Microbiology Midterm Study Guide: Immunity, Infection, and Epidemiology

Colonization and Microbial Flora

Colonization During Infancy

Infants are born with sterile tissues, but various environments become colonized rapidly after birth. Colonization is influenced by factors such as delivery method, feeding type, and environmental exposure.

• Vaginal delivery exposes infants to maternal vaginal and intestinal flora.

• Cesarean section leads to colonization by skin and environmental microbes.

• Breastfeeding introduces beneficial bacteria like Lactobacillus and Bifidobacterium.

• Environmental contact (family, hospital, pets) further diversifies the microbiota.

Resident vs. Transient Flora

• Resident flora: Microorganisms that colonize the body permanently and are usually non-pathogenic.

• Transient flora: Microbes that temporarily inhabit the body and are often removed by host defenses or competition.

Microbial Pathogenicity and Virulence

Pathogenicity vs. Virulence

• Pathogenicity: The ability of a microorganism to cause disease.

• Virulence: The degree of pathogenicity; how severe the disease is.

• Example: Streptococcus pyogenes is highly virulent, causing severe infections, while Lactobacillus is non-pathogenic.

Exotoxins vs. Endotoxins

• Exotoxins: Proteins secreted by bacteria (mainly Gram-positive) that are highly toxic and specific in action.

• Endotoxins: Lipopolysaccharide (LPS) components of Gram-negative bacterial outer membranes, released upon cell death; cause generalized effects like fever and shock.

Capsules and Virulence

• Capsules: Polysaccharide layers surrounding some bacteria, protecting them from phagocytosis and enhancing virulence.

• Example: Streptococcus pneumoniae uses its capsule to evade immune detection.

Attachment Proteins in Viruses

• Viruses use attachment proteins (spikes, glycoproteins) to bind to specific host cell receptors, determining host range and tissue tropism.

Microbial Relationships and Benefits

Symbiotic Relationships

• Mutualism: Both organisms benefit (e.g., gut flora synthesizing vitamins).

• Commensalism: One benefits, the other is unaffected (e.g., skin flora).

• Parasitism: One benefits at the expense of the other (e.g., pathogens).

Benefits of Microbes

• Digestive aid, vitamin synthesis, immune system development, bioremediation, and industrial applications.

Transmission and Infection

Types of Transmission

• Direct contact: Person-to-person (touch, droplets).

• Indirect contact: Via fomites (inanimate objects).

• Vector-borne: Biological (mosquitoes) or mechanical (flies).

• Vehicle transmission: Through air, water, or food.

Vectors

• Biological vectors: Organisms in which the pathogen develops (e.g., mosquitoes for malaria).

• Mechanical vectors: Organisms that carry pathogens on their bodies (e.g., houseflies).

Fomites

• Non-living objects that can transmit infectious agents (e.g., doorknobs, medical equipment).

Zoonoses

• Diseases transmitted from animals to humans (e.g., rabies, Lyme disease).

Opportunistic Pathogens

• Microbes that cause disease only when host defenses are compromised.

Bacteremia vs. Septicemia

• Bacteremia: Presence of bacteria in the blood.

• Septicemia: Systemic infection where bacteria multiply in the blood, often leading to sepsis.

Phases of Acute Infectious Disease

• Incubation: Time between infection and symptom onset.

• Prodromal: Early, mild symptoms.

• Illness: Most severe symptoms.

• Decline: Symptoms subside.

• Convalescence: Recovery and return to health.

Nosocomial Infections

• Hospitals and nursing homes are high-risk due to immunocompromised patients, invasive procedures, and antibiotic use.

Epidemiology

Key Terms

• Morbidity: Incidence of disease in a population.

• Mortality: Number of deaths due to a disease.

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

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

Epidemic, Endemic, Pandemic

• Endemic: Constant presence of a disease in a population.

• Epidemic: Sudden increase in cases above normal levels.

• Pandemic: Epidemic that spreads across countries or continents.

Host Defenses and Immunity

Lines of Defense

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

• Second line: Innate immune responses (phagocytes, inflammation, fever, complement).

• Third line: Adaptive immunity (B and T lymphocytes, antibodies).

Innate vs. Adaptive Immunity

• Innate immunity: Non-specific, immediate defense mechanisms.

• Adaptive immunity: Specific, slower response with memory.

Skin and Mucous Membranes

• Act as physical barriers; secrete antimicrobial substances (lysozyme, defensins).

Inflammatory Process

• Characterized by redness, heat, swelling, pain, and loss of function.

• Functions: Isolate pathogens, recruit immune cells, initiate tissue repair.

Fever

• Produced by pyrogens affecting the hypothalamus.

• Benefits: Inhibits pathogen growth, enhances immune response.

• Risks: High fever can damage tissues and organs.

Phagocytosis

• Steps: Chemotaxis, adherence, ingestion, digestion, exocytosis.

• Phagocytes originate from bone marrow and migrate via blood to tissues.

• Types: Neutrophils, macrophages, dendritic cells.

Opsonization

• Process by which opsonins (antibodies, complement proteins) coat pathogens to enhance phagocytosis.

• Main categories: Immunoglobulins (IgG) and complement proteins (C3b).

Interferons

• Type I (alpha, beta): Produced by virus-infected cells, induce antiviral state.

• Type II (gamma): Produced by T cells, activates macrophages.

Natural Killer (NK) Cells

• Unique lymphocytes that kill virus-infected and tumor cells without prior sensitization.

• Release perforin and granzymes to induce apoptosis.

Chronic Inflammation

• Dominated by macrophages and lymphocytes.

Antigens and Immune Recognition

Antigens and Epitopes

• Antigen: Substance that elicits an immune response.

• Epitope (antigenic determinant): Specific region of antigen recognized by immune cells.

• Hapten: Incomplete antigen; requires carrier to elicit response.

Effective vs. Poor Antigens

• Effective: Large, complex, foreign molecules (proteins, polysaccharides).

• Poor: Small, simple, or self-molecules.

Major Histocompatibility Complex (MHC)

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

• MHC II: Found on antigen-presenting cells (APCs); presents exogenous antigens to CD4+ T cells.

• Red blood cells lack MHC proteins.

Primary vs. Secondary Lymphoid Tissue

• Primary: Bone marrow (B cell maturation), thymus (T cell maturation).

• Secondary: Lymph nodes, spleen, tonsils; sites of immune activation.

Lymphocytes

• B lymphocytes: Produce antibodies; differentiate into plasma and memory cells.

• T lymphocytes: Mediate cellular immunity; include helper (CD4+), cytotoxic (CD8+), and regulatory T cells.

Adaptive Immunity and Antibodies

Characteristics of Adaptive Immunity

• Specificity, diversity, memory, self-tolerance.

Antibody Structure and Types

• Y-shaped molecules with variable (antigen-binding) and constant regions.

• Types: IgG (most abundant), IgM (first produced), IgA, IgE, IgD.

• Found in blood, lymph, and secretions.

Clonal Selection

• Activation and proliferation of lymphocytes specific to an antigen.

Antibody Functions

• Neutralization, opsonization, complement activation, agglutination.

Primary vs. Secondary Immune Response

• Primary: Slower, mainly IgM, less robust.

• Secondary: Faster, mainly IgG, stronger due to memory cells.

Self-Tolerance and Immunocompetence

• Immune system's ability to distinguish self from non-self.

T Cells and Cell-Mediated Immunity

T Cell Types and Markers

• CD4+: Helper T cells; recognize MHC II.

• CD8+: Cytotoxic T cells; recognize MHC I.

• Regulatory T cells: Suppress immune responses.

T Cell Activation

• Requires antigen presentation by APCs and co-stimulatory signals.

Cytokines and Interleukins

• Cytokines: General term for immune signaling molecules.

• Interleukins: Subset of cytokines, mediate communication between leukocytes.

Messenger Cells

• Cells like dendritic cells and macrophages act as messengers, presenting antigens and secreting cytokines.

Immunization and Immune Testing

Active vs. Passive Immunity

• Active: Host produces own antibodies (infection or vaccination).

• Passive: Antibodies transferred from another source (maternal, antiserum).

Immunoassays

• Direct immunoassay: Detects antigen in sample.

• Indirect immunoassay: Detects antibodies in sample.

• Positive fluorescent immunoassay: Fluorescence observed under microscope.

• Positive ELISA: Color change in assay well.

Vaccines

• Live attenuated vaccines provide longest-lasting immunity.

• Toxoid vaccines typically require boosters every 5–10 years.

Point of Care Testing Benefits

• Rapid results, minimal equipment, improved patient management.

Immune Disorders and Hypersensitivity

Types of Hypersensitivity

Type	Mechanism	Examples
I (Immediate)	IgE-mediated, mast cell degranulation	Allergies, anaphylaxis
II (Cytotoxic)	IgG/IgM against cell surface antigens	Hemolytic anemia, transfusion reactions
III (Immune Complex)	Antigen-antibody complexes deposit in tissues	Serum sickness, lupus
IV (Delayed)	T cell-mediated	Contact dermatitis, TB test

• Type II: Damage from antibodies and complement.

• Type III: Damage from immune complex deposition.

• Type IV: Damage from T cell-mediated inflammation.

• Mismatched blood transfusion: Type II reaction.

• Hemolytic disease of the newborn: Type II reaction; IgG crosses placenta.

Autoimmunity vs. Hypersensitivity

• Autoimmunity: Immune response against self-antigens.

• Hypersensitivity: Exaggerated response to non-self antigens.

Grafts

Type	Description
Autograft	From same individual
Isograft	Between genetically identical individuals
Allograft	Between genetically different individuals of same species
Xenograft	Between different species