Prokaryote Cell Structure
and Function
Background and Classification
Caulobacter
crescentus
Prokaryote Cells
No nuclear membrane
 No cellular organelles( membrane bound
organelles)
 Ribosomal size
 DNA
 RNA
 Size
 Cell wall and cell membrane

A New View of Life



Three Domains of life
Carl Woese responsible
for elucidating specific
DNA differences
between the
prokaryotes
Looked at the
relationship between
the organisms and
created a branching
tree( see chart)
Carl Woese



Studied the molecular biology of the
prokaryotes
Used 16s rRNA’s to create his Tree of Life this is interpreted as an evolutionary distance
between types of bacteria in terms of
differences in the 16s rRNA
Changes in 16s rRNA may be used as a
molecular chronometer or watch to convey
the time required to make changes in the
genes and proteins – ( Pauling 1965)
Parameters used in classification
DNA hybridization – homology of DNA
sequences – the use of probes( DNA and
m RNA)
 G+C content – DNA melting curves.
 DNA sequencing
 Protein homology
 Biochemical characteristics
 Molecular characteristics ( expression)

Prokaryote Domains
Similarities
 Bacteria and Archaea
have smaller ribosomes
( 70s)
 No membrane bound nucleus
 Generally one ds circular
chromosome- genomic DNA
( there are many exceptions)
 Many have plasmids
 Operon organization and gene
regulation mechanisms
Differences
 Cell wall differences between
Archaea and Eukarya –
Peptidoglycan
 Cell membane – ester linkage
versus ether linkage
 Ribosome sensitivity to
antibiotics
( chloramphenicol and
streptomycin
 Ribosomal sensitivity to
diptheria toxin
 RNA sequences
 RNA Polymerases
Archaea
Includes organisms regarded a
extremophiles
 Methanogens
 Halophiles
 Hyperthermophiles
 Nitrogen bacteria

Classification - Bacteria

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Proteobacteria – Five Classes – largest group. Very
diverse
Class I – Alpha proteobacteria – range from Nitrogen
fixing bacteria vital to recycling of Nitrogen to
pathogens like Rickettsiae
Class II – ( Betaproteobacteria) includes Neisseria
species ( gonnorheae and meningitidis )
Class III( Gammaproteobacteria) includes – E. coli,
Salmonella, Shigella, and other pathogens
Class IV – Organisms that are unique – Bdellovibrio
that devours gram negative bacteria
Class V – Includes Campylobacter and Helicobacter
pylori
Gram Positive Bacteria –
High G +C content
Actinomyces – Bacteria that are found
in the environment
 Mycobacterium, actinomyces, and
streptomyces
 Streptomyces and actinomyces are soil
bacteria with unusual characteristics
that have contributed to antibiotic
therapy ( Selman Waksman – Rutgers)

Spirochetes


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Unique organisms –
Treponema pallidum
Borrelia
Leptospira
Gram Positive Bacteria – Low G-C
content
Gram positive organisms
 Medically important
 Clostridium
 Mycoplasma


Bacilli, Enterococcus, and Streptococcus
Prokaryote – Cell Size


The size of bacteria
ranges from 0.1 to about
600 µm over a single
dimension
They are as small as the
largest viruses to large
enough for single cells
to be visible by the
naked eye


Mycoplasmas are about
the size of a virus with
the diameter of 0.3 µm
E. coli is a more typical
bacterium with
dimensions of 1.1-1.5 µm
wide by 2.0-6.0 µm in
length.
The range in size


Largest greater than
50 μm in diameter
Smallest less than
.3 μm
From ultra to nano
Epulopiscium fishelsoni
Nanobacteria
Shapes of bacteria
Rods
Curved spirochetes
Cocci
The Prokaryote Cell
Prokaryote Cell Structures
Prokaryote Cell Ultrastructure
Cell Wall


Rigid structure that lies just
outside the plasma membrane
Maintains shape, protects the
membrane, and regulates
transport
Basic Molecular components of
the cell wall
Peptidoglycan is a complex polymer of
sugars and amino acids
 The peptidoglycan that is unique to
bacteria is murein.
 The fact that murein is unique has made
it a target of antibiotics( an entire
class) that inhibits the synthesis of the
wall. ( Beta lactams which includes
penicillin)

The basic structure

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Glycan sugar chains linked by
peptides.
N-acetyl glucosamine
( NAG)
and N- acetyl-muramic acid(
NAM)
Linked by four peptide –
third is lysine
Cross – Linked with glycines
This structure is similar
throughout the Domain
bacteria but has variable
chemical properties in
different species
Peptidoglycan

This structure(
compared to the chain
mail of medieval
soldiers) covers the
outer surface of the
bacterial cell. This
determines the shape of
the bacterium for
instance coccus or
bacillus
Additional Cell Wall component

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An actin like protein has been found
underlying bacterial cell walls.
Cytoskeletal elements were previously
thought to be absent from bacterial cells
These proteins have been found in gram
negative bacteria
This new research indicates that the origin of
the eukaryote cell cytoskeleton may be of
prokaryote origin.
The Two Major Types of
Bacterial Cell Walls

Bacteria are divided into two major
groups based on the response to Gramstain procedure.
gram-positive bacteria stain purple
 gram-negative bacteria stain pink


staining reaction due to cell wall
structure
Teichoic Acid


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
Teichoic acids are
found in Gram
Positive Cell Walls
Polymers of glycerol
or ribitol joined by
phosphate groups
Polymers of 30 long
Extend beyond the
cell wall
Comparison of cell wall structure


The Gram Positive cell wall
is characterized by a thick
layer of Peptidoglycan.
This causes the bacterium
to stain purple with the
Gram Stain
The Gram Negative cell
wall has a layer of lipids
overlying the
Peptiodglycan layer which
is much thinner.
This results in a pinkish
color upon staining.
Gram Stain Technique
1.
2.
3.
4.
5.
6.
7.
8.
Make a smear( spread across the surface of
the slide
Air dry smear
Heat fix
Cover smear with Crystal violet – 1 minute
( gram positive) – purple and rinse
Iodine( mordant) – 1 minute and rinse
Alcohol( decolorizer) – seconds and rinse
Saffranin – gram negative – pink – 1 minute
and rinse
Gram Staining

Thought to involve constriction of the
thick peptidoglycan layer of grampositive cells


constriction prevents loss of crystal violet
during decolorization step
Thinner peptidoglycan layer of gramnegative bacteria does not prevent loss
of crystal violet
Gram Positive
Gram Positive
Gram Negative
Gram Negative
The Outer LPS Lipopolysaccharide

consist of three
parts



lipid A
core polysaccharide
O side chain (O
antigen)
Characteristics of the Gram
Negative Cell Wall
Protection from host defenses (O
antigen)
 Contributes to negative charge on cell
surface (core polysaccharide)
 Helps stabilize outer membrane
structure (lipid A)

LPS

Lipid A is an unusual glycolipid composed
of a disaccharide with attached sortchain fatty acids and phosphate groups.
This is linked to fever and shock
invertebrates and is an endotoxin
LPS
The core –A short series of sugars
attached to Lipid A
 The O antigen is a long carbohydrate
chain up to 40 sugar residues in length
which is bound to the core.
 The hydrophilic carbohydrate chains of
the O antigen exclude hydrophobic
compounds

Connections
Braun’s lipoproteins connect outer
membrane to peptidoglycan
 Adhesion sites

sites of direct contact (possibly true
membrane fusions) between plasma
membrane and outer membrane
 substances may move directly into cell
through adhesion sites

O antigen and importance
The O antigen is highly immunogenic. It
elicits a strong antibody response when
introduced when introduced into a
vertebrate host.
 E coli 157:H7 is the pathogenic form of
E. coli as compared to a commensal in
the gut. This is considered to be a
virulence factor.

LPS - significance

More permeable than plasma membrane
due to presence of porin proteins and
transporter proteins
Porin proteins form channels through which
small molecules (600-700 daltons) can pass
 These proteins and their channels are of
great complexity
 Larger molecules are translocated by
specialized protein complexes

Periplasmic space
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The two cell wall structures create an internal
compartment is the periplasm
This compartment contains degradative
enzymes such as nucleases, proteases, and
phosphatases
Binding proteins that have a high affinity for
amino acids and sugars are also present
It is space that contains the Beta lactamases
that degrade antibiotics so that they cannot
interfere with the cell wall synthesis
Function of LPS and cell wall

Osmotic lysis
Can occur when cells are in hypotonic
solutions
 Movement of water into cell causes swelling
and lysis due to osmotic pressure


Cell wall protects against osmotic lysis
Plasmolysis and Lysis

Plasmolysis

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

useful in food
preservation
e.g., dried foods and
jellies


Osmotic lysis

basis of lysozyme and
penicillin action
Osmotic lysis

can occur when cells
are in hypotonic
solutions
movement of water
into cell causes
swelling and lysis due
to osmotic pressure
Cell wall protects
against osmotic lysis
protoplast – the absence ot cell walls in
gram-positive
spheroplast – the absence of a cell wall
in gram-negative
Acid Fast Cell Wall
http://student.ccbcmd.edu/courses/bio141/lecguide/unit1/prostru
ct/afcw.html
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Mycobacterium tuberculosis is a
pathogen that has a different
solution
Their cell walls contain waxes
known as mycolic acids
These molecules are arranged in
two layers( hydrophilic tails
between them)
These are attached to the
Peptidoglycans cell wall and form
thick layers around the exterior
Proteins are interspersed within
and enable nutrients to pass
through
Acid Fact Bacteria



Acid fast stain –
The outer covering
is unaffected by
hydrochloric acid
which resulted in the
name, acid fast
Acid fast bacilli
stain red due to
carbol fuschin
Characteristics of the acid fast
cell wall
Outer waxy layer resists phagocytes
and avoids the immune system
 The permeability to nutrients is minimal
so that growth is very slow
 Mycobacterium tuberculosis may divide
only once in 24 hours

Other variants

Mycoplasmas are bacteria that lack cell

Mycoplasma pneumoniae contain sterols
walls
in the membranes which protects
against swelling and lysis
 Despite the lack of a cell wall they are
able to survive in harsh environments
and elude the defenses of the human
body
L bacteria( discovered by Lister
Institute)
Some bacteria spontaneously lose their
ability to form the cell wall
 These are wall deficient strains – that
may lose their cell wall – sometimes due
to the treatment with antibiotics


Mycobacterium paratuberculosis –is a
bacterium associated with chronic and
debilitating Crohn’s disease
Archaeal Cell walls
Lack peptidoglycan
 Can be composed of proteins,
glycoproteins, or polysaccharides
 Hyperthermophiles – these are
extremophiles that can withstand
temperatures above boiling despite the
lack of a Peptidoglycan cell wall

S layers

S-layers
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Regularly structured layers of protein or
glycoprotein
Common among Archaea, where they may be the
only structure outside the plasma membrane
In some gram-positive bacteria, the S-layer is
external to the murein wall
In gram-negative bacteria, it is external to the
outer membrane
In both the S-layer is several molecules thick
S-layers
Basically protein molecules with
carbohydrates attached
 Resistant to proteolytic enzymes and
protein denaturing agents
 In the intestinal parasite,
Campylobacter jejuni protects against
phagocytosis
 These S layers protect against invasion
from bacteriophages

S- layer of Archaean
Functions of capsules, slime
layers, and S layers
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Protection from host defenses (e.g.,
phagocytosis)
Protection from harsh environmental
conditions (e.g., desiccation)
Attachment to surfaces
Protection from viral infection or predation
by bacteria
Protection from chemicals in environment
(e.g., detergents)
Motility of gliding bacteria
Protection against osmotic stress
Additional External
Characteristics Characteristics

Layers of material lying outside the cell
wall

Capsules
 usually
composed of polysaccharides
 well organized and not easily removed from cell

Slime layers
 similar
to capsules except diffuse, unorganized
and easily removed
Capsules
Capsules and slime layers
Nutritional environment may influence
the formation of the capsule or slime
layer
 Haemophilus influenza and
Streptococcus pneumoniae are
pathogenic with capsules due to their
ability to avoid phagocytic cells of the
immune system

Slime layers
This outer covering is a major
determinant in the colonization of a
niche
 Such is the case with the bacterium,
Streptococcus mutans, this allows it to
colonize the nooks and crannies of your
teeth to cause dental caries and
participate in a biofilm on the surface
of teeth

Glycocalyx

Glycocalyx
Network of polysaccharides extending
from the surface of the cell
 A capsule or slime layer composed of
polysaccharides can also be referred to as
a glycocalyx

The Nature of Membranes
Membranes are an absolute requirement
for all living organisms
 Plasma membrane encompasses the
cytoplasm
 Some procaryotes also have internal
membrane systems

Functions of Cell Membranes


Separation of cell from its environment
Selectively permeable barrier

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some molecules are allowed to pass into or out of
the cell
transport systems aid in movement of molecules
Location of crucial metabolic processes
Detection of and response to chemicals in
surroundings with the aid of special receptor
molecules in the membrane
Lipid Bilayer

Polar ends
interact with
water
 hydrophilic


Nonpolar ends
insoluble in water
 hydrophobic

Lipids and Proteins

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

Contains Phospholipids and
proteins
 lipids usually form a bilayer
 proteins are embedded in or
associated with lipids
Highly organized, asymmetric,
flexible, and dynamic
Bacterial cell membranes are
more similar to eukaryotes than
Archaea
They have ester linkages like
eukaryotes in their phopholipids
Cell Membrane Research


http://www.rxpgnews.com/article_4916.shtml
“The discovery also demonstrated that current textbooks use
the wrong type of bacterium as a model to explain a critical
biochemical step that most disease-causing bacteria use to make
their membranes, according to Charles Rock, Ph.D., a member of
the St. Jude Department of Infectious Diseases and senior
author of the paper. As bacteria grow in size or divide, they
must make additional membrane using a series of biochemical
reactions. The first step in this process is the transfer of a
fatty acid to a molecule called G3P. Bacteria then convert this
molecule into a variety of other molecules called phospholipids,
which are the building blocks of membranes.”
Archaeal Cell Membranes



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Contain unique lipids call isoprenoids. These
are arranged in bilayers
These are also linked to glycerol by an ether
linkage instead of an ether linkage
Some membranes are single layers – The
molecules are longer than phospholipids and
have glycerol molecules at both ends
http://www.sciencemag.org/cgi/content/abst
ract/293/5527/92
Cytoplasmic Matrix



Substance between
membrane and
nucleoid
Packed with
ribosomes and
inclusion bodies
Highly organized with
respect to protein
location
Specialized Internal Membranes

Complex in-foldings of the plasma
membrane
observed in many photosynthetic bacteria
and in procaryotes with high respiratory
activity
 may be aggregates of spherical vesicles,
flattened vesicles, or tubular membranes

Internal Membranes

Mesosomes

May be invaginations of the plasma
membrane
 possible




roles
cell wall formation during cell division
chromosome replication and distribution
secretory processes
May be artifacts of chemical fixation
process
The Nucleoid Region



•
Irregularly shaped
region
Location of chromosome
 usually 1/cell
Not membrane-bound
The nucleoid region has
been isolated and
analyzed
60% DNA, 30% RNA,
and 10% protein. It has
been stained with
Feulgen that
demonstrates the
presence of DNA
Nucleoid characteristics




If stretched out the DNA of E. coli would be
1000x times the length of the cell
The folding of the DNA – its packaging forms
the nucleoid( the result of proteins )
When bacterial cells undergo lysis – and the
interior contents of the cell are released, the
viscosity or thickness is due to the nucleoid
Due to the density of the nucleoid, the
transcription of DNA takes place at the
nucleoid and cytoplasmic interface
Bacterial chromosomes

The most common form of a bacterial
chromosome is a ds circular chromosome
Exceptions
Some procaryotes have > 1 chromosome
 Some procaryotes have chromosomes
composed of linear double-stranded DNA
 A few genera have membrane-delimited
nucleoids

Bacterial chromosomes
Circular chromosomes
The circular chromosomes have ends
that are protected due to the structure
 In the linear chromosomes of
prokaryotes the ends are protected by
hairpins or by binding proteins

Eyeing Bacterial Genomes
Bacterial chromosomes can range from
580,000 base pairs to 10 million base
pairs
 The cholera bacterium has two
dissimilar chromosomes while nitrogen
fixing bacterium have three( it is
somewhat of a mystery as to the
apportioning of these chromosomes
during cell division)

Eyeing Bacterial Genomes


All species of Borrelia have
linear chromosomes ranging
in size from 900,000 to
920,000 base pairs, with an
accompaniment of circular
and linear plasmids (some
species contain up to 20
different plasmids).
Between the linear
chromosome and array of
plasmids there is a high
degree of redundancy in the
genetic sequence.
Borrelia burgdorferi
Plasmids
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Antibiotic-resistance genes
Antibiotics production genes
Heavy Metal resistance
genes
Virulence genes
Tumorigenicity (in plants)
Fertility (transfer) genes
Toxin production
Restriction / Modification
Metabolism of hydrocarbons
Ribosomes



Complex structures
consisting of protein
and RNA
Sites of protein
synthesis
Smaller than eucaryotic
ribosomes


procaryotic ribosomes 
70S
eucaryotic ribosomes 
80S

S = Svedburg unit
Bacterial Ribosome
Small Sub Unit
 30S
 16S RNA
 21 proteins
Large Subunit
 50S
 23S & 5S RNAs
 31 proteins
Inclusions




Granules of organic or inorganic material.
Used for storage of a variety of substances
like phosphates and glycogen. Most of these
inclusion bodies are free in the cytoplasm.
Some inclusion bodies are enclosed by a thin
membrane. Examples of these include
carboxysomes and gas vacuoles.
The number of inclusion bodies varies with
the nutritional status of the cells
PHB

Poly- hydroxybutyrate ( PHB) contains
hydroxybutyrate molecules joined by
ester bonds between the carboxyl and
hydroxyl of adjacent molecules. These
are common in purple sulfur bacteria
and stain with Sudan black for light
microscopy. These granules serve as
storage reservoirs for glycogen and
sugars necessary for energy and
biosynthesis.
Inclusions in Cyanobacteria


Cyanophycin granules are found in Cyanobacteria.
They are large inclusion bodies composed of
polypeptides comprised of arginine and aspartic acid.
These store additional nitrogen for the bacteria.
Cyanobacteria, thiobacilli, and nitrifying bacteria,
organisms that reduce CO2 in order to produce
carbohydrates, possess carboxysomes containing an
enzyme used for CO2 fixation.
Enterosomes




In Salmonella and E. coli have internal
structures similar to carboxysomes
Enterosomes contain enzymes required for
the metabolism of certain molecules
The existence of these molecules may be due
to the necessity of dealing with toxic
molecules
Propanediol is a metabolite of fucose which is
a sugar found on the intestinal wall of
mammals that that can be degraded by
intestinal bacteria – This is one of the
molecules metabolized in enterosomes
Gas Vacuoles
• Purple and green
photosynthetic bacteria
as well as some other
aquatic bacteria contain
gas vacuoles. These are
aggregates of hollow
protein cylinders called
gas vesicles that are
permeable to
atmospheric gas,
enabling the organism to
regulate buoyancy.
Bacteria are able to
regulate the depth at
which they float to
regulate photosynthetic
activity
Volutin

Some bacteria produce
inorganic inclusion
bodies in their
cytoplasm, including
volutin granules that
store phosphate and
sulfur granules that
store sulfur. Volutin is a
source of phosphate for
DNA. Sulfur is used by
purple photosynthetic
bacteria that use
hydrogen sulfide as a
photosynthetic electron
donor.
Magnetosomes
• Some motile aquatic
bacteria are able to
orient themselves by
responding to the
magnetic fields of the
earth because they
possess magnetosomes,
membrane-bound
crystals of magnetite or
other iron-containing
substances that
function as tiny
magnets.
Magnetosomes
Movement of bacteria in a
magnetic field
External Structures
Fimbriae
 Pili
 Flagella

Pili
• Pili are appendages that
are larger than
fimbriae. Their
presence is determined
by genes on plasmids
called sex factors.
These structures
function in conjugation
which is a genetic
exchange occurring in
bacteria with these
appendages
Fimbriae
• Fimbriae are thin, hairlie projections
extending from the cell
wall in Gram – bacteria.
They are composed of
helical protein units and
designed for
attachment to the host
cell membranes(
mucous). They also may
contribute to types of
movement in some
bacteria.
Neisseria gonorrhea
Adhesion and colonization
An essential step in the successful
colonization and production of disease is
their ability to adhere.
 Bacterial molecules utilized for adhesion
belong to a class called adhesins
 Adhesins are proteins that are found in
folds on the bacterial surface or on a
pilus or a fimbriae

Example of an Adhesin in a
Pathogen

E. coli uses an adhesion on pili to bind to
the lining of the urinary tract to cause
infection of the kidney
 The adhesin is associated with a P pilus
regarded as vital to the adhesion
process
 Receptors on the host lining of the
urinary tract are used for this adhesive
phenomenon
Flagella Motility
http://www-micro.msb.le.ac.uk/video/motility.html
Arrangement of flagella
monotrichous – one flagellum
 polar flagellum – flagellum at end of cell
 amphitrichous – one flagellum at each
end of cell
 lophotrichous – cluster of flagella at one
or both ends
 peritrichous – spread over entire
surface of cell

Arrangement of Flagella
The three parts of the flagellum

3 parts
filament
 basal body
 hook

Structure of Bacterial Flagella
The filament
Hollow, rigid cylinder
 Composed of the protein flagellin
 Some prokaryotes have a sheath around
filament
 Flagellins are highly antigenic. They are
extremely rigid in nature

The hook and basal body







Hook
links filament to basal body
The hook is a short-curved structure slightly larger
than in diameter than the filament
The hook is curved
Basal body
Series of rings that drive flagellar motor
It is composed of 15 proteins that
aggregate to form a rod to which four rings are
attached
Gram positive and gram negative bacteria have
different attachments
Flagellar complexity
Gram Positive and Gram Negative



Differ in the construction of their rings or
basal body
Gram positive have an S and M ring- an inner
ring connected to the plasma membrane and
an outer ring connected to the peptidoglycan
cell wall
Gram negative have an S and M and an L and P,
The L associates with the LPS anda the P
associates with the peptidoglycan
Flagellar Synthesis
An example of self-assembly
 Complex process involving many genes
and gene products
 New molecules of flagellin are
transported through the hollow filament
 Growth is from tip, not base

Flagellar Synthesis
Flagellar Motion

flagellum rotates like a propeller
in general, counterclockwise rotation causes
forward motion (run)
 in general, clockwise rotation disrupts run
causing a tumble (twiddle)

Tumble and Run
Other Types of Motility

Spirochetes


axial filaments cause flexing and spinning
movement
Gliding motility
cells coast along solid surfaces
 no visible motility structure has been
identified

Chemotaxis
Movement towards a chemical
attractant or away from a chemical
repellant
 Concentrations of chemoattractants and
chemorepellants detected by
chemoreceptors on surfaces of cells

Chemotaxis
Positive chemotaxis – Left ring is
caused by bacteria consuming
the amino acid serine. The right
ring a less attractive aspartate
attracts fewer bacteria
Negative chemotaxis –
Increasing concentrations of
acetate are applied to disk –
see the increasing clear zone
from right to left – suggesting
movement away
Traveling toward and Attractant


Caused by lowering
the frequency of
tumbles
Traveling away
involves similar but
opposite responses
Chemoreceptors
Bacteria detect attractants and
repellants at the molecular level
 The chemosensing system consists of
proteins that may collect in the
periplasmic space or the plasma
membrane
 The receptors may be organized in
patches on the membrane

E. coli
Has four rceptors each of which
recognize serine, aspartate, maltose,
ribose, galactose , and dipeptides.
 These chemoreceptors are called
MCP’s methyl accepting proteins
 These are found on the ends of the rod
shaped bacillus

Complexity of reaction to stimuli




Receptor and molecule bind causing
conformational changes in the receptor that
are transmitted through the membrane
The CheA protein is phosphorylated using ATP
This provides a phosphate for the Che Y
The Che Y then interacts with FliM that is at
the base of the flagella and regulates
flagellar motion
Bacterial Endospores
formed by some bacteria
 dormant
 resistant to numerous environmental
conditions

heat
 radiation
 chemicals
 desiccation

Position of endospore
Resistance of endospore
Calcium (complexed with dipicolinic acid)
 Acid-soluble, DNA-binding proteins
 Dehydrated core
 Spore coat
 DNA repair enzymes

Electron Micrograph of
endospore




CW = Vegetative cell
wall
CP= Spore Coat
SC= Spore Cortex
EX= Exosporium
Sporogenesis
Normally commences when growth
ceases because of lack of nutrients
 Complex multistage process

Formation of the Vegetative CellSporulation or Sporogenesis


Complex, multistage
process
Commences in
response to
environmental
conditions such as a
lack of nutrients
Steps




The nuclear material forms
Inward folding of the cell membrane to
enclose part of the DNA and produce the
forespore septum
The membrane continues to grow and engulfs
the immature spore in a second membrane.
The cortex is then laid down in the space
between the two membranes and dipocolinic
acid and Calcium ions are accumulated
Sporulation continued
Protein coats are then formed around
the cortex
 Maturation of the spore occurs

Steps in Activation

Activation



Germination





prepares spores for germination
often results from treatments like heating
spore swelling
rupture of absorption of spore coat
loss of resistance
increased metabolic activity
Outgrowth

emergence of vegetative cell
Protein Secretion Systems in E.
coli
Protein Secretion in Prokaryotes
 numerous protein secretion pathways
have been identified
 four major pathways are:

Sec-dependent pathway
 type II pathway
 type I (ABC) protein secretion pathway
 type III protein secretion pathway

Protein Secretion – Sec
Dependent





Sec-Dependent Pathway
Also called general secretion pathway
Translocates proteins from cytoplasm across or into
plasma membrane
Secreted proteins synthesized as preproteins having
amino-terminal signal peptide
 signal peptide delays protein folding
 chaperone proteins keep preproteins unfolded
Translocon transfers protein and removes signal
peptide
E. Coli and Sec Dependent
Pathway
In E. coli the chaperones uesed for
transport are Sec B and the Signal
Recognition particle SRP.
 Sec B is found in Gram negative
bacteria and SRP is found in all
prokaryotes

Steps in protein secretion
Sec B binds to Sec A portion of the
translocon, which is the transport
machinery
 The preprotein is transferred to SecA
 The protein can be released by
hydrolysis of GTP
 After this has occurred the protein is
transferred through the membrane

Translocon



The bacterial trnaslocaon si composed of a
membrane protein complex called SecYEG,
SecA and other proteins
It is believed that his complex forms a
channel in the membrane through which the
protein passes
Energy is required for this process in the
form of ATP hydrolysis coupled with proton
motive force.( Archaea do not possess this
mechanism)
Structure of the Sec Dependent
Pathway

Sec Dependent
Pathway
ABC Transporters





Also called ABC protein secretion pathway
Transports proteins from cytoplasm across
both plasma membrane and outer membrane
Secreted proteins have C-terminal secretion
signals
Proteins that comprise type I systems form
channels through membranes
Translocation driven by both ATP hydrolysis
and proton motive force
Type II
Transports proteins from periplasmic
across outer membrane
 Present in Pseudomonas aeruginosa and

Vibrio cholera
Observed in some gram-negative
bacteria, including some pathogens
 Complex systems consisting of up to
12-14 proteins


most are integral membrane proteins
ABC Transporters

Type I
Type III and Secretion
Secretes virulence factors of gramnegative bacteria from cytoplasm,
across both plasma membrane and outer
membrane, and into host cell
 Some type III secretion machinery is
needle-shaped


secreted proteins thought to move through
a translocation channel
Occurrence

Found in Salmonella, Pseudomonas,
Yersinia, Shigella, and E. coli
Contact between the bactgeria and the
host cells simtulates the process
 Low calcium levels may be required for
secretion

Type III and virulence factors



Type III Secretion
Pathway
Four different types of
proteins
The secretory portion,
the regulators, the
proteins that aid in the
insertion of secreted
proteins, and effectors
that alter host function
Examples of Type III
Cytotoxins
 Phagocytosis inhibitors
 Stimulators for reorganization of the
cytoskeleton
 Apoptosis promoters
