Bacterial Cell Overview

Bacteria vs. Eukaryotes

• DNA Packaging:

  • Bacteria and Archaea: nuclear material free in cytoplasm.

  • Eukaryotes: nucleus.

• Cell Wall Makeup:

  • Bacteria: peptidoglycan.

  • Archaea: distinct from bacteria/eukaryotes.

• Internal Structures:

  • Bacteria and Archaea: no membrane-bound organelles.

Bacterial Characteristics

• Life Activities: Capable of reproduction, metabolism, nutrient processing.

• Form Groups: Can exist as colonies or biofilms.

• Size: Considerable variety.

  • Average: $1 \mu m$ (micron).

  • Examples: Mycoplasma ($0.15$ to $0.30 \mu m$), Nanobacteria ($0.05$ to $0.2 \mu m$).

Bacterial Shapes and Arrangements

• General Shapes:

  • Coccus: Spheres, oval, bean-shaped, pointed.

  • Bacillus: Cylindrical, filamentous, club-shaped.

  • Vibrio: Curved.

• Arrangements of Cocci:

  • Diplococci: Pairs.

  • Tetrads: Groups of four.

  • Staphylococci: Irregular clusters.

  • Streptococci: Chains.

  • Sarcina: Cubical packets of 8, 16+ cells.

• Arrangements of Bacilli:

  • Diplobacilli: Pairs with ends attached.

  • Streptobacilli: Chains.

  • Palisades: Cells fold back, creating side-by-side rows.

External Structures: Appendages

• Motility Appendages:

  • Flagella:

    • Provide motility.

    • Arrangements:

      • Polar: Monotrichous (single), Lophotrichous (bunches), Amphitrichous (both poles).

      • Peritrichous: Dispersed randomly.

    • Taxis: Chemotaxis (chemical signals: positive/negative), Phototaxis (light).

  • Axial Filaments (Periplasmic Flagella): Internal flagellum in spirochetes, imparts twisting motion.

• Attachment/Mating Appendages:

  • Fimbriae: Small, bristle-like protein fibers; provide adhesion, often for biofilm formation.

  • Pili (Sex Pilus): Long, rigid tubular structure (pilin protein); found in gram-negative bacteria; used in conjugation (DNA transfer).

External Structures: Surface Coatings

• S layer: Single protein layer; protection in hostile environments.

• Glycocalyx: Polysaccharide/protein coating.

  • Slime layer: Loosely bound; protects from water/nutrient loss.

  • Capsule: Tightly bound, denser; protects against phagocytic white blood cells; aids biofilm formation (e.g., plaque).

The Cell Envelope

• Composition: Cell wall, cytoplasmic membrane, outer membrane (in some bacteria).

• Gram Stain: Delineates two major groups based on cell envelope structure.

  • Gram-positive: Thick peptidoglycan cell wall + inner cytoplasmic membrane.

  • Gram-negative: Outer membrane + thin peptidoglycan cell wall + inner cytoplasmic membrane.

• Function: Single protective unit; can interact with human tissues to cause disease.

Cell Wall Structure

• Function: Determines shape, provides structural support against osmotic pressure.

• Peptidoglycan: Unique macromolecule in most bacterial cell walls; glycan chains cross-linked with peptides.

• Gram-Positive Cell Wall: Thick ($20$ to $80$ nm), homogenous peptidoglycan sheath; contains teichoic and lipoteichoic acids.

• Gram-Negative Cell Wall: Single, thin sheet ($1$ to $3$ nm) of peptidoglycan; more flexible, sensitive to lysis.

• Nontypical Cell Walls:

  • Mycobacterium/Nocardia: Cell walls contain mycolic acid (cord factor); resistant to chemicals/dyes; acid-fast stain used for diagnosis.

  • Mycoplasmas: Naturally lack a cell wall; membrane stabilized by sterols; pleomorphic.

Cytoplasmic Membrane

• Structure: $5$ to $10$ nm flexible sheet of lipid bilayer with embedded proteins ($30-40$% phospholipids, $60-70$% proteins).

• Functions: Energy reactions, nutrient processing, synthesis, regulates transport (selective permeability), secretion of wastes.

Bacterial Internal Structures

• Cytoplasm: Gelatinous solution (70-80% water); site of biochemical activities; contains chromatin, ribosomes, granules, cytoskeleton.

• Bacterial Chromosome: Single circular DNA aggregated in a dense nucleoid region.

• Plasmids: Nonessential, separate, double-stranded DNA circles; confer protective traits; used in genetic engineering.

• Ribosomes: Made of RNA and protein, dispersed in cytoplasm; bacterial ribosomes are $70S$. (Eukaryotic are $80S$).

• Inclusion Bodies: Storage sites for nutrients.

• Cytoskeleton: Long polymers of proteins (similar to eukaryotic actin); contribute to cell shape.

Bacterial Endospores

• Nature: Inert, resting condition of some bacteria; withstands hostile conditions.

  • Vegetative cell: Metabolically active stage.

  • Endospore: Dormant, resistant stage.

• Sporulation: Spore formation induced by environmental stress (e.g., nutrient depletion).

• Resistance: Can resist heating, drying, freezing, radiation, chemicals.

• Medical Significance:

  • Cause diseases (e.g., Anthrax, Tetanus, Botulism).

  • Resist ordinary cleaning methods; important in sterility for hospitals and food canning industry.

Bacterial Classification

• Diagnostic Scheme: Based on Gram-stain (Gram-positive/negative, no cell walls), cell shape, arrangements, and oxygen usage (aerobic, anaerobic, facultative).

• Bacterial Species: Collection of cells with similar traits.

  • Subspecies/Strain/Type: Bacteria of same species with differing characteristics.

  • Serotype: Species variants stimulating distinct antibody responses due to surface molecules.