Bacterial Virulence and Infection Outcomes

Stages of Infection

• Exposure: the time period before infection, where the bacterium is shed by an individual
• Colonization: bacteria adhere to and multiply on a host surface
• Infection outcomes:
  • No colonization
  • Infection → disease → recovery
  • Infection → subclinical disease (carrier state)
  • Infection → disease → death/disability
  • Infection → disease → persistence (carriers)

Disease Carriers

• Incubatory carrier: carrier that is incubating the pathogen but not yet ill
• Convalescent carrier: those who are recovering but continue shedding the pathogen
• Persistent carrier: the animal recovered from the disease but could not eradicate the pathogen
• Healthy carrier (subclinical): carrier state without clinical symptoms but shed the pathogen
• Clinical carrier: infectious animal with a clinical infection in a peracute, acute, or chronic phase

Virulence Factors

• Enable processes such as:
  • Adhesion
  • Biofilm formation
  • Motility
  • Toxin secretion
  • Immune evasion
  • Colonization
  • Bacterial cell-cell communication
• Types of virulence factors:
  • Surface-associated proteins
  • Pili
  • Flagella
  • Efflux pumps
  • Siderophore secretion
  • Exotoxins
  • Capsule

Highlighted Virulence Factors

• Exotoxins:
  • Secreted by the bacteria
  • Highly antigenic proteins
  • Classified according to mechanisms and targets
  • Can modulate host cell metabolism
• Capsule:
  • Protects bacteria from the immune system
  • Example: encapsulated strains of Streptococcus are more virulent than non-encapsulated strains
• Quorum-sensing:
  • Regulation of gene expression in response to fluctuations in cell population density
  • Mediated by chemical signal molecules (autoinducers)
• Flagella:
  • Motility
  • Adhesion
  • Participates in biofilm formation
  • Modulates the immune system of eukaryotic cells
• Biofilm:
  • Clusters of bacteria attached to a surface and/or to each other and embedded in a self-produced matrix
  • Functions:
    • Protection from immune system
    • Protection from desiccation
    • Protection from antibiotics
    • Improved acquisition of nutrients
    • Improved bacterial-bacterial interactions

Transfer of Virulence Factors

• Methods of transfer:
  • Transformation (acquiring free DNA from the environment)
  • Transduction (bacteriophages transferring bacterial DNA)
  • Conjugation (transfer of plasmids between bacteria)
• Implications:
  • Emergence and dissemination of antimicrobial drug resistance
  • Selection of serious pathogenic strains in the clinics

Toxic Shock Syndrome

• Caused by bacterial toxins, particularly TSST-1, produced by Staphylococcus aureus
• Characterized by fever, hypotension, rash, diarrhea, and mucous membrane involvement
• Typically affects menstruating women using tampons
• Can be fatal if left untreated

Iron Acquisition Mechanisms

• Bacteria require iron for growth and survival
• Siderophores are secreted molecules that chelate iron, allowing bacteria to scavenge iron from host
• Hemophores bind to hemoglobin and extract iron
• Direct uptake allows bacteria to take up iron directly from the host

Infectious Disease Pathophysiology

• Inoculation: pathogen enters the host
• Colonization: pathogen multiplies and establishes itself
• Invasion: pathogen penetrates host tissues
• Toxin production: pathogen produces toxins that cause harm
• Immune response: host responds to infection
• Disease: clinical symptoms manifest

Cell Wall Components

• Bacterial cell walls are composed of peptidoglycan (also known as murein), providing structural support
• Teichoic acids are found in Gram-positive bacteria and are involved in cell wall formation
• Lipoteichoic acids are found in Gram-positive bacteria and are involved in cell wall formation
• Lipopolysaccharides are found in Gram-negative bacteria and are involved in outer membrane structure

Fungal Infections

• Candidiasis is caused by Candida species and affects skin, mucous membranes, and internal organs
• Aspergillosis is caused by Aspergillus species and affects lungs and other organs
• Cryptococcosis is caused by Cryptococcus species and affects lungs and central nervous system
• Fungal infections often occur in immunocompromised individuals

Bacterial Virulence Factors

• Adhesins are molecules that facilitate bacterial attachment to host cells
• Invasins are molecules that facilitate bacterial invasion of host cells
• Toxins are molecules that harm host cells
• Capsules are polysaccharide layers that protect bacteria from host immune system
• Biofilms are complex communities of bacteria that resist host immune system and antimicrobial agents

This quiz covers the stages of bacterial infection, from exposure to infection outcomes, including colonization and disease. Test your knowledge of bacterial virulence and its effects.