1 68 MANUAL or BACTERIOLOGY. 



The diseases of plants known to be caused by bacteria are 

 not very numerous. Among them may be mentioned pear- 

 bhght, due to micrococcus amylovorus.* Among lower 

 animals bacteria very frequently produce diseases — for example, 

 chicken-cholera, symptomatic anthrax in cattle, erysipelas of 

 swine, tuberculosis, anthrax and glanders in various animals, 

 "red leg" in frogs. f 



These are some of the diseases in which bacteria have been 

 shown definitely to be the cause. It is not enough in any case 

 merely to find bacteria to establish the connection between 

 them and the disease. Koch's postulates, as they are called, 

 given below, must be complied with in order to prove that any 

 microorganism is the cause of a particular disease: 



First. That the organism should always be found micro- 

 scopically in the bodies of animals having the disease; that 

 it should be found in that disease and no other; that it should 

 occur in such numbers and be distributed in such a manner as 

 to explain the lesions of the disease. 



Second. That the organism should be obtained from the 

 diseased animal and propagated in pure culture outside of 

 the body. 



Third. That the inoculation of these germs in pure cultures, 

 which had been freed by successive transplantations from the 

 smallest particle of matter taken from the original animal, 

 should produce the same disease in a susceptible animal. 



Fourth. That the organism should be found in the lesions 

 thus produced in the animal. 



A moment's consideration will show that it is impossible to 

 comply with all these postulates in the investigation of all in- 

 fectious diseases; for in some cases the organisms causing the 

 diseases have not yet been observed, and yet there is abundant 

 proof that they exist in certain tissues of animals suffering from 



*See E. Smith. Centralhlatt jiir Bakteriologie, etc. Zweite Abtheilung Bd 

 v., p. 271; Bd. VII., p. 88. 



t Norris and Emerson. Journal oj Experimental Medicine. Vol. VII., p 30. 



