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Prokaryotes
• Bacteria were first
discovered in the late
1600’s by Antony van
Leeuwenhoek, using the
microscope he invented.
• The first recorded
observation were of the
bacteria found in the
dental plaque of two old
men who never cleaned
their teeth.
Prokaryote Introduction
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Prokaryotes are much more
diverse in both habitat and
metabolism than the eukaryotes.
However, prokaryotes are not very
diverse in body shape or size.
Much of their classification into
different species is done by
examining their internal
biochemistry and their DNA.
Nearly all prokaryotes are singlecelled. Differentiation into different
cell types almost never occurs in
prokaryotes.
Two major groups: the Eubacteria
(sometimes just called Bacteria)
and the Archaea (or
Archaebacteria). Very different
genetically.
Prokaryote Structure
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Prokaryotes are simple cells. The
DNA is loose in the cytoplasm—
there is no separate nucleus. The
ribosomes are also in the
cytoplasm. In prokaryotes,
transcription (synthesis of RNA)
and translation (synthesis of
proteins) occurs simultaneously.
The cell is surrounded by a
membrane, but there are no
internal membranes.
Outside the membrane is a cell
wall, and sometimes an outer
capsule which can have structures
projecting form it.
Bacteria move using flagella:
whip-like hairs similar to the
flagellum of a sperm cell.
Bacterial Reproduction
• Bacteria reproduce by the
process of binary fission.
The circular chromosome
replicates its DNA. Then,
the cell splits into 2
halves, each containing a
single chromosome
• No spindle apparatus (as
exists in eukaryotic
mitosis and meiosis).
Growth of Bacteria
• Under ideal conditions,
bacteria grow very rapidly:
some double in number every
20 minutes.
• Doubling in number: 1-2-4-816-… is exponential growth. It
starts off slowly, but once
going the number of bacteria
increase very rapidly
• Usually some nutrient runs
short, or waste material builds
up, and growth ceases.
Eventually a die-off occurs,
reducing the number of live
bacteria.
Genetic Exchange in Bacteria
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Bacteria don’t have sexes or a
regular genetic exchange every
generation the way most
eukaryotes do.
However, there are several means
of sharing DNA between
individuals, even if they are not of
the same species.
Conjugation is one such
mechanism: the donor bacteria
grows tubes that project from its
surface to the surface of a
recipient. A copy of the
chromosomal DNA travels through
this tube into the recipient, where
it become incorporated into the
recipient’s genome.
Bacterial Morphology
• Bacteria only take a few basic shapes, which are found in many
different groups. Bacterial cells don’t have internal cytoskeletons, so
their shapes can’t be very elaborate.
• Shape: coccus (spheres) and bacillus (rods). Spirillum (spiral) is less
common.
• Aggregation of cells: single cells, pairs (diplo), chains (strepto),
clusters (staphylo).
• Thus we have types such as diplococcus (pair of spheres) and
streptobacillus (chain of rods).
Gram Stain
• A major distinction between
groups of bacteria is based on
the Gram stain. In this
method, bacteria are treated
with the dye “crystal violet”,
then washed. Often a second
stain, “safranin” is applies to
make the unstained bacteria
visible.
• Gram stain causes bacteria
with a lot of peptidoglycan and
very little lipid in their cells
walls to stain purple. The
presence or absence of
peptidoglycan is a fundamental
biochemical difference
between groups of bacteria
Metabolic Diversity
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Bacteria show far more metabolic diversity than eukaryotes
General classification, based on carbon (food) source and energy source.
autotroph vs. heterotroph. Autotrophs make their own food from nonorganic sources (usually carbon dioxide). Heterotrophs use organic
compounds from other organisms.
phototroph vs. chemotroph. Phototrophs get their energy from sunlight.
Chemotrophs get their energy from chemical compounds.
Photoautotrophs get energy from sunlight and synthesize their own food
from scratch, like green plants.
Photoheterotrophs ( a rare category) get energy from sunlight but need
organic compounds made by other organisms.
Chemoautotrophs get energy from chemicals such as hydrogen gas,
hydrogen sulfide or ammonia, and they use carbon dioxide as the raw
material for their organic compounds.
Chemoheterotrophs get both energy and organic compounds from other
organisms. We are chemoheterotrophs.
Relationship to Oxygen
• For more than half of Earth’s history, oxygen
wasn’t present in the atmosphere. Many
bacteria evolved under anaerobic conditions.
• Classification:
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strict aerobes (need oxygen to survive)
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strict anaerobes (killed by oxygen)
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aerotolerant (don’t use oxygen, but survive it).
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facultative anaerobes (use oxygen when it is
present, but live anaerobically when oxygen is
absent).
Spores
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Some bacteria can form very tough spores,
which are metabolically inactive and can
survive a long time under very harsh
conditions.
Allegedly, some bacterial spores that were
embedded in amber for 25 million years
have been revived. Others, trapped in salt
deposits for up to 250 million years, have
also been revived. These experiments are
viewed skeptically by many scientists.
“Extraordinary claims demand
extraordinary proof”
Spores can also survive very high or low
temperatures and high UV radiation for
extended periods.
Panspermia: the idea that life got started on
Earth due to bacterial spores that drifted in
from another solar system. (However, it
still had to start somewhere!).
Archaea
• Sometimes called “Archaebacteria”
• Genetically as different from Eubacteria as we are.
• One distinguishing characteristic: cell membranes don’t contain fatty
acids, but instead use branched molecules called isoprenes.
• Three main type: methanogens, extreme halophiles, extreme
thermophiles.
Methanogens
• Methanogens: convert
hydrogen and carbon dioxide
into methane to generate
energy anaerobically.
Methanogens are obligate
anaerobes: they are killed by
oxygen.
• Methanogens digest cellulose
in cow and termite guts. Each
cow belches 50 liters of
methane a day. A major
greenhouse gas.
• Methanogens are also in
swamps, wetlands, and
garbage dumps.
Halophiles
• Extreme halophiles. Grow in
very salty conditions. Colorful
bacteria in seawater
evaporation beds, Great Salt
Lake.
• Mostly aerobic metabolism.
• Some have a form of
photosynthesis that uses
bacteriorhodopsin, a pigment
very similar to the rhodopsin
pigment in our eyes. It is also
called “purple membrane
protein”
Thermophiles
• Extreme thermophiles. Live at
very high temperatures: ocean
hydrothermal vents (up to 113o
C, which would be boiling
except for the high pressure
under the ocean), hot springs
in Yellowstone National Park.
• Use sulfur to generate energy
just like we use oxygen:
donate electrons to sulfur to
create hydrogen sulfide. Some
generate sulfuric acid
instead—they live at very low
pHs.
Eubacteria
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The most common types of bacteria
Many categories: we will just look at a few of
them.
Enteric bacteria live in the digestive tracts of
animals. Enterics are facultative anaerobes.
Best known example: Escherichia coli (E. coli),
found in the human gut and also used as a
common experimental organism in the lab.
Most E. coli strains are harmless, but a few
pathogenic (disease-causing) strains exist,
causing food poisoning. A common source is
ground meat, but it gets on unwashed
vegetables as well.
Related enteric bacteria: Salmonella, Shigella.
Cause food poisoning. Chickens carry
Salmonella in their guts instead of E. coli.
Pyogenic Cocci
• Pyogenic cocci. “Pyogenic” means
“pus-forming”. These bacteria produce
many of the worse infections.
• Staphylococcus aureus and Neisseria
gonorrheae are 2 examples:
Staphylococcus produces pneumonia,
toxic shock syndrome, strep throat,
meninginitis, and various skin diseases,
such as impetigo. Neisseria produces
gonorrhea, a common sexually
transmitted disease
• Staphylococcus bacteria normally live
on the skin and body cavities. Only
occasionally cause disease.
• Some strains of Staphylococcus are
resistant to all known antibiotics.
Endospore-forming Bacteria
• Most of these are in the genus
Bacillus (named after their
normal shape).
• Their spores are very resistant
to environmental conditions,
and may survive millions of
years before they revive.
• Anthrax is caused by a
Bacillus species. Also is this
family are the bacteria that
cause botulism (a very bad
form of food poisoning) and
tetanus (lockjaw--the muscles
go rigid).
Rickettsia and Chlamydia
• These bacteria are intracellular
parasites of eukaryotic cells.
They can’t survive outside a
cell. Sometimes they have
very reduced genomes: they
rely on host genes for critical
functions.
• Rickettsia live in the gut linings
of insects, and they infect
mammals through mosquito
and tick bites. They cause
diseases like Rocky Mountain
spotted fever, typhus, and Q
fever.
• Chlamydia infections are the
most common sexually
transmitted disease in the US.
Nitrifying and Nitrogen-fixing
Bacteria
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All proteins contain much nitrogen. The
atmosphere is 80% nitrogen. However, we
can’t directly use atmospheric nitrogen,
because it is in the wrong form: N2. We need
it in the ammonia form: NH3.
Nitrogen fixing bacteria are able to do this
conversion. Most of them live in root
nodules of certain plants, the legumes, such
as alfalfa and soybeans. Farmers plant
these crops to enrich their soil by naturally
adding ammonia to it.
The nitrogen-fixing bacteria live in the soil
and invade the nodules of young plants.
The nitrogen-fixing enzymes are poisoned by
oxygen. The root nodules function to keep
oxygen away from the bacteria.
Plants also need nitrogen in the form of
nitrate, NO3. Nitrifying bacteria convert
ammonia into nitrate.
Cyanobacteria
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A major group of photosynthetic bacteria
The oceans contain large amounts of
cyanobacteria (called plankton), that produce
much of Earth’s oxygen.
Cyanobacteria are the source of chloroplasts
in plant cells. They also have a symbiotic
relationship in lichens: a fungus and a
cyanobacteria provide each other with
shelter and food from photosynthesis.
Cyanobacteria form cell walls to fossilize—
among the oldest forms of life known.
Some have cell differentiation: they form
filaments in which some cells become
“heterocysts”, heavily walled cells that
perform nitrogen fixation for the other cells in
the filament.