Transcript Power Point Template - Online Textbook of Bacteriology

Bacterial Cell
Structure and
Function
Part 2: cell
envelope and
cytoplasmic
constituents
Cell Walls
• Cell wall is a
structure that
completely
surrounds the cell
protoplast.
• (Almost) all bacteria
have a cell wall.
Cell Walls
•
•
•
•
Why study bacterial cell walls?
They are essential structures in bacteria.
They are made of chemical components
found nowhere else in nature.
They may cause symptoms of disease in
animals.
They are the site of action of some of our
most important antibiotics.
Profile of the bacterial cell envelope
• Gram-positive cell wall is thick homogeneous
monolayer
• Gram-negative cell wall is thin heterogeneous
multilayer
Primary function of the bacterial cell wall
• To prevent
rupture or
osmotic lysis
of the cell
protoplast
Lysis of a pair of dividing E. coli cells
Chemical nature of bacterial cell walls
• Bacterial cell walls always
contain murein, which is a
type of peptidoglycan
• Chemical nature of murein
accounts for the function of
the cell wall
• Murein is only found in the
cell walls of bacteria
E. coli peptidoglycan
Chemical nature of bacterial cell walls
Peptidoglycan is made up of
• 2 amino sugars
N-acetyl-glucosamine = G
N- acetylmuramic acid = M
• 4 amino acids
L-alanine = L-ala
D-glutamic acid = D-glu
diaminopimelic acid = DAP
D-alanine = D-ala
Chemical nature of bacterial cell walls
Gram-negative murein. Murein is a polymer of the peptidoglycan
subunit. The sugars form the glycan backbone (G-M-G-M-etc.)
and the amino acids comprise the peptide side chains of the
molecule.
Chemical nature of bacterial cell walls
Penicillin prevents
formation of this
Interpeptide bond
Lysozyme breaks this
glycoside bond between
M and G
Gram-negative murein showing the sites of action of the
antibiotic penicillin and the enzyme lysozyme
Chemical nature of bacterial cell walls
Gram-positive murein has a thicker glycan a backbone and
there are interpeptide bridges that join amino acid side chains
together.
Chemical nature of bacterial cell walls
Penicillin blocks the
Insertion of the interpeptide bridge
Lysozyme breaks the
glycoside bond between
M and G
Gram-positive murein showing the sites of action of the antibiotic
penicillin and the enzyme lysozyme
Other characteristics of bacterial cell walls
Gram-positive cell walls contain teichoic acids
Teichoic acids are thought to stabilize the
Gram positive cell wall and may be used in adherence.
Other characteristics of bacterial cell walls
Gram-negative cell walls include an outer membrane
Other characteristics of bacterial cell walls
Outer membrane of Gram-negatives has two important
properties
1. It protects the cells from permeability by many
substances including penicillin and lysozyme.
2. It is the location of lipopolysaccharide (endotoxin) which
is toxic for animals.
Table: Correlation of the Gram stain with
properties of bacterial cell walls
Property
Gram-positive
Gram-negative
Thickness of wall
thick (20-80 nm)
thin (10 nm)
Number of layers
1
2-3
Peptidoglycan
(murein) content
>50%
10-20%
Teichoic acids in wall
present
absent
Protein/lipoprotein
content
Lipopolysaccharide
content
Sensitivity to penicillin
0-3%
>50%
0
13
sensitive
resistant
Sensitivity to lysozyme sensitive
resistant
Cell (cytoplasmic) membrane
• Completely encloses
the bacterial cell
protoplast
• Composed of 60%
protein and 40%
phospholipid
• Arranged as a bilayer
Section of a cytoplasmic membrane
Membrane structure and assembly
• The
membrane
bilayer is
formed by
phospholipid
molecules
made up of
glycerol and
fatty acids
Membrane structure and assembly
• Phospholipids
arrange
themselves
spontaneously
in water: lipid
“tails” inward;
glycerol “heads”
outward
Membrane structure and assembly
The fluid mosaic model of a membrane
The proteins associate with both sides of the
membrane, or may imbed in the membrane, or
pass through the membrane.
Membrane structure and assembly
Proteins in the
cytoplasmic
membrane have a
variety of functions
including transport
and energy
transformations
The cytoplasmic membrane of E. coli
Functions of the cytoplasmic membrane
• Osmotic or
permeability
barrier: the
membrane is
impermeable to
molecules that
are charged or
greater than
molecular weight
of 100
Functions of the cytoplasmic membrane
• Location of
transport
systems to
import all the
needed
molecules that
are charged or
greater than
molecular weight
100
Transport systems in bacteria
Functions of the cytoplasmic membrane
• Energy
generation:
location of the
electron
transport system
(ETS) and the
ATP synthsizing
enzyme ATPase
Functions of the cytoplasmic membrane
• Specialized
functions
involving cell
wall synthesis,
cell division and
DNA replication.
Cytoplasmic Constituents of
Bacterial Cells
• Cytoplasm
• Genetic material: chromosome and
Plasmids (DNA)
• Ribosomes
• Inclusions
Cytoplasm of bacterial cells is gel-like and contains the
chromosome, ribosomes,various macromolecules and small
molecules in water solution.
ribosomes
DNA (chromosome)
Small molecules present in a growing bacterial cell
Molecules
Approximate number of kinds
Amino acids, their precursors and derivatives
120
Nucleotides, their precursors and derivatives
100
Fatty acids and their precursors
50
Sugars, carbohydrates and their precursors or derivatives
250
quinones, porphyrins, vitamins, coenzymes and
prosthetic groups and their precursors
300
Ions (PO4, NH3, SO4, etc.)
20
The Bacterial Chromosome or “Nucleoid”
Bacterial DNA released from
a “gently lysed” E. coli cell
DNA (deoxyribonucleic acid) is the genetic material of the cell.
It can be replicated in a semiconservative fashion and passed
on to progeny cells.
Ribosome Structure and Composition
The procaryotic ribosome (L) is 70S in size, being
composed of a 50S (large) subunit and a and 30S (small)
subunit. The eucaryotic ribosome (R) is 80S in size and is
composed of a 60S and a 40S subunit.
Ribosome Structure and Composition
Ribosomes are made of two subunits, a large subunit and a
small subunit. Each subunit is made up of RNA and various
proteins.
Ribosome Function
Ribosomes function in protein synthesis. Amino acids are
assembled into proteins according to the genetic code on
the surfaces of ribosomes during the process of translation.
Some inclusions in Bacterial Cells
Inclusion
Composition
Glycogen
poly-glucose
Reserve carbon and
energy source
Poly-betahydroxybutyric
acid (PHB)
lipid
Reserve carbon and
energy source
Poly-phosphates
polymers of PO4
Reserve phosphate,
possibly high-energy PO4
Sulfur globules
elemental S
Reserve energy and or
electrons
Magnetosomes
magnetite (iron oxide)
Provide orientation in
magnetic field
Gas vesicles
protein shells inflated with
gases
Provide buoyancy in
aquatic environments
Parasporal crystals
protein
Produced by endosporeforming Bacilli - toxic to
insects
Function
Some inclusions in Bacterial Cells
Bacterial Inclusions. A. PHB granules; b. a parasporal BT crystal in the sporangium of
Bacillus thuringiensis; c. carboxysomes in Anabaena viriabilis, showing their polyhedral
shape; d. sulfur globules in the cytoplasm of Beggiatoa.
Endospores are produced as intracellular structures within the cytoplasm
of certain bacteria, most notably Bacillus and Clostridium species.
Endospore forming bacteria left to right: Clostridium botulinum, Bacillus brevis, Bacillus thuringiensis
Endospore formation is NOT a mechanism of reproduction. Rather it is a
mechanism for survival in deleterious environments. During the process of
spore formation, one vegetative cell develops into one endospore.
The sequential steps of endospore formation in a Bacillus species. The process of endospore
formation takes about six hours. Eventually the mature endospore is released from its “mother cell”
as a free spore
Free endospore
Endospore
within mother cell
Vegetative cell
Under favorable nutritional and environmental conditions, an
endospore germinates into a vegetative cell.
Medically-important
Endospore-forming Bacteria
•
•
•
•
•
Bacillus anthracis causes anthrax
Bacillus cereus causes food poisoning
Clostridium tetani causes tetanus
Clostridium botulinum causes botulism
Clostridium perfringens causes food
poisoning and gas gangrene
• Clostridium difficile causes antibiotic-induced
diarrhea and pseudomembranous colitis
Properties of Endospores
• Resting (dormant)
cells - “cryptobiotic”
i.e., show no signs
of life…..primarily
due to lack of water
in the spore
Properties of Endospores
• Several unique
surface layers not
found in vegetative
cells: exosporium,
spore coat, cortex,
and core wall
Properties of Endospores
• Highly resistant to
heat (boiling), acids,
bases, dyes ( don’t
stain) irradiation,
disinfectants,
antibiotics, etc.
Properties of Endospores
Parasporal crystal
Endospore
• Spores and parasporal crystals produced by
some bacteria are toxic to insects