1. The suspected pathogenic organism should be present in all cases of the disease and absent from healthy animals.
Microscopy, staining Red blood cell
Observe blood/tissue under the microscope
2. The suspected organism should be grown in pure culture.
3. Cells from a pure culture of the suspected organism should cause disease in a healthy animal.
4. The organism should be reisolated and shown to be the same as the original.
Streak agar plate with sample from either diseased or healthy animal
Colonies of suspected pathogen
Inoculate healthy animal with cells of suspected pathogen
Remove blood or tissue sample and observe by microscopy
Red blood cell
No organisms present
(must be same organism as before)
Primary pathogens: Cause disease in healthy hosts
Opportunisitc pathogens: Cause disease only in immunocompromised patients
Virulence is a measure of the severity of a disease.
- Infectious dose vs. lethal dose
Streptococcus pyogenes M protein
Bordetella pertussis Pertactin
Bacteria can attach to surfaces in bulk, forming a
- Play important roles in chronic infections
B subunit: Binds to host cell
- Delivers A subunit to cytoplasm
- Often five B subunits form a pore for A entry.
A subunit: Has toxic activity
- Diphtheria toxin
- Cholera toxin
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The hemolytic alpha toxin is produced by
- Forms a transmembrane, seven-member pore in target cell membranes
Made by Bacillus anthracis
Two active toxins:
- Edema factor raises cAMP levels.
- Causes fluid secretion, tissue swelling
- Lethal factor cleaves protein kinases.
- Blocks immune from attacking
Made only by Gram-negative bacteria
Present in lipopolysaccharide of outer membrane
- Lipid A released as bacteria die
- Causes massive release of cytokine from host cells
- Can trigger fever, shock, and death