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

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 produces vitamin K.

• 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: Permanently colonize the host.

• Transient microbiota: Temporarily present; may be removed by the body's defenses.

Acquisition: Resident microbiota are acquired during birth and through contact with the environment.

Functions: The human microbiome aids in digestion, synthesizes vitamins, and protects 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 into unusual body sites

Reservoirs of Infection

Reservoirs are sources of infectious agents:

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

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

• Nonliving: 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)

Terminology: Infection, Disease, Morbidity, Pathogenicity, Virulence

• Infection: Invasion and multiplication of pathogens.

• Disease: Disruption of normal body functions.

• Morbidity: The state of being diseased.

• Pathogenicity: Ability to cause disease.

• Virulence: Degree of pathogenicity.

Signs vs Symptoms: Signs are objective (fever, rash); symptoms are subjective (pain, fatigue).

Biofilms and Infection

Biofilms are communities of microorganisms attached to surfaces. They facilitate infection by protecting microbes from the immune system and antibiotics.

Koch’s Postulates

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

• It must be isolated and grown in pure culture.

• It must cause the disease when introduced into a healthy host.

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

Function: Establish causative relationship between microbe and disease.

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

Etiology: Study of the cause of disease.

Virulence Factors

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

• Toxins: Damage host tissues (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 (flies), biological (mosquitoes)

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

Mechanical vs Biological Vectors: Mechanical vectors carry pathogens passively; biological vectors are involved in pathogen life cycle.

Types of Diseases

• Acute: Rapid onset, short duration

• Subacute: Intermediate duration

• Chronic: Slow onset, long duration

• Latent: Inactive for a period, then reactivates

Communicable vs Noncommunicable: Communicable diseases spread from person to person; noncommunicable do not.

Emerging and Re-emerging Diseases

Factors include microbial evolution, changes in human behavior, environmental changes, and global travel.

Epidemiology

Epidemiology is the study of disease distribution and determinants in populations. Methods include surveillance, statistical analysis, and outbreak investigation.

Epi curves: Graphical representation of disease cases over time (point source, continuous, propagated).

Incidence vs Prevalence

• Incidence: Number of new cases in a period.

• Prevalence: Total number of cases at a given time.

Pandemic, Epidemic, Endemic

• Pandemic: Global outbreak (e.g., COVID-19).

• Epidemic: Sudden increase in cases in a region.

• Endemic: Constant presence in a population.

Public Health

• Assessment

• Policy development

• Assurance

Nosocomial Infections

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

Innate Immunity

Three Lines of Defense

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

• Second line: Internal defenses (phagocytes, inflammation, complement)

• Third line: Adaptive immunity (lymphocytes, antibodies)

Skin and Mucous Membranes

• Skin: Physical barrier, acidic pH, antimicrobial peptides

• Mucous membranes: Trap pathogens, contain lysozyme and IgA

Microbial Antagonism

Normal microbiota compete with pathogens, preventing their colonization.

Blood Components

• Plasma: Contains proteins, antibodies

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

WBCs neutralize pathogens via phagocytosis, production of reactive oxygen species, and release of antimicrobial substances.

Natural Killer (NK) Cells

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

Complement Pathways

• Classical pathway: Activated by antigen-antibody complexes

• Alternative pathway: Activated by microbial surfaces

Interferons

Interferons are cytokines that inhibit viral replication and activate immune cells.

Toll-like Receptors (TLRs)

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

Adaptive Immunity

Attributes of Adaptive Immunity

• Specificity

• Inducibility

• Clonality

• Unresponsiveness to self

• Memory

Types of White Blood Cells

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

• T lymphocytes: Responsible for cell-mediated immunity

Specificity: Caused by unique antigen receptors on B and T cells.

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

The lymphatic system transports lymph and houses immune cells. Peyer's patches are lymphatic tissue in the small intestine. Lymph flow is directed by muscle contractions and valves.

Thymus and T Lymphocytes

The thymus is essential for T cell development. 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 in response

• IgA: Found in mucosal areas

• IgE: Involved in allergies

• IgD: Functions mainly as a BCR

Antigen-antibody complexes eliminate antigens via 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

Immune Responses and Memory

Cell-mediated response involves cytotoxic T cells killing infected cells. Memory T and B cells provide rapid response upon re-exposure.

Primary vs Secondary Response: Secondary response is faster and stronger due to memory cells.

Acquired Immunity

• Active: Host produces own antibodies (leads to memory)

• Passive: Host receives antibodies (no memory)

• Natural: Through infection or maternal antibodies

• Artificial: Through vaccination or antibody therapy

Immunization and Immune Testing

Types of Vaccines

• Attenuated: Live, weakened organisms; strong immunity, risk of reversion

• Inactivated: Killed organisms; safer, weaker immunity

• Subunit: Contains parts of pathogen; safe, may require adjuvant

• Toxoid: Inactivated toxins; used for diseases like tetanus

• Conjugate: Linked to carrier proteins; improves response in children

Adjuvants: Enhance immune response; often used with subunit vaccines.

Routine Vaccination

Benefits: Prevents disease, herd immunity. Risks: Adverse reactions, rare complications.

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

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 test (e.g., pregnancy test)

• Fluorescent antibody tests: Use labeled antibodies to detect antigens

• Direct: Detects antigen

• Indirect: Detects antibody

Immune Disorders: HIV/AIDS

AIDS and HIV

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

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

HIV: Retrovirus infecting helper T cells; replicates via reverse transcription, evades immunity by mutating.

Helper T cell population: Declines as HIV progresses, leading to immunodeficiency.

Risk and Prevention Behaviors

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

• Prevention behaviors: Safe sex, sterile needles, screening blood, antiretroviral therapy

Summary Table: Types of Disease Transmission

Mode	Example	Key Features
Contact	Touch, droplets	Direct or indirect, short-range
Vehicle	Water, food, air	Nonliving medium, long-range
Vector	Mosquito, fly	Mechanical (passive) or biological (active)

Summary Table: Types of Immunity

Type	Source	Memory?	Example
Active, Natural	Infection	Yes	Chickenpox recovery
Active, Artificial	Vaccination	Yes	MMR vaccine
Passive, Natural	Maternal antibodies	No	Placental transfer
Passive, Artificial	Antibody therapy	No	Rabies post-exposure

Key Equations

Incidence and Prevalence

• Incidence:

$\text{Incidence} = \frac{\text{Number of new cases}}{\text{Population at risk}} \times 100$

• Prevalence:

$\text{Prevalence} = \frac{\text{Total number of cases}}{\text{Total population}} \times 100$

Example: The spread of influenza in a community can be tracked using incidence and prevalence rates, and controlled by vaccination to achieve herd immunity.

Additional info: Academic context was added to expand brief points and clarify terminology, examples, and mechanisms.