Pathogenic organism: the organism able to produce disease.

Bacteria Divisions according to degree of Pathogenicity

1. Pathogenic bacteria: bacteria live in a host and cause tissue damage and disease

2. Commensal bacteria: live on surface of host body, do not cause disease = normal flora, sometimes benefit as they compete with pathogen and inhibit their growth.

3. Saprophytoc bacteria: live free in nature, do not cause disease

4. Opportunistic bacteria: when commensal bacteria cause disease, if hosts defense decreased or if it changes its habitat.

Pathogenicity: the ability of an infectious agent to cause disease.

Virulence: measure the quantitative ability of pathogen to cause disease.

Symbiotic Relationships

Commensalism: one of the organisms benefits without harming the other

Mutualism: both organisms benefit (e.g. E.Coli in intestines produce Vit. K and some B vitamins)

Parasitism: one organism benefits at the expense of the other

Infection

the process by which parasite enter the body (bacteria, virus, fungus).

The result may be:

• Sub-clinical or silent infection: infection without manifest clinical disease

• Clinical disease: signs and symptoms

Requirement steps of Infection to occur

1. Source of infection

2. Mode of transmission

3. Multiplication inside host

4. Site of exit from host

Vertical transmission: pathogen can cross the placenta, transmission during birth (post natal)

Spread of Infection

1. Human: human body, carriers

2. Animal: zoonosis (diseases that occur primarily in wild and domestic animals, and can be transmitted to humans (e.g rabies))

3. Nonliving Reservoirs: soil, water

Reservoirs of Infection

1. Contact transmission

2. Vehicle transmission

3. Vectors

Contact transmission

• Direct contact: person - person

• Indirect contact: person - nonliving object

• Droplet transmission

Vehicle transmission

• Waterborne transmission

• Airborne transmission

• Blood, body fluids, drugs and intravenous fluids

Vectors

• animals that carry pathogens from one host to another (mostly arthropods)

• Mechanical transmission: via simple contact

• Biological transmission: via bites and blood

Carriers

Healthy individual carries pathogenic organism without clinical manifestation and can transmit this organism to other individual.

Bacterial virulence factors

certain structures or product that helps bacteria to cause disease

Virulence factors:

• adherence factor

• invasive factor

• anti-phagocytic factors

• intracellular pathogenicity

• enzymes

• antigenic heterogeneity

• toxic production

Anti-phagocytic factors: factors that prevent phagocytosis

• capsule (best)

• protein A of staph.

• M-protein of strept.

Toxin production

Exotoxin: produces gram positive ( + )

Endotoxin produces gram negative ( - ) on its cell wall

Immunogens or Antigens

• Immunogens: any foreign substances which can stimulate immune response

• Haptens (Incomplete antigen): low molecular weight substances not capable of inducing an immune response. When they are coupled to a larger carrier protein they become capable of inducing an immune reponse.

• Epitopes (Antigenic determinants): part on antigen that bind with antibody or immune cell receptors. = antigen binding site

Antigen characters that induce immune response (to be immunogenic)

1. Foreignness

2. Molecular size

3. Protein

4. Methods of Administration of Antigen

5. Adjuvant

   • substances when mixed with an antigen before its administration will increase the immune response to that antigen

Types of Antigens

1. bacterial

2. viral

3. superantigens

4. human tissue antigens (isoantigens):

   • Blood group antigens: A, B and D

   • Histocompatibility antigens: glycoprotein on membranes of tissue cells.

     • called: Histocompatibility complex (MHC) antigens or human leukocytic antigens (HLA)

Two classes of MHC:

MHC I: present on all nucleated cells and APCs

MHC II: present on antigen presenting cells (APCs)

Antigen presenting cells (APCs)

• dendritic cell

• macrophages

• B cell

• T cell