Download Bacterial Cell Structure

Bacterial Cell Structure
Assist. Prof. Emrah Ruh
NEU Faculty of Medicine
Department of Medical Microbiology
Bacterial cell structure
 Cytoplasmic structures
 Nucleoid
 Ribosome
 Cytoplasmic membrane
 Cell wall
 External structures
Bacterial cell structure
Nucleoid
 Prokaryotes:
 No true nucleus; no
nuclear membrane,
no nucleolus
 Bacterial
chromosome:
 Single, double-
stranded circle found
in the nucleoid
Bacterial cell structure
Ribosome
 Prokaryotic cell: 70S ribosome (30S + 50S
subunits)
 Eukaryotic cell: 80S ribosome (40S + 60S
subunits)
Bacterial cell structure
Cell membrane
 Lipid bilayer
structure
 Does not
usually contain
sterols
Bacterial cell structure
Cell membrane – Functions
 Selective permeability and transport of
solutes
 Electron transport and oxidative
phosphorylation
 Excretion of hydrolytic exoenzymes
 Functioning in DNA and cell wall synthesis
 Bearing the receptors of the chemotactic and
other sensory transduction systems
Bacterial cell structure
Cell membrane
 Mesosome:
 Folded invaginations in the cytoplasmic membrane
 Bind and pull apart daughter chromosomes during
cell division
Bacterial cell structure
Cell membrane – Transport of substances
1. Passive transport
•
Simple diffusion, faciliated diffusion, channel
proteins
2. Active transport
•
Ion-coupled transport, ATP-binding cassette
(ABC) transport
3. Group translocation
4. Special transport process
Bacterial cell structure
Cell membrane – Transport of substances
1. Passive transport: (Diffusion; no energy)
A. Simple diffusion: Not selective; (eg,
dissolved O2, CO2, and H2O)
B. Faciliated diffusion: Selective
C. Channel proteins: Rare in prokaryotes;
selective channels  passage of specific
molecules (eg, glycerol)
Bacterial cell structure
Cell membrane – Transport of substances
1. Passive transport: (Diffusion; no energy)
Bacterial cell structure
Cell membrane – Transport of substances
2. Active transport:
A. Ion-coupled transport:
I.
Uniport: single transport of a solute
II.
Symport: cotransport of a solute and H+ in
same direction
III. Antiport: transport of two similar solutes in
opposite directions
Bacterial cell structure
Cell membrane – Transport of substances
2. Active transport:
A. Ion-coupled
transport
Bacterial cell structure
Cell membrane – Transport of substances
2. Active transport:
B. ATP-binding cassette (ABC) transport:
 Uses ATP
 Binding proteins

Gram-negative bacteria  periplasmic space

Gram-positive bacteria  outer surface of the
cell membrane
Bacterial cell structure
Cell membrane – Transport of substances
2. Active transport:
B. ATP-binding cassette (ABC) transport:
 Bound substrate is transferred to a membrane-
bound protein complex
 Hydrolysis of ATP
 Energy  membrane pore open  movement of
substrate into the cell
Bacterial cell structure
Cell membrane – Transport of substances
2. Active transport:
B. ATP-binding cassette (ABC) transport
Bacterial cell structure
Cell membrane – Transport of substances
3. Group translocation:
 Uptake of certain sugars (glucose, mannose…)
 Not active transport
 Phosphotransferase system:
 Membrane carrier protein  phosphorylated
(phosphoenolpyruvate)  binds the free sugar 
transport into the cytoplasm: sugar phosphate
Bacterial cell structure
Cell membrane – Transport of substances
3. Group translocation:
Bacterial cell structure
Cell membrane – Transport of substances
4. Special transport process:
 Iron (Fe): essential nutrient for bacteria
 Siderophores  transport Fe into the cell
Bacterial cell structure
Cell membrane – Transport of substances
4. Special transport process:
Bacterial cell structure
 Cytoplasmic structures
 Nucleoid
 Ribosome
 Cytoplasmic membrane
 Cell wall
 External structures
Bacterial cell structure
Cell wall
 Distinguish Gram-positive from Gram-negative
bacteria
 Most prokaryotes  Peptidoglycan (murein)
layer
 Rigidity, and the shape of the bacterial cell
Bacterial cell structure
Cell wall
Bacterial cell structure
Cell wall
 Gram-positive
 Gram-negative
bacteria
bacteria
 Peptidoglycan
 Peptidoglycan
 Teichoic acid
 Lipoteichoic acid
 Periplasmic space
 Outer membrane
 Proteins
 Lipopolysaccharide
Bacterial cell structure
Cell wall – Peptidoglycan
 Gram-positive
bacteria:
 Thick peptidoglycan;
~40 sheets (50% of
the cell wall)
 Gram-negative
bacteria:
 Thin peptidoglycan;
1 or 2 sheets (5-10%
of the cell wall)
Bacterial cell structure
Cell wall – Peptidoglycan
 Peptidoglycan layer;
 Provides the strength to the bacterial cell
wall
 Provides the osmotic stability to the bacterial
cell
 Synonyms:
 Murein, mucopeptide
Bacterial cell structure
Cell wall – Peptidoglycan
 Lysozyme (enzyme in tears, saliva and nasal
secretions) degrades the peptidoglycan
 Hypotonic media  Osmotic pressure
differences  water flows into the cell 
cell lysis
Bacterial cell structure
Cell wall – Peptidoglycan
 Isotonic media:
 Gram-positive bacteria  Lysozyme 
Protoplasts
 Gram-negative bacteria  EDTA-lysozyme 
Spheroplasts
 If protoplasts/spheroplasts are able to grow
and divide, they are called L-forms.
Bacterial cell structure
Cell wall – Peptidoglycan
 Lysozyme treatment – protoplasts
Bacterial cell structure
Cell wall – Peptidoglycan synthesis
 Peptidoglycan:
 Glycan portion:
 N-acetylglucosamine (GlcNAc, NAG)
 N-acetylmuramic acid (MurNAc, NAM)
 Peptide portion:
 Tetrapeptide side chains
 Peptide cross-bridges
Bacterial cell structure
Cell wall – Peptidoglycan synthesis
 Transglycosylation:
 NAM-NAG are linked
together (b-1,4 glycosidic
bond)
 Enzyme:
Transglycosylase
Bacterial cell structure
Cell wall – Peptidoglycan synthesis
 Transpeptidation:
 Peptide chains attach to N-acetylmuramic acid
 Enzyme: Transpeptidase
Bacterial cell structure
Cell wall – Peptidoglycan synthesis
 Transpeptidation:
 Pentapeptide:
 L-Alanine (L-Ala)
 D-Glutamate (D-Glu)
 L-Lysine (L-Lys) / Diaminopimelic acid (DAP)
 D-Alanine (D-Ala)
 D-Alanine (D-Ala)
Bacterial cell structure
Cell wall – Peptidoglycan synthesis
 Transpeptidation:
 Pentapeptide:
 The terminal D-Alanine (D-Ala) from
pentapeptide is removed by transpeptidase
and carboxypeptidase enzymes
 In mature peptidoglycan, peptide chains are
tetrapeptides
Bacterial cell structure
Cell wall – Peptidoglycan synthesis
 Transglycosylase, transpeptidase and
carboxypeptidase enzymes are called
“Penicillin-binding proteins (PBPs)”
Bacterial cell structure
Cell wall – Peptidoglycan synthesis
 Transpeptidation:
 Gram-negative
bacteria:
 Diaminopimelic
acid of one
tetrapeptide and
terminal D-alanine
of second
tetrapeptide are
directly linked
Bacterial cell structure
Cell wall – Peptidoglycan synthesis
 Transpeptidation:
 Most Gram-positive
bacteria:
 L-lysine of one
tetrapeptide and
D-alanine of
second
tetrapeptide are
linked by
pentaglycine
(Gly5) cross-bridge
Bacterial cell structure
Cell wall – Peptidoglycan synthesis
L-Ala
D-Glu
DAP / L-Lys
D-Ala
Gly5
Bacterial cell structure
Cell wall – Teichoic acid / Lipoteichoic acid
 Gram-positive bacteria
 Common surface antigens (distinguish
bacterial serotypes)
 Attachment (adherence) to the host cell
 Important factors in virulence
 Teichoic acids  peptidoglycan
 Lipoteichoic acids  cytoplasmic membrane
Bacterial cell structure
Cell wall – Teichoic acid / Lipoteichoic acid
Bacterial cell structure
Cell wall – Periplasmic space
 Gram-negative bacteria
 The space between the inner and outer
membranes
 Contains the peptidoglycan layer
 Possess hydrolytic enzymes (proteases,
lipases,...) (breakdown of large molecules for
metabolism)
Bacterial cell structure
Cell wall – Periplasmic space
Bacterial cell structure
Cell wall – Outer membrane
 Gram-negative bacteria
 Bilayered structure:
 Inner leaflet  similar composition with the
cell membrane
 Outer leaflet  contains lipopolysaccharide
(LPS)
Bacterial cell structure
Cell wall – Outer membrane
Bacterial cell structure
Cell wall – Outer membrane
 Exclude hydrophobic molecules (unusual
feature among biological membranes!)
 Protect the bacterium (eg, digestive system)
 Connected to both the peptidoglycan layer
and the cytoplasmic membrane
 Lipoprotein: connects peptidoglycan with
outer membrane
Bacterial cell structure
Cell wall – Outer membrane
 Possess special channels (porins): passive
diffusion of low-molecular-weight
hydrophilic compounds (sugars, amino acids
and ions)
 Large antibiotic molecules penetrate slowly:
antibiotic resistance!
Bacterial cell structure
Cell wall – Outer membrane
Bacterial cell structure
Cell wall – Lipopolysaccharide
 Lipopolysaccharide (LPS)  Endotoxin of
Gram-negative bacteria
Bacterial cell structure
Cell wall – Lipopolysaccharide
 O-specific
polysaccharide:
 Induces specific
immunity (O-antigen)
 Common core
polysaccharide:
 Same in all Gramnegative bacteria
 Lipid A:
 Responsible for primary
toxicity
Bacterial cell structure
 Cytoplasmic structures
 Nucleoid
 Ribosome
 Cytoplasmic membrane
 Cell wall
 External structures
Bacterial cell structure
External structures
 Capsule/slime layer
 Flagella
 Fimbriae (pili)
Bacterial cell structure
External structures – Capsule/slime layer
 Distinct bacterial species
 Polysaccharide
 Capsule of Bacillus anthracis  Polypeptide
(poly-D-glutamic acid)
Bacterial cell structure
External structures – Capsule/slime layer
 Capsule  condensed
layer; closely surrounds
the bacterium
 Slime layer  loosely
adherent; nonuniform
in density and thickness
 Capsule/slime layer:
also called glycocalyx
Bacterial cell structure
External structures – Capsule/slime layer
 Protects bacteria against phagocytosis
Bacterial cell structure
External structures – Capsule/slime layer
 Plays a role in adherence (biofilm formation)
 Artificial valves, catheters,…
Bacterial cell structure
External structures
 Capsule/slime layer
 Flagella
 Fimbriae (pili)
Bacterial cell structure
External structures – Flagella
Bacterial cell structure
External structures – Flagella
 Thread-like appendages
 Composed of protein subunits called flagellin
 Subunits aggregate and form a helical
structure
 Highly antigenic (H antigens)
 If removed by mechanical agitating; new
flagella are rapidly formed and motility is
rapidly restored
Bacterial cell structure
External structures – Flagella
 Types of arrangements of flagella:
 Monotrichous
single polar flagellum
 Amphitrichous
single polar flagella –
opposite sites
 Lophotrichous
multiple polar flagella –
opposite sites
 Peritrichous
flagella distributed over
the entire cell
Bacterial cell structure
External structures – Flagella
Bacterial cell structure
External structures – Flagella
 Hook:
 Short curved structure
 Joint between the basal body and the
flagellum
 Basal body:
 Set of rings  1 pair (M-S ring) in Gram-
positive bacteria; 2 pairs (L-P, M-S) in Gramnegative bacteria
Bacterial cell structure
External structures – Flagella
Gram-negative
Gram-positive
Bacterial cell structure
External structures – Flagella
Bacterial cell structure
External structures – Flagella
 Electric field and
proton gradient across
the membrane:
Proton motive force
 Flow of protons
through the motor;
 From the periplasmic
space, to the cell
membrane and the
cytoplasm
Bacterial cell structure
External structures – Flagella
 Chemotaxis:
 Moving toward (run,
swim) food and away
(tumble) poisons
 Rotation of the
flagellum:
 Counterclockwise (run);
clockwise (tumble)
Bacterial cell structure
External structures – Flagella
Bacterial cell structure
External structures
 Capsule/slime layer
 Flagella
 Fimbriae (pili)
Bacterial cell structure
External structures – Fimbriae (pili)
 Gram-negative bacteria
 Shorter and finer than
flagella
 Composed of protein
subunits called pilins
 Adhesins: minor proteins
at the tips of pili;
responsible for
attachment
Bacterial cell structure
External structures – Fimbriae (pili)
 Ordinary pili:
 Adherence of bacteria to
host cells
 Sex pili:
 Attachment of donor and
recipient cells in
bacterial conjugation
Bacterial cell structure
Endospores (spores)
 Distinct bacterial genera;
the most commons:
 Bacillus (Gram-positive
aerobic rod)
 Clostridium (Gram-
positive anaerobic rod)
 Response to
environmental conditions
(depletion of nutrients)
Bacterial cell structure
Endospores (spores)
 Cannot be stained with Gram (seen colourless)
Bacillus
Spore
Clostridium
Bacterial cell structure
Endospores (spores)
 Special staining techniques (eg, malachite
green stain)
Bacillus
Spore
Bacterial cell structure
Endospores (spores)
 Sporulation  formation of spore
 Liberated when the mother cell undergoes
autolysis
 Resting cell; highly resistant to desiccation,
heat, and chemical agents
 Germination  favorable nutritional
conditions; activation of spore to produce a
single vegetative cell
Bacterial cell structure
Endospores (spores) – Sporulation
Bacterial cell structure
Endospores (spores)
 Parts of spores (from in
to outwards):
 Core
 Spore wall
 Cortex
 Coat
 Exosporium
Core
Bacterial cell structure
Endospores (spores) – Core
 The core: is the spore protoplast
 Contains chromosome
 Does not contain ATP (energy of germination
is stored as 3-phosphoglycerate)
 Heat resistance of spores;
 Dehydrated state
 Calcium dipicolinate found in the core
Bacterial cell structure
Endospores (spores) – Spore wall
Spore wall:
 Contains normal peptidoglycan
 Becomes the cell wall of the germinating
vegetative cell
Bacterial cell structure
Endospores (spores) – Cortex
Cortex:
 The thickest layer of the spore envelope
 Contains an unusual type of peptidoglycan
 Fewer cross-links
 Extremely sensitive to lysozyme
 Autolysis plays a role in spore germination
Bacterial cell structure
Endospores (spores) – Coat
Coat:
 Composed of a keratin-like protein
 Impermeable  resistance of spores against
antibacterial chemical agents
Bacterial cell structure
Endospores (spores) – Exosporium
Exosporium:
 The outermost layer of spore envelope
 Composed of proteins, lipids, and
carbohydrates
 Consists of a paracrystalline basal layer and
a hairlike outer region
Bacterial cell structure
Classification
 Gram-staining feature
 Gram-positive
 Gram-negative
 Morphology
 Coccus, bacillus, coccobacillus,…
Bacterial cell structure
Classification – Gram staining
 Introduced by Hans
Christian Joachim Gram
 Based on the differences
of Gram-positive and
Gram-negative cell wall
Bacterial cell structure
Classification – Gram staining
 Basic principle:
 Crystal violet gets trapped in the thick and
cross-linked peptidoglycan of Gram-positive
bacteria
 Gram-negative bacteria (thin peptidoglycan)
are easily decolorized by alcohol and do not
retain crystal violet
Bacterial cell structure
Gram staining
Crystal violet
Lugol (iodine)
Alcohol
Safranin (fuchsin)
Bacterial cell structure
Classification – Gram staining
Bacterial cell structure
Classification – Gram staining
Gram-positive
Gram-negative
Bacterial cell structure
Classification – Morphology (basic)
 Coccus
spherical
 Bacillus
rod-shaped
 Coccobacillus
 ……
transition form
Bacterial cell structure
Classification – Morphology
Gram-positive cocci
Gram-negative cocci
Bacterial cell structure
Classification – Morphology
Gram-positive bacilli
Gram-negative bacilli
Bacterial cell structure
Classification – Morphology
Gram-negative coccobacilli