Study Guide: Disease Processes, Pathogenicity, and Host Defenses in Microbiology

Principles of Disease

Symbiosis and Its Forms

Symbiosis refers to the close association between two different species living together. In microbiology, it describes the relationship between microorganisms and their hosts.

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

• Commensalism: One organism benefits, while the other is neither helped nor harmed. Example: Skin microbiota living on human skin.

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

Key Terms in Disease and Epidemiology

• Normal microbiota: Microorganisms that colonize the body without causing disease under normal conditions.

• 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 in a population.

• Epidemiology: Study of where and when diseases occur and how they are transmitted.

Streptococcus pneumoniae and Pneumonia

• Condition for Disease: Streptococcus pneumoniae can cause typical pneumonia when the host's immune defenses are compromised, or when the bacteria evade immune responses (e.g., via capsule formation).

Classification of Infectious Diseases

• Acute disease: Develops rapidly but lasts a short time (e.g., influenza).

• Chronic disease: Develops slowly and persists over a long period (e.g., tuberculosis).

• Infectious disease: Caused by pathogenic microorganisms.

• Communicable disease: Can be spread from one host to another.

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

• Local infection: Pathogens are limited to a small area of the body.

• Systemic infection: Infection spreads throughout the body.

• Primary infection: Acute infection that causes the initial illness.

• Secondary infection: Caused by an opportunistic pathogen after the primary infection has weakened the host.

Koch’s Postulates

• Set of criteria to establish a causative relationship between a microbe and a disease:

  1. The microorganism must be found in all organisms suffering from the disease, but not in healthy organisms.

  2. The microorganism must be isolated from a diseased organism and grown in pure culture.

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

  4. The microorganism must be re-isolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.

• Exceptions: Some pathogens cannot be cultured in the lab, some diseases are caused by multiple pathogens, and some pathogens cause different diseases in different hosts.

Stages of Disease

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

• Prodromal period: Early, mild symptoms.

• Period of illness: Disease is most severe; symptoms are evident.

• Period of decline: Symptoms subside.

• Period of convalescence: Recovery occurs; body returns to pre-diseased state.

• Transmission: Can occur during incubation, prodromal, illness, and sometimes convalescence.

• Secondary infections: Often occur during illness or decline when host defenses are weakened.

Reservoirs, Hosts, and Vectors

• Reservoir: Natural habitat of a pathogen (human, animal, or nonliving).

• Host: Organism that harbors the pathogen.

• Vector: Living organism (often an arthropod) that transmits pathogens between hosts.

• Zoonotic: Diseases that are transmitted from animals to humans.

Reservoirs and Rabies Transmission

• Reservoirs (e.g., bats, raccoons) maintain rabies virus in nature and are critical for transmission to humans.

Transmission of Disease

• Direct transmission: Physical contact between infected and susceptible host (e.g., touching, kissing).

• Indirect transmission: Via a nonliving object (fomite), vector, or airborne droplets.

• Classification: Transmission can be classified as direct (person-to-person) or indirect (fomites, vectors, vehicles).

Nosocomial Infections

• Definition: Infections acquired in healthcare settings.

• Contributing factors:

  • Microorganisms in hospital environment

  • Compromised host

  • Chain of transmission

• Control: Hospital practices can control chain of transmission (e.g., hand hygiene, sterilization).

Microbial Mechanisms of Pathogenicity

Portals of Entry and Exit

• Portals of entry: Mucous membranes, skin, parenteral route (injection, bites, wounds).

• Portals of exit: Respiratory tract, gastrointestinal tract, genitourinary tract, skin, blood.

Adhesion Factors

• Microorganisms use adhesion factors (e.g., fimbriae, pili, adhesins) to attach to host tissues, a critical step in pathogenesis.

Virulence Factors

• Capsules: Prevent phagocytosis by host cells.

• Hyaluronidase and collagenase: Enzymes that degrade host tissues, facilitating spread.

• Coagulase: Clots fibrinogen, protecting bacteria from immune cells.

• Kinase: Dissolves clots, allowing spread of bacteria.

Exotoxins vs. Endotoxins

Feature	Exotoxins	Endotoxins
Source	Mostly Gram-positive bacteria	Gram-negative bacteria (outer membrane)
Structure	Proteins	Lipopolysaccharide (LPS)
Release	Secreted during growth	Released upon cell death
Immunogenicity	Highly immunogenic (can form toxoids)	Weakly immunogenic

Types of Exotoxins

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

• Membrane-disrupting toxins: Cause cell lysis by disrupting plasma membranes.

• Superantigens: Cause excessive immune response.

Infection vs. Intoxication

• Infection: Pathogen grows and multiplies in the host.

• Intoxication: Disease results from ingestion of preformed toxins.

Exotoxin-Producing Bacteria

• Clostridium tetani: Produces tetanospasmin (neurotoxin causing tetanus).

• Clostridium botulinum: Produces botulinum toxin (causes flaccid paralysis).

• Staphylococcus aureus: Produces enterotoxins (food poisoning) and toxic shock syndrome toxin.

• Vibrio cholerae: Produces cholera toxin (causes watery diarrhea).

Cytopathic Effects (CPE)

• Definition: Visible effects of viral infection on host cells.

• Types:

  • Syncytia formation: Fusion of cells into multinucleated giant cells.

  • Inclusion bodies: Aggregates of viral particles or altered host cell components.

  • Cell lysis and rounding up: Destruction and morphological changes in host cells.

Interferons and Antigenic Change

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

• Antigenic change: Alteration of surface antigens on infected cells, helping viruses evade immune detection.

Co-evolution

• Process by which hosts and pathogens adapt to each other over time, influencing each other's evolution.

Host Defenses: Innate and Adaptive Immunity

Immune Cell Types and Functions

Cell Type	Function
Mast cells	Release histamine; involved in inflammation and allergy
Basophils	Release histamine; involved in allergic responses
Eosinophils	Combat parasites; involved in allergic responses
Neutrophils	Phagocytosis of pathogens
Monocytes/Macrophages	Phagocytosis; antigen presentation
Dendritic cells	Antigen presentation; activate T cells
B cells	Produce antibodies
T cells	Cell-mediated immunity
Natural Killer (NK) cells	Destroy infected or abnormal cells

Innate vs. Adaptive Immunity

• Innate immunity: Non-specific, immediate defense mechanisms (e.g., skin, phagocytes).

• Adaptive immunity: Specific, slower response involving lymphocytes and memory.

Physical Barriers

• Skin: Physical barrier; secretes antimicrobial substances.

• Mucous membranes: Trap and remove microbes; contain immune cells.

Phagocytosis

• Six steps: Chemotaxis, adherence, ingestion, formation of phagosome, fusion with lysosome, digestion and exocytosis.

• Pattern recognition receptors (PRRs): Detect pathogen-associated molecular patterns (PAMPs).

• Opsonization: Coating of pathogens with molecules (e.g., antibodies, complement) to enhance phagocytosis.

• Evasion: Some microbes (e.g., Leishmania donovani) avoid phagocytosis by surviving inside phagocytes.

Complement System

• Classical pathway: Triggered by antibodies bound to antigen.

• Alternative/Lectin pathways: Triggered by microbial surfaces or lectin binding.

• Functions:

  • Opsonization (C3b coats pathogens)

  • Inflammation (C3a, C5a recruit immune cells)

  • Cytolysis (formation of membrane attack complex)

Other Innate Defenses

• Inflammation: Local response to infection or injury; increases blood flow and immune cell recruitment.

• Interferon production: Inhibits viral replication.

• Fever: Elevated body temperature inhibits pathogens and enhances immune responses.

Cytokine Storm

• Excessive, uncontrolled release of cytokines (e.g., IL-1, IL-6, TNF-α) leading to tissue damage.

Lymphoid Organs

Organ/Site	Function
Bone marrow	Origin of all blood cells; B cell maturation
Thymus	T cell maturation
Lymph nodes	Filter lymph; site of immune activation
Spleen	Filters blood; immune responses to blood-borne antigens
MALT	Immune responses at mucosal surfaces

Antibody Structure

• Composed of two heavy chains and two light chains.

• Variable region: Antigen-binding site (Fab region).

• Constant region: Determines antibody class (Fc region is the stem).

Generation of Antibody Diversity

• Genetic recombination (V(D)J recombination) creates diverse antibody specificities.

Classes of Antibodies

Class	Main Features
IgG	Most abundant; crosses placenta; opsonization
IgM	First produced; pentamer; agglutination
IgA	Secreted in mucosal areas; dimer
IgD	On B cell surface; function unclear
IgE	Allergic responses; defense against parasites

Functions of Antibodies

• Agglutination: Clumping of antigens.

• Neutralization: Blocks pathogen binding or toxin activity.

• Opsonization: Enhances phagocytosis.

• Complement fixation: Activates complement system.

• Antibody-dependent cellular cytotoxicity (ADCC): NK cells destroy antibody-coated cells.

Major Histocompatibility Complex (MHC)

Feature	MHC I	MHC II
Distribution	All nucleated cells	Antigen-presenting cells
Antigen type	Endogenous	Exogenous
T cell interaction	CD8+ (cytotoxic T cells)	CD4+ (helper T cells)

T Helper vs. T Cytotoxic Cells

• T helper cells (CD4+): Interact with MHC II; coordinate immune response via cytokines.

• T cytotoxic cells (CD8+): Interact with MHC I; kill infected cells.

Immunological Synapse

• Contact point between T cell and antigen-presenting cell, involving T cell receptor and MHC-peptide complex.

Primary Cell-Mediated (Th1) Response

• Lymphoid organs: Lymph nodes, spleen.

• Cell types: Th1 cells, cytotoxic T cells (Tc), macrophages.

• Cytokines: IFN-γ, IL-2.

• Function: Tc cells kill infected cells; Th1 cells activate macrophages.

Primary Humoral (Th2) Response

• Lymphoid organs: Lymph nodes, spleen.

• Cell types: Th2 cells, B cells.

• Clonal selection: B cells with specific receptors proliferate.

• Antibodies: Produced by plasma cells.

• Cytokines: IL-4, IL-5, IL-6.

• B cell maturation: Into plasma cells (antibody production) or memory cells.

• Class switching: B cells change antibody class (e.g., IgM to IgG).

Memory Cells and Secondary Response

• Memory cells enable a faster and stronger response upon re-exposure to the same antigen.

Immune Evasion by Pathogens

• Trypanosoma brucei: Changes surface glycoproteins to evade antibodies.

• Influenza virus type A: Undergoes antigenic drift and shift to escape immune detection.

Types of Acquired Immunity

Type	How Acquired	Example
Naturally acquired active	Infection	Recovery from measles
Naturally acquired passive	Maternal antibodies	Placental transfer of IgG
Artificially acquired active	Vaccination	MMR vaccine
Artificially acquired passive	Injection of antibodies	Antitoxin administration

Applications in Immunology

Vaccines and Herd Immunity

• Vaccine: Preparation of antigens used to induce immunity.

• Herd immunity: Protection of non-immune individuals due to high prevalence of immunity in the population.

Whole Agent Vaccines

• Attenuated: Live, weakened pathogens; strong, long-lasting immunity but risk for immunocompromised.

• Inactivated: Killed pathogens; safer but may require boosters.

Salk vs. Sabin Polio Vaccines

• Salk (inactivated): Safe, no risk of vaccine-derived polio; used in polio-free regions.

• Sabin (attenuated): Oral, induces gut immunity, but rare risk of reversion to virulence; used in endemic areas.

Subunit Vaccines

• Toxoid vaccines: Inactivated toxins (e.g., tetanus toxoid).

• Acellular vaccines: Contain purified components of pathogens.

• Recombinant vaccines: Produced by genetic engineering.

• DNA/mRNA vaccines: Use genetic material to induce antigen production in host cells.

Agglutination and Antibody Titer

• Agglutination tests measure the concentration of antibodies by observing clumping of antigen-antibody complexes.

ELISA Techniques

Feature	Antibody-Sandwich ELISA	Indirect ELISA
What is tested	Antigen	Antibody
What is adsorbed	Capture antibody	Antigen
Enzyme-linked antibody recognizes	Antigen	Primary antibody