17 - Host/Microbe Interactions

• Mutualism - Both benefit
• Commensalism - One benefits, other is unharmed
• Parasitism - One benefits at expense of host

• Protects us from harmful organisms by covering binding sites, consuming available nutrients and producing toxic compounds, and stimulating our immune systems.
• Acquired at birth.

• 1. Chemicals/Chemo
• 2. Burns
• 3. Auto-immune Defeciencies

• If the colonizing organism has a parasitic relationship with the host.
• An infection that results in disease is termed an "infections disease."

• Symptoms - Experienced by patient, ie pain/nausea
• Signs - Can be observed through examination, ie Rash, Swelling, Pus

• Primary - Initial infection
• Secondary - Additional infections resulting from the primary infection

The ability of a pathogen to produce an infectious disease in an organism.

• Term referring to a pathogens disease causing ability.
• Highly virulent organisms have a high degree of pathogenicity.

Microbes that cause disease when the body's defenses are down.

• Number of organisms required to establish infection
• Diseases with SMALL infectious doses are more easily spread than diseases with HIGH infectious doses because less of the microbe is needed.

• Incubation - Time between infection and onset of symptoms
• Illness - Signs and symptoms
• Convalescence - Recuperation and recovery; may still be contagious

• Acute - Rapid onset, last a short time
• Chronic - Develop slowly and persist
• Latent - Never completely eliminated, may be reactive

• Limited to a small area.
• Example - Boil

• Microbe spreads throughout body
• Example - Measles

Toxins circulating in the blood stream

Viruses cirulating in the blood stream

Life-threatening illness caused by infectious agent or its products circulating in the blood

Their ability to overcome the body's immune system

• Mechanism used to overcome the body's immune responses
• More than one mechanism is called "Virulence Determinants"

• Adherence - Binding to the hosts cells
• Colonization - Must successfully multiply, compete for nutrients
• Delivery of Effector Molecules - Must be able to deliver colonized cells to host

• Penetration of skin - most difficult
• Penetration of mucous membranes - most common

• Hide in a host cell - some can transfer from cell to cell
• Avoid being killed by complement protiens - Gram negative cells more susceptible
• Avoiding antibodies -

• IgA protease
• Antigenic Variation - alternation of surface antigens
• Mimicking Host Molecules

• Toxins that are leaked or secreted by bacterium
• Among most potent toxins known
• Can act locally or systemically

• Neurotoxins - damage nervous system
• Enterotoxins - damage intestines/digestive tract
• Cytotoxins - Damage to variety of cells, cause interference with cell function and lysis

Disrupt the plasma membrane of a cell

Antigens that override T-Cell response and cause toxis effects due to massive release of cytokines by helper t-cells.

Example: TSS

Endotoxin

• Fever
• Drop in blood pressure
• Inflammation

• Binding to host cell/Invasion
• Avoiding immune responses

• Fungi - Infection begins with inhalation of airborne particles, Spores lodge in lung tissues
• Eukaryotic Parasites - Most live in intestinal tract, or enter body via bite of an arthropod, attach to hosts via specialized receptors, some hide within host cells.