BACTEEIAL DISEASES OF MAN AND ANIMALS 91 



After twenty-four to thirty-six hours Uft the layer of agar and 

 determine whether the tissues of the turnip have undergone any 

 softening. If so, how does the softened area correspond with the 

 size and shape of the superimposed bacterial colony ? 



Transfer bits of the softened tissue by means of a sterilized 

 needle to tubes of sterile media. If no organisms develop in 

 these tubes, what do you conclude as to the diffusibility of the 

 enzyme ? Make a microscopic examination of the softened tissue. 



4. Study the characters of the organisms on gelatin stab, agar 

 slope, potato, milk, and Dunham's solution. Test the effect of 

 desiccation and exposure to strong light. 



SECTION XV 

 BACTERIAL DISEASES OF MAN AND ANIMALS 



The first great advance of bacteriology was in the study and 

 treatment of disease in the animal body. Its importance to 

 medicine and to civilization can hardly be overestimated. The 

 proof that a given disease is caused by bacteria depends upon 

 the fulfillment of four postulates laid down by Robert Koch. 

 They are as follows : (1) the organism must be demonstrated in 

 the circulation or tissues of the diseased animal ; (2) this organ- 

 ism must be isolated and grown in pure culture for successive 

 generations ; (3) the pure culture of the organism, when intro- 

 duced into the body of a healthy, susceptible animal, must produce 

 the disease in question ; (4) the organism must be found and re- 

 isolated from the circulation or tissues of the inoculated animal. 



It is not assumed that the following exercises give an exten- 

 sive survey of the subject of animal diseases; they are merely 

 intended to be illustrative. An extensive survey of the subject 

 is to be had from works on medical or veterinary bacteriology, 

 among which the following may be mentioned : 



JoEDAN. General Bacteriology. Philadelphia, 1910. 

 Frost. Laboratory Bacteriology. New York, 1903. 

 Sternberg. Manual of Bacteriology. New York, 1901. 

 Buchanan. Veterinary Bacteriology. Philadelphia, 1911. 

 Herzog. Veterinary Micro-organisms. Philadelphia, 1912. 



