50 MICRO-ORGANISMS AND FERMENTATION. 



He at first prepared his pure cultures by means of streak 

 infections in nutrient gelatine. He afterwards devised a far 

 better method, the plate-culture method (1883). The process 

 is as follows : A trace of the crude culture is transferred to 

 a large proportion of sterilised water. From this a small 

 quantity is transferred to a test-tube containing, for instance, 

 a mixture of meat-broth and gelatine warmed to 30 C. The 

 tube is shaken in order to distribute the germs, and the con- 

 tents poured on to a large glass plate, which is then covered 

 with a bell-jar. The gelatine quickly sets and the germs 

 are enclosed in the solid mass. In a few days they develop 

 to colonies dots or specks which are visible to the naked 

 eye. The purity of the bacterial growths in the gelatine is 

 ascertained, according to Koch, partly by their appearance. 



An improvement in the method consists in the use of 

 glass dishes with lids instead of glass plates, the Petri dishes 

 (introduced by Salomonsen), into which the liquefied gelatine 

 is poured : or the " roll-tubes " of Esmarch may be used, 

 prepared by continuously rotating a test-tube round its longer 

 axis until the inoculated gelatine has set in the tube, so that 

 the whole of the inner surface is covered. 



When species are being developed which require a high 

 temperature (at which gelatine would be liquefied), plates are 

 made of agar, or of agar and gelatine. The growth can be 

 mixed with the liquefied material, or else spread over the 

 surface of the solid, either by strokes of a platinum pencil, 

 or by stabs with an inoculated needle. After selecting colonies, 

 which appear to be pure, from a plate prepared in any one of 

 these ways, a new plate-culture may be prepared from one 

 colony. If all the colonies that develop on this plate are pure, 

 it is probable that we are dealing with a pure culture.* 



When regarded more closely it will be seen, however, that 

 there is no essential difference between the distribution of 



* Great importance is ascribed to the appearance of colonies of bacteria on 

 gelatine, to their colour, shape, the nature of the edge, etc., and whether they 

 liquefy gelatine or not. Other characters of the gelatine cultures are taken from 

 the streak- and stab-cultures. According to Hansen, colonies of many species of 

 yeast on gelatine plates exhibit characters of great value. The giant cultures of 

 Lindner are also used. 



