Virology and Mycology

• Virology involves studying viruses and mycology involves studying fungi.

Viruses

• Non-cellular entities
• Biologically inert outside a host.
• Obligate intracellular parasites

Viral infection

• Can infect bacteria, animals, plants, and fungi
• Too small to be seen with a light microscope

Viruses vs Bacteria

• Viruses are obligate intracellular parasites
• Viruses lack plasma membranes, binary fission, ATP-generating metabolism, and ribosomes
• Viruses possess either RNA or DNA
• Viruses are not sensitive to antibiotics.

Viral Structure

• A virion is a complete, infectious viral particle with two components: a core of nucleic acid (DNA or RNA) and a capsid (protein coat).
• Capsids are made of protein units called capsomeres.
• The nucleocapsid includes the nucleic acid and capsid.
• Some nucleocapsids are surrounded by an envelope.

Virus classification

• Viruses are classified based on capsid architecture.
• Icosahedral viruses are spherical with 20-sided geometric shapes.
• Enveloped viruses are surrounded by a membrane of lipids, proteins, and carbohydrates.
• Helical viruses resemble long rods with a hollow cylindrical capsid and a helical structure.
• Complex viruses have more complicated structures, like bacteriophages.

Viral Genomes

• Viral genetic material can be DNA or RNA, but not both simultaneously.
• Some viruses use both DNA and RNA at different life cycle stages, but never at the same time.
• Viral nucleic acids can be single-stranded (ss) or double-stranded (ds), and either linear or circular.

Baltimore Classification System

• A classification scheme based on the nature of the viral nucleic acid.
• The system has seven groups (I-VII):
• Group I: dsDNA viruses
• Group II: ssDNA viruses
• Group III: dsRNA viruses
• Group IV: (+) sense ssRNA viruses
• Group V: (-) sense ssRNA viruses
• Group VI: ss (+) sense RNA viruses with a DNA intermediate.
• Group VII: dsDNA viruses with an RNA intermediate.

Bacteriophages

• Viruses that infect only bacteria.
• They are diverse.

Bacteriophage Lifecycle

• Two alternative mechanisms: the lytic cycle and the lysogenic cycle.
• The lytic cycle results in host cell lysis and death.
• The lysogenic cycle allows the host cell to remain alive
• Virulent phages replicate only via the lytic cycle.
• Temperate phages can replicate via the lytic or lysogenic cycle.

Lytic Cycle

• Attachment: Bacteriophage attaches and injects linear DNA into the host cell, which then circularizes.
• Biosynthesis: Phage nucleic acids use the host cell machinery to make viral parts, degrading host DNA via phage-encoded enzymes.
• Maturation/Assembly: New viral components assembly into virions
• Release: The host cell is lysed, and virions are released

Lysogenic Cycle

• Attachment: Bacteriophage attaches and injects DNA into the host cell.
• Phage DNA integrates into the host chromosome via recombination, becoming a prophage.
• The prophage replicates with the host genome, and most viral genes are unexpressed.
• The phage doesn't kill the host cell.
• Bacteria containing a prophage are called lysogens, representing a latent infection.
• Under certain conditions, the prophage can reactivate the lytic pathway and produce new viral particles.
• The lysogenic cycle can be stimulated by nutrient deprivation or by phages attaching to cells
• Reactivation of the lytic cycle can occur due to host cell DNA damage or cell stress.

Animal Viruses

• Infect plants and animals, they share properties with bacterial viruses, but differ.
• Differences: entire virion enters the host cell where eukaryotic cells contain a nucleus where many animal viruses replicate.

Culturing Viruses

• Viruses need a host cell due to not replicating by themselves
• To test for bacteriophages, bacterial cells need to be cultured.
• To grow an animal virus, tissue cultures are required.

Plaque Assay

• Used to quantify infectious virions in a volume of fluid, expressed as titre.
• When a virus infects a host cell and causes lysis, a plaque forms, that is a clear area on the lawn of host cells, used to calculate PFU/ml.

Fungi

• Eukaryotic
• Spore-forming organisms
• Can be unicellular or multicellular.

Fungi cell structure

• Cell walls are made of chitin.
• Membranes contain ergosterol (targeted by antifungal drugs).

Types of fungi

• Yeasts are unicellular.
• Moulds are multicellular with hyphae
• Dimorphic fungi can switch between yeast and mould forms.

Fungal Life Cycle

• Asexual Reproduction: Through spores
  • Examples: Aspergillus, Penicillium.
• Sexual Reproduction: Involves meiosis to produce genetically diverse spores.
  • Examples: Rhizopus, Saccharomyces.

Fungal infection

• Mycoses are fungal infections are classified based on the depth of infection of the affected tissues.
• Superficial mycoses affect skin and hair.
• Cutaneous mycoses affect the keratinized layers of skin, hair, and nails.
• Subcutaneous mycoses involve deeper skin layers, subcutaneous tissue, and sometimes muscle.
• Systemic mycoses involve internal organs, often originating in the lungs.
• Opportunistic mycoses occur in immunocompromised individuals, caused by non-pathogenic fungi.

Fungal Toxins

• Mycotoxins are toxic secondary metabolites produced by certain fungi for ecological roles like deterring predators, competing, or aiding survival.
• Mycotoxins contaminate food, water, posing risks to humans, animals, and plants.

Benefits of Fungi

• Used in pulp, paper, textile industry (amylases, proteases, cellulases, lipases, xylanases, etc)
• Used in food, beverage industry (cellulases, xylanases, pectinases, tannases)
• Used as biocatalysts (amylases, proteases, cellulases, xylanases, lipases)
• Used in animal feed (phytases, cellulases, lipases, keratinases)
• Used in environmental management (laccase, phenoloxidase, lipases, keratinases)