Microbiology Study Guide: Infection, Immunity, and Adaptive Responses

Chapter 14: Infection, Infectious Diseases, and Epidemiology

Stages of Infection and Disease

The progression of infectious disease follows a series of stages, each characterized by specific clinical and microbiological features.

• Incubation Period: Time between exposure to pathogen and onset of symptoms.

• Prodromal Period: Early, mild symptoms appear.

• Illness: Disease is most severe; characteristic signs and symptoms are present.

• Decline: Symptoms begin to subside as immune response or treatment reduces pathogen numbers.

• Convalescence: Recovery and return to normal health.

Pathogen vs Microbe

• Microbe: Any microscopic organism, including bacteria, viruses, fungi, and protozoa.

• Pathogen: A microbe capable of causing disease in a host.

• Example: Escherichia coli is a microbe; certain strains are pathogens.

Epidemiology and Etiology

• Epidemiology: Study of the distribution, frequency, and determinants of diseases in populations.

• Etiology: Study of the cause or origin of disease.

• Example: Identifying the etiological agent of tuberculosis as Mycobacterium tuberculosis.

Symbiotic Relationship Terminology

• Mutualism: Both organisms benefit.

• Commensalism: One benefits, the other is unaffected.

• Parasitism: One benefits at the expense of the other.

Modes of Transmission

• Contact Transmission: Direct (physical contact), indirect (via fomites), or droplet.

• Vehicle Transmission: Via air, water, or food.

• Vector Transmission: Via animals (usually arthropods) that carry pathogens from one host to another.

Microbiome

• Microbiome: The collection of microorganisms living in and on the human body, contributing to health and disease.

Contamination, Infection, and Disease

• Contamination: Presence of microbes on surfaces or tissues.

• Infection: Invasion and multiplication of microbes in host tissues.

• Disease: Disruption of normal body functions due to infection.

Sporadic, Endemic, Epidemic, Pandemic

• Sporadic: Occurs occasionally and irregularly.

• Endemic: Constantly present in a population.

• Epidemic: Sudden increase in cases above expected levels.

• Pandemic: Epidemic that spreads across countries or continents.

Koch's Postulates

• 1. The suspected pathogen must be present in every case of the disease.

• 2. The pathogen must be isolated and grown in pure culture.

• 3. The cultured pathogen must cause disease when introduced into a healthy host.

• 4. The same pathogen must be re-isolated from the experimentally infected host.

Signs vs Symptoms

• Signs: Objective, measurable indicators of disease (e.g., fever, rash).

• Symptoms: Subjective experiences reported by the patient (e.g., pain, fatigue).

Virulence and Pathogenicity

• Pathogenicity: Ability of a microbe to cause disease.

• Virulence: Degree of pathogenicity; how severe the disease is.

Chapter 15: Innate Immunity

Innate Immunity

Innate immunity is the body's first line of defense against pathogens, providing immediate, non-specific protection.

• Characteristics: Present at birth, rapid response, no memory.

• Components: Physical barriers, chemical barriers, cellular defenses.

First and Second Lines of Defense

• First Line: Skin, mucous membranes, secretions (e.g., lysozyme, acid).

• Second Line: Phagocytes, inflammation, fever, antimicrobial proteins.

Microbial Antagonism

• Definition: Normal microbiota compete with pathogens, preventing their colonization.

Leukocytes Involved in Innate Immunity

• Neutrophils: Phagocytosis and killing of microbes.

• Macrophages: Phagocytosis, antigen presentation.

• Natural Killer (NK) Cells: Destroy infected or abnormal cells.

• Eosinophils: Defense against parasites.

• Basophils: Release histamine, involved in inflammation.

Pathogen-Associated Molecular Patterns (PAMPs)

• PAMPs: Molecules found on pathogens (e.g., LPS, peptidoglycan) recognized by host immune cells.

Fever and Inflammation

• Fever: Elevated body temperature, inhibits pathogen growth, enhances immune response.

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

Steps of Phagocytosis (in order)

• 1. Chemotaxis: Movement toward chemical signals.

• 2. Adherence: Attachment to pathogen.

• 3. Ingestion: Engulfment of pathogen.

• 4. Digestion: Fusion with lysosome and destruction of pathogen.

• 5. Exocytosis: Release of debris.

Endotoxins vs Exotoxins

• Endotoxins: Lipopolysaccharide (LPS) from Gram-negative bacteria; released upon cell death.

• Exotoxins: Proteins secreted by bacteria; highly specific and potent.

Endogenous vs Exogenous Antigens

• Endogenous Antigens: Generated within cells (e.g., viral proteins).

• Exogenous Antigens: Enter the body from outside (e.g., bacteria, pollen).

Chapter 16: Adaptive Immunity

MHC I and II

Major Histocompatibility Complex (MHC) molecules are essential for antigen presentation and immune recognition.

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

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

T Cells and B Cells

• T Cells: Develop in thymus; mediate cellular immunity.

• B Cells: Develop in bone marrow; mediate humoral immunity (antibody production).

Antigen Presenting Cells (APCs)

• Function: Process and present antigens to T cells.

• Examples: Dendritic cells, macrophages, B cells.

Antibody Classes and Functions

• IgG: Most abundant; crosses placenta; opsonization, neutralization.

• IgM: First produced; pentamer; agglutination, complement activation.

• IgA: Found in mucosal areas; protects surfaces.

• IgE: Allergic responses; defense against parasites.

• IgD: B cell receptor; function not fully understood.

Apoptosis

• Definition: Programmed cell death; eliminates damaged or unnecessary cells.

Primary vs Secondary Immune Responses

• Primary Response: First exposure; slower, lower antibody levels.

• Secondary Response: Subsequent exposure; faster, higher antibody levels due to memory cells.

Antibody Functions

• Opsonization: Coating pathogens to enhance phagocytosis.

• Neutralization: Blocking pathogen attachment or toxin activity.

• Complement Activation: Triggering complement cascade for pathogen lysis.

• Inflammation: Promoting inflammatory response.

• Agglutination: Clumping pathogens for easier clearance.

• Antibody-Dependent Cellular Cytotoxicity (ADCC): Targeting cells for destruction by immune cells.

Cytokines

• Cytokines: Signaling proteins that regulate immune responses, inflammation, and cell communication.

Clonal Expansion, Deletion, and Selection

• Clonal Expansion: Proliferation of lymphocytes after activation.

• Clonal Deletion: Removal of self-reactive lymphocytes to prevent autoimmunity.

• Clonal Selection: Activation of lymphocytes specific to an antigen.

Lymphocyte Functions

• Cytotoxic T Cells (CD8+): Destroy infected or abnormal cells.

• B Cells: Produce antibodies.

• Helper T Cells (CD4+): Activate B cells and other immune cells.

• Regulatory T Cells: Suppress immune responses to maintain tolerance.

• Natural Killer Cells: Kill infected or cancerous cells without antigen specificity.