Chapter 15 - Pathogenicity

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I.          How Microorganisms Enter the Host

            A.        Portals of Entry – areas were a pathogen can gain entry into the host.

1.         Mucous Membranes – The lining of mucous that covers the gastrointestinal tract, respiratory tract, genitourinary tract, and conjunctiva

2.         Skin – unbroken skin is a good barrier to most microorganisms, though some (certain helminthes and fungi) can actively overcome the barrier.

3.         Parenteral Route – direct deposition of pathogens through a barrier through injury, such as a needle stick.

            B.         The Preferred Portal of Entry

1.         Most pathogens have a preferred portal of entry that allows them to infect, while other entry points don’t.

2.         Some pathogens (i.e. Bacillus anthracis) have more than one preferred portal of entry.

            C.        Number of Invading Microbes

1.         The likelihood of infection increases with the number of microorganisms introduced.

2.         Virulence of an organism is reported in the measure ID₅₀ – the infectious dose for fifty-percent of the same population.

3.         Potency of a toxin is reported in the measure LD₅₀ – the lethal dose for fifty-percent of the sample population.

            D.        Adherence – the ability of a pathogen to attach to host tissue.

II.        How Bacterial Pathogens Penetrate Host Defenses

A.        Capsules – the bacterial capsule is utilized to help prevent Phagocytosis by immune cells.

B.         Cell Wall Components – various intricate components of the cell wall can be used for attachment (i.e. fimbriae) or preventing phagocytosis (i.e. mycolic acid in Mycobacterium) 

C.        Enzymes

1.         Coagulases – causes clots that isolate a pathogen from host defenses.

2.         Kinases – break down clots and allow passage of pathogens away from the initial site of infection.

3.         Hyaluronidase – digests connective tissue, allowing spread.

4.         Collagenase – digest collagen, allowing for spread.

D.        Antigenic Variation – some organisms have the ability to alter their surface proteins, tricking the immune system (i.e. N. gonorrhoeae)

III.       How Bacterial Pathogens Damage Host Cells

A.        Siderophores – secreted proteins that sequester iron (a highly regulated nutrient in most hosts) from the host, depriving the host of the resource.

B.         Direct Damage – Some pathogens by their very growth cause host cell damage, though most of this is done more by toxins than directly by growth.

C.        Toxins – poisonous substances that are produced by certain microorganisms.

1.         Exotoxins – toxins produced inside the pathogen (mostly gram-positives) and secreted or released into the surrounding media.  The actions of exotoxins are typically very specific.

2.         Endotoxins – parts of the outer cell wall of gram-negative bacteria that are toxic to host cells.

D.        Many pathogenicity components (toxins, resistance factors, etc.) are borne on plasmids or other transferable elements that allow their exchange between bacteria.

IV.       Pathogenic Properties of Viruses

A.        Since viruses are intracellular, they hide from a large portion of the host’s immune system.

B.         Cytopathic effects – the visible signs of a viral infection in cells.

V.        Portals of Exit

A.        All hosts are dead ends for the pathogen, they must have a method of exiting the host and moving on to the next.

B.         The typical portals of exit are the mucous lined tracts of the respiratory, gastrointestinal, or genital systems.

C.        Some organisms can cause the host to “bleed-out” near and after dead so that the organism has a chance to escape.

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