The Bacterial Cell – Key Vocabulary

Cell Theory

• Postulates combining ideas of Schleiden, Schwann, and Virchow

  • All living things are composed of cells.

  • All living cells arise from pre-existing cells (biogenesis).

• Historical contributors

  • Matthias Schleiden – plants are cellular.

  • Theodor Schwann – animals are cellular.

  • Rudolf Virchow – cellular reproduction principle.

Eukaryotic vs. Prokaryotic Cells

• Eukaryotes

  • Membrane-bound nucleus & organelles.

  • Larger genome and cell size.

• Prokaryotes (Bacteria & Archaea)

  • No membrane-bound nucleus; DNA concentrated in nucleoid region.

  • Generally smaller, simpler internal structure.

• Visual comparison slide showed stylized prokaryotic vs. eukaryotic silhouettes.

Bacterial & Eukaryotic DNA

• Genome size: bacterial genomes are \approx 10^3 times smaller than humans.

• Form: usually single, circular chromosome.

• Localisation: aggregated in non-membranous nucleoid.

• Nucleoid ≠ true nucleus (no envelope).

Plasmids

• Small, circular, extra-chromosomal DNA; copy number varies.

• Carry few, often non-essential, but advantageous genes:

  • Antibiotic resistance (e.g., \text{R}\text{-} factors).

  • Novel metabolic pathways (pollutant degradation, bacteriocin production).

  • Virulence determinants that enhance pathogenicity.

• Replicate independently; can be transferred between cells (conjugation).

Genetic Recombination & Gene Transfer

• Definition: exchange of genetic material producing new allele combinations.

• In eukaryotes: crossing-over during meiosis.

• In prokaryotes: accomplished through horizontal gene transfer (HGT).

Vertical Gene Transfer

• Parent → daughter via binary fission.

• Offspring genetically identical (clonal).

• NOT recombination.

Horizontal Gene Transfer (recombination)

• DNA moves between contemporaneous cells.

• Three classical mechanisms:

1. Transformation

• Competent bacteria pick up naked DNA from environment.

• Incorporated fragments may recombine with chromosome.

• Requires physiological competence; usually closely related species.

• Examples: Bacillus, Neisseria, Acinetobacter, Haemophilus.

2. Conjugation

• DNA transfer via sex pilus; plasmid-mediated.

• Steps:

  1. Donor with conjugative plasmid forms pilus.

  2. Single DNA strand passes through pilus.

  3. Strand circularises, complementary strand synthesized in recipient.

• Common in E. coli.

• Genes often moved: R-factors (antibiotic resistance), toxin genes, other virulence loci.

3. Transduction

• Bacteriophage packages bacterial DNA and injects it into another cell.

• Indirect; virus acts as vector.

• Two types (not detailed in slide): generalized & specialized.

Morphology & General Anatomy

Common Shapes

• Cocci (spheres).

• Bacilli (rods).

  • Short/oval rods = coccobacilli.

• Spirilla (rigid spirals).

• Vibrios (slightly curved rods).

• Spirochetes (flexible spirals).

Whole-Cell Overview

• Labeled diagram components:

  • Chromosome (nucleoid).

  • Plasmid.

  • Ribosomes.

  • Food/storage granules.

  • Plasma membrane.

  • Cell wall.

  • Capsule/slime layer (glycocalyx).

  • Fimbriae, pili.

  • Flagellum.

Functional Grouping

• Cell envelope (≤3 layers).

• Appendages (flagella, fimbriae, pili).

• Internal structures (DNA, ribosomes, inclusions, endospore).

Internal Structures (Cytoplasmic)

Ribosomes

• Composition: large & small subunits of rRNA + protein; distinct from eukaryotic 80S.

• Function: \text{protein synthesis (translation)}.

Cytoplasm

• Highly aqueous: 70\text{–}80\% water.

• Contains sugars, amino acids, salts; universal metabolic solvent.

Inclusions & Granules

• Non-membranous nutrient storage bodies.

• Content varies (glycogen, poly-β-hydroxybutyrate, sulfur, gas vesicles).

• Gas vesicles confer buoyancy.

Photosynthetic Adaptations

• Cyanobacteria possess internal thylakoid-like membrane folds with chlorophyll.

• Perform oxygenic photosynthesis → major \mathrm{O_2} contributor, generate glucose from sunlight.

Endospores

• Dormant, highly resistant structures (sporulation induced by nutrient stress).

• Endospore ⇆ vegetative cell via germination.

• Withstand heat, desiccation, freezing, radiation, chemicals; considered “hardiest life form.”

• Clinically relevant spore-formers & diseases:

  • Bacillus anthracis → Anthrax.

  • Clostridium tetani → Tetanus.

  • Clostridium botulinum → Botulism.

Cell Envelope Detail

Layer Overview

1. Glycocalyx (outermost).

2. Cell wall (middle).

3. Plasma membrane (innermost).

Glycocalyx

• Composition: polysaccharides ± proteins; hydrophilic.

• Forms:

  • Slime layer – loose, easily removed.

  • Capsule – dense, firmly attached, organized.

• Functions / significance:

  • Adherence to surfaces (e.g., dental plaque, medical implants).

  • Biofilm formation scaffold.

  • Prevents desiccation.

  • Immune evasion: inhibits phagocytosis (“slippery”).

  • Virulence factor: encapsulated forms of Streptococcus pneumoniae, Bacillus anthracis are more pathogenic.

Cell Wall

• Semi-rigid mesh of peptidoglycan (murein).

• Maintains morphology; counters osmotic pressure (prevents lysis when outside \pi > inside \pi).

• Antimicrobial target (unique to bacteria):

  • β-lactams (Penicillin).

  • Glycopeptides (Vancomycin).

Gram Reaction

• Gram-positive

  • Thick peptidoglycan.

  • Teichoic acids for wall reinforcement & adherence.

  • Crystal violet retained → purple.

  • Generally more permeable.

• Gram-negative

  • Thin peptidoglycan + outer membrane with lipopolysaccharide (LPS).

  • Porins in outer membrane allow selective entry.

  • Periplasmic space between membranes.

  • Decolorized primary stain, counterstained safranin → pink.

  • Lower permeability to many drugs.

Plasma (Cell) Membrane

• Phospholipid bilayer + proteins.

• Selectively permeable:

  • Passive diffusion for small non-polar molecules (\text{H}2\text{O}, \text{CO}2, \text{O}_2).

  • Transport systems for large or polar molecules.

• Anchors respiratory chain, secretion systems, flagellar base.

Cellular Appendages

General Notes

• Proteinaceous external fibers.

• Not universal.

• Two chief roles:

  1. Motility (flagella).

  2. Adherence (fimbriae, pili).

Flagella

• Act like rotating propellers; basal body rotation drives filament spin.

• Structure

  • Basal body (rod + rings) embedded through membranes/wall.

  • Hook (universal joint).

  • Filament (flagellin).

• 360° rotation; opposite rotation of cell body relative to filament.

• Distribution patterns (laboratory identification): monotrichous, lophotrichous, amphitrichous, peritrichous, endoflagella (axial filaments in spirochetes).

• Occurrence by shape:

  • Spirilla: all; some internal.

  • Bacilli: ~50 %.

  • Cocci: rare.

Motility Mechanics

• Run (counter-clockwise) – straight movement lasting seconds.

• Tumble (clockwise) – brief, random re-orientation.

• Chemotaxis, phototaxis, thermotaxis achieved by biasing run length:

  • Positive taxis = toward stimulus.

  • Negative taxis = away from harmful cue.

Fimbriae

• Short, numerous bristles of protein.

• Mediate tight adhesion to surfaces/other cells (e.g., epithelial colonization).

• Important in biofilm architecture.

Pili

• Longer, fewer than fimbriae.

• Composed of pilin.

• Specialized “sex” pilus enables conjugation.

• Often carry transmissible antibiotic-resistance plasmids; critical in spread of MDR pathogens.

Virulence, Clinical & Ecological Connections

• Capsule, fimbriae, pili, flagella, toxins, endospores, plasmid-borne traits all contribute to pathogenic potential.

• Horizontal gene transfer accelerates antibiotic resistance emergence (e.g., MRSA).

• Gram-negative outer membrane LPS = endotoxin; clinical concern for sepsis.

• Endospores demand rigorous sterilization protocols (autoclaving \ge 121^\circ\text{C}, \ge 15 psi, \ge 15 min).

• Cyanobacteria’s photosynthetic output crucial for global oxygen cycle; early Earth oxygenation.

These bullet-point notes capture major & minor details, terminology, mechanisms, clinical examples, and quantitative references from the original transcript, suitable for exam preparation without needing to consult the slides.