Microbial Mechanisms of Pathogenicity

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Chapter 15

• Microbial Mechanisms of Pathogenicity

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Microbial Mechanisms of Pathogenicity

• Pathogenicity The ability to cause disease.
• Virulence The extent of pathogenicity.

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Portals of Entry

• Mucous membranes
• Skin
• Parenteral route

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Numbers of Invading Microbes

• ID50 Infectious dose for 50 of the test
  population.
• LD50 Lethal dose (of a toxin) for 50 of the
  test population.

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Bacillus Anthracis
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Toxins
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Adherence

• Adhesions/ligands bind to receptors on host cells
• Glycocalyx Streptococcus mutans
• Fimbriae Escherichia coli
• M protein Streptococcus pyogenes
• Opa protein Neisseria gonorrhoeae
• Tapered end Treponema pallidum

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Adherence
Figure 15.1
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Capsules

• Prevent phagocytosis
• Streptococcus pneumoniae
• Haemophilus influenzae
• Bacillus anthracis

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Cell Wall Components

• M protein resists phagocytosis
• Streptococcus pyogenes
• Opa protein inhibits T helper cells
• Neisseria gonorrhoeae
• Mycolic acid (waxy lipid) resists digestion
• Mycobacterium tuberculosis

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Enzymes

• Coagulase Coagulate blood
• Kinases Digest fibrin clots
• Hyaluronidase Hydrolyses hyaluronic acid
• Collagenase Hydrolyzes collagen
• IgA proteases Destroy IgA antibodies
• Siderophores Take iron from host iron-binding
  proteins
• Antigenic variation Alter surface proteins

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Penetration into the Host Cell
Figure 15.2
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Toxins

• Toxin Substances that contribute to
  pathogenicity.
• Toxigenicity Ability to produce a toxin.
• Toxemia Presence of toxin in the host's blood.
• Toxoid Inactivated toxin used in a vaccine.
• Antitoxin Antibodies against a specific toxin.

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Endotoxins
Figure 15.4b
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Exotoxin
Figure 15.4a
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The Action of an A-B Exotoxin
Figure 15.5
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Exotoxins

• Membrane-disrupting toxins
• Lyse hosts cells by
• Making protein channels in the plasma membrane
  (e.g., leukocidins, hemolysins).
• Disrupting phospholipid bilayer.

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Exotoxins

• Superantigens
• Cause an intense immune response due to release
  of cytokines from host cells.
• Fever, nausea, vomiting, diarrhea, shock, and
  death.

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Exotoxins

• Specific for a structure or function in host cell

Figure 15.4a
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Exotoxins
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Endotoxins
Figure 15.6
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Cytopathic Effects of Viruses
Table 15.4
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Pathogenic Properties of Fungi

• Fungal waste products may cause symptoms.
• Chronic infections provoke an allergic response.
• Tichothecene toxins inhibit protein synthesis.
• Fusarium
• Proteases
• Candida, Trichophyton
• Capsule prevents phagocytosis.
• Cryptococcus
• Ergot toxin
• Claviceps

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Pathogenic Properties of Fungi

• Aflatoxin
• Aspergillus
• Mycotoxins
• Neurotoxins Phalloidin, amanitin
• Amanita

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Pathogenic Properties of Protozoa

• Presence of protozoa.
• Protozoan waste products may cause symptoms.
• Avoid host defenses by
• Growing in phagocytes
• Antigenic variation

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Pathogenic Properties of Helminths

• Use host tissue.
• Presence of parasite interferes with host
  function.
• Parasite's metabolic waste can cause symptoms.

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Pathogenic Properties of Algae

• Paralytic shellfish poisoning
• Dinoflagellates
• Domoic acid intoxication
• Diatoms

Figure 27.15
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Portals of Exit

• Respiratory tract
• Coughing and sneezing
• Gastrointestinal tract
• Feces and saliva
• Genitourinary tract
• Urine and vaginal secretions
• Skin
• Blood
• Biting arthropods and needles or syringes

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Mechanisms of Pathogenicity
Figure 15.9