1e. Bacterial Pathogenicity Virulence Factors and Genetics 22

Relationships between host and microbes:
- Commensal
- Benefit received by microbe with no harm to the host.
- Normal Flora (the “microbiome”):
- Community of organisms that normally exist on a body surface.
- Constituents vary according to the site (e.g., normal respiratory flora, normal enteric flora, normal skin flora).
- These are typically commensal or even beneficial.
- Opportunist:
- Microbe receives benefit and can cause disease if host defenses are weakened (e.g., flea on a dog).
- Parasitism:
- Only one side benefits, while the other is harmed.

Pathogenicity:
- The ability of an organism to cause disease.
- Example: Staphylococcus aureus (S. aureus) causes disease more often than coagulase-negative staphylococci, thus having greater pathogenicity.
Virulence:
- The extent to which an organism can cause severe disease.
- Example: Yersinia pestis causes plague (life-threatening), while Escherichia coli (E. coli) commonly causes urinary tract infections (not usually life-threatening), indicating Y. pestis is more virulent than E. coli.

Sources of infection may be:
- From the microbiome.
- From other sources:
- People: Transmission through contact, droplets, fecal-oral routes, blood borne, and sexual transmission.
- Animals: Contact and food borne transmission.
- Environment: Ingestion and airborne transmission.
- Vectors and fomites: Object or organism that can carry infectious agents.

Vector:
- A small organism (e.g., insect or tick) that transmits an infectious agent; infection occurs upon contact (typically biting) with the vector.
Fomite:
- An inanimate object that transmits infection when contaminated (e.g., doorknob); commonly serves as a vector.

Definition: Properties that enable an organism to cause infection.
Functions:
- Enables evasion of host defenses (e.g., capsule or slime layer avoids phagocytosis).
- Improves access to the body's nutrients (e.g., colonization factors like fimbriae).
- Adhesins: Proteins that allow organisms to adhere to host cells.

Exotoxins:
- Toxins excreted from the bacterial cell.
- A wide variety of enzymes and toxic proteins are released.
- Disrupt cells or interfere with their function, active at both site of infection and distant sites (e.g., Tetanus).

Invading substances:
- Hemolysins: Cause lysis of red blood cells and damage to other cells.
- Leukocidins: Kill white blood cells.
- Hyaluronidase: Breaks down connective tissue, allowing spread of organisms.
- Collagenase: Breaks down collagen, a structural protein (e.g., Streptococcal hemolysins).