Microbiology Exam 3 Study Guide: Infection, Immunity, and Epidemiology

Chapter 14: Infection, Infectious Diseases, and Epidemiology

Types of Symbiosis

Symbiosis refers to the close association between two different organisms. There are several types:

• 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 microbiota.

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

Normal Microbiota

The human body hosts a variety of microorganisms known as the normal microbiota, which are classified as:

• Resident microbiota: Permanent members, typically acquired during birth and early life.

• Transient microbiota: Temporary members, present for days or weeks.

Acquisition: Resident microbiota are acquired from the environment, food, and contact with others.

Functions: The human microbiome aids in digestion, synthesizes vitamins, and provides protection against pathogens (microbial antagonism).

Opportunistic Pathogens

Normal microbiota can cause disease under certain conditions:

• Immune suppression

• Changes in the normal microbiota (e.g., antibiotics)

• Introduction of microbiota into unusual sites

Reservoirs of Infection

Reservoirs are sources of infectious agents:

• Human reservoirs: Carriers of disease (e.g., HIV, tuberculosis)

• Animal reservoirs: Zoonoses (e.g., rabies, Lyme disease)

• Nonliving reservoirs: Soil, water, food (e.g., botulism, cholera)

Portals of Entry and Exit

Pathogens enter and exit the body through specific portals:

• Skin

• Mucous membranes (respiratory, gastrointestinal, genitourinary tracts)

• Placenta

• Parenteral route (injuries)

Definitions and Comparisons

• Infection: Invasion of host by pathogens

• Disease: Change from normal health due to infection

• Morbidity: Illness or disease state

• Pathogenicity: Ability to cause disease

• Virulence: Degree of pathogenicity

• Signs: Objective, measurable changes (e.g., fever)

• Symptoms: Subjective, felt by patient (e.g., pain)

Biofilms and Infection

Biofilms are communities of microbes attached to surfaces, protected by extracellular matrix. They facilitate infection by resisting immune responses and antibiotics.

Koch’s Postulates

• Microbe must be found in every case of disease

• Microbe must be isolated and grown in pure culture

• Microbe must cause disease when introduced to healthy host

• Microbe must be re-isolated from experimentally infected host

Limitations: Some pathogens cannot be cultured; some diseases are caused by multiple agents.

Etiology: Study of disease causation.

Virulence Factors

• Extracellular enzymes: Aid invasion (e.g., hyaluronidase)

• Toxins: Damage host (exotoxins, endotoxins)

• Adhesion factors: Allow attachment to host cells

• Antiphagocytic factors: Prevent destruction by immune cells

Stages of Infectious Disease

1. Incubation period

2. Prodromal period

3. Illness

4. Decline

5. Convalescence

Modes of Transmission

• Contact transmission: Direct, indirect, droplet

• Vehicle transmission: Air, water, food

• Vector transmission: Mechanical (passive), biological (active)

Droplet vs Airborne: Droplet is short-range; airborne is long-range.

Types of Diseases

• Acute: Rapid onset, short duration

• Subacute: Intermediate

• Chronic: Slow onset, long duration

• Latent: Inactive for a period

Communicable: Spread person-to-person; Noncommunicable: Not spread between people.

Emerging and Re-emerging Diseases

Factors include mutation, travel, environmental changes, and antibiotic resistance.

Epidemiology

The study of disease distribution and determinants in populations. Used to track outbreaks, identify risk factors, and guide public health interventions.

• Incidence: Number of new cases

• Prevalence: Total cases (new + existing)

• Pandemic: Global outbreak

• Epidemic: Sudden increase in cases

• Endemic: Constant presence in population

Public Health

• Assessment

• Policy development

• Assurance

Nosocomial Infections

Hospital-acquired infections. Influenced by patient susceptibility, invasive procedures, and antibiotic use. Prevention includes hygiene, sterilization, and infection control protocols.

Chapter 15: Innate Immunity

Lines of Defense

The human body has three lines of defense against pathogens:

1. First line: Physical and chemical barriers (skin, mucous membranes)

2. Second line: Internal defenses (phagocytes, inflammation, complement, interferons)

3. Third line: Adaptive immunity (lymphocytes, antibodies)

Skin and Mucous Membranes

• Skin: Physical barrier, acidic pH, antimicrobial peptides

• Mucous membranes: Trap microbes, contain lysozyme, secrete mucus

Microbiota and Microbial Antagonism

Normal microbiota compete with pathogens, produce inhibitory substances, and stimulate immune responses.

First vs Second Line of Defense

• Physical defenses: Skin, mucous membranes

• Chemical defenses: Lysozyme, acid, salt

• Second line: Phagocytes, inflammation, fever, complement

Blood Components

• Plasma: Contains proteins, antibodies

• White blood cells (WBCs): Neutrophils, lymphocytes, monocytes, eosinophils, basophils

WBCs neutralize pathogens by phagocytosis, releasing toxic substances, and producing antibodies.

Natural Killer (NK) Cells

NK cells destroy infected or abnormal cells by releasing cytotoxic granules.

Complement Pathways

• Classical pathway: Activated by antibodies

• Alternative pathway: Activated by microbial surfaces

Interferons

Proteins produced by cells in response to viral infection; inhibit viral replication and activate immune cells.

Toll-like Receptors (TLRs)

TLRs recognize pathogen-associated molecular patterns (PAMPs) and trigger innate immune responses.

Chapter 16: Adaptive Immunity

Attributes of Adaptive Immunity

• Specificity

• Inducibility

• Clonality

• Unresponsiveness to self

• Memory

White Blood Cells in Adaptive Immunity

• B lymphocytes: Responsible for antibody-mediated (humoral) immunity

• T lymphocytes: Responsible for cell-mediated immunity

Specificity is caused by unique antigen receptors.

Divisions of Adaptive Immunity

• Humoral immunity: Targets extracellular pathogens; mediated by B cells and antibodies

• Cell-mediated immunity: Targets intracellular pathogens; mediated by T cells

Lymphatic System

Includes lymph nodes, spleen, and lymphatic tissue (e.g., Peyer's patches in small intestine). Lymph flow is directed by muscle contractions and valves.

Thymus and T Lymphocytes

The thymus is essential for T cell development and maturation. Types of T cells:

• Helper T cells (CD4+)

• Cytotoxic T cells (CD8+)

• Regulatory T cells

B Lymphocytes and Immunoglobulins

B cell specificity is furnished by unique B cell receptors (BCRs). Five classes of immunoglobulins:

• IgG: Most abundant, crosses placenta

• IgM: First produced

• IgA: Found in mucosal areas

• IgE: Allergic responses

• IgD: B cell receptor

Antigen-antibody complexes eliminate antigens by neutralization, opsonization, and complement activation.

Antigen Characteristics

Effective antigens are large, complex, and foreign to the host.

MHC Proteins

• Class I MHC: Present on all nucleated cells; alert cytotoxic T cells

• Class II MHC: Present on antigen-presenting cells; alert helper T cells

Cell-Mediated Immune Response

Involves activation of cytotoxic T cells to destroy infected cells.

Memory Cells

• Memory T cells: Provide rapid response upon re-exposure

• Memory B cells: Produce antibodies quickly upon re-exposure

Primary vs Secondary Immune Response

Primary response is slower and weaker; secondary response is faster and stronger due to memory cells.

Active vs Passive Immunity

• Active immunity: Host produces own antibodies; leads to memory

• Passive immunity: Host receives antibodies; no memory

• Natural: Exposure through infection or maternal antibodies

• Artificial: Vaccination or antibody therapy

Chapter 17: Immunization and Immune Testing

Types of Vaccines

• Attenuated (live): Weakened pathogens; strong immunity, risk of reversion

• Inactivated (killed): Safer, weaker immunity, often require adjuvants

• Subunit: Only parts of pathogen; safe, less immunogenic

• Toxoid: Inactivated toxins; require boosters

• Conjugate: Linked antigens; improved response in children

First vaccinations: Edward Jenner (smallpox).

Germ theory of disease: Diseases are caused by microorganisms.

Routine Vaccination

Benefits: Prevents disease, herd immunity. Risks: Side effects, rare adverse reactions.

Contact immunity: Immunity in unvaccinated individuals due to spread of vaccine virus.

Herd immunity: Protection of population when enough individuals are immune.

Active vs Passive Immunization

• Active: Long-lasting, memory

• Passive: Immediate, short-term, no memory

Passive immunotherapy involves administration of pre-formed antibodies.

Serology and Immune Testing

Serology is the study of blood serum for antibodies. Uses include diagnosis and monitoring immunity.

• Immunochromatographic assay: Rapid tests (e.g., pregnancy test)

• Fluorescent antibody tests: Use fluorescent tags to detect antigens or antibodies

• Direct: Detects antigen

• Indirect: Detects antibody

Chapter 18: Immune Disorders (HIV/AIDS Focus)

AIDS and HIV

• AIDS: Acquired Immunodeficiency Syndrome; defined by severe immune deficiency

• Disease vs Syndrome: Disease has specific cause; syndrome is a collection of symptoms

• HIV: Human Immunodeficiency Virus; retrovirus that infects helper T cells

• Replication: HIV uses reverse transcriptase to integrate into host genome

• Immune evasion: High mutation rate, attacks immune cells

• Helper T cell population: Declines as AIDS progresses, leading to immunodeficiency

Risk and Prevention Behaviors

• Risk behaviors: Unprotected sex, sharing needles, blood transfusions, mother-to-child transmission

• Prevention behaviors: Safe sex, not sharing needles, screening blood, antiretroviral therapy during pregnancy

Table: Comparison of Disease Transmission Modes

Mode	Definition	Example
Contact	Direct or indirect transfer	Touching, sexual contact
Vehicle	Via air, water, food	Cholera (water), influenza (air)
Vector	Via animals (mechanical or biological)	Malaria (mosquito), plague (fleas)

Table: Types of Immunity

Type	Source	Memory	Example
Active, Natural	Infection	Yes	Chickenpox
Active, Artificial	Vaccination	Yes	MMR vaccine
Passive, Natural	Maternal antibodies	No	IgG crossing placenta
Passive, Artificial	Antibody therapy	No	Antivenom

Table: Five Classes of Immunoglobulins

Class	Function	Location
IgG	Main antibody, crosses placenta	Blood, tissues
IgM	First produced, activates complement	Blood
IgA	Protects mucosal surfaces	Mucus, saliva, tears
IgE	Allergic responses, defense against parasites	Blood
IgD	B cell receptor	Surface of B cells

Key Equations

Incidence and Prevalence

• Incidence:

• $\text{Incidence} = \frac{\text{Number of new cases}}{\text{Population at risk}}$

• Prevalence:

• $\text{Prevalence} = \frac{\text{Total number of cases}}{\text{Total population}}$

Additional info: Academic context and explanations were added to expand brief learning objectives into comprehensive study notes. Tables were inferred and constructed for clarity. Equations were provided for epidemiological calculations.