MICROBIOLOGY
Chapter 15
Microbial Mechanisms of Pathogenicity
Dr. Abdelraouf A. Elmanama
Ph. D Microbiology
Medical Technology Department, Faculty of Science, Islamic University-Gaza
2008
•
Microbial Mechanisms of
Pathogenicity Pathogenicity
The ability to cause disease
• Virulence
2008
The extent of pathogenicity
Portals of Entry
• Mucous membranes
• Skin
• Parenteral route
2008
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
2008
Bacillus anthracis
2008
Portal of entry
Skin
ID50
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
2008
• Glycocalyx
Streptococcus mutans
• Fimbriae
Escherichia coli
• M protein
Streptococcus pyogenes
• Opa protein
Neisseria gonorrhoeae
• Tapered end
Treponema pallidum
Enzymes
2008
• Coagulase
Coagulate blood
• Kinases
Digest fibrin clots
• Hyaluronidase
Hydrolyses hyaluronic
acid
• Collagenase
Hydrolyzes collagen
• IgA proteases
Destroy IgA antibodies
• Siderophores
Take iron from host ironbinding proteins
• Antigenic variation
Alter surface proteins
Penetration into the Host Cell
2008
Figure 15.2
Toxins
• Toxin
Substances that contribute to
pathogenicity
• Toxigenicity
Ability to produce a toxin
• Toxemia
Presence of toxin the host's blood
• Toxoid
Inactivated toxin used in a vaccine
• Antitoxin
Antibodies against a specific toxin
2008
Endotoxin
2008
Figure 15.4b
Exotoxin
Source
Metabolic product
Chemistry
By-products of growing cell
Protein
Fever?
No
Neutralized by antitoxin
Yes
LD50
2008
Mostly Gram +
Small
Exotoxins
• A-B toxins or
type III toxins
2008
Figure 15.5
Exotoxins
• Superantigens or type I toxins
• Cause an intense immune response due to release
of cytokines from host cells
• Fever, nausea, vomiting, diarrhea, shock, death
2008
Exotoxins
• Membrane-disrupting toxins or type II toxins
• Lyse host’s cells by:
• Making protein channels in the plasma membrane
(e.g., leukocidins, hemolysins)
• Disrupting phospholipid bilayer
2008
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
• Corynebacterium diphtheriae
• Staphylococcus aureus
2008
Superantigen. Enterotoxin.
+
Exotoxins
2008
Figure 15.4a
Endotoxins
2008
Figure 15.6
Endotoxins
2008
Source
Gram–
Metabolic product
Present in LPS of outer membrane
Chemistry
Lipid
Fever?
Yes
Neutralized by antitoxin
No
LD50
Relatively large
Cytopathic Effects of Viruses
2008
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
2008
Pathogenic Properties of Fungi
• Aflatoxin
• Aspergillus
• Mycotoxins
• Neurotoxins: Phalloidin, amanitin
• Amanita
2008
Pathogenic Properties of Protozoa
• Presence of protozoa
• Protozoan waste products may cause symptoms
• Avoid host defenses by
• Growing in phagocytes
• Antigenic variation
2008
Pathogenic Properties of Helminths
• Use host tissue
• Presence of parasite interferes with host function
• Parasite's metabolic waste can cause symptoms
2008
Pathogenic Properties of Algae
• Neurotoxins produced by dinoflagellates
• Saxitoxin
• Paralytic shellfish poisoning
2008
Portals of Exit
• Respiratory tract
• Coughing, sneezing
• Gastrointestinal tract
• Feces, saliva
• Genitourinary tract
• Urine, vaginal secretions
• Skin
• Blood
• Biting arthropods, needles/syringes
2008
Mechanisms of Pathogenicity
2008
Figure 15.9