Biology 119 Microbiology Unit V Notes

Chapter 15: Microbial Mechanism of Pathogenicity

• Definitions:

  • Infection: Entry and multiplication of a pathogen in body tissues.

  • Disease: Any change from a state of health; often associated with signs and symptoms.

  • Signs: Observable or measurable elevations that indicate disease (e.g., fever, rash).

  • Symptoms: Subjective experiences reported by the patient (e.g., pain, fatigue).

  • Syndrome: A collection of signs and symptoms associated with a particular disease.

• Infection vs. Disease:

  • Infection: Pathogens enter and multiply.

  • Disease: Symptoms and signs manifest due to the infection.

• Types of Infections:

  • Infectious: Caused by infectious agents.

  • Communicable: Can be transmitted between hosts.

  • Contagious: Easily spread (e.g., flu).

  • Noncommunicable: Not transmitted (e.g., tetanus).

  • Noninfectious: Not caused by pathogens (e.g., genetic disorders).

  • Zoonotic: Transmitted from animals to humans.

• Iatrogenic vs. Nosocomial Infections:

  • Iatrogenic: Resulting from medical procedures/treatment.

  • Nosocomial: Acquired in healthcare settings.

• Stages/Phases of Disease:

  1. Incubation: Pathogen multiplies without symptoms.

  2. Prodromal: Non-specific symptoms appear.

  3. Illness: Disease is most severe.

  4. Decline: Symptoms subside.

  5. Convalescence: Recovery from the illness.

• Disease Types:

  • Acute Diseases: Rapid onset and short duration.

  • Chronic Diseases: Long-lasting or recurrent.

  • Latent Diseases: Pathogen is present but inactive.

• Koch’s Postulates:

  1. Pathogen must be found in diseased individuals.

  2. Pathogen must be isolated and grown in pure culture.

  3. The cultured pathogen should cause disease when introduced to a healthy host.

  4. Pathogen must be re-isolated from the experimentally infected host.

• Virulence and Pathogenicity:

  • Virulence refers to the severity or harmfulness of a disease caused by a microorganism.

  • It is directly related to pathogenicity, indicating how well a pathogen can cause disease.

• LD50 and ID50 Definitions:

  • LD50: Lethal dose for 50% of the population.

  • ID50: Infectious dose for 50% of the population.

• Pathogen Types:

  • Primary: Causes disease in a healthy individual.

  • Secondary: Only causes disease in immunocompromised individuals.

  • Opportunistic: Causes disease when the opportunity arises (e.g., weakened immune system).

Pathogenesis

• Stages of Pathogenesis:

  1. Entry into the host.

  2. Adherence to host tissues.

  3. Invasion and multiplication.

  4. Damage to host tissues.

• Types of Infection:

  • Local: Confined to a specific area.

  • Systemic: Spreads throughout the body.

  • Focal: Starts in one location but spreads to others.

• Portals of Entry and Exit:

  • Entry: mucous membranes, skin, respiratory tract.

  • Exit: secretions, excretions, skin flakes.

• Pathogen Attachment Mechanisms:

  • Capsid: Protein shell of a virus.

  • Fimbriae: Hair-like projections for attachment.

  • Flagella: Helps with motility and attachment.

  • Protein Spikes: Viral proteins that facilitate attachment.

  • Glycocalyx: A sticky layer that helps adhere to surfaces.

• Virulence Factors:

  • Adhesins: Assist in binding (e.g., pili on bacteria).

  • Biofilms: Bacterial communities that adhere together.

  • Toxins: Harmful substances produced by pathogens.

  • Extracellular Enzymes: Break down host tissues to facilitate pathogen spread.

• Endotoxins vs. Exotoxins:

  • Endotoxins: Part of the bacterial cell wall; can cause fever and shock.

  • Exotoxins: Secreted proteins; more toxic and can cause damage at lower doses.

• Pathogen Virulence Factors:

  • Bacteria: Produce toxins, form biofilms.

  • Fungi: Produce enzymes for tissue invasion.

  • Protozoa: Vary in pathogenic strategies depending on the species.

  • Helminths: May include hookworm which can invade tissue.

Disease & Epidemiology

• Epidemiology Terms:

  • Epidemiology: Study of how disease spreads.

  • Incidence: The number of new cases in a specific time.

  • Prevalence: The total number of cases at a given time.

  • Endemic: Disease constantly present in a region.

  • Epidemic: Sudden increase in disease incidence.

  • Pandemic: Worldwide spread of a disease.

  • Morbidity: The state of being diseased.

  • Etiological Agent: The cause of a disease.

• Pioneers of Epidemiology:

  • Snow: Father of epidemiology, mapped cholera outbreak.

  • Nightingale: Emphasized sanitation in hospitals.

  • Lister: Promoted antiseptic techniques in surgery.

• Sources of Infection:

  • Reservoir: Natural habitat of a pathogen.

  • Passive Carrier: Carries a pathogen without symptoms.

  • Active Carrier: Infected individual who shows symptoms.

  • Asymptomatic Carrier: Infected but shows no symptoms.

  • Definitive Host: Host where the pathogen sexually reproduces.

  • Intermediate Host: Host where the pathogen undergoes development but not reproduction.

• Transmission Types:

  • Vertical: Transmission from parent to offspring.

  • Horizontal: Among individuals of the same generation.

• Modes of Transmission:

  • Direct Contact: Skin-to-skin or physical touch.

  • Indirect Contact: Pathogen transferred through an intermediate object.

  • Droplet: Transmission via droplets from a sneeze or cough.

  • Airborne: Suspension in air for long distances.

  • Vehicle: Transmission via food, water, or air.

  • Biological Vector: Living organisms that carry pathogens (e.g., mosquitoes).

  • Mechanical Vector: Passive transport of pathogens on surfaces (e.g., flies).

• Significance of Nosocomial Infections:

  • Occur in hospitals; emphasize the need for universal precautions and hygiene protocols.

  • Universal Precautions: Guidelines to prevent disease transmission in healthcare settings.

  • Etiology: Study of the cause of diseases.

• Role of CDC and WHO:

  • CDC: National public health institute in the USA; focuses on disease prevention and control.

  • WHO: Specialized agency of the UN, coordinates international health efforts.

Innate Nonspecific Host Defenses

• Three Lines of Host Defenses:

  1. First Line: Mechanical barriers (skin, mucous membranes).

  2. Second Line: Innate immune responses (phagocytosis, inflammation).

  3. Third Line: Adaptive immunity (specific responses).

• Barriers to Infection:

  • Cell Junctions: Prevent pathogen entry by sealing gaps between cells.

  • Skin: Physical barrier; secretes antimicrobial substances.

  • Mucous Membranes: Trap pathogens and contain antimicrobial factors.

• Mechanical Barriers:

  • Cilia: Sweep out pathogens in respiratory tract.

  • Microbiome: Competes with pathogens for resources and space.

  • Chemical Defenses: Enzymes and acids inhibit pathogen growth.

• Antimicrobial Peptides (AMPs):

  • Function in the second line of defense, disrupting bacterial membranes (e.g., defensins).

• Acute Phase Proteins (APPs):

  • Respond to inflammation and can enhance phagocytosis (e.g., C-reactive protein).

• Complement Pathways:

  • Classical Pathway: Activated by antibodies.

  • Alternative Pathway: Activated by pathogen surfaces.

  • Lectin Pathway: Activated by binding of lectins to specific sugars on pathogens.

• Complement System:

  • A series of proteins that work to opsonize pathogens, induce inflammation, and lyse pathogens.

  • Steps include: Activation > Opsonization > Membrane Attack Complex formation.

• Cytokines Role:

  • Interleukins: Communicate between cells in immune response.

  • Chemokines: Attract immune cells to inflammation sites.

  • Interferons: Protective proteins against viral infections.

• Inflammation Mediators:

  • Histamine: Causes vasodilation and increased permeability.

  • Leukotrienes: Involved in inflammatory response and bronchoconstriction.

  • Prostaglandins: Involved in pain and fever response.

  • Bradykinin: Increases blood vessel permeability and causes pain.

• Cellular Defenses:

  • Innate: Neutrophils, macrophages, dendritic cells.

  • Adaptive: B and T lymphocytes.

• Blood Cells:

  • Erythrocytes: Red blood cells for oxygen transport.

  • Leukocytes: White blood cells for immune response.

  • Platelets: Involved in blood clotting.

• Leukocytes Types:

  • Granulocytes: Neutrophils (phagocytic), basophils (involved in allergies), eosinophils (combat parasites).

  • Agranulocytes: Monocytes (become macrophages), macrophages (major phagocytic cells), dendritic cells (activate T cells).

• Natural Killer Cells:

  • Part of the innate immune response; they kill virus-infected cells and tumors by releasing cytotoxic substances.

• PAMPs, PRRs, and TLRs:

  • PAMPs (Pathogen-Associated Molecular Patterns): Unique patterns found on pathogens.

  • PRRs (Pattern Recognition Receptors): Receptors on host cells that recognize PAMPs.

  • TLRs (Toll-Like Receptors): A specific type of PRR that activates immune responses when bound to PAMPs.

• Phagocytosis Phases:

  • Recognition and Attachment: The pathogen recognized by PRRs.

  • Engulfment: The pathogen is engulfed into vesicles.

  • Digestion: Lysosomes fuse with phagosome to destroy pathogen.

  • Exocytosis: Waste is expelled from the phagocyte.

• Inflammation Signs and Symptoms:

  • Redness, heat, swelling, pain (cardinal signs).

  • Stages: Vasodilation, increased permeability, attraction of leukocytes.

• Benefits of Inflammation:

  • Localizes infection, prevents spread, initiates healing processes.

• Pyrogen Definition:

  • Fever-inducing substance; causes elevated body temperature through hypothalamus response.

  • Benefits of Fever: Enhances immune function and inhibits pathogen growth.