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Chapter 15
Microbial Mechanisms
of Pathogenicity
Biology 205
Chandler-Gilbert Community College
Microbial Mechanisms of Pathogenicity
 Pathogenicity: The ability to cause disease.
 Virulence: The extent of pathogenicity.
Portals of Entry
 Mucous membranes
 Skin
 Parenteral route
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.
Bacillus Anthracis
Portal of Entry
ID50
Skin
10-50 endospores
Inhalation
10,000-20,000 endospores
Ingestion
250,000-1,000,000
endospores
Adherence
 Adhesions/ligands bind to receptors on host cells
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
Glycocalyx: Streptococcus mutans
Fimbriae: Escherichia coli
M protein: Streptococcus pyogenes
Opa protein: Neisseria gonorrhoeae
Tapered end: Treponema pallidum
Enzymes

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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
Penetration into the Host Cell
Figure 15.2
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.
Endotoxins
Source:
Relation to microbe:
Gram –
Present in LPS of outer membrane
Chemistry:
Lipid
Fever?
Yes
Neutralized by
antitoxin?
No
LD50:
Relatively large
Figure 15.4b
Exotoxin
Source:
Relation to microbe:
Chemistry:
Mostly Gram +
By-products of growing cell
Protein
Fever?
No
Neutralized by
antitoxin?
LD50:
Yes
Small
Figure 15.4a
Exotoxins
 A-B toxins
Figure 15.5
Exotoxins
 Membrane-disrupting toxins

Lyse host’s cells by


Making protein channels in the plasma membrane
(e.g., leukocidins, hemolysins).
Disrupting phospholipid bilayer.
Exotoxins
 Superantigens


Cause an intense immune response due to
release of cytokines from host cells.
Fever, nausea, vomiting, diarrhea, shock, and
death.
Exotoxins
 Specific for a structure or function in host cell
Figure 15.4a
Exotoxins
Exotoxin
Lysogenic
conversion
A-B toxin.
Inhibits protein
synthesis.
+
Streptococcus pyogenes
Membrane-disrupting.
Erythrogenic.
+
Clostridium botulinum
A-B toxin. Neurotoxin
+
C. tetani
A-B toxin. Neurotoxin
Vibrio cholerae
A-B toxin. Enterotoxin
+
Superantigen.
+
Corynebacterium
diphtheriae
Staphylococcus aureus
Endotoxins
Figure 15.6
Cytopathic Effects of Viruses
Table 15.4
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
Pathogenic Properties of Fungi
 Aflatoxin

Aspergillus
 Mycotoxins

Neurotoxins: Phalloidin, amanitin

Amanita
Pathogenic Properties of Protozoa
 Presence of protozoa.
 Protozoan waste products
may cause symptoms.
 Avoid host defenses by


Growing in phagocytes
Antigenic variation
Pathogenic Properties of Helminths
 Use host tissue.
 Presence of parasite
interferes with host
function.
 Parasite's metabolic
waste can cause
symptoms.
Pathogenic Properties of Algae
 Paralytic shellfish
poisoning

Dinoflagellates
 Domoic acid
intoxication

Diatoms
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
Mechanisms of Pathogenicity
Figure 15.9