97 



PATHOGENIC BACTERIA. 



By the application of the gelatin plate method it is 

 possible to readily separate a given organism from other 

 forms which may be present and thus obtain a. pure cul- 

 ture. The isolated colony as it develops on a plate fur- 

 nishes the first pure cultivation since it is derived from a 

 single micro-organism. Transplantations made from a 

 colony, if made with proper piecautions, in turn yield 

 pure cultures or growths containing 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 chemical products of bacteria, flask cultures can 

 be made. 



It is evident that in order to demonstrate that a given 

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

 production of some pigment or of phosphorescence, etc., it 

 is necessary that it should, first, be isolated and obtained 

 in pure cultures, and that, second, pure cultures of the 

 organism grown under the same or similar conditions, 

 should give rise to the original phenomena the produc- 

 tion of the same fermentation, pigment, phosphorescence, 

 etc. Having thus demonstrated that a given organism is 

 the cause of certain changes it does not follow that this 

 organism has the exclusive power to do so. Thus, in alco- 

 holic fermentation the yeast plant is commonly said to be 

 the cause, but a large number of different species of yeasts 

 are known which have this power, and not only the 

 yeasts but many bacteria possess similar properties. 

 Again a considerable number of bacteria have been shown 

 to be capable of inducing acetic, lactic, butyric acid fermen- 

 tations, the ammoniacal and hydrogen sulphide fermen- 

 tations of urine, the phosphorescence of sea-water, etc. 



The most that can be said of a given organism which 



