Biology 2 chapter 25 NOTES
Appearing about 3.5
billion years ago, prokaryotes (bacteria)
were the earliest living organisms and the
only forms of life for
2 billion years.
1. They're
(almost) everywhere! an overview ofprokaryotic life
Prokaryotes dominate
the biosphere; they are the most numerous organisms and can be found in
all habitats.
Approximately 4,000
species are currently recognized, however, estimates of the actual
diversity range from
400,000 ‑ four million species
Are structurally and
metabolically diverse.
Prokaryotes and the Origins of Metabolic Diversity 449
Prokaryotic cells
differ from eukaryotic cells in several ways:
Prokaryotes are smaller and lack membrane‑bound
organelles.
Most
have cell walls but the composition and structure differ from those found in
plants,
fungi and protists.
Prokaryotes have simpler genomes.
synthesis and recombination.
They also differ in
genetic replication, protein
Prokaryotes, while
very small, have a tremendous impact on the Earth.
A small percentage cause disease.
Some are decomposers, key organisms in life‑sustaining
chemical cycles.
Many
form symbiotic relationships with other prokaryotes and eukaryotes.
Mitochondria
and chloroplasts may have evolved from such
symbioses.
11. Archaea
and Bacteria are the two main branches of prokaryotic evolution
The traditional five‑kingdom
system recognizes one kingdom of prokaryotes (Monera) and four
kingdoms of eukaryotes
(Protista, Plantae, Fungi, and Animalia).
This
system emphasizes the structural differences between prokaryotic and eukaryotic
cells.
Recent research in
systematics has resulted in questions about the placement of a group as
diverse as the
prokaryotes in a single kingdom. Two major branches of prokaryotic evolution
have been indicated by
comparing ribosomal RNA and other genetic products:
One
branch is called the archaehacteria.
Believed
to have evolved from the earliest cells.
Inhabit
extreme environments which may resemble the Earth's early habitats (I
springs
and salt ponds).
The
second branch is called the eubacteria.
Considered
the more "modem" prokaryotes, having evolved later in Earth's
history.
More
numerous than archaebacteria.
=> Differ
from archaebacteria in structural, biochemical, and physiological characters.
This recently
acquired molecular data has led to new proposals for the systematic
relationships
of organisms.
Carl Woese has
proposed a six‑kingdom system that includes:
=> Two
prokaryotic kingdoms.
=:> Four
eukaryotic kingdoms.
An eight‑kingdom
system has also been proposed. (Covered in more detail in Chapter 26.)
=:> This
system also contains two kingdoms of prokaryotic organisms.
Domain
BACTERIA
(Eubacteria)
Domain
ARCHAEA
(Archaebacteria)
N/
450 Prokaryotes
and the Origins of Metabolic Diversity
X
In addition to the
kingdoms systems proposed, many systernatists now favor an organization of
the diversity of life
which includes three domains, with
the domain being a taxonomic level
higher than kingdom.
Domain
EUKARYA
(Eukaryotes)
Prokaryotes
comprise two of the three domains, Archaea and Bacteria, while eukaryotes
fill the third domain, Eukarya.
The
domain Archaea includes the archaebacteria, the Bacteria includes the
eubacteria,
and
the Eukarya includes all eukaryotic organisms.
The Eukarya and Archaea shared a common ancestor
which lived more recently than
the ancestor shared between the Archaea and
Bacteria.
Although systematic debates continue, the
archaebacteria and eubacteria are structurally
organized at the
prokaryotic level while differing on structural, genetic, and metabolic
levels.
111. The
success of prokaryotic life is based on diverse adaptations of form and
function
A. Morphological Diversity of Prokaryotes
A majority of
prokaryotes are single‑celled, although some aggregate into two‑celled
to
several celled
groups. Others form true, permanent aggregates and some bacterial species
have a simple
multicellular form with a division of labor between specialized cells.
Cells
have a diversity of shapes, the most common being spheres (cocci), rods
(bacilli),
and helices (spirilla and
spirochetes).
One
rod‑shaped species measures ;t:0.5 mm in length, and is the largest
prokaryotic
cell known.
Most have diameters of 1‑5mm (compared
to eukaryotic diameters of 10 ‑ I 00mm).
B. The Cell Surface of Prokaryotes
A majority of
prokaryotes have external cell walls that:
Maintain the cell shape.
Protect the cell.
Prevent the cell from bursting in a
hypotonic environment.
In
eubacteria, contain peptidoglycan; archaebacteria
lack peptidoglycan in their cell
walls
Prokaryotes and the Origins of Metabolic Diversity 451
Pel2tidoglycan =
Modified sugar polymers cross‑linked by short polypeptides.
Exact composition varies among species.
Some
antibiotics work by preventing formation of the cross‑links in
peptidoglycan,
thus preventing the formation of a functional
cell wall.
Gram stai = A stain used
to distinguish two groups of eubacteria by virtue of a structural
difference in their cell walls.
1. Gram‑positive
bacteria.
Have simple cell walls with large amounts of
peptidoglycan.
Stain blue.
2. Gram‑negative
bacteria.
Have
more complex cell walls with smaller amounts of peptidoglycan.
An
outer lipopolysaccharide‑containing membrane covers the cell wall.
Stain
pink.
These
cells are more often disease‑causing (pathogenic)
than gram‑positive
bacteria.
Lipopolysaccharides
are often toxic and the outer membrane helps protect these
bacteria
from host defense systems.
Lipopolysaccharides
also impede entry of drugs into the cells, making gram
negative
bacteria more resistant to antibiotics.
Capsule = A gelatinous
secretion of some prokaryotes which provides cells with additional
protection, helps them adhere to hosts and helps
form aggregates.
Pili = Surface appendages
used for adherence to a host (in the case of a pathogen), or for
transferring DNA when bacteria conjugate.
C. The Motility of
Prokaryotes
Motile bacteria (,,z~
50% of known species) use one of three mechanisms to move:
1.
Flagella.
Prokaryotic flagella differ from
eukaryotic flagella in that they are:
Unique in structure and
function. Prokaryotic flagella lack the "9 + 2"
microtubular structure and
rotate rather than whip back and forth like
eukaryotic flagella.
=> Not covered by an extension of
the plasma membrane.
=> One‑tenth the width of
eukaryotic flagella.
Filaments, composed of chains of the
protein flagellin, are attached to
another
protein hook which is inserted into
the basal apparatus.
The basal apparatus consist of 35
different proteins arranged in a system of rings
which sit in the various cell wall
layers.
Their rotation is powered by the
diffusion of protons into the cell. The proton
gradient is maintained by an ATP‑driven
proton pump.
452 Prokaryotes
and the Origins of Metabolic Diversity
2. Filaments which are
characteristic of spirochetes, helical‑shaped bacteria.
Several filaments spiral around the cell
inside the cell wall.
Similar
to prokaryotic flagella in structure, axial filaments are attached to basal
motors at either end of the cell. Filaments
attached at opposite ends move
relative to each other, rotating the cell like a
corkscrew.
3. Gliding.
Some
bacteria move by gliding through a layer of slimy chemicals secreted by
the organism.
The movement may result from flagellar motors
that lack flagellar filaments.
Prokaryotic movement
is fairly random in homogenous environments but may become
directional in a
heterogenous environment.
Taxis
= Movement to or away from a stimulus.
The
stimulus can be light (phototaxis), a chemical (chemotaxis), or a magnetic
field
(magnetotaxis).
Movement
toward a stimulus is a positive taxis (i.e. positive phototaxis = toward
light)
while movement away from a stimulus is a negative taxis (i.e. negative
phototaxis
= away from light).
During taxis
(directed movement), bacteria move by running and
tumbling
movements:
Enabled
by rotation of flagella either counterclockwise or clockwise respectively.
Caused
by flagella moving coordinately about each other (for a run), or in separate
and
randomized movements (for a tumble).
D. Internal
Membranous Organization
Prokaryotes
lack the diverse internal membranes characteristic of eukaryotes. Some
prokaryotes,
however, do have specialized membranes, formed by invaginations of the
plasma
membranes.
Infoldings
of the plasma membrane function in cellular respiration of aerobic
bacteria.
Cyanobacteria
have thylakoid membranes that contain chlorophyll and that
function
in photosynthesis.
E. Prokaryotic
Genomes
The prokaryotic genome
has only 1/1000 as much DNA as the eukaryotic genome.
Genophore = The bacterial
chromosome, usually one double‑stranded, circular DNA
molecule.
This
DNA is concentrated in the nucleoid region, and is not surrounded by a
membrane; therefore, there is no true nucleus.
Has very little protein associated with the
DNA.
Prokaryotes and the Origins
of Metabolic Diversity 453
Many bacteria
also have plasmids.
Plasmi = Smaller rings of DNA
having supplemental (usually not essential) genes for
functions such as antibiotic resistance or
metabolism of unusual nutrients.
Replicate independently of the genophore.
Can be transferred between partners during
conjugation.
While
prokaryotic and eukaryotic DNA replication and translation are similar, there
are
some
differences. For example,
Bacterial
ribosomes are smaller and have different protein and RNA content than
eukaryotic
ribosomes.
=> This
difference permits some antibiotics (i.e. tetracycline) to block bacterial
protein synthesis while not inhibiting the
process in eukaryotic cells.
F. Growth,
Reproduction, and Gene Exchange
Neither mitosis
nor meiosis occur in the prokaryotes.
Reproduction is asexual by hinaryfission.
DNA synthesis is almost continuous.
Growth in the numbers
of cells is geometric in an environment with unlimited resources.
Generation
time is usually I ‑ 3 hours, although some can be 20 minutes in optimal
environments.
At high concentrations of cells, growth slows
due to accumulation of toxic wastes,
lack of nourishment, etc.
Competition in natural environments is reduced
by the release of antibiotic
chemicals which inhibit the growth of other
species.
Optimal growth requirements vary depending
upon the species.
Some bacteria survive
adverse environmental conditions and toxins by producing
endospores.
Endospore =
Resistant cell formed by some bacteria; contains one chromosome copy
surrounded by a thick wall.
When endospores form, the original cell
replicates its chromosome and surrounds
one copy with a durable wall. The original
surrounding cell disintegrates,
releasing the resistant endospore.
Since
some endospores can survive boiling water for a short time, home canners
and
food canning industry must take special precautions to kill endospores of
dangerous
bacteria.
May remain dormant for many years until proper
environmental conditions
return.
454 Prokaryotes
and the Origins of Metabolic Diversity
Although
meiosis and syngamy do not occur in prokaryotes, genetic recombination can
take
place through three mechanisms that transfer variable amounts of DNA:
Transformation =
The process by which external DNA is incorporated by bacterial
cells.
Cog‑jugation
= The direct transfer of genes from one bacterium to
another.
Transduction
= The transfer of genes between bacteria by viruses.
Short
generation times allow prokaryotic populations to adapt to rapidly changing
environmental
conditions.
New mutations and genomes (from
recombination) are screened by natural selection
very
quickly.
Has
resulted in the current diversity and success of prokaryotes as well as the
variety
of nutritional and metabolic mechanisms found in
this group.
IV. All
major types of nutrition and metabolism evolved among prokaryotes
The
prokaryotes exhibit some unique modes of nutrition as well as every type of
nutrition found
in
eukaryotes.
A. Major Modes of Nutrition
Prokaryotes
exhibit a great diversity in how they obtain the necessary resources (energy
and
carbon) to synthesize organic compounds.
Some
obtain energy from light (phototrophs),
while others use chemicals taken from
the environment (chemotrophs).
Many
can utilized C02 as a carbon source (autotrophs)
and others require at least
one organic nutrient as a carbon source (heterotrophs).
Depending
upon the energy source and the carbon source, prokaryotes have four possible
nutritional
modes:
I . Photoautotrophs: Use
light energy to synthesize organic compounds from C02.
Includes
the cyanobacteria. (Actually all photosynthetic eukaryotes fit in this
category.)
2.
Chemoautotrophs: Require only C02 as a carbon source and
obtain energy by
2+
oxidizing inorganic compounds such as H2S, NI‑13
and Fe . This mode of nutrition
is unique to certain prokaryotes (i.e.
archaebacteria of the genus Su~fobolus).
3. Photoheterotrophs: Use
light to generate ATP from an organic carbon source. This
mode
of nutrition is unique to certain prokaryotes.
4.Chemoheterotrophs: Must obtain organic
molecules for energy and as a source of
carbon. Found in many
bacteria as well as most eukaryotes.
Prokaryotes and the Origins
of Metabolic Diversity 455
B. Nutritional
Diversity Among Chemoheterotrophs
Most bacteria
are chemoheterotrophs and can be divided into two subgroups: saprobes
andparasites.
Saprobes are decomposers that absorb nutrients from dead organic matter.
Parasites are bacteria that absorb nutrients from body fluids of living hosts.
The
chemoheterotrophs are a very diverse group, some have very strict requirements
while
others
are extremely versatile. 0 Lactobacillus will grow well only when
the medium contains all 20 amino acids,
several vitamins, and
other organic compounds.
9 E.
coli will grow on a medium which contains only a single organic ingredient
(i.e.
glucose or some other
substitute).
Almost
any organic molecule can serve as a carbon source for some species
9 Some
bacteria are capable of degrading petroleum and are used to clean oil spills.
* Those
compounds that cannot be used as a carbon source by bacteria are considered non‑biodegradable (e.g. some
plastics).
C. Nitrogen
Metabolism
While
eukaryotes can only use some forms of nitrogen to produce proteins and nucleic
acid,
prokaryotes can metabolize most nitrogen compounds.
Prokaryotes are
extremely important to the cycling of nitrogen through ecosystems.
Some
chemoautotrophic bacteria (Nitrosomonas) convert NH3 ‑> N02‑‑
Other bacteria,
such as Pseudomonas, denitrify N02‑ or N03‑ to atmospheric N2.
Nitrogen
fixation (N2 ‑> NH3) is unique to certain prokaryotes (cyanobacteria)
and
is the only
mechanism that makes atmospheric nitrogen available to organisms for
incorporation
into organic compounds.
The nitrogen
fixing cyanobacteria are very self‑sufficient, they need only light
energy, C02,
N2, water and a few minerals to grow.
D. Metabolic
Relationships to Oxygen
Prokaryotes differ in
their growth response to the presence of oxygen.
Obligate
aerobes = Prokaryotes needing 02 for cellular
respiration.
Facultative anaerobes =
Prokaryotes that use 02 when present, but in its absence can
grow using fermentation.
Obligate anaerobes =
Prokaryotes that are poisoned by oxygen.
Some species live exclusively by fermentation.
Other species use inorganic molecules (other
than 02) as electron acceptors
during anaerobic respiration.
456 Prokaryotes and the Origins of Metabolic
Diversity
V. The
evolution of prokaryotic metabolism was both cause and effect of changing
environments on Earth
Prokaryotes
evolved all forms of nutrition and most metabolic pathways eons before
eukaryotes
arose.
Evolution
of these new metabolic capabilities were a response to the changing
environment
of the early atmosphere.
As these new capabilities evolved, they changed
the environment for subsequent
prokaryotic communities.
Information
from molecular systematics, comparisons of energy metabolism, and geological
studies
about Earth's early atmosphere have resulted in many hypotheses about the
evolution of
prokaryotes
and their metabolic diversity.
A. The Origin of Glycolysis
The
first prokaryotes, which evolved 3.5 billion years ago, were probably
chemoheterotrophs
that absorbed free organic compounds (including ATP) generated by
abiotic
synthesis.
The
universal role of ATP implies that prokaryotes used that molecule for energy
very
early
in their evolution.
As
ATP supplies were depleted, natural selection favored those prokaryotes that
could
regenerate ATP from ADP, leading to step by step evolution of glycolysis and
other
catabolic pathways.
Glycolysis
is the only metabolic pathway common to all modem organisms and does not
require
02
(which was not abundant on early Earth).
Some
extant archaebacteria and other obligate anaerobes that live by fermentation
have
forms of nutrition believed to be similar to those of the original prokaryotes.
B. The Origin of Electron Transport Chains
and Cheiniosmosis
Chemiosmotic
ATP synthesis probably evolved in early prokaryotes as it is a common
mechanism
in all three domains.
Early
prokaryotes may have used the transmembrane pumps to help regulate their
internal
pH by expelling hydrogen ions produced by fermentation. Energy (ATP)
would
have been necessary to drive these pumps.
ATP may have been saved by the first electron
transport chains by coupling
oxidation of organic acids to the transport of
H+ out of the cell.
Some
bacteria may have evolved electron transport chains so efficient that more H+
was
extruded than was necessary for pH regulation. These cells could then utilize
the
influx of H+ to reverse the proton pump and generate ATP.
=> Some
modem bacteria use this form of energy metabolism (= anaerobic
respiration).
For
example, members of the genus Pseudomonas pass
electrons down transport
chains
from organic substrates to N03
Prokaryotes and the Origins of Metabolic Diversity 457
C. The Origin of
Photosynthesis
As
the supply of free ATP and abiotically produced organic molecules was depleted,
natural
selection may have favored organisms that could make their own organic
molecules
from inorganic resources.
Light
absorbing pigments in the earliest prokaryotes may have provided protection to
the
cells
by absorbing excess light energy, especially ultraviolet, that could be
harmful.
These
energized pigments may have then been coupled with electron transport
systems to power ATP synthesis.
Bacteriorhodopsin, the light‑energy
capturing pigment in the membrane of extreme
halophiles (a group of
archaebacteria), uses light energy to pump H+ out of the cell
to produce a gradient
of hydrogen ions. This gradient provides the power for
production of ATP.
This mechanism is being studied as a model
system of solar energy conversion.
Components
of electron transport chains that functioned in anaerobic respiration in other
prokaryotes
may have been co‑opted to also provide reducing power. For example, H2S
could
be used as a source of electrons and hydrogen for fixing C02,
The
nutritional modes of modem purple and green sulfur bacteria are believed the
most similar to early prokaryotes.
The
colors of these bacteria are due to bacteriochlorophyll, their main
photosynthetic
pigment.
D. Cyanobacteria,
the Oxygen Revolution, and the Origins of Cellular Respiration
Eventually,
some prokaryotes evolved that could use H20 as the electron source. Thus
evolved
cyanobacteria, which released oxygen.
Cyanobacteria evolved between 2.5 and 3.4
billion years ago.
They
lived with other bacteria in colonies that resulted in the formation of the
stromatolites. (See Campbell, Figure 24.3.)
Oxygen
released by photosynthesis may have first reacted with dissolved iron ions to
precipitate
as iron oxide (supported by geological evidence of deposits), preventing
accumulation
of free 02‑
Precipitation of iron oxide would have
eventually depleted the supply of dissolved
iron and 02 would have accumulated in the seas.
As
seas became saturated with 02, the gas was released to the atmosphere.
As
02 accumulated, many species became extinct while others survived in anaerobic
environments
(including some archaebacteria) and others evolved with antioxidant
mechanisms.
Aerobic
respiration may have originated as a modification of electron transport
chains
used in photosynthesis. The purple non‑sulfur bacteria are
photoheterotrophs
which
still use a hybrid electron transport system between a photosynthetic and
respiratory
system.
Other
bacterial lineages reverted to chemoheterotrophic nutrition with electron
transp
ort chains adapted only to aerobic respiration.
All major forms of
nutrition evolved among prokaryotes before the first eukaryotes arose.
458 Prokaryotes and the Origins of Metabolic
Diversity
VI. Molecular
systematics is leading to a phylogenetic classification of prokaryotes
The use of
molecular systematics (especially ribosomal RNA comparisons) has shown that
prokaryotes
diverged into the archaebacteria and eubacteria lineages very early in
prokaryotic
evolution.
Studies of r1bosomal
RNA indicate the presence of signature
sequences.
Signature sequences =
Domain‑specific base sequences at comparable locations in
ribosomal RNA or other nucleic acids.
Numerous other
characteristics differentiate these two domains. (See Campbell, Table
25.2.)
A somewhat surprising
result of these types of studies has been the realization that the
archaebacteria have at
least as much in common with the eukaryotes as they do with the
eubacteria.
A. Domain
Archaea (Archaebacteria)
Some unique
characteristics of archaebacteria include:
Cell walls lack peptidoglycan.
Plasma membranes have a unique lipid
composition.
RNA
polymerase and ribosomal protein are more like those of eukaryotes than of
cubacteria.
The archaebacteria
inhabit the most extreme environments of the Earth. Studies of these
organisms have
identified three main groups:
1. Methanogens are
named for their unique form of energy metabolism.
Use H2 to
reduce C02 to CH4 and are strict anaerobes.
Some
species are important decomposers in marshes and swamps (form marsh
gas) and some are used in sewage treatment.
Other
species are important digestive system symbionts in termites and
herbivores that subsist on cellulose diets.
2. Extreme halophiles inhabit high salinity
(15‑20%) environments (e.g. Dead Sea).
Some
species simply tolerate extreme salinities while others require such
conditions.
They
have the pigment bacteriorhodopsin in their plasma membrane which
absorbs light to pump H+ ions out of the cell.
This pigment is also responsible for the purple‑red
color of the colonies.
3. Extreme thermophiles inhabit hot
environments.
Live in habitats of 60 ‑ 80'C.
One
sulfur‑metabolizing thermophile inhabits water of 105'C near deep sea
hydrothermal vents
Prokaryotes and the Origins
of Metabolic Diversity 459
B. Domain Bacteria (Eubacteria)
The
major groups of eubacteria include a very diverse assemblage of organisms.
Among
the
thousands of known species are forms which exhibit every known mode of
nutrition
and
energy metabolism.
Molecular
systematics has provided an increased understanding of the once hazy
relationships
among members of this taxon. While most prokaryotic systematists
recognize
a dozen groups of eubacteria, the following figure shows the relationship of
five
with
the other domains.
Domain
BACTERIA (Eubacteria)
Gram
Positive
Cyano‑ Proteo
Spirochetes Chlamydias Bacteria bacteria
bacteria
Earliest
Prokaryotes
1. Proteobacteria
Domain Domain7
F‑do‑m
ARCHAEA
(Archeabacteria) JEUKARYAJ
i
Extreme Extreme
Methanogens halophiles thennophiles Eukaryotes
The most diverse group of
bacteria, containing 3 main subgroups:
9 Purple bacteria
=> Photoauto‑
and photolieterotrophic organisms with bacteriochlorophylls
built into plasma membrane invaginations.
=> Extract electrons from molecules other than
H20 (i.e. H2S), thus they release
no
oxygen.
=> Most
are obligate anaerobes found in sediments of ponds, lakes, and mud
flats.
=> Many species are flagellated.
Chemoautotrophic
proteobacteria
=::> Includes both free‑living and
symbiotic species.
=> Many play key roles in the nitrogen cycle,
including nitrogen fixation.
=:> Example: Rhizobiurn
Chemoheterotrophic
proteobacteria
=:> Enteric bacteria found in the intestinal
tracts of animals.
=:> Most are rod‑shaped
facultative anaerobes.
=> Examples: E.
coli and Salmonella.
460 Prokaryotes
and the Origins of Metabolic Diversity
2. Gram‑positive eubacteria
Most are gram‑positive
while a few are gram‑negative. Includes many
photosynthetic
members, but most are chemoheterotrophs. Many form endospores.
Mycoplasmas
=> Smallest of all known cells, 0. 10 ‑
0.25 ~Lm.
=> Only eubacteria that lack cell walls.
=> Common in soil and some are
pathogenic (i.e. Mycoplasmapneumoniae)
Actinomycetes
=> Soil bacteria that form branching
colonies which resemble fungi.
=> Many are important sources of
antibiotics (i.e. Streptomyces).
3. Cyanobacteria
Photoautotrophs
with plantlike photosynthesis; possess chlorophyll a and use two
photosystems to
split water and yield 02 as a product.
Most
species inhabit fresh water, but some are marine and others form symbiotic
relationships with fungi (lichens).
Cell walls are often thick and gelatinous.
Motile forms move by gliding.
Many
species are single‑celled forms, others are colonial, and some are truly
multicellular with a division of labor between
specialized cells.
4. Spirochetes
Helical
cells which are sometimes very long (up to 0.25 mm) but very thin.
Internal flagellar filaments function in corkscrew‑like movements.
Chemoheterotrophs which include both free‑living
species and pathogens.
=> Includes Treponema pallidum (causes syphilis) and Borrelia burgdorferi
(causes Lyme disease).
Chlamydias
Obligate
intracellular parasites of animals.
Obtain all of their ATP from host cells.
Have gram‑negative walls but lack
peptidogylcan found in other eubacteria.
Chlamydia trachomatis is the
most common cause of blindness in the world and
causes the most common sexually transmitted
disease (non‑gonococcal
urethritis) in the U.S.
Prokaryotes and the Origins
of Metabolic Diversity 461
VIL Prokaryotes
continue to have an enormous ecological impact
A. Prokaryotes and
Chemical Cycles
Prokaryotes are
critical links in the recycling of chemical elements between the biological
and physical components
of ecosystems ‑ a critical element in the continuation of life.
Decomposers =
Prokaryotes that decompose dead organisms and waste of live organisms.
Return
elements such as carbon and nitrogen to the environment in inorganic forms
needed
for reassimilation by other organisms, many of which are also prokaryotes.
Autotrophic bacteria =
Bacteria that fix C02, thus supporting food chains through which
organic nutrients pass.
Cyanobacteria
supplement plants in restoring oxygen to the atmosphere as well as fixing
nitrogen into
nitrogenous compounds used by other organisms.
Other
prokaryotes also support cycling of nitrogen, sulfur, iron and hydrogen.
B. Symbiotic Bacteria
Most
prokaryotes form associations with other organisms; usually with other bacterial
species
possessing complementary metabolisms.
Symbiosis =
Ecological relationships between organisms of different species that are in
direct
contact.
9 Usually the smaller
organism, the symbiont, lives
within or on the larger host.
Three Categories
of Symbiosis:
Mutualism =
Symbiosis in which both symbionts benefit.
For
example, nitrogen‑fixing bacteria in root nodules of certain plants fix
nitrogen
to be used by the plant, which in turn furnishes sugar and other nutrients
to
the bacteria.
Commensalism = Symbiosis in
which one symbiont benefits while neither helping nor
harming the other symbiont.
Parasitism = Symbiosis in
which one symbiont (the parasite)
benefits at the expense of
the host.
Symbiosis is believed
to have played a major role not only in the evolution of prokaryotes,
but also in the origin
of early eukaryotes.
C. Bacteria and Disease
About 1/2 of
human disease is caused by bacteria. To cause a disease, the bacteria must
invade the
host, evade or resist the host's internal defenses long enough to grow, and
harm
the host
Some pathogens
are opportunistic.
Opportunistic =
Normal inhabitants of the body that become pathogenic only when
defenses are weakened by other factors
such as poor nutrition or other infections.
For
example, Streptococcus
pneumoniae lives in the
throat of most healthy humans,
but
can cause pneumonia if the host's defenses are weakened.
While
Louis Pasteur, Joseph Lister and others began linking disease to pathogenic
microbes
in the late 1800s, Robert Koch was the first to determine a direct connection
between
specific bacteria and certain diseases.
Koch
identified the bacteria responsible for anthrax and tuberculosis, and his
methods
established the four criteria (Koch's postulates) used as guidelines in
medical
microbiology.
Koch's postulates =
Four criteria to substantiate a specific pathogen as the cause for a
disease are:
1. Find the same pathogen in each diseased
individual.
2. Isolate the pathogen from a diseased subject
and grow it in a pure culture.
3. Use cultured pathogen to induce the disease
in experimental animals.
4. Isolate the same pathogen in the diseased
experimental animal.
Some
pathogens cause disease by growth and invasion of tissues which disrupts the
physiology
of the host while others cause disease by production of a toxin. Two major
types
of toxins have been found:
Exotoxins =
Proteins secreted by bacterial cells.
Can cause disease wjJLW the organism
itself being present; the‑toxin ‑is, enough.
Among the most potent poisons known.
Elicits specific symptoms.
For example, botulism
toxin from Clostridium
botulinum and cholera toxin from
Vibrio
cholerae.
Endotoxins =
Toxic component of outer membranes of some gram‑negativ
* All induce the general symptoms of fever and
aches.
Examples
are Salmonella typhi (typhoid
fever) and other species of Salmonella
which
cause food poisoning.
Improved
sanitation measures and development of antibiotics have greatly reduced
mortality
due to bacterial diseases over the last half of the century.
Many
of the antibiotics now in use are produced naturally by members of the genus
Streptomyces. In
its natural habitat (soil) such materials would reduce competition
from
other prokaryotes.
Although
beneficial, the excessive and improper use of antibiotics has resulted in the
evolution
of many antibiotic‑resistant bacterial species which now pose a major
health
problem.
D. Putting
Bacteria to Work
Humans use the
metabolic diversity of bacteria for a multitude of purposes. The range of
these purposes
has increased through the application of recombinant DNA technology.
Pharmaceutical
companies use cultured bacteria to make vitamins and antibiotics.
More than half of the antibiotics used to treat
bacterial diseases corne from cultures
of various species of Streptomyces
maintained by pharmaceutical companies.
As simple models of life to learn about
metabolism and molecular biology. (E. coli
is
the best understood of all organisms.