Epidemiology, Pathogenicity, Immunity, and Immune Disorders: Study Notes for Microbiology

Ch14 - Principles of Disease and Epidemiology

Symbiotic Relationships

Microorganisms interact with hosts in various ways, forming symbiotic relationships:

• Commensalism: One organism benefits, the other is unaffected. Example: Staphylococcus epidermidis on skin.

• Mutualism: Both organisms benefit. Example: Escherichia coli in the gut produces vitamins for the host.

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

Koch’s Postulates

Koch’s postulates are criteria used to establish a causal relationship between a microbe and a disease:

• Microorganism must be found in all cases of the disease.

• It must be isolated and grown in pure culture.

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

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

Communicable vs. Noncommunicable Diseases

• Communicable disease: Can be transmitted from one host to another (e.g., influenza).

• Noncommunicable disease: Not spread between hosts (e.g., tetanus).

Classification of Diseases

• Frequency: Sporadic (occasional), Endemic (constant presence), Epidemic (sudden increase), Pandemic (global epidemic).

• Severity: Acute (rapid onset), Chronic (long duration), Subacute (intermediate), Latent (inactive, then active).

Incidence vs. Prevalence

• Incidence: Number of new cases in a given time period.

• Prevalence: Total number of cases (new and existing) at a given time.

Predisposing Factors for Disease

• Genetic background

• Age

• Gender

• Lifestyle (e.g., smoking, nutrition)

Patterns of Disease

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

• Vector: Organism that transmits pathogens (e.g., mosquitoes).

Transmission of Disease

• Contact transmission: Direct (person-to-person), indirect (via fomites), droplet (coughing/sneezing).

• Vehicle transmission: Via water, food, air.

• Mechanical transmission: Passive transport by vectors (e.g., flies).

• Biological transmission: Pathogen reproduces in vector (e.g., malaria in mosquitoes).

Healthcare-Associated Infections (HAIs)

• Infections acquired in healthcare settings (e.g., hospitals).

• Common types: surgical site infections, pneumonia, urinary tract infections.

Definition of Epidemiology

• Study of the distribution, determinants, and control of diseases in populations.

Ch15 - Microbial Mechanisms of Pathogenicity

Portals of Entry

Microbes enter the host through specific portals:

• Skin

• Mucous membranes (respiratory, gastrointestinal, genitourinary tracts)

• Parenteral route (injuries, bites)

Adherence to Host Cells

• Microbes use adhesins (surface molecules) to attach to host cells.

• Example: Streptococcus pyogenes uses fimbriae to adhere to throat cells.

Capsules and Cell Wall Components

• Capsules: Prevent phagocytosis (e.g., Streptococcus pneumoniae).

• Cell wall components: M protein, mycolic acid, etc., aid in evading host defenses.

Effects of Enzymes

• Enzymes such as coagulase, hyaluronidase, and collagenase help microbes invade tissues.

Mechanisms to Avoid Phagocytosis

• Capsules

• Cell wall proteins

• Enzyme production

Direct Damage

• Microbes damage host cells by using nutrients, producing waste, or physically disrupting cells.

Exotoxins and Endotoxins

• Exotoxins: Secreted proteins, highly specific, potent (e.g., botulinum toxin).

• Endotoxins: Lipid A component of Gram-negative bacteria, released upon cell death, causes fever and shock.

Cytopathic Effects of Viral Infections

• Cell death, inclusion bodies, syncytia formation, transformation to cancerous cells.

Chapter 16 - Innate Immunity: Nonspecific Defenses of the Host

Immunity Overview

• Immunity: Ability to resist infection.

• Innate immunity: Nonspecific, present at birth.

• Adaptive immunity: Specific, acquired after exposure.

PAMPs and PRMs

• PAMPs: Pathogen-Associated Molecular Patterns (e.g., LPS, flagellin).

• PRMs: Pattern Recognition Molecules (e.g., Toll-like receptors) recognize PAMPs.

First Line of Defense

• Physical factors: Skin, mucous membranes, cilia, tears.

• Chemical factors: Lysozyme, acidic pH, sebum.

Second Line of Defense

• Phagocytes: Neutrophils, macrophages ingest pathogens.

• Natural killer cells: Destroy infected or abnormal cells.

Innate Immune Cells

• Neutrophils

• Macrophages

• Natural killer cells

Inflammatory Response

• Vasodilation (increased blood flow)

• Erythema (redness)

• Edema (swelling)

• Recruitment of immune cells

Phagocytosis

• Process by which cells ingest and destroy microbes.

• Steps: Chemotaxis, adherence, ingestion, digestion.

Fever

• Systemic response to infection.

• Elevated body temperature inhibits pathogens and enhances immune function.

Complement System

• Group of proteins that enhance immune responses.

• Outcomes: Opsonization, inflammation, cell lysis.

Chapter 17 - Adaptive Immunity: Specific Defenses of the Host

Types of T Cells and Their Roles

• Helper T cells (Th): Activate B cells and other T cells.

• Cytotoxic T cells (Tc): Destroy infected cells.

• Regulatory T cells: Suppress immune responses.

Antigen Presentation and MHC

• MHC I: Presents antigens to CD8+ cytotoxic T cells.

• MHC II: Presents antigens to CD4+ helper T cells.

CD4 and CD8 Receptors

• CD4: Found on helper T cells.

• CD8: Found on cytotoxic T cells.

Cell-Mediated Immunity

• Cytotoxic T cells attack infected, cancerous, or foreign cells.

• Destruction via perforin and granzymes.

Antibody-Mediated (Humoral) Immunity

• B cells produce antibodies.

• MHC II present on B cells.

• Helper T cells (Th2) activate B cells.

• Memory B cells provide long-term immunity.

Immunoglobulin Classes

Class	Function/Characteristic
IgG	Most abundant; crosses placenta
IgM	First produced upon initial exposure
IgA	Present in secretions (tears, breast milk)
IgE	Involved in hypersensitivity (allergies)
IgD	Functions mainly as a B cell receptor

Structure of Immunoglobulin

• Y-shaped molecule with two heavy and two light chains.

• Variable region binds antigen; constant region determines class.

Primary vs. Secondary Immune Response

• Primary: First exposure, slower, mainly IgM.

• Secondary: Subsequent exposure, faster, mainly IgG.

Opsonization

• Process by which antibodies or complement coat pathogens, enhancing phagocytosis.

Natural vs. Artificial Immunity

• Natural: Acquired through infection or maternal antibodies.

• Artificial: Acquired through vaccination or injection of antibodies.

Active vs. Passive Immunity

Type	Source	Example
Active	Own immune response	Vaccination, infection
Passive	Received antibodies	Maternal IgG, antiserum

Vaccines

• Preparations that stimulate active immunity against specific pathogens.

Chapter 18 - Practical Applications of Immunology

Biochemical Tests

• Based on detection of specific microbial products or reactions.

Serology

• Study of blood serum and immune responses.

Agglutination and Precipitation

• Agglutination: Clumping of cells or particles by antibodies.

• Precipitation: Formation of insoluble complexes between antibodies and soluble antigens.

Chapter 19 - Disorders Associated with the Immune System

Hypersensitivity

• Exaggerated immune response causing tissue damage.

Types of Hypersensitivity

Type	Mechanism	Example
I	IgE-mediated, immediate	Allergies, anaphylaxis
II	Cytotoxic, IgG/IgM + complement	Hemolytic anemia
III	Immune complex, IgG/IgM/IgA	Serum sickness
IV	Cell-mediated, delayed (T cells)	Contact dermatitis

• Type I: IgE, mast cells, histamine release.

• Type II: Complement activation, cell lysis.

• Type III: Immune complexes lodge in tissues.

• Type IV: T cells, cytokines, delayed response.

Additional info: Academic context and examples were added to clarify and expand brief points from the original notes.