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Bacterium

A bacterium (plural form: bacteria) is a               Bacteria
certain type of single celled
micro-organism characterized by the lack       Scientific classification

of a membrane-bound nucleus and             Domain*: Bacteria or Eubacteria
membrane-bound organelles, the other type
being the archaea. Bacteria are among the           Phyla/Divisions
oldest and most numerous living things and Aquificae
are found in the soil, water, and inside   Thermotogae
and outside most multicellular organisms.  Thermodesulfobacteria
                                           Deinococci
Bacteria are minute, with physical         Chrysogenetes
dimensions typically in the range of 0.5   Chloroflexi
to 5.0 micrometers (one micrometer is      Thermomicrobia
about 1/25,400 inch). Their study,         Nitrospira
bacteriology, is part of microbiology.     Deferribacteres
                                           Cyanobacteria
                                           Proteobacteria
                                           Firmicutes
                                           Chlorobi
                                           Actinobacteria
                                           Planctomycetes
                                           Chlamydiae
                                           Spirochaetes
                                           Fibrobacteres
                                           Acidobacteria
                                           Bacterioidetes
                                           Fusobacteria
                                           Verrucomicrobia
                                           Dictyoglomi
                                           * or kingdom, see text

Etymology

The term bacterium was introduced by German scientist C.G. Ehrenberg in 1828
as a representative name for some bacterial types.

In 1866, E.H. Haeckel, a German zoologist, suggested the name Protista to
include all unicellular organisms (bacteria, algae, fungi, and protozoa).

In 1878, French surgeon Charles Emmanuel Sedillot coined the term microbe
which is used to describe a bacterial cell or, more generally, any
micro-organism.

History

Since bacteria are unicellular microscopic organisms, they are not visible
with the naked eye and require the use of a microscope to be seen. In 1683,
Antony van Leeuwenhoek was the first to report viewing bacteria with the aid
of a single-lens microscope of his design, at a magnification of about 200
times actual size. Louis Pasteur (1822-1895) and Robert Koch (1843-1910)
described the role of bacteria in causing disease.

Reproduction

Bacteria reproduce both asexually and by genetic recombination. The primary
means of reproduction in bacteria is binary fission, an asexual process. In
binary fission, one bacterial cell divides into two daughter cells with the
development of a transverse cell wall. However, genetic variations can occur
within individual cells through recombinant events such as mutation (random
genetic change within a cell's own genetic code), transformation (the
transfer of naked DNA from one bacterial cell to another in solution),
transduction (the transfer of viral, bacterial, or both bacterial and viral
DNA from one cell to another via bacteriophage) and bacterial conjugation
(the transfer of DNA from one bacterial cell to another via a special
protein structure called a conjugation pilus). Bacteria, having acquired DNA
from any of these events, can then undergo fission and pass the recombined
genome to new progeny cells. Many bacteria harbor plasmids that contain
extrachromosomal DNA.

Under favourable conditions, bacteria may form aggregates visible to the
naked eye, such as bacterial mats.

Metabolisms

Bacteria show a wide variety of different metabolisms. Some bacteria require
only carbon dioxide for their carbon source and are called autotrophs. Those
that obtain their energy in the form of light, via photosynthesis, are
called photoautotrophs. Those that obtain energy by oxidizing chemical
compounds are called chemoautotrophs. Another group of bacteria is dependent
on an organic form of carbon and they are called heterotrophs.

The photoautotrophs include the cyanobacteria, which are some of the oldest
known organisms and probably played an important role in creating the
Earth's oxygen atmosphere. Other photosynthetic bacteria undergo different
processes which do not produce oxygen. These comprise the green sulfur,
green non-sulfur, purple sulfur, purple non-sulfur bacteria and heliobacteria.

Other nutritional requirements include nitrogen, sulfur, phosphorous,
vitamins and metallic elements such as sodium, potassium, calcium,
magnesium, manganese, iron, zinc, and cobalt for normal growth.

Based on their response to oxygen, most bacteria can be placed into one of
three groups: Some bacteria can grow only in the presence of oxygen and are
called aerobes; others can grow only in the absence of oxygen and are called
anaerobes; and some can grow in the presence or absence of oxygen and are
called facultative anaerobes. Bacteria also thrive in environments that are
considered extreme for mankind. These organisms are called extremophiles.
Some bacteria inhabit hot springs and are called thermophiles; others
inhabit highly saltine lakes and are called halophiles; yet others inhabit
acidic or alkaline environments and are called acidophiles and alkaliphiles,
repectively; and still others inhabit alpine glaciers and are called psychrophiles.

Movement

Motile bacteria can move about, either using flagella, bacterial gliding, or
changes of buoyancy. A unique group of bacteria, the spirochaetes, have
structures similar to flagella, called axial filaments, between two
membranes in the periplasmic space. They have a distinctive helical body
which twists about as it moves.

Bacterial flagella are arranged in many different ways. Bacteria can have a
single polar flagellum at one end of a cell, or they can have clusters of
many flagella at one end. Peritrichous bacteria have flagella scattered all
over the cell.

Motile bacteria are attracted or repelled by certain stimuli, behaviors
called taxes - for instance, chemotaxis, phototaxis, and mechanotaxis. In
one peculiar group, the myxobacteria, individual bacteria attract to form
swarms and may differentiate to form fruiting bodies.

Taxonomy

The classification of bacteria has changed radically to reflect thoughts
about phylogeny, and many groups and even species undergo frequent
alteration or renaming. However, this places bacteriology in an ideal
position to exploit recent advances in gene sequencing, genomics,
bioinformatics and computational biology.

Originally the bacteria were considered a group of fungi, except the
cyanobacteria, which were not considered bacteria at all but rather
blue-green algae. The discovery of their common prokaryotic cell structure,
as distinct from all other organisms (all of them eukaryotes), led to their
treatment as a single and separate group, variously called Monera, Bacteria,
and Prokaryota. It was generally believed that this was a grade, in that the
eukaryotes arose from prokaryotes.

Looking at RNA, Woese found that the prokaryotes comprised two separate
groups. These he called the Eubacteria and Archaebacteria, but they have
since become renamed the Bacteria and Archaea, which is the usage followed
here. Woese argued that these two groups, together with the eukaryotes,
comprised separate domains which had originated separately from a primordial
organism. Researchers have abandoned this model, but the three-domain system
has gained general acceptance. In this case the Bacteria, so restricted, may
be divided into several kingdoms, though in other systems they are treated
as a single kingdom. They are generally considered a monophyletic group,
though this has been disputed.

Groupings of bacteria

[Bacteria come in a wide variety of shapes.]
A. Rod-shaped
B. Round-shaped or spherical.
C. Round-shaped in clusters.
D. Round-shaped in twos.
E. Spiral-shaped.
F. Comma-shaped.

Bacteria are grouped in a number of different ways. Bacteria exist in a
number of shapes. Most bacteria are of one of three shapes: The Bacillus is
rod-shaped; the Coccus is spherical in shape (e.g. Streptococcus or
Staphylococcus); and the Spirillum is spiral-shaped. An additional group,
the Vibrio, is comma-shaped.

The structure of bacteria is very simple--that of a prokaryotic cell, which
does not have membrane-bound organelles such as mitochondria and
chloroplasts, but does have cell walls. On the basis of the composition of
the cell walls, that is, the number and placement of cell membranes,
bacteria are divided into two groups, gram positive and gram negative. The
name gram comes from Hans Christian Gram, who developed the technique of
gram staining.

The cell wall usually includes a second membrane surrounding the cell, but
in a few groups this is absent, and instead the cell wall is composed mostly
of glycoproteins. A few bacteria without the second membrane, however, lack
the glycoproteins and show up as gram-negative despite belonging to
gram-positive groups.

Some bacterial cells have capsules outside their cell walls, which are made
up of polysaccharides, and form a covering or envelope around the cell.
These capsules help the bacteria to remain dormant during dry seasons and to
store food and dispose of waste substances.

Many bacteria move from one place to another with the help of thin,
hair-like structures called flagella. Bacteria that possess flagella are
categorized as motile; those without flagella are categorized as immotile.

Benefits and dangers

Bacteria are both harmful and useful to the environment, humans, and
animals. Some bacteria act as pathogens and cause tetanus, typhoid fever,
pneumonia, syphilis, cholera, influenza, and tuberculosis. In plants,
bacteria cause leaf spot, fire blight, and wilts. The mode of infection
includes contact, air, food, water, and insect-borne microorganisms.
Pathogens may be treated by antibiotics, which can be classified as
bacteriocidal and bacteriostatic, which at concentrations that can be
reached in bodily fluids either kill bacteria or hamper their growth,
respectively.

In soil, microorganisms help in the transformation of nitrogen to ammonia
with enzymes secreted by these microbes, which reside in the rhizosphere (a
zone that includes the root surface and the soil that adheres to the root
after gentle shaking). Some bacteria are able to use molecular nitrogen as
their source of nitrogen, converting it to nitrogenous compounds, a process
known as nitrogen fixation. Many other bacteria are found as symbionts in
humans and other organisms. For example, their presence in the large
intestine can help prevent the growth of potentially harmful microbes.

The ability of bacteria to degrade a variety of organic compounds is
remarkable. Highly specialized groups of microorganisms play important roles
in the mineralization of specific classes of organic compounds. For example,
the decomposition of cellulose, which is one of the most abundant
constituents of plant tissues, is mainly brought about by aerobic bacteria
that belong to the group Cytophaga.

Bacteria, often in combination with yeasts and molds, are used in the
preparation of fermented foods such as cheese, pickles, soy sauce,
sauerkraut, vinegar, wine, and yogurt. Using biotechnology techniques,
bacteria can be bioengineered for the production of medical compounds, like
insulin, or for the bioremediation of toxic wastes.

Misc

In terms of evolution, bacteria are thought to be very old organisms,
appearing about 3.7 billion years ago.

Two organelles, mitochondria and chloroplasts, are generally believed to
have been derived from endosymbiotic bacteria.

Microorganisms are widely distributed and are most abundant where they have
food, moisture, and the right temperature for their multiplication and
growth. Bacteria can be carried by air currents from one place to another.
The human body is home to billions of microorganisms; they can be found on
skin surfaces, in the intestinal tract, in the mouth, nose, and other body
openings. They are in the air one breathes, the water one drinks, and the
food one eats.

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