Microbiology and Host Defenses: Study Guide (Unit 3)

Chapter 11: Interactions Between Microbes and Humans

Key Concepts and Definitions

• Normal Microbiota: Microorganisms normally present on and in the body that typically have a symbiotic relationship with the host (e.g., skin, lower intestines).

• Sites with Normal Flora: Skin, respiratory tract, GI tract, external ear, external eye, and external genitalia.

• Microbial Pathogenicity: The ability of microbes to cause disease. Pathogenicity is influenced by factors such as virulence, portal of entry, and host defenses.

• Acquiring Microbiota: Colonization begins at birth (vaginal vs. C-section) and through environment (family, pets, food, air, surfaces, etc.).

• Portal of Entry: The specific route by which a pathogen gains entry to the host (e.g., respiratory, gastrointestinal, urogenital tracts, skin, placenta).

• Pathogens in Pregnancy: Some pathogens can cross the placenta and infect the fetus, leading to congenital infections (e.g., T.O.R.C.H. infections: Toxoplasma, Other [syphilis, varicella-zoster, parvovirus B19], Rubella, Cytomegalovirus, Herpes simplex virus).

• Toxins: Substances produced by microbes that damage host tissues or trigger host responses that cause damage. Exotoxins are secreted proteins (e.g., botulinum toxin), while endotoxins are lipopolysaccharides from Gram-negative cell walls.

• Latency: The ability of a pathogen to remain dormant within host tissues and reactivate later (e.g., herpesviruses, HIV, tuberculosis bacteria).

• Communicable Disease: A disease that can be transmitted from one host to another (directly or indirectly).

• Non-communicable Disease: A disease not spread from host to host (e.g., tetanus from environment).

• Etiologic Agent: The specific microorganism that causes a disease.

• Koch's Postulates: A set of criteria for establishing that a specific disease is caused by a specific microorganism.

Chapter 12: Epidemiology and Innate Immunity (Host Defenses I)

A. Epidemiology: Disease Transmission and Pathogenicity

• Epidemiology: The science of tracking disease occurrence and distribution.

• Incidence: Number of new cases in a population during a specific time period.

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

• Reservoir: Where pathogens persist (e.g., animals, humans, environment).

• Transmission: Can be biological (via vector), direct, or indirect (fomites, aerosols).

• Florence Nightingale: Pioneered epidemiological methods in public health in the mid-1800s.

B. Lines of Host Defense

• The immune system is divided into innate (nonspecific) and adaptive (specific) components.

Line of Defense	Description	Components/Characteristics
First	Surface protection (anatomical and physiological barriers)	Nonspecific; Physical barriers (skin, mucous membranes), chemical barriers (lysozyme, stomach acid), genetic barriers
Second	Cellular and chemical system that comes into play if pathogens pass the first line	Nonspecific; Phagocytes, inflammation, fever, antimicrobial proteins
Third	Specific host defenses developed for each microbe	Acquired, specific; Lymphocytes (B cells, T cells), antibodies

C. Second Line of Defense Components

• Phagocytosis: The process by which professional phagocytes (neutrophils, macrophages, dendritic cells) engulf and destroy pathogens.

• Inflammation: Localized response to infection or injury, characterized by redness, heat, swelling, and pain. Mediated by cytokines and other immune factors.

• Fever: Systemic increase in body temperature, which can inhibit pathogen growth and enhance immune responses.

• Complement System: Group of serum proteins that enhance phagocytosis, lyse pathogens, and promote inflammation.

Chapter 13: Specific Immunity (Adaptive Response/Host Defenses II)

A. Humoral Immunity (B Cells)

• B Cells: Function as antibody-secreting machines, each with over 100,000 antibody copies on its surface.

• Activation: An immunocompetent B cell matures in peripheral tissues and is stimulated by antigen exposure. This leads to proliferation (making many copies).

• Outcomes of Activation:

  • Memory B cells: Store large quantities of antibodies.

  • Regulatory B cells: Modulate immune response.

• Antibody Structure: The Y-shaped structure has variable and constant regions.

• Ig (Immunoglobulin) Types:

  • IgG: Most common in serum (80%), crosses the placenta, and neutralizes toxins/viruses.

  • IgM: First antibody during infection, a pentamer structure that is highly effective at agglutination.

  • IgA: Found in secretions (mucus, saliva, tears, breast milk).

  • IgE: Involved in allergic reactions and defense against parasites.

  • IgD: Found on B cell surfaces, function not well understood.

B. Cell-Mediated Immunity (T Cells)

• T Cell Activation: Occurs when antigen is presented with MHC molecules on antigen-presenting cells (APCs).

• MHC I: Found on all nucleated cells; presents endogenous antigens to CD8+ cytotoxic T cells.

• MHC II: Found on professional APCs (macrophages, dendritic cells, B cells); presents exogenous antigens to CD4+ helper T cells.

• Cytotoxic T Cells (CD8+): Destroy virus-infected cells and cancer cells.

• Helper T Cells (CD4+): Coordinate the immune response by activating other immune cells.

C. Immune Response and Vaccination

• Primary Response: The first response to an antigen, characterized by a latent period and the initial production of IgM.

• Secondary Response (Anamnestic Response): Occurs upon re-exposure, characterized by a much faster and stronger response, primarily due to a rapid increase in IgG.

• Vaccine Types:

  • Whole cell/virus: Live, attenuated (weakened) cells or viruses, or killed/inactivated cells or viruses.

  • Antigenic molecules (subunits): Selected derived from cultures of cells, viruses, or synthesized biochemically.

  • Toxoid: Inactivated toxins that have been chemically altered so no longer cause disease (e.g., tetanus toxoid).

• Herd Immunity: The collective immunity in a population that reduces indirect protection to non-immune individuals when a large percentage are immune.

Chapter 15: Specimen Collection, Laboratory Diagnosis, and Serology

Key Concepts and Methods

Concept	Description
Analytical Microbial Diagnosis	Disease diagnosis is achieved through biochemical, genotypic, chemical, and immunologic methods. Proper collection, handling, and storage of specimens are essential for accurate results.
Specimen Collection	Includes swabs, blood, urine, sputum, and tissue samples. Key to preventing contamination and ensuring diagnostic accuracy.
STAT	Latin for "immediately"; used for urgent laboratory tests.
Urine Collection	Midstream (clean-catch) urine is preferred to avoid contamination.
UTI Pathogens	Urinary tract infections are commonly caused by Escherichia coli and other enteric bacteria.
Phases of Disease	Incubation, prodromal, acute, convalescent, and continuation phases describe the clinical course of infectious diseases.
Serology	Study of serum antibodies to detect exposure to pathogens.
ELISA	Enzyme-linked immunosorbent assay; used for the culture, tracking, and quantification of specific antibodies or antigens in patient samples.
Hepatitis B Virus	Diagnosis focuses on serological markers. Early infection is detected by surface antigen (HBsAg); late infection (clearing phase) is detected by IgG antibodies.
HIV	Serum tests for HIV antigens or IgM antibodies. Late infection (clearing phase) is detected by IgG.