedded in the xylem: Such phloem strands are called included phloem or interxylary phloem.^[6]

[edit]Nutritional use

Stripping the inner bark from a pine branch.

Phloem of pine trees has been used in Finland as a substitute food in times of famine, and even in good years in the northeast, where supplies of phloem from earlier years helped stave off starvation somewhat in the great famine of the 1860s. Phloem is dried and milled to flour (pettu in Finnish) and mixed with rye to form a hard dark bread. The least appreciated was silkko, a bread made only from buttermilkand pettu without any real rye or cereal flour. Recently, pettu has again become available as a curiosity, and some have made claims of health benefits. However, its food energy content is low relative to rye or other cereals.

Microorganisms

                               Microorganisms

                                    

• A microorganism (from the Greek: μικρός, mikrós, "small" andὀργανισμός, organismós, "organism") or microbe is amicroscopic organism that can be a single cell (unicellular),  or a multicellular organism. The study of microorganisms is calledmicrobiology, a subject that began with Anton van Leeuwenhoek's discovery of microorganisms in 1675, using a microscope of his own design.
  Microorganisms are very diverse; they include all of theprokaryotes, namely the bacteria and archaea; and various forms of eukaryote, comprising the protozoa, fungi, algae, microscopicplants (green algae), and animals such as rotifers and planarians. Some microbiologists also classify viruses as microorganisms, but others consider these as nonliving.^[2][3] Most microorganisms are microscopic, but there are some like Thiomargarita namibiensis, which are macroscopic and visible to the naked eye.^[4]
  Microorganisms live in all parts of the biosphere including soil, hot springs, on the ocean floor, high in theatmosphere and deep inside rocks within the Earth's crust. Microorganisms are critical to nutrient recycling inecosystems as they act as decomposers. As some microorganisms can fix nitrogen, they are a vital part of thenitrogen cycle, and recent studies indicate that airborne microbes may play a role in precipitation and weather.^[5]
  Microbes are also exploited by people in biotechnology, both in traditional food and beverage preparation, and in modern technologies based on genetic engineering. However there are many pathogenic microbes which are harmful and can even cause death in plants and animals
  
  

  History:

  Single-celled microorganisms were the first forms of life to develop on Earth, approximately 3-4billion years ago.Further evolution was slow and for about 3 billion years in the Precambrian eon, all organisms were microscopic.^[11] So, for most of the history of life on Earth the only forms of life were microorganisms.^[12] Bacteria, algae and fungi have been identified in amber that is 220 million years old, which shows that the morphology of microorganisms has changed little since the Triassic period.^[13]
  Microorganisms tend to have a relatively fast rate of evolution. Most microorganisms can reproduce rapidly, and bacteria are also able to freely exchange genes through conjugation, transformation and transduction, even between widely-divergent species.^[14] This horizontal gene transfer, coupled with a high mutation rate and many other means of genetic variation, allows microorganisms to swiftly evolve (via natural selection) to survive in new environments and respond to environmental stresses. This rapid evolution is important in medicine, as it has led to the recent development of 'super-bugs' — pathogenic bacteria that are resistant to modern antibiotics.^[15]

  Pre-microbiology

  The possibility that microorganisms exist was discussed for many centuries before their actual discovery in the 17th century. The existence of unseen microbiological life was postulated by Jainism, which is based onMahavira's teachings as early as 6th century BCE.^[16] Paul Dundas notes that Mahavira asserted existence of unseen microbiological creatures living in earth, water, air and fire.^[17] Jain scriptures also describe nigodas, which are sub-microscopic creatures living in large clusters and having a very short life and are said to pervade each and every part of universe, even in tissues of plants and flesh of animals.^[18] However, the earliest known idea to indicate the possibility of diseases spreading by yet unseen organisms was that of the Roman scholar Marcus Terentius Varro in a 1st century BC book titled On Agriculture in which he warns against locating a homestead near swamps:

  … and because there are bred certain minute creatures that cannot be seen by the eyes, which float in the air and enter the body through the mouth and nose and they cause serious diseases.^[19]

  In The Canon of Medicine (1020), Abū Alī ibn Sīnā (Avicenna) hypothesized that tuberculosis and other diseases might be contagious^[20][21]
  In 1546, Girolamo Fracastoro proposed that epidemic diseases were caused by transferable seedlike entities that could transmit infection by direct or indirect contact, or even without contact over long distances.
  All these early claims about the existence of microorganisms were speculative and were not based on any data or science. Microorganisms were neither proven, observed, nor correctly and accurately described until the 17th century. The reason for this was that all these early studies lacked the microscope.

  History of microorganisms' discovery

  
  

  

  Antonie van Leeuwenhoek, the first microbiologist and the first to observe microorganisms using amicroscope
  
  

  

  Lazzaro Spallanzani showed that boiling a broth stopped it from decaying
  
  

  

  Louis Pasteur showed that Spallanzani's findings held even if air could enter through a filter that kept particles out
  
  

  

  Robert Koch showed that microorganisms caused disease
  Antonie Van Leeuwenhoek (1632–1723) was one of the first people to observe microorganisms, using a microscope of his own design, thereby making one of the most important contributions to biology.^[22] Robert Hooke was the first to use a microscope to observe living things; his 1665 bookMicrographia contained descriptions of plant cells.
  Before Leeuwenhoek's discovery of microorganisms in 1675, it had been a mystery why grapes could be turned intowine, milk into cheese, or why food would spoil. Leeuwenhoek did not make the connection between these processes and microorganisms, but using a microscope, he did establish that there were forms of life that were not visible to the naked eye.^[23][24]Leeuwenhoek's discovery, along with subsequent observations by Spallanzani and Pasteur, ended the long-held belief that lifespontaneously appeared from non-living substances during the process of spoilage.
  Lazzaro Spallanzani (1729–1799) found that boiling broth wouldsterilise it, killing any microorganisms in it. He also found that new microorganisms could only settle in a broth if the broth was exposed to air.
  Louis Pasteur (1822–1895) expanded upon Spallanzani's findings by exposing boiled broths to the air, in vessels that contained a filter to prevent all particles from passing through to the growth medium, and also in vessels with no filter at all, with air being admitted via a curved tube that would not allow dust particles to come in contact with the broth. By boiling the broth beforehand, Pasteur ensured that no microorganisms survived within the broths at the beginning of his experiment. Nothing grew in the broths in the course of Pasteur's experiment. This meant that the living organisms that grew in such broths came from outside, as spores on dust, rather than spontaneously generated within the broth. Thus, Pasteur dealt the death blow to the theory of spontaneous generation and supported germ theory.
  In 1876, Robert Koch (1843–1910) established that microbes can cause disease. He found that the blood of cattle who were infected with anthrax always had large numbers of Bacillus anthracis. Koch found that he could transmit anthrax from one animal to another by taking a small sample of blood from the infected animal and injecting it into a healthy one, and this caused the healthy animal to become sick. He also found that he could grow the bacteria in a nutrient broth, then inject it into a healthy animal, and cause illness. Based on these experiments, he devised criteria for establishing a causal link between a microbe and a disease and these are now known as Koch's postulates.^[25] Although these postulates cannot be applied in all cases, they do retain historical importance to the development of scientific thought and are still being used today.^[26]

  Classification and structure

  
  

  

  Evolutionary tree showing the common ancestry of all three domains of life.^[27] Bacteria are colored blue, eukaryotes red, and archaea green. Relative positions of some phyla are shown around the tree.
  Microorganisms can be found almost anywhere in the taxonomic organization of life on the planet. Bacteria and archaea are almost always microscopic, while a number of eukaryotes are also microscopic, including most protists, somefungi, as well as some animals and plants.Viruses are generally regarded as not living and therefore not considered as microbes, although the field of microbiology also encompasses the study of viruses.

  [edit]Prokaryotes

  Main article: Prokaryote
  Prokaryotes are organisms that lack a cell nucleus and the other membrane bound organelles. They are almost always unicellular, although some species such asmyxobacteria can aggregate into complex structures as part of their life cycle.
  Consisting of two domains, bacteria and archaea, the prokaryotes are the most diverse and abundant group oforganisms on Earth and inhabit practically all environments where the temperature is below +140 °C. They are found in water, soil, air, animals' gastrointestinal tracts, hot springs and even deep beneath the Earth's crust inrocks.^[28] Practically all surfaces that have not been specially sterilized are covered by prokaryotes. The number of prokaryotes on Earth is estimated to be around five million trillion trillion, or 5 × 10³⁰, accounting for at least half the biomass on Earth.^[29]

  [edit]Bacteria

  Main article: Bacteria
  
  

  

  Staphylococcus aureus bacteria magnified about 10,000x
  Almost all bacteria are invisible to the naked eye, with a few extremely rare exceptions, such as Thiomargarita namibiensis.^[30] They lack a nucleus and other membrane-bound organelles, and can function and reproduce as individual cells, but often aggregate in multicellular colonies.^[31] Their genome is usually a single loop of DNA, although they can also harbor small pieces of DNA called plasmids. These plasmids can be transferred between cells through bacterial conjugation. Bacteria are surrounded by a cell wall, which provides strength and rigidity to their cells. They reproduce by binary fission or sometimes bybudding, but do not undergo sexual reproduction. Some species form extraordinarily resilient spores, but for bacteria this is a mechanism for survival, not reproduction. Under optimal conditions bacteria can grow extremely rapidly and can double as quickly as every 20 minutes.^[32]

  Archaea

  Main article: Archaea
  Archaea are also single-celled organisms that lack nuclei. In the past, the differences between bacteria and archaea were not recognised and archaea were classified with bacteria as part of the kingdom Monera. However, in 1990 the microbiologist Carl Woese proposed the three-domain system that divided living things into bacteria, archaea and eukaryotes.^[33] Archaea differ from bacteria in both their genetics and biochemistry. For example, while bacterial cell membranes are made from phosphoglycerides with ester bonds, archaean membranes are made of ether lipids.^[34]
  Archaea were originally described in extreme environments, such as hot springs, but have since been found in all types of habitats.^[35] Only now are scientists beginning to realize how common archaea are in the environment, with crenarchaeota being the most common form of life in the ocean, dominating ecosystems below 150 m in depth.^[36][37] These organisms are also common in soil and play a vital role in ammonia oxidation.^[38]

  Eukaryotes

  Main article: Eukaryote
  Most living things that are visible to the naked eye in their adult form are eukaryotes, including humans. However, a large number of eukaryotes are also microorganisms. Unlike bacteria and archaea, eukaryotes containorganelles such as the cell nucleus, the Golgi apparatus and mitochondria in their cells. The nucleus is an organelle that houses the DNA that makes up a cell's genome. DNA itself is arranged in complexchromosomes.^[39] Mitochondria are organelles vital in metabolism as they are the site of the citric acid cycle andoxidative phosphorylation. They evolved from symbiotic bacteria and retain a remnant genome.^[40] Like bacteria,plant cells have cell walls, and contain organelles such as chloroplasts in addition to the organelles in other eukaryotes. Chloroplasts produce energy from light by photosynthesis, and were also originally symbioticbacteria.^[40]
  Unicellular eukaryotes are those eukaryotic organisms that consist of a single cell throughout their life cycle. This qualification is significant since most multicellular eukaryotes consist of a single cell called a zygote at the beginning of their life cycles. Microbial eukaryotes can be either haploid or diploid, and some organisms have multiple cell nuclei (see coenocyte). However, not all microorganisms are unicellular as some microscopic eukaryotes are made from multiple cells.

  Protists

  Main article: Protista
  Of eukaryotic groups, the protists are most commonly unicellular and microscopic. This is a highly diverse group of organisms that are not easy to classify.^[41][42] Several algae species are multicellular protists, and slime moldshave unique life cycles that involve switching between unicellular, colonial, and multicellular forms.^[43] The number of species of protozoa is uncertain, since we may have identified only a small proportion of the diversity in this group of organisms.^[44][45]
  
  

  

  A microscopic mite Lorryia formosa.

  Animals

  Main article: Micro-animals
  Most animals are multicellular,^[46] but some are too small to be seen by the naked eye. Microscopic arthropods include dust mites and spider mites. Microscopic crustaceans include copepods and the cladocera, while many nematodes are too small to be seen with the naked eye. Another particularly common group of microscopic animals are therotifers, which are filter feeders that are usually found in fresh water. Micro-animals reproduce both sexually and asexually and may reach new habitats as some eggs can survive harsh environments that would kill the adult animal. However, some simple animals, such as rotifers and nematodes, can dry out completely and remain dormant for long periods of time.^[47]

  Fungi

  Main article: Fungus
  The fungi have several unicellular species, such as baker's yeast (Saccharomyces cerevisiae) and fission yeast (Schizosaccharomyces pombe). Some fungi, such as the pathogenic yeast Candida albicans, can undergophenotypic switching and grow as single cells in some environments, and filamentous hyphae in others.^[48] Fungi reproduce both asexually, by budding or binary fission, as well by producing spores, which are called conidiawhen produced asexually, or basidiospores when produced sexually.

  Plants

  Main article: Plant
  The green algae are a large group of photosynthetic eukaryotes that include many microscopic organisms. Although some green algae are classified as protists, others such as charophyta are classified with embryophyteplants, which are the most familiar group of land plants. Algae can grow as single cells, or in long chains of cells. The green algae include unicellular and colonial flagellates, usually but not always with two flagella per cell, as well as various colonial, coccoid, and filamentous forms. In the Charales, which are the algae most closely related to higher plants, cells differentiate into several distinct tissues within the organism. There are about 6000 species of green algae.^[49]

  Habitats and ecology

  Microorganisms are found in almost every habitat present in nature. Even in hostile environments such as thepoles, deserts, geysers, rocks, and the deep sea. Some types of microorganisms have adapted to the extreme conditions and sustained colonies; these organisms are known as extremophiles. Extremophiles have been isolated from rocks as much as 7 kilometres below the Earth's surface,^[50] and it has been suggested that the amount of living organisms below the Earth's surface may be comparable with the amount of life on or above the surface.^[28] Extremophiles have been known to survive for a prolonged time in a vacuum, and can be highly resistant to radiation, which may even allow them to survive in space.^[51] Many types of microorganisms have intimate symbiotic relationships with other larger organisms; some of which are mutually beneficial (mutualism), while others can be damaging to the host organism (parasitism). If microorganisms can cause disease in a host they are known as pathogens.

  [edit]Extremophiles

  Main article: Extremophile
  Extremophiles are microorganisms that have adapted so that they can survive and even thrive in conditions that are normally fatal to most life-forms. For example, some species have been found in the following extreme environments:
  • Temperature: as high as 130 °C (266 °F),^[52] as low as −17 °C (1 °F)^[53]
  • Acidity/alkalinity: less than pH 0,^[54] up to pH 11.5^[55]
  • Salinity: up to saturation^[56]
  • Pressure: up to 1,000-2,000 atm, down to 0 atm (e.g. vacuum of space)^[57]
  • Radiation: up to 5kGy^[58]
  Extremophiles are significant in different ways. They extend terrestrial life into much of the Earth's hydrosphere,crust and atmosphere, their specific evolutionary adaptation mechanisms to their extreme environment can be exploited in bio-technology, and their very existence under such extreme conditions increases the potential forextraterrestrial life.^[59]

  Soil microbes

  The nitrogen cycle in soils depends on the fixation of atmospheric nitrogen. One way this can occur is in the nodules in the roots of legumes that contain symbiotic bacteria of the genera Rhizobium, Mesorhizobium,Sinorhizobium, Bradyrhizobium, and Azorhizobium.^[60]

  Symbiotic microbes

  Symbiotic microbes such as fungi and algae form an association in lichen. Certain fungi form mycorrhizalsymbioses with trees that increase the supply of nutrients to the tree.

  Importance

  Microorganisms are vital to humans and the environment, as they participate in the Earth's element cycles such as the carbon cycle and nitrogen cycle, as well as fulfilling other vital roles in virtually all ecosystems, such as recycling other organisms' dead remains and waste products through decomposition. Microbes also have an important place in most higher-order multicellular organisms as symbionts. Many blame the failure of Biosphere 2on an improper balance of microbes.^[61]

  Use in digestion

  Some forms of bacteria that live in animals' stomachs help in their digestion.For example,cows have a variety of different microbes in their stomachs that aid them in their digestion of grass and hay.

  Use in food

  Microorganisms are used in brewing, winemaking, baking, pickling and other food-making processes.
  They are also used to control the fermentation process in the production of cultured dairy products such as yogurtand cheese. The cultures also provide flavour and aroma, and inhibit undesirable organisms.^[62]

  Use in water treatment

  Specially-cultured microbes are used in the biological treatment of sewage and industrial waste effluent, a process known as bioaugmentation.^[63]

  Use in energy

  Main articles: Algae fuel, Cellulosic ethanol, and Ethanol fermentation
  Microbes are used in fermentation to produce ethanol,^[64] and in biogas reactors to produce methane.^[65]Scientists are researching the use of algae to produce liquid fuels,^[66] and bacteria to convert various forms of agricultural and urban waste into usable fuels.^[67]

  ]Use in production of chemicals, enzymes etc.

  Many microbes are used for commercial and industrial production of chemicals, enzymes and other bioactive molecules.
  Examples of organic acid produced include
  • Acetic acid: Produced by the bacterium Acetobacter aceti and other acetic acid bacteria (AAB)
  • Butyric acid (butanoic acid): Produced by the bacterium Clostridium butyricum
  • Lactic acid: Lactobacillus and others commonly called as lactic acid bacteria (LAB)
  • Citric acid: Produced by the fungus Aspergillus niger
  Microbes are used for preparation of bioactive molecules and enzymes.
  • Streptokinase produced by the bacterium Streptococcus and modified by genetic engineering is used as a clot buster for removing clots from the blood vessels of patients who have undergone myocardial infarctions leading to heart attack.
  • Cyclosporin A is a bioactive molecule used as an immunosuppressive agent in organ transplantation
  • Statins produced by the yeast Monascus purpureus are commercialised as blood cholesterol lowering agents which act by competitively inhibiting the enzyme responsible for synthesis of cholesterol.^[68]

  Use in science

  Microbes are also essential tools in biotechnology, biochemistry, genetics, and molecular biology. The yeasts (Saccharomyces cerevisiae) and fission yeast (Schizosaccharomyces pombe) are important model organisms in science, since they are simple eukaryotes that can be grown rapidly in large numbers and are easily manipulated.^[69] They are particularly valuable in genetics, genomics and proteomics.^[70][71] Microbes can be harnessed for uses such as creating steroids and treating skin diseases. Scientists are also considering using microbes for living fuel cells,^[72] and as a solution for pollution.^[73]

  Use in warfare

  Main article: Biological warfare
  In the Middle Ages, diseased corpses were thrown into castles during sieges using catapults or other siege engines. Individuals near the corpses were exposed to the deadly pathogen and were likely to spread that pathogen to others.^[74]

  Importance in human health

  Human digestion

  
  
  Microorganisms can form an endosymbiotic relationship with other, larger organisms. For example, the bacteria that live within the human digestive system contribute to gut immunity, synthesise vitamins such as folic acid andbiotin, and ferment complex indigestible carbohydrates

  Diseases caused by microbes

  Microorganisms are the cause of many infectious diseases. The organisms involved include pathogenic bacteria, causing diseases such as plague, tuberculosis and anthrax; protozoa, causing diseases such as malaria,sleeping sickness and toxoplasmosis; and also fungi causing diseases such as ringworm, candidiasis orhistoplasmosis. However, other diseases such as influenza, yellow fever or AIDS are caused by pathogenic viruses, which are not usually classified as living organisms and are not, therefore, microorganisms by the strict definition. As of 2007, no clear examples of archaean pathogens are known,^[76] although a relationship has been proposed between the presence of some archaean methanogens and human periodontal disease.^[77]

  Importance in ecology

  Microbes are critical to the processes of decomposition required to cycle nitrogen and other elements back to the natural world.

  Hygiene

  Hygiene is the avoidance of infection or food spoiling by eliminating microorganisms from the surroundings. As microorganisms, in particular bacteria, are found virtually everywhere, the levels of harmful microorganisms can be reduced to acceptable levels. However, in some cases, it is required that an object or substance be completely sterile, i.e. devoid of all living entities and viruses. A good example of this is a hypodermic needle.
  In food preparation microorganisms are reduced by preservation methods (such as the addition of vinegar), clean utensils used in preparation, short storage periods, or by cool temperatures. If complete sterility is needed, the two most common methods are irradiation and the use of an autoclave, which resembles a pressure cooker.
  There are several methods for investigating the level of hygiene in a sample of food, drinking water, equipment, etc. Water samples can be filtrated through an extremely fine filter. This filter is then placed in a nutrient medium. Microorganisms on the filter then grow to form a visible colony. Harmful microorganisms can be detected in food by placing a sample in a nutrient broth designed to enrich the organisms in question. Various methods, such asselective media or PCR, can then be used for detection. The hygiene of hard surfaces, such as cooking pots, can be tested by touching them with a solid piece of nutrient medium and then allowing the microorganisms to grow on it.
  There are no conditions where all microorganisms would grow, and therefore often several different methods are needed. For example, a food sample might be analyzed on three different nutrient mediums designed to indicate the presence of "total" bacteria (conditions where many, but not all, bacteria grow), molds (conditions where the growth of bacteria is prevented by, e.g., antibiotics) and coliform bacteria (these indicate a sewage contamination).