Lecture 1 Pathogenesis of microbial diseases

BIOL 2010 Lecture 1 - Pathogenesis of Microbial Disease

• Course: BIOL 2010
• Lecture: Pathogenesis of Microbial Disease
• Textbook: Bailey and Scott's, Chapter 3
• Topics covered in lecture:
  • Infection versus infectious disease
  • Normal flora and their role in infection
  • Primary and secondary infections
  • Phases of infectious process
  • Characteristics and pathogenesis of common microorganisms (grouped by body system)
  • Bacterial virulence
  • Endotoxins and exotoxins

Thought Questions

• What makes some microbes pathogenic and others not?
• If microbes are everywhere, why don't we get more infections?

Host-Microbe Relationships

• Symbiosis: Two organisms living together.
  • Commensalism: Microorganism benefits, host is not harmed.
  • Mutualism: Both microorganism and host benefit.
  • Parasitism: Microorganism benefits, host is harmed.
• Indigenous (normal) flora: Microorganisms commonly found on or in healthy people.
  • Colonization: Growth of microbiota without causing damage or symptoms.
  • Resident microbiota: Microorganisms colonizing for months or years.
  • Transient flora: Microorganisms temporarily colonizing a host.
  • Carrier state: Condition of a host capable of transmitting the infection.

Host-Microbe Relationships (continued)

• Pathogenic: Organism causes disease
  • Ex: Salmonella
• Nonpathogenic: Organism doesn't cause disease
  • Ex: Corynebacteria (skin flora)
• Opportunistic pathogen: Organism causes disease only when host defenses are compromised
  • Ex: Indigenous flora

Terms

• Pathogenicity: Degree to which a microorganism causes disease.
  • Pathogens: Highly pathogenic, cause significant harm to host.
  • Non-pathogens: Low pathogenicity.
• Virulence: Degree of pathogenicity.

Common Microbiota in Humans

• Includes a variety of microorganisms, grouped by body system (e.g., ears, circulatory, GI tract, skin, LRT, GU).
• Locations that are normally sterile (e.g., serous fluids, CSF, deeper tissues)

Host-Microbe Interactions

• Infection: Growth and multiplication of microorganisms causing damage to the host.
• Disease: Infection causing notable changes in human physiology.
• Infectious disease: Transmissible disease.

Host-Microbe Encounters

• Reservoirs: The origin of the etiologic agent.
  • Humans (including indigenous flora, infected individuals)
  • Animals
  • Environment (water, soil)
• Transmission Types:
  • Direct: Host directly encounters the microbial reservoir.
  • Indirect: Host encounters the microbe through an agent.
    • Fomites (non-living): Objects.
    • Vectors (living): Animals and insects.

Colonization of Host Surfaces

• Colonization: Microorganisms settle and grow on host surfaces without causing noticeable symptoms.
• Harmless colonization: Microorganisms can become part of the resident flora or transient colonizers.

Entry, Invasion, and Dissemination

• Microorganisms must penetrate or circumvent host physical barriers (skin, mucosal surfaces).
• Overcoming these defenses depends on host and microbial factors.

Overview of Host Barriers

• Physical barriers.
• Organ-specific defenses.
• Phagocytes, inflammation, and antibodies.

Host Barriers - Skin

• Physical barrier (dry, acidic, cool outer layer).
• Antimicrobial substances (sebum, sweat).
• Indigenous flora.
• Skin-associated lymphoid tissue (SALT).

Host Barriers - Mucous Membranes

• Rapid sloughing of mucosal cells and mucus production.
• Antimicrobial substances (lactoferrin, lysozymes, lactoperoxidase).
• Mucosa-associated lymphoid tissue (MALT).

Host Barriers - Other Nonspecific Barriers

• Oral cavity (saliva, IgA, lysozyme).
• Stomach (low pH, proteolytic enzymes).
• Small intestines (bile salts, microbiota).
• Upper respiratory tract (cilia).
• Vaginal lining (low pH, microbiota, mucus plug).

Host Barriers - Phagocytes

• Phagocytes (white blood cells) ingest and destroy foreign particles (bacteria, other pathogens).
• Phagocytosis: The process by which this occurs.
• Major phagocytes include: Neutrophils, Monocytes/Macrophages, and Dendritic cells.

Host Barriers - Inflammation

• Inflammation: A complex response to tissue injury or infection.
• Cellular components, swelling, redness, heat, pain.
• Complement system: Helps enhance phagocyte activity and directly kills bacteria.
• Coagulation system: Walls off the site of infection.
• Chemical messengers (cytokines): Enhance immune response.

Host Barriers - Antibodies

• Antibodies (immunoglobulins): Proteins secreted by plasma cells (activated B cells).
• Respond to foreign substances such as chemicals and toxins.
• Protect host upon subsequent encounters (memory).
• Found in blood and secretions.

Overcoming Host Barriers

• Colonization (first step of infection).
• Harmless colonization vs. infectious disease.
• Microorganisms have defenses to overcome host barriers (ex: virulence factors).

Virulence Factors

• Virulence factors: Traits that allow pathogens to cause harm to host.
  • Survival against host defenses.
• Attachment to host cells.
• Invasion of tissues.

Virulence Factors (cont.)

• Toxins: Biochemically active substances that damage host cells
  • Endotoxins
    • Gram negative bacteria
    • Released upon bacterial death
    • Cause generalized effects leading to a number of issues.
  • Exotoxins
    • Produced by living bacteria
    • Bacterial species specific in their effect
    • Can kill host cells or disrupt intracellular processes.
• Pathogenicity islands: Parts of pathogens' DNA holding virulence factors.
• Biofilms: Complex communities with resistant properties.

Outcomes of Infectious Diseases

• Dependent on host health and virulence of the pathogen.
  • Infection vs disease.
• Types of infections (acute, chronic, latent, localized, systemic).
• Stages of infection (incubation, prodromal, illness, convalescent).

Test your knowledge on exotoxins, endotoxins, and microbial strategies to overcome host defenses. This quiz covers various aspects of pathogenicity islands and the characteristics of indigenous microbiota. Discover how these factors influence the interaction between microbes and their hosts.