Everything about Unicellular totally explained
A
microorganism (also can be spelled as
micro organism) or
microbe is an
organism that's
microscopic (too small to be seen by the naked human eye). The study of microorganisms is called
microbiology, a subject that began with
Anton van Leeuwenhoek's discovery of microorganisms in 1675, using a
microscope of his own design.
Microorganisms are incredibly diverse and include
bacteria,
fungi,
archaea, and
protists, as well as some microscopic
plants and
animals such as
plankton, and popularly-known animals such as the
planarian and the
amoeba. They don't include
viruses and
prions, which are generally classified as non-living. Most microorganisms are single-
celled, or
unicellular, but some multicellular organisms are microscopic, while some unicellular protists, and a bacteria called
Thiomargarita namibiensis are visible to the naked eye.
Microorganisms live in all parts of the
biosphere where there's liquid
water, including
hot springs, on the
ocean floor, high in the
atmosphere and deep inside rocks within the Earth's
crust. Microorganisms are critical to nutrient recycling in
ecosystems as they act as
decomposers. As some microorganisms can
fix nitrogen, they're a vital part of the
nitrogen cycle, and recent studies indicate that airborne microbes may play a role in
precipitation and
weather.
Microbes are also exploited by people in
biotechnology, both in traditional
food and beverage preparation, as well as modern technologies based on
genetic engineering. However,
pathogenic microbes are harmful, since they invade and grow within other organisms, causing
diseases that kill millions of people, other animals, and plants.
History
Evolution
Single-celled microorganisms were the
first forms of life to develop on earth, approximately
3–4 billion years ago. Further evolution was slow, and for about 3 billion years in the
Precambrian eon, all organisms were microscopic. So, for most of the history of
life on Earth the only form of life were microorganisms. Bacteria, algae and fungi have been identified in
amber that's 220 million years old, which shows that the morphology of microorganisms has changed little since the
triassic period.
Most microorganisms can reproduce rapidly and microbes such as bacteria can also freely exchange genes by
conjugation,
transformation and
transduction between widely-divergent species. 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.
Pre-Microbiology
The possibility that microorganisms might exist was discussed for many centuries before their actual discovery in the 17th century. The first ideas about microorganisms were those of the
Roman scholar Marcus Terentius Varro in a book titled
On Agriculture in which he warns against locating a homestead near swamps:
This passage seems to indicate that the ancients were aware of the possibility that diseases could be spread by yet unseen organisms.
In
The Canon of Medicine (1020),
Abū Alī ibn Sīnā (Avicenna) stated that bodily
secretion is contaminated by foul foreign earthly bodies before being infected. He also hypothesized that
tuberculosis and other diseases might be contagious,
for example that they were
infectious diseases, and used
quarantine to limit their spread.
When the
Black Death bubonic plague reached
al-Andalus in the 14th century, Ibn Khatima wrote that infectious diseases were caused by "contagious entities" that enter the human body. Later, 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 in nature and 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 inquiries lacked the most fundamental tool in order for
microbiology and
bacteriology to exist as a science, and that was the
microscope.
Discovery
Anton van Leeuwenhoek was the first person to observe microorganisms, using a
microscope of his own design, thereby making him the first
microbiologist. In doing so Leeuwenhoek would make one of the most important contributions to
biology and open up the fields of
microbiology and
bacteriology. Prior to Leeuwenhoek's discovery of microorganisms in 1675, it had been a mystery as to why
grapes could be turned into
wine,
milk into
cheese, or why food would spoil. Leeuwenhoek didn't 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. Leeuwenhoek's discovery, along with subsequent observations by
Lazzaro Spallanzani and
Louis Pasteur, ended the long-held belief that life
spontaneously appeared from non-living substances during the process of spoilage.
Lazzarro Spallanzani found that microorganisms could only settle in a broth if the broth was exposed to the air. He also found that boiling the broth would
sterilise it and kill the microorganisms. Louis Pasteur 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 wouldn't 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 established that microbes can cause disease. He did this by finding that the blood of cattle who were infected with
anthrax always had large numbers of
Bacillus anthracis. Koch also 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, causing the healthy animal to become sick. He also found that he could grow the bacteria in a nutrient broth, inject it into a healthy animal, and cause illness. Based upon these experiments, he devised criteria for establishing a causal link between a microbe and a disease in what are now known as
Koch's postulates. Though these postulates can't be applied in all cases, they do retain historical importance in the development of scientific thought and can still be used today.
Classification and structure
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, some
fungi, as well as some
animals and plants.
Viruses are generally regarded as not living and therefore are not microbes, although the field of
microbiology also encompasses the study of viruses.
Prokaryotes
Prokaryotes are organisms that lack a
cell nucleus and the other
organelles found in
eukaryotes. Prokaryotes are almost always unicellular, although some species such as
myxobacteria can aggregate into complex structures as part of their
life cycle. These organisms are divided into two groups, the archaea and the bacteria.
Bacteria
Bacteria are the most diverse and abundant group of
organisms on
Earth. Bacteria inhabit practically all environments where some liquid water is available and the temperature is below +140 °C. They are found in
sea water,
soil,
air, animals'
gastrointestinal tracts,
hot springs and even deep beneath the Earth's crust in
rocks. Practically all surfaces which have not been specially sterilized are covered in bacteria. The number of bacteria in the world is estimated to be around five million trillion trillion, or 5 × 10
30.
Bacteria are practically all invisible to the naked eye, with a few extremely rare exceptions, such as
Thiomargarita namibiensis. They are
unicellular organisms and lack membrane-bound organelles. 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 by
budding, but don't 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 10 minutes.
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. 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.
Archaea were originally described in extreme environments, such as
hot springs, but have since been found in all types of habitats. Only now are scientists beginning to appreciate 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. These organisms are also common in soil and play a vital role in
ammonia oxidation.
Eukaryotes
All living things which are
individually visible to the naked eye are
eukaryotes (with few exceptions, such as
Thiomargarita namibiensis), including
humans. However, a large number of eukaryotes are also microorganisms. Unlike
bacteria and
archaea, eukaryotes contain
organelles such as the
cell nucleus, the
Golgi apparatus and
mitochondria in their
cells. The nucleus is an organelle which houses the
DNA that makes up a cell's
genome. DNA itself is arranged in complex
chromosomes.
Mitochondria are organelles vital in
metabolism as they're the site of the
citric acid cycle and
oxidative phosphorylation. They evolved from
symbiotic bacteria and retain a remnant genome. 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 symbiotic
bacteria. Several
algae species are
multicellular protists, and
slime molds have unique life cycles that involve switching between unicellular, colonial, and multicellular forms. 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.
Animals
All animals are multicellular, 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 the
rotifers, which are filter feeders that are usually found in fresh water. Micro-animals reproduce both sexually and asexually and may reach new habitats as eggs that 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.
Fungi
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 undergo
phenotypic switching and grow as single cells in some environments, and
filamentous hyphae in others. Fungi reproduce both asexually, by budding or binary fission, as well by producing spores, which are called
conidia when produced asexually, or
basidiospores when produced sexually.
Plants
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
embryophyte plants, 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.
Habitats and ecology
Microorganisms are found in almost every
habitat present in nature. Even in hostile environments such as the
poles,
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, 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. 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're known as
pathogens.
Extremophiles
Extremophiles are microorganisms which have adapted so that they can survive and even thrive in conditions that are normally fatal to most lifeforms. For example, some species have been found in the following extreme environments:
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 for
extraterrestrial life.
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.
Symbiotic microbes
Symbiotic microbes
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 2 on an improper balance of microbes.
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
yogurt and
cheese. The cultures also provide flavour and aroma, and inhibit undesirable organisms.
Use in water treatment
Microbes are used in the biological treatment of sewage and industrial waste effluents..
Use in energy
Microbes are used in fermentation to produce ethanol.
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're simple eukaryotes that can be grown rapidly in large numbers and are easily manipulated. They are particularly valuable in
genetics,
genomics and
proteomics.
Microbes can be harnessed for uses such as creating steroids and treating skin diseases. Scientists are also considering using microbes for living
fuel cells, and as a solution for pollution.
Use in warfare
In the Middle Ages, dead corpses were thrown over walls during sieges, this meant that any bacteria carrying the disease that killed the person/creature would multiply in the vicinity of the opposing side.
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 and
biotin, and ferment complex indigestible
carbohydrates.
Diseases and immunology
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 or
histoplasmosis. However, other diseases such as
influenza,
yellow fever or
AIDS are caused by
pathogenic viruses, which are not living organisms and are not therefore microorganisms. As of 2007, no clear examples of archaean pathogens are known, although a relationship has been proposed between the presence of some methanogens and human
periodontal disease.
Hygiene
Hygiene is the avoidance of
infection or
food spoiling by eliminating microorganisms from the surroundings. As microorganisms, particularly
bacteria, are found practically everywhere, this means in most cases the reduction of harmful microorganisms to acceptable levels. However, in some cases it's required that an object or substance be completely sterile, for example 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 as
selective 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 for example
antibiotics) and
coliform bacteria (these indicate a sewage contamination).
In fiction
Microorganisms have frequently played an important part in
science fiction, both as agents of disease, and as entities in their own right.
Some notable uses of microorganisms in fiction include:
The War of the Worlds, where microorganisms play important thematic and plot-related roles.
Fantastic Voyage, in which some scientists are miniaturised to microscopic size and observe micro-organisms from a new perspective
Blood Music, in which a colony of microorganisms is given intelligence
The Andromeda Strain, in which extraterrestrial microorganisms kill several people
The White Plague, is created and released in vengeance by John Roe O'Neill for the death of his wife and children, it's designed to kill only women.
Twelve Monkeys, James Cole (Bruce Willis) searches for a pure germ in the past, which creates a deadly plague in the future. Also, Brad Pitt (as Jeffery Goines) discusses his germaphobia.Further Information
Get more info on 'Unicellular'.
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