Unit 4 Microbiology Study Guide: Viruses, Disease, Epidemiology, and Immunity

Chapter 13: Viruses, Viroids, and Prions

Overview of Viruses

Viruses are acellular infectious agents that require a host cell to replicate. They are distinct from living organisms due to their lack of cellular structure and metabolism.

• Obligate intracellular parasites: Viruses can only reproduce inside living host cells.

• Host range: Each virus infects specific hosts and cell types, determined by viral surface proteins and host cell receptors.

• Structure: Viruses consist of genetic material (DNA or RNA), a protein coat (capsid), and sometimes a lipid envelope.

• Size: Viruses are much smaller than bacteria, typically measured in nanometers.

Key Terms:

• Virion: Complete, infectious viral particle.

• Capsid: Protein shell surrounding the viral genome.

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

• Spikes: Glycoprotein projections involved in host cell attachment.

Viral Classification

• Based on nucleic acid type (DNA or RNA), capsid structure, presence of envelope, and replication strategy.

• Family names often end in -viridae; genus names end in -virus.

Viral Multiplication

• Lytic cycle: Virus replicates and lyses host cell.

• Lysogenic cycle: Viral genome integrates into host DNA (prophage) and replicates with host cell.

• Animal virus replication: Involves attachment, entry, uncoating, biosynthesis, maturation, and release.

Viroids and Prions

• Viroids: Infectious RNA molecules without protein coat, cause plant diseases.

• Prions: Infectious proteins causing neurodegenerative diseases (e.g., mad cow disease).

Chapter 14 & 15: Principles of Disease, Epidemiology, and Pathogenicity

Definitions and Concepts

• Pathology: Study of disease.

• Etiology: Cause of disease.

• Infection: Invasion and colonization of the body by pathogens.

• Disease: Abnormal state in which the body is not functioning normally.

Types of Diseases

• Communicable: Spread from one host to another (e.g., influenza).

• Non-communicable: Not spread between hosts (e.g., tetanus).

• Contagious: Easily spread from person to person.

Occurrence and Spread

• Incidence: Number of new cases in a population during a specific time.

• Prevalence: Total number of cases at a given time.

• Epidemic: Disease acquired by many hosts in a short time.

• Pandemic: Worldwide epidemic.

• Endemic: Constantly present in a population.

Reservoirs and Transmission

• Reservoirs: Continual sources of infection (human, animal, nonliving).

• Transmission: Direct contact, indirect contact (fomites), droplet, vehicle, vector (mechanical/biological).

Stages of Disease

• Incubation period: Interval between infection and first symptoms.

• Prodromal period: Early, mild symptoms.

• Period of illness: Disease is most severe.

• Period of decline: Signs and symptoms subside.

• Period of convalescence: Recovery occurs.

Nosocomial (Healthcare-Associated) Infections

• Acquired in hospitals or healthcare settings.

• Result from interaction of microorganisms, compromised host, and chain of transmission.

Mechanisms of Pathogenicity

• Portal of entry: Mucous membranes, skin, parenteral route.

• Adherence: Attachment to host tissues via adhesins/ligands.

• Penetration/Evasion: Capsules, cell wall components, enzymes, antigenic variation.

• Damage to host: Direct damage, toxins (exotoxins, endotoxins), hypersensitivity reactions.

Exotoxins vs. Endotoxins

Feature	Exotoxins	Endotoxins
Source	Mostly Gram-positive bacteria	Gram-negative bacteria
Chemical Nature	Proteins	Lipopolysaccharide (LPS)
Heat Stability	Unstable (destroyed by heat)	Stable
Toxicity	High	Low
Effect	Specific (e.g., neurotoxin)	General (fever, shock)

Chapter 16: Innate Immunity (Nonspecific Defenses)

Overview

Innate immunity provides immediate, nonspecific defense against pathogens. It includes physical, chemical, and cellular barriers.

• First line of defense: Skin, mucous membranes, secretions (tears, saliva, mucus), normal microbiota.

• Second line of defense: Phagocytes (neutrophils, macrophages), inflammation, fever, antimicrobial substances.

Phagocytosis

• Process by which phagocytes ingest and destroy microbes.

• Steps: Chemotaxis, adherence, ingestion, digestion.

Inflammation

• Redness, heat, swelling, pain; functions to contain infection and promote healing.

• Involves release of cytokines and recruitment of immune cells.

Fever

• Systemic response to infection; increases body temperature to inhibit pathogens and enhance immune response.

Antimicrobial Substances

• Include interferons, complement proteins, iron-binding proteins, and antimicrobial peptides.

Complement System

• Group of proteins that enhance immune responses via classical, alternative, and lectin pathways.

• Functions: Opsonization, inflammation, cell lysis.

Chapter 17: Adaptive Immunity (Specific Defenses)

Overview

Adaptive immunity is a specific defense system that develops after exposure to antigens. It involves lymphocytes (B and T cells) and produces memory for future responses.

• Humoral immunity: Mediated by B cells and antibodies; targets extracellular pathogens.

• Cell-mediated immunity: Mediated by T cells; targets intracellular pathogens and abnormal cells.

Antigens and Antibodies

• Antigen: Substance that provokes an immune response.

• Antibody: Protein produced by B cells that binds specifically to antigens.

• Antibody structure: Two heavy chains, two light chains, variable and constant regions.

Types of Immunity

• Active immunity: Results from exposure to antigen (infection or vaccination).

• Passive immunity: Transfer of antibodies (e.g., maternal antibodies, antiserum).

Major Histocompatibility Complex (MHC)

• Cell surface proteins important for antigen presentation and immune recognition.

• MHC I: Present on all nucleated cells; present endogenous antigens to CD8+ T cells.

• MHC II: Present on antigen-presenting cells; present exogenous antigens to CD4+ T cells.

Immune Responses

• Primary response: First exposure to antigen; slower, lower antibody production.

• Secondary response: Subsequent exposure; faster, stronger due to memory cells.

Types of Adaptive Immunity

Type	Source	Example
Natural Active	Infection	Recovery from measles
Artificial Active	Vaccination	MMR vaccine
Natural Passive	Maternal antibodies	Antibodies from placenta or breast milk
Artificial Passive	Injection of antibodies	Antiserum for snakebite

Additional info: These notes expand on the provided outline by adding definitions, examples, and context for each major topic, ensuring a comprehensive review for exam preparation.