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

Introduction to Microbiology

Definition of Strain

• Strain: A genetic variant or subtype of a microorganism (e.g., bacteria, virus, fungus). Strains differ from each other in minor genetic or phenotypic ways, but belong to the same species.

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

Introduction to Prokaryotic Cells

Universal Cell Structures

• All cells possess the following structures:

  • Plasma membrane

  • Cytoplasm

  • Genetic material (DNA)

  • Ribosomes

Prokaryotic vs. Eukaryotic Cells

• Prokaryotic cells: Lack a nucleus and membrane-bound organelles; DNA is in the nucleoid region; reproduce by binary fission.

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

• Examples: Bacteria and Archaea are prokaryotes; Fungi, Protozoa, Algae, Plants, and Animals are eukaryotes.

Prokaryotic Cell Morphologies

• Coccus: Spherical shape

• Bacillus: Rod-shaped

• Spirillum: Spiral-shaped

• Vibrio: Comma-shaped

• Spirochete: Flexible, spiral-shaped

Structures External to the Cell Wall

Glycocalyx

• A sticky, gelatinous layer outside the cell wall composed of polysaccharides and/or polypeptides.

• Capsule: Organized and firmly attached; Slime layer: Unorganized and loosely attached.

• Importance: Protects against desiccation, phagocytosis, and aids in adherence to surfaces; contributes to virulence.

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: Rotate like propellers; chemotaxis (movement toward/away from chemicals).

• Axial Filament: Found in spirochetes (e.g., Borrelia); enables corkscrew movement through viscous environments.

• Pili and Fimbriae:

  • Fimbriae: Numerous, short; used for attachment.

  • Pili: Longer, fewer; involved in conjugation (DNA transfer).

The Cell Wall

• Main Purpose: Maintains cell shape, prevents osmotic lysis, and provides protection.

• Major Component: Peptidoglycan (murein) in bacteria.

Gram-Positive vs. Gram-Negative Cell Walls

Feature	Gram-Positive	Gram-Negative
Peptidoglycan	Thick layer	Thin layer
Teichoic acids	Present	Absent
Outer membrane	Absent	Present (contains lipopolysaccharide, LPS)
Periplasmic space	Small or absent	Large
Staining	Retains crystal violet (purple)	Counterstained (pink/red)

• Pathogenicity: LPS in Gram-negative bacteria acts as an endotoxin; teichoic acids in Gram-positive aid in adherence.

• Lab Identification: Gram stain differentiates based on cell wall structure.

Atypical Cell Walls

• Acid-Fast Cell Walls: Contain mycolic acid (waxy lipid); found in Mycobacterium species.

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

Lysozyme Action

• Lysozyme: Enzyme that hydrolyzes bonds in peptidoglycan, leading to cell wall damage and 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: Reserve deposits (e.g., glycogen, polyphosphate, sulfur granules).

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

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

Introduction to Eukaryotic Cells

Plasma Membrane Structure and Function

• 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.

Eukaryotic Cell Walls

• Examples: Plants (cellulose), Fungi (chitin), Algae (cellulose, silica, or other polysaccharides).

• Differences: Composition varies among kingdoms; animals and protozoa lack cell walls.

Eukaryotic Glycocalyx

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

Eukaryotic Flagella and Cilia

• Flagella: Long, whip-like; 9+2 arrangement of microtubules; move in a wave-like motion.

• Cilia: Short, numerous; also 9+2 microtubule structure; move substances along cell surface or propel cell.

• Comparison: Prokaryotic flagella rotate; eukaryotic flagella undulate.

Four Kingdoms of Eukaryotes

• Protista (includes protozoa and algae)

• Fungi

• Plantae

• Animalia

Fungi

Main Features and Structures

• Eukaryotic, non-photosynthetic, cell walls of chitin.

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

Fungal Spores

• Asexual spores: Formed by mitosis (e.g., conidia, sporangiospores).

• Sexual spores: Formed by fusion of nuclei from two opposite mating types (e.g., zygospores, ascospores, basidiospores).

Mycosis

• Mycosis: Any fungal infection in or on the body.

Protozoa

Terminology and Classification

• Trophozoite: Active, feeding stage.

• Cyst: Dormant, resistant stage.

• Protozoan: Unicellular, eukaryotic organisms; classified by mode of locomotion.

• Four main groups:

  • Amoeboids: Move by pseudopodia

  • Flagellates: Move by flagella

  • Ciliates: Move by cilia

  • Apicomplexans: Non-motile; complex life cycles

Algae

• Photosynthetic, eukaryotic organisms; can be unicellular or multicellular.

• Clinical relevance: Some produce toxins (e.g., red tide); source of agar and other products.

Helminths

Main Features and Life Cycle

• Multicellular, eukaryotic parasitic worms.

• Platyhelminthes: Flatworms (e.g., flukes, tapeworms)

• Nematodes: Roundworms

• Hosts: Intermediate host harbors larval stage; Definitive host harbors adult, sexually mature stage.

Viruses and Prions

General Features of Viruses

• Acellular, obligate intracellular parasites; lack cellular structure.

• Contain either DNA or RNA (not both), surrounded by a protein coat (capsid); some have lipid envelope.

Viral Structures

• Capsid: Protein shell enclosing nucleic acid.

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

• Spikes: Glycoproteins for attachment to host cells.

Viral Genomic Variations

• Genomes can be DNA or RNA, single- or double-stranded, linear or circular, segmented or non-segmented.

Viral Genome Expression

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

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

Viral Genome Evolution

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

• Consequences: Rapid evolution, emergence of new strains, drug resistance.

Antigenic Shift vs. Antigenic Drift

Feature	Antigenic Drift	Antigenic Shift
Definition	Minor, gradual mutations in viral genes	Major, abrupt genetic changes (reassortment)
Impact	Seasonal epidemics	Pandemics
Example	Influenza A annual changes	Emergence of new influenza subtypes

Host Range and Tropism

• Host range: Spectrum of host species a virus can infect.

• Tropism: Specificity for particular host tissues or cell types.

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

• General steps: Attachment, penetration, uncoating, biosynthesis, assembly, release.

• Enveloped viruses: Often released by budding.

• Naked viruses: Usually released by cell lysis.

Persistent Viral Infections

• Chronic: Virus continuously present (e.g., hepatitis B).

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

Oncogenic Viruses

• Viruses that can cause cancer by integrating into host genome or altering cell regulation.

• Examples: Human papillomavirus (HPV) – cervical cancer; Epstein-Barr virus – Burkitt's lymphoma; Hepatitis B and C – liver cancer.

Prions

• Infectious proteins lacking 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 the original guide listed only brief points, academic context and definitions have been expanded for clarity and completeness.