Microbial Mechanism of Pathogenicity and Host Defenses

Microbial Mechanism of Pathogenicity

Key Definitions

• Infection: Invasion and multiplication of pathogens in a host.

• Disease: A disorder in functioning, often accompanied by symptoms.

• Sign: Objective evidence of disease observable by others.

• Symptom: Subjective evidence of disease experienced by the patient.

• Syndrome: A group of signs and symptoms that occur together.

Differences between Concepts

• Infection vs. Disease: Infection refers to the presence of pathogens, while disease refers to the adverse effects of infection on body function.

• Signs vs. Symptoms: Signs are measurable or observable (e.g., rash), while symptoms are reported by the patient (e.g., pain).

Types of Infections

• Infectious: Caused by pathogens.

• Communicable: Can be transmitted from host to host.

• Contagious: Easily spread through contact.

• Noncommunicable: Not transmitted between hosts.

• Noninfectious: Not caused by pathogens.

• Zoonotic: Transmitted from animals to humans.

Infection Types

• Iatrogenic: Resulting from medical interventions.

• Nosocomial: Acquired in healthcare settings.

Stages of Disease (5 Phases)

1. Incubation: Pathogen entry and initial growth.

2. Prodromal: Non-specific symptoms appear.

3. Illness: Maximum impact of disease, clear symptoms.

4. Decline: Symptoms subside and immune response fights infection.

5. Convalescence: Recovery and return to health.

Types of Diseases

• Acute: Rapid onset, severe symptoms, short duration.

• Chronic: Long-lasting symptoms, can be controlled but not cured.

• Latent: Inactive infection that can reactivate later.

Koch's Postulates

1. The organism must be found in abundance in all organisms suffering from the disease.

2. The organism must be isolated and cultured outside the host.

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

4. The organism must be re-isolated from the inoculated diseased experimental host.

Virulence vs. Pathogenicity

• Virulence: The degree of pathogenicity, measured by the number of organisms required to cause disease (LD50 and ID50).

• Pathogenicity: The ability to cause disease.

Definitions of LD50 and ID50

• LD50: Lethal dose required to kill 50% of a test population.

• ID50: Infectious dose required to cause disease in 50% of a test population.

Types of Pathogens

• Primary: Causes disease in healthy hosts.

• Secondary: Causes disease in hosts already compromised by primary pathogens.

• Opportunistic: Causes disease in immunocompromised hosts.

Pathogenesis Process (4 Stages)

1. Exposure: Entry of pathogens into the host.

2. Adhesion: Pathogen attaches to the host cells.

3. Invasion: Pathogen invades tissues and begins to multiply.

4. Infection: Pathogen spreads and leads to disease.

Portals of Entry and Exit

• Entry: Skin, mucous membranes, respiratory tract, gastrointestinal tract.

• Exit: Respiratory droplets, bodily fluids, feces.

Pathogen Attachment Mechanisms

• Capsid: Viral protein shell that helps in adhesion.

• Fimbrae: Hair-like structures for sticking to host.

• Flagella: Helps in motility and adherence.

• Glycocalyx: Polysaccharide layer aiding in attachment.

Virulence Factors

• Adhesins: Structures for attachment (e.g., fimbriae).

• Biofilm: Community of microorganisms adhering to surfaces.

• Extracellular Enzymes: Enzymes that damage host tissues.

• Toxins: Substances that can harm host cells.

• Antigenic Drift/Shift: Changes in pathogen antigens that affect host immune recognition.

Endotoxins vs. Exotoxins

• Endotoxins: Components of bacterial cell wall (gram-negative) that trigger immune responses (e.g., fever).

• Exotoxins: Toxins secreted by bacteria that can cause damage to host cells and tissues.

Virulence Factors by Pathogen Type

• Bacteria: Toxins, adhesion factors, evasion mechanisms.

• Fungi: Enzymes, toxins, allergenic proteins.

• Protozoa: Phospholipases, enzymes, antigenic variation.

• Helminth: Enzymes, immunomodulatory molecules.

Disease & Epidemiology

Epidemiological Terms

• Epidemiology: Study of disease distribution and determinants in populations.

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

• Prevalence: Total number of existing cases.

• Endemic: Constant presence of a disease within a particular area.

• Epidemic: Sudden increase in disease cases within a population.

• Pandemic: Global epidemic.

Pioneers of Epidemiology

• Snow: Mapped cholera cases, founding father of epidemiology.

• Nightingale: Focused on sanitation in nursing and public health.

• Lister: Introduced antiseptic techniques to surgery.

Sources of Infection

• Reservoir: Natural habitat of a pathogen.

• Carrier: Individual infected but showing no symptoms (passive vs. active).

• Hosts: Definitive host carries the sexually mature stage, while intermediate host carries other developmental stages.

Transmission Types

contact

• Vertical: From mother to child.

• Horizontal: From one individual to another.

Modes of Transmission

• Direct Contact: Person to person.

• Indirect Contact: Through intermediate objects.

• Aerosol: Droplets from respiratory tract.

• Vehicle Transmission: Through contaminated inanimate objects.

• Vector Transmission: Carried by an organism (biological or mechanical).

Nosocomial Infections

• Healthcare-associated infections; significant for patient safety. Universal precautions are essential to prevent their spread.

Roles of CDC and WHO

• CDC: National public health institute in the U.S., focuses on disease control and prevention.

• WHO: Global health authority, coordinates international response to disease outbreaks.

Innate Nonspecific Host Defenses

Host Defenses (3 Lines)

1. Physical Barriers: Skin, mucous membranes.

2. Innate Immunity: Phagocytic cells, antimicrobial proteins.

3. Adaptive Immunity: Specific responses involving antibodies and lymphocytes.

Barriers to Infection

• Cell Junctions, Skin: Act as physical barriers preventing pathogen entry.

• Mucus Memoranes & Endothelia: Trap pathogens and facilitate their removal.

Mechanical and Chemical Defenses

• Mechanical Barriers: Skin, cilia, tears, saliva.

• Microbiome: Normal flora that competes against pathogens.

• Chemical Defenses: pH, enzymes, antimicrobial compounds.

Antimicrobial Peptides (AMPs)

• Small proteins that disrupt bacterial membranes; play a role in innate immunity.

• Example: Defensins.

Acute Phase Proteins (APPs)

• Proteins produced in response to inflammation; assist in pathogen elimination.

• Example: C-reactive protein.

Complement Pathways (3 Types)

1. Classical Pathway: Activated by antibodies.

2. Alternative Pathway: Activated by pathogen surfaces.

3. Lectin Pathway: Activated by carbohydrates on pathogens and hn.

Cytokines and Immune Response

• Cytokines: Signaling proteins influencing immune responses.

• Interleukins: Mediate communication between white blood cells.

• Chemokines: Attract immune cells to sites of infection.

• Interferons: Proteins that inhibit viral replication.

Inflammation Mediators

• Histamine, Leukotrienes, Prostaglandins: Increase blood flow and permeability, recruiting immune cells.

• Bradykinin: Causes vasodilation and pain, enhancing the inflammatory response.

Cellular Defenses in Immunity

• Innate Defenses: Neutrophils and macrophages for immediate response.

• Adaptive Defenses: Lymphocytes for specific recognition of pathogens.

Phagocytosis Phases

1. Chemotaxis: Movement toward the pathogen.

2. Adherence: Attachment to the pathogen.

3. Ingestion: Engulfing the pathogen.

4. Digestion: Enzymatic breakdown of the pathogen.

Inflammation Signs and Benefits

• Cardinal Signs: Redness, heat, swelling, pain.

• Benefits of Inflammation: Localizes infection, prevents spread, promotes tissue repair.

Fever and Pyrogens

• Pyrogens: Substances that cause fever, typically cytokines.

• Beneficial effects: Increased immune efficiency and pathogen inhibition.