Learning Objectives Unit 3

Principles of Disease

Symbiosis and Its Forms

Symbiosis refers to the close association between two different species, often with at least one benefiting from the relationship. The three main forms are:

• Mutualism: Both organisms benefit. Example: Escherichia coli in the human gut synthesizes vitamins for the host and receives nutrients.

• Commensalism: One organism benefits, the other is unaffected. Example: Skin bacteria that feed on secretions without harming the host.

• Parasitism: One organism benefits at the expense of the other. Example: Pathogenic bacteria causing disease in humans.

Key Epidemiological Terms

Understanding disease spread and occurrence requires kno wledge of several terms:

• Normal microbiota: Microorganisms that reside in or on the body without causing disease.

• Endemic disease: Disease constantly present in a population.

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

• Pandemic disease: Worldwide epidemic.

• Sporadic disease: Disease that occurs occasionally.

• Epidemiology: Study of disease distribution and determinants in populations.

Streptococcus pneumoniae and Pneumonia

Streptococcus pneumoniae is capable of causing typical pneumonia under conditions such as impaired host immunity, pre-existing lung disease, or exposure to large inocula.

Classification of Infectious Diseases

• Acute disease: Rapid onset, short duration.

• Chronic disease: Develops slowly, persists over time.

• Infectious disease: Caused by pathogenic microorganisms.

• Communicable disease: Can be transmitted from person to person.

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

• Local infection: Confined to a small area.

• Systemic infection: Spread throughout the body.

• Primary infection: Initial infection in a healthy host.

• Secondary infection: Subsequent infection by a different pathogen.

Koch's Postulates

Koch's Postulates are criteria used to establish a causative relationship between a microbe and a disease:

1. The microorganism must be found in all cases of the disease.

2. It must be isolated and grown in pure culture.

3. The cultured microorganism should cause disease when introduced into a healthy host.

4. It must be re-isolated from the experimentally infected host.

Exceptions: Some pathogens cannot be cultured, and some diseases are caused by multiple organisms.

Stages of Disease and Transmission

• Incubation period: Time between exposure and appearance of symptoms.

• Prodromal period: Early, mild symptoms.

• Period of illness: Most severe symptoms.

• Period of decline: Symptoms subside.

• Period of convalescence: Recovery.

Transmission can occur during any stage, especially illness and convalescence.

Reservoirs and Transmission

• Reservoir: Source of infection (human, animal, environment).

• Host: Organism harboring the pathogen.

• Vector: Organism transmitting the pathogen (e.g., mosquito).

Example: Rabies reservoirs include wild mammals; transmission occurs via bites.

Types of Transmission

• Direct transmission: Physical contact (touch, droplets).

• Indirect transmission: Via fomites, vectors, or airborne particles.

Nosocomial (Hospital-Acquired) Infections

• Definition: Infections acquired in healthcare settings.

• Contributing factors: Microorganisms in hospital, compromised host, chain of transmission.

• Control: Hand hygiene, sterilization, isolation procedures.

Pathogenicity and Virulence Factors

Portals of Entry and Exit

• Entry: Skin, mucous membranes, parenteral route.

• Exit: Respiratory tract, gastrointestinal tract, urogenital tract.

Adhesion Factors

Adhesion factors (e.g., pili, fimbriae) enable pathogens to attach to host tissues, a critical step in infection.

Virulence Factors

• Capsules: Prevent phagocytosis.

• Hyaluronidase and Collagenase: Break down host tissues.

• Coagulase and Kinase: Manipulate blood clotting to evade immune response.

Exotoxins vs. Endotoxins

Feature	Exotoxin	Endotoxin
Source	Gram-positive and Gram-negative bacteria	Gram-negative bacteria
Structure	Proteins	Lipopolysaccharide (LPS)
Release	Secreted	Released upon cell death
Immunogenicity	High	Low

Types of Exotoxins

• A-B toxins: Two-part toxins with active and binding components.

• Membrane-disrupting toxins: Cause cell lysis.

• Superantigens: Trigger excessive immune response.

Infection vs. Intoxication

• Infection: Pathogen grows and causes disease.

• Intoxication: Disease caused by ingestion of preformed toxins.

Exotoxin-Producing Bacteria

• Clostridium tetani: Tetanus toxin

• Clostridium botulinum: Botulinum toxin

• Staphylococcus aureus: Enterotoxins

• Vibrio cholerae: Cholera toxin

Cytopathic Effects (CPE)

• Definition: Observable changes in host cells due to viral infection.

• Types: Syncytia formation, inclusion bodies, cell lysis, rounding up.

Interferon and Antigenic Change

• Interferon: Proteins produced by host cells in response to viral infection; inhibit viral replication.

• Antigenic change: Alteration of viral surface antigens to evade immune detection.

Co-evolution

Co-evolution describes the reciprocal evolutionary adaptations between hosts and pathogens.

Immunity and Host Defense

Cells of the Immune System

• Myeloid cells: Mast cells, basophils, eosinophils, neutrophils, monocytes/macrophages, dendritic cells.

• Lymphoid cells: B cells, T cells, Natural Killer (NK) cells.

Innate vs. Adaptive Immunity

• Innate immunity: Non-specific, immediate defense (skin, mucous membranes, phagocytes).

• Adaptive immunity: Specific, acquired defense (B and T lymphocytes, antibodies).

Skin and Mucous Membranes

Act as physical and chemical barriers to infection; produce antimicrobial substances.

Phagocytosis and Pattern Recognition

• Steps of phagocytosis: Chemotaxis, adherence, ingestion, digestion, exocytosis.

• Pattern Recognition Receptors (PRRs): Detect pathogen-associated molecular patterns (PAMPs).

• Opsonization: Coating of pathogens to enhance phagocytosis.

Complement Pathways

• Classical pathway: Activated by antigen-antibody complexes.

• Alternative/Lectin pathway: Activated by microbial surfaces.

Functions of Complement

• Opsonization: Enhances phagocytosis.

• Inflammation: Attracts immune cells.

• Cytolysis: Lysis of pathogens.

Host Defense Mechanisms

• Inflammation: Local response to injury/infection.

• Interferon production: Antiviral response.

• Fever: Elevated body temperature inhibits pathogens.

Cytokine Storm

Excessive release of cytokines leading to severe inflammation; key cytokines include IL-1, IL-6, TNF-α.

Lymphoid Organs

• Bone marrow: Hematopoiesis.

• Thymus: T cell maturation.

• Lymph nodes: Immune response initiation.

• Spleen: Filters blood, immune surveillance.

• Mucosa-associated lymphoid tissue (MALT): Immune defense at mucosal surfaces.

Antibody Structure and Classes

• Heavy chain: Determines antibody class.

• Light chain: Contributes to antigen binding.

Class	Main Function
IgG	Most abundant, crosses placenta
IgM	First produced, pentamer
IgA	Secretions, mucosal immunity
IgE	Allergic responses, parasites
IgD	B cell receptor

Antibody Functions

• Agglutination

• Neutralization

• Opsonization

• Complement fixation

• Antibody-dependent cellular cytotoxicity

Genetic Recombination in Antibody Diversity

Antibody diversity is generated by recombination of gene segments encoding variable regions.

Major Histocompatibility Complex (MHC)

Feature	MHC I	MHC II
Cell type	All nucleated cells	Antigen-presenting cells
Antigen source	Endogenous	Exogenous
Interacts with	CD8+ T cells	CD4+ T cells

T Helper Cells vs. T Cytotoxic Cells

• T Helper (Th) cells: Express CD4, interact with MHC II, coordinate immune response.

• T Cytotoxic (Tc) cells: Express CD8, interact with MHC I, kill infected cells.

Immunological Synapse

Specialized junction between T cell and antigen-presenting cell, facilitating signal transduction.

Primary Immune Responses

• Cell-mediated (Th1): Involves T cells, cytokines, targets intracellular pathogens.

• Humoral (Th2): Involves B cells, antibody production, targets extracellular pathogens.

Clonal selection: Activation and proliferation of specific lymphocytes.

Class switching: Change in antibody class produced by B cells.

Memory Cells and Secondary Response

Memory cells enable rapid and robust response upon re-exposure to antigen.

Microbial Evasion of Immunity

• Trypanosoma brucei: Antigenic variation.

• Influenza virus type A: Antigenic drift and shift.

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard microbiology curriculum.