Ch. 14/15 Principles of Disease and Microbial Pathogenicity

Introduction to Disease and Pathogenicity

Understanding how diseases develop and spread is fundamental in microbiology. This section covers the definitions, classifications, and mechanisms of infectious diseases.

• Infection: Entry, establishment, and multiplication of a pathogen in a host.

• Disease: A change from a state of good health; the result of an infection that impairs normal body function.

• Pathology: The scientific study of disease, including its cause, development, and effects on the body.

• Etiology: The cause of a disease.

• Pathogenesis: The manner in which a disease develops.

• Infectious disease: Disease caused by pathogenic microorganisms.

• Communicable disease: Disease that can be spread from one host to another (e.g., influenza, chickenpox).

• Noncommunicable disease: Disease not spread from one host to another (e.g., tetanus).

• Contagious disease: Easily spread from one person to another.

• Pandemic: Worldwide epidemic (e.g., COVID-19, 1918 influenza).

• Reservoir: Continual source of infection (human, animal, or nonliving).

• Carrier: Individuals who harbor pathogens without showing symptoms but can transmit them to others.

• Vector: Animals (often arthropods) that transmit pathogens between hosts.

• Transmission: The way a pathogen spreads (direct, indirect, droplet, vehicle, vector).

Mechanisms of Pathogenicity

• Portal of entry: The route by which a pathogen enters the body (e.g., mucous membranes, skin, parenteral route).

• Virulence: The degree of pathogenicity of a microorganism.

• Virulence factors: Traits that enhance a pathogen's ability to cause disease (e.g., toxins, enzymes, capsules).

• Pathogen strategies: Include adherence to host tissues, evasion of host defenses, and damage to host cells.

• Antigenic variation: Pathogens alter their surface antigens to evade immune detection (e.g., influenza virus).

Host Responses and Disease Symptoms

• Exotoxins: Toxic proteins secreted by bacteria that cause damage to host tissues.

• Endotoxins: Lipopolysaccharide components of the outer membrane of Gram-negative bacteria, released upon cell death.

• Symptoms: Subjective changes felt by the patient (e.g., pain, fatigue).

• Signs: Objective changes observed by others (e.g., fever, rash).

Ch. 16 Innate Immunity: Nonspecific Defenses of the Host

Overview of Innate Immunity

Innate immunity provides the first line of defense against pathogens, using physical, chemical, and cellular mechanisms that are not specific to particular pathogens.

• Surface barriers: Skin, mucous membranes, and their secretions act as physical and chemical barriers.

• Second line of defense: Includes phagocytic cells, inflammation, fever, and antimicrobial substances.

• Phagocytes: White blood cells (e.g., neutrophils, macrophages) that ingest and destroy pathogens.

• Inflammation: Localized response to injury or infection, characterized by redness, heat, swelling, and pain.

• Fever: Elevated body temperature that enhances immune responses and inhibits pathogen growth.

• Interferons: Proteins produced by virus-infected cells that inhibit viral replication in neighboring cells.

• Complement system: Group of proteins that enhance immune responses by promoting phagocytosis and cell lysis.

Leukocytes and Their Functions

• Neutrophils: First responders that engulf and destroy bacteria.

• Eosinophils: Combat multicellular parasites and are involved in allergic responses.

• Basophils: Release histamine during allergic reactions.

• Monocytes: Circulate in the blood and differentiate into macrophages in tissues.

Ch. 17 Adaptive Immunity: Specific Defenses of the Host

Overview of Adaptive Immunity

Adaptive immunity is characterized by specificity and memory, allowing the immune system to recognize and respond more effectively to previously encountered pathogens.

• Antigen: Any substance that elicits an immune response; typically proteins or polysaccharides on the surface of pathogens.

• Epitope: The specific part of an antigen recognized by antibodies.

• Antibody (immunoglobulin): Protein produced by B cells that binds specifically to an antigen.

• Humoral immunity: Involves B cells and the production of antibodies to neutralize extracellular pathogens.

• Cell-mediated immunity: Involves T cells that target infected or abnormal cells.

• Helper T cells (CD4+): Activate other immune cells by releasing cytokines.

• Cytotoxic T cells (CD8+): Directly kill infected or abnormal cells.

Antibody Structure and Function

• Variable region: Binds to specific antigens.

• Constant region: Determines the antibody class (IgG, IgM, IgA, IgE, IgD).

• Neutralization: Antibodies block pathogen binding sites.

• Opsonization: Antibodies enhance phagocytosis.

• Complement activation: Antibodies trigger the complement system to destroy pathogens.

Ch. 18 Practical Applications of Immunology

Vaccines and Immunization

Vaccines stimulate the immune system to develop protection against specific pathogens, providing immunity without causing disease.

• Active immunity: The body produces its own antibodies in response to exposure to antigens (natural infection or vaccination).

• Passive immunity: Antibodies are transferred from another source (e.g., maternal antibodies, antibody therapy).

• Herd immunity: When a large portion of the population is immune, the spread of disease is reduced.

Types of Vaccines

• Live attenuated vaccines: Contain weakened pathogens that replicate without causing disease.

• Inactivated vaccines: Contain killed pathogens.

• Subunit, recombinant, and conjugate vaccines: Contain only parts of the pathogen (e.g., proteins, polysaccharides).

• Toxoid vaccines: Contain inactivated toxins.

• mRNA vaccines: Contain messenger RNA encoding a pathogen protein; host cells produce the protein, triggering immunity.

History and Impact of Vaccination

• Edward Jenner introduced the first vaccine (smallpox) in 1796.

• Vaccines have eradicated or controlled many infectious diseases (e.g., smallpox, polio).

Ch. 20 Antimicrobial Drugs

Overview of Antimicrobial Agents

Antimicrobial drugs are used to treat infections by killing or inhibiting the growth of microorganisms. They can be classified by their mode of action and spectrum of activity.

• Antibiotics: Substances produced by microorganisms that inhibit or kill other microbes.

• Bactericidal: Kill bacteria.

• Bacteriostatic: Inhibit bacterial growth.

• Broad-spectrum antibiotics: Effective against a wide range of bacteria.

• Narrow-spectrum antibiotics: Target specific types of bacteria.

Mechanisms of Action

• Inhibition of cell wall synthesis: Prevents bacteria from forming peptidoglycan cell walls (e.g., penicillins, cephalosporins).

• Inhibition of protein synthesis: Blocks bacterial ribosomes (e.g., tetracyclines, macrolides).

• Inhibition of nucleic acid synthesis: Interferes with DNA/RNA replication (e.g., quinolones, rifamycins).

• Injury to plasma membrane: Disrupts membrane integrity (e.g., polymyxins).

• Inhibition of essential metabolite synthesis: Blocks metabolic pathways (e.g., sulfonamides).

Antibiotic Resistance

• Horizontal gene transfer: Bacteria share resistance genes via plasmids, transduction, or transformation.

• Superbugs: Bacteria resistant to multiple antibiotics (e.g., MRSA).

• Antibiotic misuse: Overuse or inappropriate use of antibiotics accelerates resistance.

• Phage therapy: Use of bacteriophages to treat bacterial infections as an alternative to antibiotics.

Tables

Key Equations and Concepts

• Basic Reproductive Number (R0): Indicates the average number of secondary infections produced by one infected individual in a susceptible population.

• Herd Immunity Threshold: The proportion of immune individuals needed to prevent disease spread.

Additional info:

• Some definitions and examples were expanded for clarity and completeness.

• Table entries were inferred and supplemented for study purposes.