LEC 25: Pathogenesis Pt. 1: Adhesion, colonization, and invasion

Host-microbe interactions

1. Removal/destruction by host defenses

2. Colonization without disease

-Subclinical infection (carrier)

3. Replication and tissue damage

-Clinical infectious disease

Infection

Persistence of an organism in or on the host

Infectious disease

Colonization or invasion of the host resulting in tissue damage

Pathogenicity

The ability or potential for an organism to cause disease

Virulence

The degree or intensity of an organism's pathogenicity

Virulence factors

Any component of a microorganism that contributes to:

1. Invasion

2. Evasion or subversion of host defenses

3. Tissue damage

Primary pathogen

A microorganism that causes disease even in healthy hosts

Opportunistic pathogen:

A microorganism that normally causes disease only in hosts that are compromised in some way.

Host defense status

1. Immune status

-Immunocompromised

*Genetic defects

*Other diseases

*Stress levels

-Immunosuppressive therapy

2. Co-evolution of hosts and pathogens

-Malaria and sickle cell anemia

-Tuberculosis

*European vs Asian populations

Routes of entry for pathogens

1. Ingestion

2. Inhalation

3. Trauma

4. Needlestick

5. Arthropod bite

6. Sexual transmission

5 Steps to Infectious Diseases

1. Adherence

2. Replication

3. Invasion

4. Evasion of host defenses

5. Tissue Damage

Adhesins

Microbial surface structures that attach to a host cell receptor (integrin).

Integrins

Host cell receptors that bind to microbial adhesins

-Host cell receptors typically comprised of glycolipids or glycoproteins

-Are important for host cell function

-determine host tropism!

Types of Adhesins

Pili (frimbriae) on mostly gram negative bacteria

Non-pili adhesins

Bacterial Capsules

Functions of a bacterial capsules

1. Promotes adherence to host cells

2. Prevents phagocytosis

3. Aids in adherence to other bacteria to form (biofilms)

Host tropism

The susceptibility of a host to infection by a microorganism

Tissue tropism

The cells or tissues of a host that support the growth of a particular microorganism.

Replication

Microbes typically have to replicate to cause infectious disease.

Sites of replication within a host

1. Extracellular growth (outside of host cells)

2. Intracellular growth (growth inside of a host cell)

**Some bacterial pathogens like Salmonella enterica are facultative intracellular pathogens, and can grow on either mucosal surfaces or within host cells.

Invasion: Strategies for survival and replication

1. Evasion of host defenses

2. Source of nutrients

*Crucial for obligate intracellular pathogens like Chlamydia and Rickettsia

3. Less competition

4. Access to host cell machinery

*Replication of viruses

How might pathogens get out of a host cell?

-Viruses may bud from a host cell, or escape via lysis of the host cell

-Some bacteria like Listeria and Shigella exploit host cell proteins to propel themselves into adjacent healthy host cells.