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 Cell membrane Lipid bilayer structure
Does not usually contain sterols
Slide 6
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
Slide 7
Bacterial cell structure Cell membrane Mesosome: Folded
invaginations in the cytoplasmic membrane Bind and pull apart
daughter chromosomes during cell division
Slide 8
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
Slide 9
Bacterial cell structure Cell membrane Transport of substances
1. Passive transport: (Diffusion; no energy) A. Simple diffusion:
Not selective; (eg, dissolved O 2, CO 2, and H 2 O) B. Faciliated
diffusion: Selective C. Channel proteins: Rare in prokaryotes;
selective channels passage of specific molecules (eg,
glycerol)
Slide 10
Bacterial cell structure Cell membrane Transport of substances
1. Passive transport: (Diffusion; no energy)
Slide 11
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
Slide 12
Bacterial cell structure Cell membrane Transport of substances
2. Active transport: A. Ion-coupled transport
Slide 13
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
Slide 14
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
Slide 15
Bacterial cell structure Cell membrane Transport of substances
2. Active transport: B. ATP-binding cassette (ABC) transport
Slide 16
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
Slide 17
Bacterial cell structure Cell membrane Transport of substances
3. Group translocation:
Slide 18
Bacterial cell structure Cell membrane Transport of substances
4. Special transport process: Iron (Fe): essential nutrient for
bacteria Siderophores transport Fe into the cell
Slide 19
Bacterial cell structure Cell membrane Transport of substances
4. Special transport process:
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 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)
Slide 25
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
Slide 26
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
Slide 27
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 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
Slide 34
Bacterial cell structure Cell wall Peptidoglycan synthesis
Transglycosylase, transpeptidase and carboxypeptidase enzymes are
called Penicillin-binding proteins (PBPs)
Slide 35
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
Slide 36
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 (Gly 5 ) cross-bridge
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)
Slide 41
Bacterial cell structure Cell wall Periplasmic space
Slide 42
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)
Slide 43
Bacterial cell structure Cell wall Outer membrane
Slide 44
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
Slide 45
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 Lipopolysaccharide
O-specific polysaccharide: Induces specific immunity (O-antigen)
Common core polysaccharide: Same in all Gram- negative bacteria
Lipid A: Responsible for primary toxicity O-specific
polysaccharide: Induces specific immunity (O-antigen) Common core
polysaccharide: Same in all Gram- negative bacteria Lipid A:
Responsible for primary toxicity
Bacterial cell structure External structures Capsule/slime
layer Distinct bacterial species Polysaccharide Capsule of Bacillus
anthracis Polypeptide (poly-D-glutamic acid)
Slide 52
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
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
Slide 58
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
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
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
Slide 64
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 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
Slide 68
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
Slide 69
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)
Slide 70
Bacterial cell structure Endospores (spores) Cannot be stained
with Gram (seen colourless) BacillusClostridium Spore
Slide 71
Bacterial cell structure Endospores (spores) Special staining
techniques (eg, malachite green stain) Bacillus Spore
Slide 72
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) Parts of spores
(from in to outwards): Core Spore wall Cortex Coat Exosporium
Core
Slide 75
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
Slide 76
Bacterial cell structure Endospores (spores) Spore wall Spore
wall: Contains normal peptidoglycan Becomes the cell wall of the
germinating vegetative cell
Slide 77
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
Slide 78
Bacterial cell structure Endospores (spores) Coat Coat:
Composed of a keratin-like protein Impermeable resistance of spores
against antibacterial chemical agents
Slide 79
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
Introduced by Hans Christian Joachim Gram Based on the differences
of Gram-positive and Gram-negative cell wall
Slide 82
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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