Comprehensive Study Notes: Prokaryotic and Eukaryotic Cells, Viruses, and Prions

Introduction to Microbiology: Cell Types and Microbial Diversity

Definition of Strain

• Strain: A genetic variant or subtype of a microorganism (e.g., bacteria, virus, fungus). Strains arise from a single species but differ in genetic makeup and sometimes in phenotype.

• Example: Escherichia coli O157:H7 is a pathogenic strain of E. coli.

Cell Structures in All Cells

• All cells (prokaryotic and eukaryotic) share these structures:

  • Plasma membrane

  • Cytoplasm

  • Genetic material (DNA)

  • Ribosomes

Prokaryotic vs. Eukaryotic Cells

Main Structures and Reproductive Processes

• Prokaryotic cells: Lack a nucleus and membrane-bound organelles; reproduce by binary fission.

• Eukaryotic cells: Have a true nucleus and membrane-bound organelles; reproduce by mitosis or meiosis.

• Examples: Bacteria (prokaryotes), Fungi and Protozoa (eukaryotes).

Cell Morphologies in Prokaryotes

• Coccus: Spherical shape

• Bacillus: Rod-shaped

• Spirillum: Spiral-shaped

• Vibrio: Comma-shaped

• Spirochete: Flexible, spiral-shaped

Structures External to the Cell Wall

Glycocalyx

• Definition: A gelatinous, sticky substance surrounding the outside of some bacterial cells.

• Importance: Protects against desiccation, helps evade immune response, aids in attachment to surfaces, and contributes to virulence.

• Types: Capsule (organized, firmly attached) and Slime layer (unorganized, loosely attached).

Appendages

• Flagella: Long, whip-like structures for motility.

  • Arrangements: Monotrichous (single), Lophotrichous (tuft at one end), Amphitrichous (one at each end), Peritrichous (all over).

  • Movement: Bacteria move by rotating flagella, allowing "runs" and "tumbles" to navigate their environment (chemotaxis).

• Axial Filament: Found in spirochetes (e.g., Borrelia).

  • Located between cell wall and outer sheath; enables corkscrew movement through viscous environments.

• Pili and Fimbriae:

  • Fimbriae: Numerous, short; used for attachment to surfaces.

  • Pili: Fewer, longer; involved in conjugation (DNA transfer) and sometimes motility.

The Cell Wall

Main Purpose and Components

• Purpose: Maintains cell shape, protects against osmotic lysis, and provides structural support.

• Major Component in Bacteria: Peptidoglycan (murein).

Gram-Positive vs. Gram-Negative Cell Walls

• Gram-Positive: Thick peptidoglycan layer, teichoic acids, no outer membrane.

• Gram-Negative: Thin peptidoglycan layer, outer membrane with lipopolysaccharide (LPS), periplasmic space.

• Pathogenicity: LPS (endotoxin) in Gram-negative bacteria can trigger strong immune responses.

• Lab Differentiation: Gram stain—Gram-positive cells retain crystal violet (purple); Gram-negative cells take up safranin (pink/red).

Atypical Cell Walls

• Acid-Fast Cell Walls: Contain mycolic acid (waxy lipid); resist decolorization (e.g., Mycobacterium).

• Mycoplasma: Lack cell walls; have sterols in plasma membrane for stability.

Lysozyme Action

• Lysozyme: Enzyme that breaks the β-1,4-glycosidic bonds in peptidoglycan, leading to cell lysis.

Internal Bacterial Cell Structures

• Bacterial Chromosome: Single, circular DNA molecule; contains essential genes.

• Plasmids: Small, circular DNA; carry non-essential genes (e.g., antibiotic resistance).

• Inclusions: Storage granules for nutrients (e.g., glycogen, polyphosphate).

• Endospores: Dormant, highly resistant structures formed by genera such as Bacillus and Clostridium.

  • Clinical Relevance: Endospores resist heat, desiccation, chemicals; important in infection control.

Eukaryotic Cell Features

Plasma Membrane

• Composed of phospholipid bilayer with embedded proteins; contains sterols (e.g., cholesterol in animals, ergosterol in fungi).

• Functions: Selective permeability, cell signaling, and interaction with environment.

Cell Walls in Eukaryotes

• Examples: Fungi (chitin), Plants (cellulose), Algae (varied polysaccharides).

• Structure and composition differ among kingdoms.

Eukaryotic Glycocalyx

• Carbohydrate-rich layer outside plasma membrane; involved in cell recognition, adhesion, and protection.

Eukaryotic Flagella and Cilia

• Flagella: 9+2 arrangement of microtubules; whip-like motion.

• Cilia: Short, numerous; move fluid or cells over surfaces.

• Comparison: Prokaryotic flagella are made of flagellin and rotate; eukaryotic flagella are made of microtubules and undulate.

Four Kingdoms of Eukaryotes

• Protista

• Fungi

• Plantae

• Animalia

Fungi

• Main Features: Eukaryotic, chitin cell walls, heterotrophic, reproduce by spores.

• Forms: Yeasts (unicellular), Molds (multicellular, filamentous), Dimorphic (can switch forms).

• Spores: Asexual (conidia, sporangiospores), Sexual (zygospores, ascospores, basidiospores).

• Mycosis: Fungal infection in animals or humans.

Protozoa

• Feeding Form: Trophozoite

• Dormant Form: Cyst

• Definition: Unicellular, eukaryotic organisms, often motile.

• Classification: By motility structures:

  • Amoeboids (pseudopodia)

  • Flagellates (flagella)

  • Ciliates (cilia)

  • Sporozoans (non-motile, complex life cycles)

Algae

• Features: Photosynthetic, eukaryotic, aquatic; cell walls vary (cellulose, silica, etc.).

• Clinical Relevance: Some produce toxins (e.g., red tide), important in food webs.

Helminths

• Main Features: Multicellular, eukaryotic, parasitic worms.

• Groups: Platyhelminthes (flatworms), Nematodes (roundworms).

• Life Cycle: Often involves multiple hosts.

• Hosts: Intermediate (larval development), Definitive (adult, sexual reproduction).

Viruses and Prions

General Features of Viruses

• Non-cellular, obligate intracellular parasites.

• Contain DNA or RNA (not both), protein coat (capsid), sometimes envelope.

• Lack ribosomes and metabolic machinery.

Viral Structures

• Capsid: Protein shell protecting nucleic acid.

• Envelope: Lipid membrane derived from host; present in some viruses.

• Spikes: Glycoproteins for attachment to host cells.

Viral Genomic Variations

• DNA or RNA; single- or double-stranded; linear or circular; segmented or non-segmented.

Viral Genome Expression

• DNA viruses: Use host machinery for transcription/translation.

• RNA viruses: May carry their own enzymes (e.g., RNA-dependent RNA polymerase) to make mRNA.

Viral Genome Evolution

• High mutation rates, especially in RNA viruses (lack proofreading).

• Consequences: Rapid adaptation, emergence of new strains.

Antigenic Shift vs. Antigenic Drift

Feature	Antigenic Drift	Antigenic Shift
Definition	Minor mutations in viral genes	Major genetic reassortment
Impact	Seasonal epidemics	Pandemics
Example	Influenza A annual changes	New influenza A subtypes

Host Range and Tropism

• Host Range: Spectrum of hosts a virus can infect.

• Tropism: Specific cell types or tissues a virus targets.

Bacteriophage Replication

• Lytic Cycle: Virus replicates and lyses host cell.

• Lysogenic Cycle: Viral genome integrates into host DNA (prophage); can later enter lytic cycle.

Phage Conversion

• Acquisition of new traits by bacteria due to prophage genes (e.g., toxin production).

• Medical Importance: Can increase bacterial virulence.

Animal Virus Replication

• Attachment, penetration, uncoating, biosynthesis, assembly, release.

• Enveloped viruses: Often released by budding.

• Naked viruses: Released by cell lysis.

Chronic and Latent Infections

• Chronic: Continuous low-level production of virus (e.g., hepatitis B).

• Latent: Virus remains dormant, can reactivate (e.g., herpesviruses).

Oncogenic Viruses

• Viruses that can cause cancer by disrupting cell cycle regulation.

• Examples: Human papillomavirus (HPV) – cervical cancer; Epstein-Barr virus – Burkitt lymphoma.

Prions

• Infectious proteins; lack nucleic acids.

• Cause neurodegenerative diseases (e.g., Creutzfeldt-Jakob disease, mad cow disease).

• Mechanism: Induce misfolding of normal proteins in the brain.

Additional info: Where details were not explicit in the original file, standard academic context was added for completeness and clarity.