CHAPTER X. 



RELATION OF BACTERIA TO DISEASE. METHODS OF INFEC- 

 TION AND EXAMINATION. 



By the application of the gelatin plate method, or its 

 several modifications, it is possible to readily separate a 

 given organism from the other forms which may be present, 

 and to obtain thus a pure culture. The isolated colony as 

 it develops on a plate, furnishes the first pure cultivation, 

 since it is derived from a single micro-organism. Trans- 

 plantations made from a colony, if made with proper pre- 

 cautions, in turn yield pure cultures, or growths contain- 

 ing but a single species. Tube cultures can thus be made 

 in gelatin, bouillon, agar, blood-serum, potato, etc., and 

 where it is desired, as in the study of the chemical products 

 of bacteria, flask cultures can be made. 



Relation of Bacteria to Disease. 



It is evident that in order to demonstrate that a given 

 bacterium is the cause of a certain fermentation, or of the 

 production of a pigment, or of phosphorescence, etc., it is 

 necessary that it should, firsl, be isolated and obtained in 

 a pure culture. And that, second, a pure culture of the 

 organism grown under the same or under similar condi- 

 tions, should give rise to the original phenomenon the 

 production of the same fermentation, pigment, phosphor- 

 escence, etc. Having thus demonstrated that a given or- 

 ganism is the cause of a certain change, it does not follow 

 that this organism has the exclusive power to do this. 



