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]
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 on
Mahavira'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 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
Louis Pasteur showed that Spallanzani's findings held even if air could enter through a filter that kept particles out
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 book
Micrographia contained descriptions of plant cells.
Before Leeuwenhoek's discovery of microorganisms in 1675, it had been a mystery why
grapes could be turned into
wine,
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 life
spontaneously appeared from non-living substances during the process of spoilage.
Lazzaro Spallanzani (1729–1799) found that boiling broth would
sterilise 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
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 not considered as microbes, although the field of
microbiology also encompasses the study of viruses.
[edit]Prokaryotes
Prokaryotes are organisms that lack a
cell nucleus and the other membrane bound organelles. They are almost always unicellular, although some species such as
myxobacteria 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 of
organisms 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 in
rocks.
[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 × 10
30, accounting for at least half the
biomass on Earth.
[29]
[edit]Bacteria
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 by
budding, 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
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
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
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]
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 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 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
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
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 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,
[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:
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.
[59]
Soil microbes
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
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
]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
Microbes are used for preparation of bioactive molecules and enzymes.
Use in science
Use in 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 and
biotin, 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 or
histoplasmosis. 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 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, e.g.,
antibiotics) and
coliform bacteria (these indicate a sewage contamination).
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