Fig. 1-15


The Postulates:

1. The suspected pathogenic organism should be present in all cases of the disease and absent from healthy animals.


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



- Play important roles in chronic infections

Figure 25.15A

Figure 25.16

AB Toxins

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

Figure 25.17B

Figure 25.17A

Figure 13.7A

Figure 25.20

ADP-Ribosylating Toxins

Animation: Cholera Toxin Mode of Action

Click box to launch animation

Alpha Toxin

The hemolytic alpha toxin is produced by

Staphylococcus aureus.

- Forms a transmembrane, seven-member pore in target cell membranes

Figure 25.18

Anthrax Toxin

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

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