Chapter 13: Viruses and Prions

Components of Mature Viruses

Viruses are acellular infectious agents composed of genetic material and protein, sometimes surrounded by a lipid envelope.

• Capsid: Protein coat surrounding the viral genome, providing protection and shape.

• Nucleocapsid: Combination of the capsid and the nucleic acid.

• Envelope: Lipid membrane surrounding some viruses, derived from the host cell membrane.

• Naked Virus: Lacks an envelope, often more resistant to environmental factors.

General Structure of a Virus

Viral genomes may be DNA or RNA, single- or double-stranded, and are enclosed in a protein capsid. Some viruses have additional structures such as envelopes or specialized enzymes.

Differences Between Naked vs. Enveloped Viruses

• Naked Viruses: More stable in the environment, resistant to desiccation and disinfectants.

• Enveloped Viruses: Susceptible to desiccation and disinfectants, but may evade immune detection more easily.

Lytic Replication Cycle in Bacteriophage

The lytic cycle is a process by which bacteriophages replicate and destroy the host cell.

1. Attachment: Virus attaches to the host cell.

2. Penetration: Viral DNA is injected into the host.

3. Biosynthesis: Viral components are produced.

4. Maturation: New viruses assemble.

5. Release: Host cell bursts, releasing new virions.

• Plaque Formation: Clear zones where cells have been lysed by viruses.

• Cytopathic Effect: Visible effects of viral infection on cells (e.g., cell lysis or fusion).

Culturing Viruses

• Methods include using live animals, embryonated eggs, and cell cultures.

Viral Life Cycle

• Adsorption → Penetration → Uncoating → Synthesis (early/late) → Assembly → Release.

Lytic vs. Lysogenic Cycle

• Lytic: Virus kills the host cell.

• Lysogenic: Viral DNA integrates into the host genome as a prophage, which can later initiate the lytic cycle.

Animal Viruses

Animal viruses have similar life cycle stages as bacteriophages, but with differences in penetration and uncoating.

Retroviruses and Reverse Transcriptase

• Retroviruses use reverse transcriptase to convert RNA into DNA within the host cell.

Prions

• Cellular PrPC vs. Prion PrPSc: Prion PrP misfolds and induces other proteins to misfold, causing diseases such as Creutzfeldt-Jakob disease, mad cow disease, and scrapie.

Chapter 14: Pathogens and Disease Terminology

Definitions

• Pathogen: Any microorganism that can cause disease.

• Etiology: Study of the cause of a disease.

• Pathology: Study of diseases, including their nature and effects on body functions.

• Pathogenesis: Sequence of events in the development of a disease.

• Infection: When a pathogen invades and multiplies in a host.

• Disease: When infection causes symptoms and harm to the host.

Resident Flora and Microbial Relationships

• Resident Flora: Microbes that normally inhabit various body sites without causing disease.

• Antagonism: Competition between resident flora and pathogens for colonization.

Types of Symbiosis

• Mutualism: Both host and microbe benefit.

• Commensalism: Microbe benefits, host is unaffected.

• Parasitism: Microbe benefits at the host's expense.

• Opportunistic Pathogens: Typically non-harmful microbes that can cause disease when the host's immunity is compromised or if they enter unusual sites.

Types of Diseases

• Communicable Disease: Easily spread from one person to another.

• Noncommunicable Disease: Not spread between hosts; typically originates from environmental factors or host conditions (e.g., genetic or metabolic disorders).

Epidemiology Terms

• Incidence: Number of new cases in a specified time frame.

• Prevalence: Total number of cases (new and existing) at a given time.

• Sporadic Disease: Occurs infrequently and irregularly.

• Endemic Disease: Constantly present in a particular population or region.

• Epidemic Disease: Occurs at a higher-than-normal rate in a specific area.

• Pandemic Disease: Epidemic that has spread across multiple countries or continents, typically affecting a large population.

Stages of Disease Development

1. Incubation Period: Time between pathogen entry and first symptoms.

2. Prodromal Period: Early, mild symptoms appear.

3. Period of Illness: Severe symptoms; the immune response is in full effect.

4. Period of Decline: Symptoms begin to subside as immune response or treatment takes effect.

5. Convalescence: Recovery phase; host regains strength, but may still be infectious in some cases.

Modes of Transmission

• Contact Transmission:

  • Direct Contact: Person-to-person physical contact.

  • Indirect Contact: Via inanimate objects (fomites).

  • Droplet Transmission: Short-range droplets (e.g., sneezing, coughing).

  • Airborne Transmission: Pathogens travel through air over distances (e.g., on dust particles).

  • Vehicle Transmission: Through contaminated items like food, water, or blood.

  • Vector Transmission: Spread by animals (vectors), especially insects (e.g., mosquitoes transmitting malaria).

Chapter 15: Virulence Factors and Toxicology

LD50 and ID50

• LD50 (Lethal Dose 50): The dose of a pathogen that will kill 50% of a test population.

• ID50 (Infectious Dose 50): The dose of a pathogen required to infect 50% of a test population.

Types of Toxins

• Exotoxins: Proteins secreted by bacteria with powerful effects.

• AB Exotoxins: Consist of two parts—B binds to host cells, and A exerts toxic effects inside the host cell.

• Superantigens: Exotoxins that cause excessive immune activation, potentially leading to shock.

• Endotoxins: Components of Gram-negative bacterial cell walls, typically lipopolysaccharides (LPS), that can cause fever and inflammation when released into the host.

Chapter 16: Innate Immunity

First Line of Defense

• Skin, Mucous Membranes, Lacrimal Apparatus: Provide physical barriers and antimicrobial secretions.

• Normal Microbiota: Competes with pathogens, maintaining balance.

• Defensins: Antimicrobial peptides that inhibit pathogen growth.

Cellular Elements of Blood

• Neutrophils: Phagocytic, attack bacteria.

• Basophils: Release histamine, involved in allergic responses.

• Eosinophils: Attack parasites and allergens.

• Monocytes/Macrophages: Phagocytic, antigen-presenting.

• Lymphocytes: Key in adaptive immunity.

• Dendritic Cells: Present antigens to T cells.

Second Line of Defense

• Phagocytosis: Stages include chemotaxis, adherence, ingestion, digestion, exocytosis.

• Interferons: Proteins that inhibit viral replication.

• Complement System: Classical pathway activated by antibodies, leading to pathogen destruction.

• Pluripotent Stem Cells: Found in bone marrow; give rise to various blood cells.

Chapter 17: Adaptive Immunity

Humoral Immunity (B-cell mediated)

• Clonal Selection: B cells specific to antigens are selected and multiply.

Cellular Immunity (T-cell mediated)

• Cytotoxic T Cells (Tc): Destroy infected cells.

• Helper T Cells (Th): Aid in immune responses.

• Activation of T cells requires antigen presentation and co-stimulation.

Independent vs. Dependent Humoral Immunity

• T-Independent: Directly activates B cells without T-cell help.

• T-Dependent: Requires T-cell assistance.

Antibodies

• Types: IgG, IgM, IgA, IgE, IgD.

• Structure and Function: Variable and constant regions, antigen binding.

• Antigen-Antibody Binding Results: Agglutination, opsonization, neutralization, complement activation, ADCC (antibody-dependent cellular cytotoxicity).

Classes of T Cells

• Helper T Cells (CD4+): Coordinate responses.

• Cytotoxic T Cells (CD8+): Target infected cells.

• Regulatory T Cells (Treg): Regulate immune responses.

Chapter 18: Vaccines and Immunological Testing

Herd Immunity

• Protection is provided when a high percentage of a population is vaccinated.

Types of Vaccines

• Live attenuated, inactivated, subunit, and conjugate vaccines.

Monoclonal Antibodies (MAbs)

• Produced using hybridoma technology for specific antigen recognition.

Immunological Tests

• Precipitation: Detects soluble antigens.

• Agglutination: Detects particulate antigens.

• Neutralization: Blocks toxin/virus action.

• Fluorescent-antibody Technique: Uses fluorescent labels.

• Complement Fixation: Detects antibody-antigen complexes.

• ELISA: Uses enzyme-linked antibodies for sensitivity.