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Chapter 3: Bacterial Structure

Bacterial DNA

• Structure: Typically a single, circular chromosome. Some bacteria also contain plasmids, which are small, circular DNA molecules.

• Location: Found in the nucleoid region, not enclosed by a membrane.

Glycocalyx

• Structure: An outer coating of polysaccharides, proteins, or both. Can exist as:

• Capsule: Well-organized, tightly attached.

• Slime layer: Loosely organized, easily washed off.

• Function:

• Protection: Against phagocytosis (e.g., Streptococcus pneumoniae).

• Adherence: Helps bacteria stick to surfaces (e.g., biofilm formation by Pseudomonas aeruginosa).

• Prevents dehydration: Retains moisture.

Flagella

• Function: Provides motility.

• Structure:

1. Filament: The long, whip-like part.

2. Hook: Connects the filament to the basal body.

3. Basal Body: Anchors the flagellum to the cell wall and plasma membrane.

• Arrangements:

• Monotrichous: Single flagellum.

• Lophotrichous: Tuft of flagella at one end.

• Amphitrichous: Flagella at both ends.

• Peritrichous: Flagella all over the cell.

Bacterial Movement

• Types:

• Run: Straight-line movement.

• Tumble: Random changes in direction.

• Directional Responses:

• Chemotaxis: Movement toward or away from chemicals.

• Phototaxis: Movement toward or away from light.

• Spirochete Movement: Use axial filaments (endoflagella) that rotate, causing a corkscrew motion (e.g., Treponema pallidum).

Fimbriae and Sex Pili

• Fimbriae: Short, hair-like structures for attachment to surfaces (e.g., Escherichia coli).

• Sex Pili: Longer, involved in conjugation, transferring DNA between bacteria.

Gram-Positive vs. Gram-Negative Cell Walls

• Gram-Positive:

• Thick peptidoglycan layer.

• Contains teichoic acids.

• Stains purple in Gram stain.

• Gram-Negative:

• Thin peptidoglycan layer, surrounded by an outer membrane.

• Contains lipopolysaccharides (LPS).

• Stains pink in Gram stain.

• Has a periplasmic space between membranes.

Atypical Bacteria

• Mycoplasma:

• Lack a cell wall, making them resistant to antibiotics like penicillin.

• Mycolic Acid Layer:

• Found in acid-fast bacteria like Mycobacterium tuberculosis.

• Provides resistance to certain chemicals and dyes.

Plasma Membrane

• Function: Selective barrier, controls substance movement.

• Components: Phospholipid bilayer, proteins, cholesterol (in eukaryotes), glycoproteins.

• Transport Mechanisms:

• Simple Diffusion: Movement of small, nonpolar molecules.

• Facilitated Diffusion: Via transport proteins.

• Active Transport: Requires ATP, moves molecules against the concentration gradient.

• Osmosis: Movement of water through a semi-permeable membrane.

Efflux Pumps

• Function: Expel toxic substances (e.g., antibiotics) out of the cell.

• Importance: Contribute to antibiotic resistance.

Secretion in Prokaryotes

• Mechanism: Secretion systems transport proteins and toxins out of the cell.

• Benefits: Helps bacteria invade host tissues and evade immune responses.

Cytoplasmic Contents

• Nucleoid: Contains bacterial DNA.

• Ribosomes: Sites of protein synthesis.

• Inclusions: Storage of nutrients and other compounds.

Binomial Nomenclature

• Format: Genus species (e.g., Escherichia coli).

• Rules: Genus is capitalized, species is lowercase, and both are italicized or underlined.

Bacterial Shapes and Arrangements

• Shapes:

• Coccus: Spherical.

• Bacillus: Rod-shaped.

• Spiral: Includes spirilla and spirochetes.

• Arrangements:

• Strepto-: Chains.

• Staphylo-: Clusters.

• Diplo-: Pairs.

• Pleomorphic: Bacteria with variable shapes (e.g., Mycoplasma).

Eukaryotic vs. Prokaryotic Cells

• Eukaryotic Cells:

• Nucleus: Present.

• Organelles: Membrane-bound (e.g., mitochondria, ER, Golgi).

• Cell Walls: Made of cellulose (plants) or chitin (fungi).

• Prokaryotic Cells:

• No nucleus.

• No membrane-bound organelles.

• Cell Walls: Usually peptidoglycan (bacteria).

Endocytosis vs. Exocytosis

• Endocytosis:

• Phagocytosis: “Cell eating.”

• Pinocytosis: “Cell drinking.”

• Receptor-mediated: Specific uptake of molecules.

• Exocytosis:

• Releases substances outside the cell.

Endosymbiotic Theory

• Theory: Mitochondria and chloroplasts originated from ancient bacteria.

• Evidence:

• Own DNA: Circular, like bacteria.

• Double membrane: Suggesting engulfment.

• Reproduce independently through binary fission.

Endospores

• Purpose: Allow bacteria to survive in harsh conditions.

• Structure:

• Core: Contains DNA and ribosomes.

• Cortex: Peptidoglycan layer.

• Coat: Protein layers providing durability.

• Vegetative vs. Dormant:

• Vegetative: Active, growing form.

• Dormant: Inactive, spore form.

• Germination: When spores return to vegetative state.

• Sporulation: Formation of endospores under stress.

• Example: Bacillus anthracis (causes anthrax).

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