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Bacterial Cell Structure
Assist. Prof. Emrah Ruh NEU Faculty of Medicine Department of Medical Microbiology
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Bacterial cell structure
Cytoplasmic structures Nucleoid Ribosome Cytoplasmic membrane Cell wall External structures
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Bacterial cell structure Nucleoid
Prokaryotes: No true nucleus; no nuclear membrane, no nucleolus Bacterial chromosome: Single, double- stranded circle found in the nucleoid
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Bacterial cell structure Ribosome
Prokaryotic cell: 70S ribosome (30S + 50S subunits) Eukaryotic cell: 80S ribosome (40S + 60S subunits)
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Bacterial cell structure Cell membrane
Lipid bilayer structure Does not usually contain sterols
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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
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Bacterial cell structure Cell membrane
Mesosome: Folded invaginations in the cytoplasmic membrane Bind and pull apart daughter chromosomes during cell division
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Bacterial cell structure Cell membrane – Transport of substances
Passive transport Simple diffusion, faciliated diffusion, channel proteins Active transport Ion-coupled transport, ATP-binding cassette (ABC) transport Group translocation Special transport process
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Bacterial cell structure Cell membrane – Transport of substances
Passive transport: (Diffusion; no energy) Simple diffusion: Not selective; (eg, dissolved O2, CO2, and H2O) Faciliated diffusion: Selective Channel proteins: Rare in prokaryotes; selective channels passage of specific molecules (eg, glycerol)
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Bacterial cell structure Cell membrane – Transport of substances
Passive transport: (Diffusion; no energy)
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Bacterial cell structure Cell membrane – Transport of substances
Active transport: Ion-coupled transport: Uniport: single transport of a solute Symport: cotransport of a solute and H+ in same direction Antiport: transport of two similar solutes in opposite directions
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Bacterial cell structure Cell membrane – Transport of substances
Active transport: Ion-coupled transport
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Bacterial cell structure Cell membrane – Transport of substances
Active transport: ATP-binding cassette (ABC) transport: Uses ATP Binding proteins Gram-negative bacteria periplasmic space Gram-positive bacteria outer surface of the cell membrane
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Bacterial cell structure Cell membrane – Transport of substances
Active transport: 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
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Bacterial cell structure Cell membrane – Transport of substances
Active transport: ATP-binding cassette (ABC) transport
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Bacterial cell structure Cell membrane – Transport of substances
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
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Bacterial cell structure Cell membrane – Transport of substances
Group translocation:
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Bacterial cell structure Cell membrane – Transport of substances
Special transport process: Iron (Fe): essential nutrient for bacteria Siderophores transport Fe into the cell
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Bacterial cell structure Cell membrane – Transport of substances
Special transport process:
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Bacterial cell structure
Cytoplasmic structures Nucleoid Ribosome Cytoplasmic membrane Cell wall External structures
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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
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Bacterial cell structure Cell wall
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Bacterial cell structure Cell wall
Gram-positive bacteria Peptidoglycan Teichoic acid Lipoteichoic acid Gram-negative bacteria Peptidoglycan Periplasmic space Outer membrane Proteins Lipopolysaccharide
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Bacterial cell structure Cell wall – Peptidoglycan
Gram-positive bacteria: Thick peptidoglycan; ~40 sheets (50% of the cell wall) Gram-negative bacteria: Thin peptidoglycan; or 2 sheets (5-10% of the cell wall)
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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
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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
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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.
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Bacterial cell structure Cell wall – Peptidoglycan
Lysozyme treatment – protoplasts
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Bacterial cell structure Cell wall – Peptidoglycan synthesis
Glycan portion: N-acetylglucosamine (GlcNAc, NAG) N-acetylmuramic acid (MurNAc, NAM) Peptide portion: Tetrapeptide side chains Peptide cross-bridges
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Bacterial cell structure Cell wall – Peptidoglycan synthesis
Transglycosylation: NAM-NAG are linked together (b-1,4 glycosidic bond) Enzyme: Transglycosylase
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Bacterial cell structure Cell wall – Peptidoglycan synthesis
Transpeptidation: Peptide chains attach to N-acetylmuramic acid Enzyme: Transpeptidase
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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)
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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
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Bacterial cell structure Cell wall – Peptidoglycan synthesis
Transglycosylase, transpeptidase and carboxypeptidase enzymes are called “Penicillin-binding proteins (PBPs)”
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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
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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
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Bacterial cell structure Cell wall – Peptidoglycan synthesis
L-Ala D-Glu DAP / L-Lys D-Ala Gly5
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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
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Bacterial cell structure Cell wall – Teichoic acid / Lipoteichoic acid
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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)
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Bacterial cell structure Cell wall – Periplasmic space
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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)
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Bacterial cell structure Cell wall – Outer membrane
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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
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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!
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Bacterial cell structure Cell wall – Outer membrane
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Bacterial cell structure Cell wall – Lipopolysaccharide
Lipopolysaccharide (LPS) Endotoxin of Gram-negative bacteria
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Bacterial cell structure Cell wall – Lipopolysaccharide
O-specific polysaccharide: Induces specific immunity (O-antigen) Common core polysaccharide: Same in all Gram- negative bacteria Lipid A: Responsible for primary toxicity
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Bacterial cell structure
Cytoplasmic structures Nucleoid Ribosome Cytoplasmic membrane Cell wall External structures
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Bacterial cell structure External structures
Capsule/slime layer Flagella Fimbriae (pili)
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Bacterial cell structure External structures – Capsule/slime layer
Distinct bacterial species Polysaccharide Capsule of Bacillus anthracis Polypeptide (poly-D-glutamic acid)
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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
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Bacterial cell structure External structures – Capsule/slime layer
Protects bacteria against phagocytosis
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Bacterial cell structure External structures – Capsule/slime layer
Plays a role in adherence (biofilm formation) Artificial valves, catheters,…
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Bacterial cell structure External structures
Capsule/slime layer Flagella Fimbriae (pili)
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Bacterial cell structure External structures – Flagella
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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
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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
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Bacterial cell structure External structures – Flagella
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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 Gram- negative bacteria
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Bacterial cell structure External structures – Flagella
Gram-negative Gram-positive
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Bacterial cell structure External structures – Flagella
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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
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Bacterial cell structure External structures – Flagella
Chemotaxis: Moving toward (run, swim) food and away (tumble) poisons Rotation of the flagellum: Counterclockwise (run); clockwise (tumble)
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Bacterial cell structure External structures – Flagella
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Bacterial cell structure External structures
Capsule/slime layer Flagella Fimbriae (pili)
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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
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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
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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)
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Bacterial cell structure Endospores (spores)
Cannot be stained with Gram (seen colourless) Bacillus Clostridium Spore
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Bacterial cell structure Endospores (spores)
Special staining techniques (eg, malachite green stain) Bacillus Spore
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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
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Bacterial cell structure Endospores (spores) – Sporulation
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Bacterial cell structure Endospores (spores)
Parts of spores (from in to outwards): Core Spore wall Cortex Coat Exosporium Core
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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
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Bacterial cell structure Endospores (spores) – Spore wall
Contains normal peptidoglycan Becomes the cell wall of the germinating vegetative cell
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Bacterial cell structure Endospores (spores) – 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
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Bacterial cell structure Endospores (spores) – Coat
Composed of a keratin-like protein Impermeable resistance of spores against antibacterial chemical agents
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Bacterial cell structure Endospores (spores) – Exosporium
The outermost layer of spore envelope Composed of proteins, lipids, and carbohydrates Consists of a paracrystalline basal layer and a hairlike outer region
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Bacterial cell structure Classification
Gram-staining feature Gram-positive Gram-negative Morphology Coccus, bacillus, coccobacillus,…
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Bacterial cell structure Classification – Gram staining
Introduced by Hans Christian Joachim Gram Based on the differences of Gram-positive and Gram-negative cell wall
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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
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Bacterial cell structure Gram staining
Crystal violet Lugol (iodine) Alcohol Safranin (fuchsin)
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Bacterial cell structure Classification – Gram staining
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Bacterial cell structure Classification – Gram staining
Gram-positive Gram-negative
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Bacterial cell structure Classification – Morphology (basic)
Coccus spherical Bacillus rod-shaped Coccobacillus transition form ……
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Bacterial cell structure Classification – Morphology
Gram-positive cocci Gram-negative cocci
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Bacterial cell structure Classification – Morphology
Gram-positive bacilli Gram-negative bacilli
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Bacterial cell structure Classification – Morphology
Gram-negative coccobacilli
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