A
microorganism (from the ,
mikrós,
"small" and ,
organismós, "organism"; also spelled
micro organism or
micro-organism)
or
microbe is an
organism
that is
microscopic (usually 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 very diverse; they include
bacteria,
fungi,
archaea, and
protists;
microscopic
plants (called
green algae); and
animals such as
plankton and the
planarian. Some microbiologists also include
viruses, but others consider these as
non-living. Most microorganisms are unicellular (single-celled),
but this is not universal, since some
multicellular organisms are
microscopic, while some unicellular protists and
bacteria, like
Thiomargarita namibiensis,
are
macroscopic and visible to the naked
eye.
Microorganisms live in all parts of the
biosphere where there is liquid
water, including
soil,
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 are 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, and in
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 forms of life
were microorganisms. 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.
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 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 1st
century BC 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,
i.e. that they
were
infectious diseases, and
used
quarantine to limit their
spread.
When the
Black Death bubonic plague reached Andalusia
in Spain, in the 14th century, Ibn Khatima wrote
that infectious diseases were caused by contagious "minute bodies"
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 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
Anton van Leeuwenhoek was the
one of the first people to observe microorganisms, and used a
microscope of his own design, and made one of the most important
contributions to
biology.
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.
Leeuwenhoek's discovery, along with subsequent observations by
Lazzaro Spallanzani and
Louis Pasteur, ended the long-held belief that
life
spontaneously appear from
non-living substances during the process of spoilage.
Lazzaro Spallanzani found that boiling broth would
sterilise it and kill any
microorganisms in it. He also found that new microorganisms could
only settle in a broth if the broth was exposed to the air. 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 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 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. 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.
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 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 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
rock. Practically all
surfaces which 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.
Bacteria
250 px
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 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 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
Most living things which 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 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 are 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.
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. 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
Mostly 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,
rock, 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.
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. 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.
Extremophiles
Extremophiles are microorganisms which
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.
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 such as fungi and algae
form an association in
lichen. Certain fungi
form
mycorhizzal symbioses 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
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
Specially-cultured microbes are used in the biological treatment of
sewage and industrial waste effluent, a process known as
bioaugmentation.
Use in energy
Microbes are used in fermentation to produce ethanol, and in
biogas reactors to produce
methane. Scientists are researching the use of
algae to produce liquid fuels, and
bacteria to convert various forms of agricultural and urban waste
into
usable fuels.
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. 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, 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.
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 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, although a
relationship has been proposed between the presence of some
methanogens and human
periodontal
disease.
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, particularly
bacteria, are found practically everywhere, this
means in most cases the reduction of harmful microorganisms 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).
See also
References
External links
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