Fig. 1-15
KOCH’S POSTULATES
The Postulates:
1. The suspected pathogenic organism should be present in all cases of the disease and absent from healthy animals.
Tools:
Microscopy, staining Red blood cell
Suspected pathogen
Diseased animal
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.
Laboratory culture Streak agar plate with sample from either diseased or healthy animal
Colonies of suspected pathogen
Inoculate healthy animal with cells of suspected pathogen
Experimental animal
Laboratory reisolation
Suspected pathogen
Diseased animal
Remove blood or tissue sample and observe by microscopy
Laboratory culture
Healthy animal
Red blood cell
No organisms present
Pure culture
(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
Figure 25.5

Figure 25.6

Figure 25.11
Figure 25.13A

Table 28-3

Streptococcus pyogenes M protein
Bordetella pertussis Pertactin
Bacteria can attach to surfaces in bulk, forming a biofilm .
- Play important roles in chronic infections
Figure 25.15A

Figure 25.16

•
•
• 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
ADPribosyltransferase
- Diphtheria toxin
- Cholera toxin
Figure 25.17B
Figure 25.17A

Figure 13.7A

Figure 25.20

Click box to launch animation

•
• The hemolytic alpha toxin is produced by
Staphylococcus aureus.
- Forms a transmembrane, seven-member pore in target cell membranes
Figure 25.18

• Made by Bacillus anthracis
•
•
•
•
• Two active toxins:
- Edema factor raises cAMP levels.
- Causes fluid secretion, tissue swelling
- Lethal factor cleaves protein kinases.
- Blocks immune system from attacking
Figure 25.21C

• 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
Figure 25.22

Figure 25.31

Figure 25.32

Figure 25.29