Host defenses

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Host defenses
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Host vs Microbe

• S. aureus
• Intact skin and no infection.
• Broken skin and infection.
• Toxoplasmosis
• normal host and mild flu-like illness.
• Neonate - severe disease.
• HIV - life threatening

Organism is the same, but disease different. Why?
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Summary
microbial virulence
host defenses
Infection disease
NO infection No disease
The microbe is nothing the terrain is
everything. Louis Pasteur
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• For a given HOST meeting an organism.
• For a given ORGANISM meeting a host.

Virulence of organism
Chance of infection
Host defenses
Chance of infection
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Host defenses

• Surface defenses - mechanical
• - chemical
• - microbial
• Non specific defenses - random phagocytosis.
• Specific - directed specifically directed
  antibodies and immune cells.

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Mechanical surface defenses

• Intact integument is your primary defense.
• Skin - dry
• - keratinised outer layer.
• Mucous membranes - mucous.
• - cilia
• - flushing
• - shedding

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Chemical surface defenses

• Skin - fatty acids and oils.
• Lysozyme - in tears and sweat.
• Stomach acid - major entry point for orgs
• Bile - small intestine.
• Prostatic fluid.

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Microbial surface defenses

• Normal flora has an ANTAGONISTIC role.
• Block receptors.
• Create adverse environment.
• Compete for nutrients

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Compromised surface defenses 1

• Break in the inegument
• Cuts in the skin and S. aureus.
• CVC and S. epidermidis.
• Urinary catheters - 50 in 24 hrs
• - 95 in 4 days
• Burst appendix - bowel flora
• Tooth extraction oral flora / endocarditis

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Compromised surface defenses 2

• Decreased muco-ciliary clearance
• - viral inf / smoking / cystic fibrosis
• Decreased stomach acidity
• - malnutrition and cholera.
• - helicobacter and cholera
• Dispalced normal flora
• - vaginal and oral thrush, C. difficile.

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A healthy host is remarkably resistant to
infection

• In most cases of infection, a predisposing event
  or decrease in host defenses can be identified.
• Immunocompromised
• Decreased host defenses
• Immunospuppressed