Transcript Chapter 27

CHAPTER 27
PROKARYOTES AND THE ORIGINS OF
METABOLIC DIVERSITY
E. coli on surface of human skin
They’re (almost) everywhere!
• Prokaryotes were the
earliest organisms on
Earth and evolved alone
for 1.5 billion years.
Vibrio cholerae - Gram-negative,
facultatively anaerobic, causes
Asiatic cholera.
Current taxonomy recognizes
two prokaryotic domains:
domain Bacteria and domain
Archaea.
Domain Archae (Archaebacteria)
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Lack peptidoglycan in c.w.
•
More than one RNA poly-ase
•
Histones assoc. with DNA.
•
Introns in some genes
Halophilic bacterium
Domain: Archae
Methanogens: form CH4 from H2 and CO2 or
acetate. Requires anaerobic conditions (bottom of
pond, landfill, sewage digester)
Halophiles: require extremes in salt (> 10%),
contain pigment (bacteriorhodopsin) that can
absorb light and create H+ gradient.
Thermoacidophiles: found in geothermal springs
(high heat) and low pHs.
Alicyclobacillus spp. - an
acidophilic, thermophilic,
spore forming bacterium
Domain: Bacteria (Eubacteria)
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Contain peptidoglycan in c.w.
•
One type of RNA poly-ase.
•
Lack introns in genome.
•
Sensitive to antibiotics (streptomycin and
chlroamphinicol)
•
Ex. Purple bacteria, free-living, enteric,
mycoplasmas, actinomycetes (soil
bacteria), cyanobacteria, spirochetes,
chlamydias.
Structure and Morphology
• Most prokaryotes are
unicellular; some colonial
• In nearly all prokaryotes, a
cell wall maintains the shape
of the cell, affords physical
protection, and prevents the
cell from bursting in a
hypotonic environment.
• The most common
shapes among prokaryotes
are spheres (cocci), rods
(bacilli), and helices.
Staphylococcus
Streptococcus
Diplococcus
Nearly all prokaryotes have a cell wall
external to the plasma membrane
• Most bacterial cell walls contain peptidoglycan, a
polymer of modified sugars cross-linked by short
polypeptides.
• The walls of archaea lack peptidoglycan.
• Chemistry of cell wall allows for identification and
classification. Gram staining.
• Gram-positive bacteria have simpler cell walls,
with large amounts of peptidoglycans and only
one membrane. Stains purple.
• Gram-negative bacteria have more complex cell
walls and less peptidoglycan. Have a double
lipopolysaccharide membrane. Stains Pink.
• An outer membrane on the cell wall contains
lipopolysaccharides, carbohydrates bonded to lipids.
• Among pathogenic bacteria, gram-negative
species are generally more threatening than grampositive species.
• The lipopolysaccharides on the walls are often toxic
and the outer membrane protects the pathogens from
the defenses of their hosts.
• Gram-negative bacteria are commonly more resistant
than gram-positive species to antibiotics because the
outer membrane impedes entry of antibiotics.
• Many prokaryotes secrete another sticky
protective layer, the capsule, outside the cell wall.
• Composed of protein or polysaccharides.
• Capsules adhere the cells to their substratum.
• They may increase resistance to host defenses.
• They glue together the cells of those prokaryotes that
live as colonies.
• i.e. R and S strains of Griffith experiment.
• Another way for prokaryotes to adhere to one
another or to the substratum is by surface
appendages called pili (or fimbriae).
• Pili/fimbriae can fasten pathogenic bacteria to the
mucous membranes of its host.
• Formed from protein Pilin
• Sex Pili
Many prokaryotes are motile
• About half of all
prokaryotes are capable of
directional movement.
• The action of flagella,
scattered over the entire
surface or concentrated at
one or both ends, is the
most common method of
movement.
• The flagella of
prokaryotes differ in
structure and function
from those of eukaryotes.
• In a prokaryotic flagellum, chains of a globular protein
wound in a tight spiral from a filament which is attached to
another protein (the hook), and the basal apparatus.
• Rotation of the filament is driven by the diffusion of protons into
the cell through the basal apparatus after the protons have been
actively transported by proton pumps in the plasma membrane.
• A second motility mechanism is found in
spirochetes, helical bacteria.
• Two or more helical filaments under (internalized) the
cell wall are attached to a basal motor attached to the
cell.
• When the filaments rotate, the cell moves like a
corkscrew. Useful in thick fluids.
• A third mechanism occurs in cells that secrete a
jet of slimy threads that anchors the cells to the
substratum.
• The cell glides along at the growing end of threads.
• In a relatively uniform environment, a flagellated
cell may wander randomly.
• In a heterogenous environment, many prokaryotes
are capable of taxis, movement toward or away
from a stimulus.
• With chemotaxis, binding between receptor cells on
the surface and specific substances results in
movement toward the source (positive chemotaxis) or
away (negative chemotaxis).
• Other prokaryotes can detect the presence of light
(phototaxis) or magnetic fields (magnetotaxis).
Growth, reproduction and genetic
exchange
Endospore
•
Binary fission; Conjugation;
Transduction; Transformation
•
Endospore formation.
Resting structures that
withstand high heat, radiation,
desiccation, toxins. Allow
survival for hundreds of years.
Form during unfavorable
conditions. Low in water
Clostridium botulinum content (15% vs. 90% of
normal cell). Tough wall forms Causes botulism.
that is one of the toughest
biological structures.
Metabolic Diversity
• Nutritional modes
depend on how an
organism obtains
energy and a
carbon source
from the
environment to
build the organic
molecules of cells.
• Photoautotrophs are photosynthetic organisms that
harness light energy to drive the synthesis of organic
compounds from carbon dioxide.
• Among the photoautotrophic prokaryotes are the
cyanobacteria.
• Among the photosynthetic eukaryotes are plants and algae.
Cyanobacteria
• Chemoautotrophs need only CO2 as a carbon
source, but they obtain energy by oxidizing
inorganic substances, rather than light.
• These substances include hydrogen sulfide (H2S),
ammonia (NH3), and ferrous ions (Fe2+) among others.
Use these to form H+ gradient then ATP.
• This nutritional mode is unique to prokaryotes.
• Symbionts with tubeworms at hydrothermal vents
• Photoheterotrophs use light to generate ATP but
obtain their carbon in organic form.
• This mode is restricted to prokaryotes.
• Chemoheterotrophs must consume organic
molecules for both energy and carbon.
• This nutritional mode is found widely in prokaryotes,
protists, fungi, animals, and even some parasitic
plants.
The majority of known prokaryotes are
• These include saprobes,
chemoheterotrophs.
decomposers that absorb
nutrients from dead
organisms, and parasites,
which absorb nutrients
from the body fluids of
living hosts.
Lactobacillus sp.
• Some of these organisms
(such as Lactobacillus)
have very exacting
nutritional requirements,
while others (E. coli) are
less specific in their
requirements.
• Prokaryotes are responsible for the
key steps in the cycling of nitrogen
through ecosystems.
• Some chemoautotrophic bacteria
convert ammonium (NH4+) to nitrite
(NO2-).
• Others “denitrify” nitrite or nitrate
(NO3-) to N2, returning N2 gas to the
atmosphere.
• A diverse group of prokaryotes,
including cyanobacteria, can use
atmospheric N2 directly.
• During nitrogen fixation, they
convert N2 to NH4+, making
atmospheric nitrogen available to
other organisms for incorporation into
organic molecules.
Nitrosomonas spp
• Nitrogen fixing cyanobacteria are the most selfsufficient of all organisms.
• They require only light energy, CO2, N2, water and some
minerals to grow.
• The presence of oxygen has a positive impact on
the growth of some prokaryotes and a negative
impact on the growth of others.
• Obligate aerobes require O2 for cellular respiration.
• Facultative anerobes will use O2 if present but can
also grow by fermentation in an anaerobic
environment.
• Obligate anaerobes are poisoned by O2 and use either
fermentation or anaerobic respiration.
• In anaerobic respiration, inorganic molecules
other than O2 accept electrons from electron
transport chains.
•Gram-negative,
facultatively anaerobic,
rod.
Yersinia pestis
•Transmitted by the rat
flea to humans, it
caused the bubonic
plague.
•Gram-negative, tiny flagella at the
end of the cell.
•main cause of chronic superficial
gastritis, associated with both gastric
and duodenal ulcers
•lives in the interface between the
surface of gastric epithelial cells (the
lining of the stomach).
Helicobacter pylori