Microbiology Study Notes: Eukaryotic Microbes and Viruses

Chapter 5 Review: Eukaryotic Microbes

Endosymbiotic Theory

The endosymbiotic theory explains the origin of eukaryotic cells from prokaryotic organisms. It proposes that certain organelles, such as mitochondria and chloroplasts, originated as free-living bacteria that were engulfed by ancestral eukaryotic cells and developed a symbiotic relationship.

• Key Point: Mitochondria and chloroplasts have their own DNA and double membranes, supporting this theory.

• Example: Mitochondria are similar to modern alpha-proteobacteria in structure and genetics.

Eukaryotic Cell Structure

Eukaryotic cells are complex, containing membrane-bound organelles and a defined nucleus. They are found in organisms such as fungi, protozoa, algae, and helminths.

• Nucleus: Contains genetic material (DNA) and is surrounded by a nuclear envelope.

• Organelles: Includes mitochondria (energy production), endoplasmic reticulum (protein/lipid synthesis), Golgi apparatus (modification and transport), and lysosomes (digestion).

• Plasma Membrane: Phospholipid bilayer with embedded proteins.

Fungi

• Dimorphic Fungus: A fungus that can exist in two forms—yeast (unicellular) and mold (multicellular)—depending on environmental conditions.

• Macroscopic vs. Microscopic Fungi: Macroscopic fungi (e.g., mushrooms) are visible to the naked eye, while microscopic fungi (e.g., yeasts, molds) require a microscope for observation.

• Heterotrophic: Organisms that obtain nutrients by consuming organic matter; fungi are heterotrophic and absorb nutrients from their environment.

• Technical Term for Fungal Infections: Mycoses (singular: mycosis).

• Hyphae vs. Pseudohyphae: Hyphae are long, branching filamentous structures of a fungus, while pseudohyphae are chains of elongated yeast cells that remain attached after budding.

Benefits and Adverse Impacts of Fungi

• Benefits: Decomposition, nutrient cycling, production of antibiotics (e.g., penicillin), food production (e.g., bread, cheese).

• Adverse Impacts: Cause of diseases (mycoses), food spoilage, production of toxins (mycotoxins).

Protozoa

• Classification: Protozoa are divided into two main groups based on their mode of movement: those with flagella (flagellates) and those with pseudopodia or cilia (amoeboids and ciliates).

• Trophozoite vs. Cyst: The trophozoite is the active, feeding, and motile stage, while the cyst is the dormant, resistant stage that allows survival in harsh conditions.

• Identification: Protozoa are identified by their morphology, movement, and sometimes by molecular methods.

Chapter 6: Introduction to Viruses, Viroids, and Prions

Discovery and Nature of Viruses

Viruses are acellular infectious agents that require a host cell to replicate. They are much smaller than bacteria and are considered obligate intracellular parasites.

• Historical Milestones: Robert Koch identified bacterial pathogens; Loeffler and Frosch (1898) discovered filterable agents (viruses).

• Role in Evolution: Viruses have influenced the evolution of Bacteria, Archaea, and Eukarya.

General Properties of Viruses

• Obligate Intracellular Parasites: Infect bacteria, protists, fungi, plants, and animals.

• Size: Range from 20 nm to 750 nm; most are

  • Structure: Not cellular; consist of a protein shell (capsid) surrounding nucleic acid (DNA or RNA).

  • Cannot independently perform life functions: Inactive outside host cells; active only inside host cells.

  • Lack metabolic enzymes and protein synthesis machinery.

Viral Structure

• Capsid: Protein shell made of subunits called capsomers; protects nucleic acid.

• Nucleic Acid: Either DNA or RNA, never both. Can be single- or double-stranded.

• Envelope: Some viruses have an outer lipid envelope derived from the host cell membrane; those without are called naked viruses.

• Spikes: Glycoproteins on the envelope essential for attachment to host cells.

• Enzymes: Some viruses carry enzymes (e.g., polymerases, reverse transcriptase) necessary for replication.

Types of Capsids

• Helical: Capsomers form a helical structure around the nucleic acid.

• Icosahedral: Symmetrical, 20-sided structure with 12 corners.

• Complex: Found in some viruses (e.g., bacteriophages), combining icosahedral and helical features.

Viral Genomes

• DNA Viruses: Usually double-stranded, may be single-stranded.

• RNA Viruses: Usually single-stranded, may be double-stranded or segmented.

• Positive-sense RNA: Can be directly translated by host ribosomes.

• Negative-sense RNA: Must be converted to positive-sense RNA before translation.

Viral Multiplication Cycle (Animal Viruses)

1. Adsorption: Virus binds to specific receptors on host cell membrane.

2. Penetration: Entry of viral genome into host cell (fusion or endocytosis).

3. Uncoating: Release of viral nucleic acid from capsid.

4. Synthesis: Viral components are produced using host machinery.

5. Assembly: New viral particles are constructed.

6. Release: Viruses exit the cell by budding (enveloped) or lysis (naked).

Host Range and Specificity

• Host Range: Spectrum of host cells a virus can infect (e.g., hepatitis B infects human liver cells).

• Attachment: Determined by specific interactions between viral spikes and host cell receptors.

Medical Moment: Influenza Virus

• Causative Agent: Influenza virus types A, B, and C; segmented ssRNA genome; enveloped with hemagglutinin (HA) and neuraminidase (NA) spikes.

• Antigenic Drift: Gradual accumulation of mutations in HA and NA genes, leading to new strains.

• Antigenic Shift: Major genetic reassortment, resulting in new subtypes (only in Influenza A).

• Symptoms: Fever, headache, fatigue, sore throat, runny nose, muscle aches, and sometimes gastrointestinal symptoms.

• Pathogenesis: Virus targets respiratory epithelium; HA binds to host cells, NA aids in release of new virions.

• Treatment: Neuraminidase inhibitors (oseltamivir, zanamivir); supportive care; vaccination for prevention.

Viral Classification and Naming

• Classification: Based on structure, chemical composition, and genetic makeup.

• Nomenclature: Families end in -viridae, genera in -virus.

• Species: Defined by host range, pathogenicity, and genetic characteristics.

Bacteriophage Replication

• Bacteriophages: Viruses that infect bacteria; often have complex structures.

• Lytic Cycle: Adsorption, penetration, replication, assembly, maturation, and lysis/release.

• Lysogeny: Viral DNA integrates into host genome as a prophage; can be reactivated to enter lytic cycle.

• Lysogenic Conversion: Prophage genes may confer new properties (e.g., toxin production) to the host bacterium.

Viral Cultivation and Identification

• Methods: Cell (tissue) culture, embryonated eggs, and live animal inoculation.

• Identification: Observation of cytopathic effects, detection of viral components, and serological tests for antibodies.

Prions and Viroids

• Prions: Infectious proteins causing neurodegenerative diseases (e.g., Creutzfeldt-Jakob disease, mad cow disease); extremely resistant to sterilization.

• Viroids: Infectious RNA molecules affecting plants.

Table: Comparison of Virus, Viroid, and Prion

Agent	Genetic Material	Protein Coat	Host(s)	Example Disease
Virus	DNA or RNA	Yes	Animals, plants, bacteria	Influenza, rabies
Viroid	RNA	No	Plants	Potato spindle tuber disease
Prion	None	No	Animals, humans	Mad cow disease, CJD

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard microbiology curriculum.