Bacteria - REMC 8 / Kent ISD Moodle VLE

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Transcript Bacteria - REMC 8 / Kent ISD Moodle VLE

Chapter 27
 All bacteria are prokaryotes - cells without a nuclear
 Bacteria are the most abundant organisms on Earth.
 The biomass of bacteria is significantly larger than all other
organism groups combined.
 Prokaryotes are the most ancient of all living things
The Structure of Prokaryotes
 1. Prokaryotic cells are smaller in size (1-10 μm) than
eukaryotic cells (10 - 100 μm). μ = "micro"
 2. Most prokaryotes are all strictly unicellular.
 3. Prokaryotes do not have a nucleus, nor do they have
histone proteins. Their DNA, which is circular, is free in
the cytoplasm.
 4. Cell division in prokaryotes is a simple binary fission
and does not involve spindles made up of microtubules.
 5. Prokaryotes lack the ability to form zygotes by way of
sexual reproduction (although some species may
exchange genetic information.)
 6. Prokaryotes lack membrane-bound organelles and lack
a cytoskeleton.
 7. Bacterial flagella are simple and composed of a single
fiber of the protein flagellin.(Eukaryotes have the complex
9+2 arrangement of microtubules.) These flagella are for
 8. The metabolic processes in prokaryotes are much
more varied than they are in eukaryotes. Only bacteria
can fix atmospheric nitrogen.
 9. There are no cytoplasmic organelles except ribosomes
in prokaryotes.
 10. Prokaryotic cells have a cell wall that is different in
structure from the cell wall of eukaryotic plant and fungal
cells. Most bacterial cell walls contain peptidoglycan
(not cellulose or lignin).
 11. Many prokaryotes have a sticky wall of
polysaccharides (a capsule) surrounding the cell wall.
 12. Many prokaryotes have hairlike appendages called
fimbriae that they use to stick to their substrate or to one
 13. Some prokaryotes have pili, appendages that pull
two bacterial cells together and promote the exchange of
Cell-Surface Structure of Bacteria
 Nearly all bacteria contain a cell wall to protect the cell,
maintain cell shape, and prevent it from bursting in a
hypotonic environment.
 Bacterial cell walls contain peptidoglycan, a polymer of
nitrogen-enforced sugars (NAG and NAM) that are crosslinked with short polypeptides
 Archae have cell walls composed of polysaccharides and
protein, but they lack peptidoglycan.
 The eubacteria that have peptidoglycan cell walls can be
separated into two categories of bacteria:
 (1) Gram-positive bacteria have simple walls with a thick
peptidoglycan layer.
 (2) Gram-negative bacteria have complex cell walls with 2
membranes surrounding a thin peptidoglycan layer.
• Eubacteria having thick
peptidoglycan cell walls readily
absorb crystal violet - a deep
purple cell dye - making them
Gram positive .
 •Gram positive bacteria are
highly susceptible to penicillin.
 •Penicillin affects the
bacterium's ability to form cell
walls by acting as an enzyme
inhibitor. Without a regulatory
cell wall, water rushes in and
the bacterium lyses.
 •It is easy to see the medical
benefit of the Gram stain. Gram
positive cells may be treated
with penicillin.
 •Other eubacteria have thin
peptidoglycan cell walls that
 do not absorb crystal violet these bacteria are
 Gram negative.
 •These bacteria are not
susceptible to penicillin.
 •The bacteria appear red
because when they are
 counterstained (after crystal
violet has been added
 and then rinsed), they absorb the
red counterstain.
Bacterial Motility
 •About 1/2 of bacteria demonstrate motility - purposeful
movement in response to stimuli.
 •What is taxis? Positive taxis? Negative taxis?
 •What is kinesis?
 •The most common locomotory structure is the flagellum.
 •Flagella are found arranged in a multitude of ways on the
bacterial body.
 •Bacterial and archael flagella operate in the same way,
but are made of different proteins.
 •They both function differently that eukaryotic flagella.
 •This evidence supports the idea that flagella developed
independently in all three domains. (ie. They are
analogous structures!)
The Internal Structure and
Organization of Bacteria
 •Bacteria lack internal membranes and
 •They can, however, infold their plasma
membrane and use it for ATP synthesis via
 •The DNA is much shorter (fewer genes), circular, and
largely lacks introns (some archae do have introns).
 •The DNA is restricted to an area called the nucleoid.
There is no nuclear envelope.
 •In addition to their single, circular chromosome, some
bacteria have accessory DNA called a plasmid. Each
 plasmid contains only a few genes.
 •Bacteria DO have ribosomes, although the ribosomes are
different in structure than eukaryotic ribosomes.
 •Bacterial ribosomes are smaller than eukaryotic
ribosomes, and have different protein and rRNA.
 •However, archae have rRNA that is more similar in
structure to eukaryotes than eubacteria, and this is the
principle basis for establishing closer relatedness between
the archae and eukaryotes than between eubacteria
 and eukaryotes.
Bacterial Reproduction
 •Bacteria reproduce by binary fission, not mitosis.
 •Optimal reproductive rates double populations every 1-3
 •Bacterial populations grow rapidly, exponentially.
 •Some
bacteria can withstand
hostile environmental
conditions by forming spores
around themselves.
•During spore formation, the
original bacterial cell produces
a copy of its chromosome and
surrounds it and a small bit of
cytoplasm with a protective
capsule. Most of the water
is removed, and the metabolic
machinery grinds to a halt.
When the original cell dies, the
spore is released.
•Spores can remain viable for
years, even decades and
Genetic Recombination In Bacteria
 •Bacteria populations demonstrate substantial genetic
variation because there are multiple mechanisms that
can lead to high variation within a bacterial population.
 •The most important of these mechanisms are (1)
mistake-making during replication and binary fission and
the mutation of the naked bacterial DNA, and (2)
bacterial recombintion.
Rapid Reproduction and Mutation
 •If an organism group
reproduces sexually, most
recombination is due to the
random union
 of gametes.
 •However, prokaryotes do not
reproduce sexually.
 •Because bacteria reproduce
so rapidly, even low mutation
rates during replication can
lead to substantial variation in
a bacterial population.
 •This rapid recombination can
lead to accelerated
Bacterial Recombination
 •There are three
mechanisms that occur
naturally in bacterial
populations that lead to
genetic recombination combining DNA in new
 1. Bacterial
 2. Bacterial
 3. Bacterial Conjugation
Bacterial Transformation
 •Bacterial transformation
occurs when bacteria uptake
foreign DNA from their
 •Often, the DNA that they
absorb from their surroundings
is bacterial DNA from dead
 •Bacteria are most likely to
absorb DNA that comes from
closely related species - they
have cell surface proteins that
recognize "similar species"
DNA, and they absorb it
 •Once inside the cell, the "new
DNA" can be incorporated into
the host DNA by homologous
 DNA exchange.
Bacterial Transduction
 •Phages (bacteria-infectious
viruses) carry bacterial genes
from one bacterium to another.
 •Transduction usually occurs
when a virus incorporates a bit
of bacterial DNA into its own
DNA during replication.
 •The virus with the host cell
DNA manages to transport this
bit of bacterial DNA to a new
host, but the host bacterium
survives and incorporates the
transducted DNA into its own
Bacterial Conjugation
 •During conjugation, DNA is
exchanged between two bacterial
cells (usually of the same species).
 •In this process, the movement of
DNA is one-way. There is a donor
bacterium and a recipient.
 •Exchange is facilitated by use of a
conjugation pilus.
 •The pilus pulls the two bacterial
cells together, and may serve as the
conduit through which bacterial DNA
moves from one cell to the other.
 •The ability to form pili and donate
DNA results from a piece of DNA on
the donor genome called the F
 •The F factor can reside in either the
bacterial DNA or the bacterium's
 If the F factor is part of plasmid DNA, a DNA donating
bacterium is designated as the F+ cell, and the recipient
cell is the F- cell during conjugation. Following
conjugation, the F- cell becomes an F+ cell.
 •If the F factor is found on the bacterial chromosome, the
cell containing the F factor is called an Hfr cell. (Hfr =
High frequency of recombination)
 •An Hfr cell serves as a donor during conjugation, and
the recipient cell is still called an F- cell.
 •When DNA from the Hfr cell enters, it is spliced into the
recipient DNA at its homologous site,
 replacing that portion of the recipient cell genome.