20 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



they are therefore well described by Percy Frankland as ' our 

 secret friends and foes.' 



For the purpose of studying the precise chemical changes 

 effected by a single organism it is necessary to obtain it in 

 pure culture, that is, free from admixture of any other organism. 

 The earliest method for accomplishing this, such as was 

 used by Pasteur and Lister, was the method of dilution. A 

 small portion of the solution containing the mixture of 

 organisms was transferred to a second portion of the same 

 solution rendered sterile by heat, and after development of 

 the organisms, a small portion of this solution was again trans- 

 planted, and so on, until a growth was obtained consisting 

 of only one species of organism, arrived at through a process 

 of natural selection. Such a method is exceedingly tedious, 

 but it is surprising what great advances in knowledge were 

 made by its means. The method of plate culture described 

 by Koch in 1881 is much more rapid and certain. Koch 

 introduced a solution containing bacteria into a mixture 

 of suitable nutritive substances thickened with gelatine, the 

 mixture being kept at a temperature slightly above the melt- 

 ing point of gelatine ; on pouring the gelatine culture medium 

 on to a plate and allowing the gelatine to set, wherever a 

 micro-organism was present it developed in situ, forming a 

 small cluster or colony, which could be picked out and trans- 

 ferred to a similar gelatine culture medium, and if necessary 

 re-plated until only one species of organism was found to 

 be present upon the gelatine plate. The form of plate now 

 generally used is known from its inventor as a Petri dish, 

 and consists, as shown in Fig. 5 a, of two shallow glass dishes, 

 fitting into one another, the larger serving as a cover for the 

 smaller, into which the gelatine is poured. 



Different culture media have been found to be necessary 

 for different organisms, but all require nitrogen in some form 

 together with certain mineral salts, especially phosphates. 



It is of the greatest importance in the preparation of 



