4 APPLIED BACTERIOLOGY 



well to describe the general lines on which he worked. He 

 first carefully observed the nature of the organic material 

 in which certain fermentations took place, studying both 

 synthetically and analytically the best medium for the 

 purpose, and then by careful microscdpical study deter- 

 mined what organisms developed most rapidly during the 

 fermentation process. After making a solution of the 

 substance to be fermented, he added a small quantity of 

 albuminous material and a trace of the ash of the yeast 

 under examination, so that there should be a sufficient 

 quantity of the necessary mineral constituents present. 

 The medium was then carefully sterilised by being boiled 

 in flasks to which only filtered air had access. To the 

 germ-free solution he added a small trace of the special 

 yeast which he wished to examine. By this means, after 

 growing the organism through two or three generations, he 

 obtained pure cultures. Pasteur also employed the ' dilu- 

 tion,' or ' fractional,' method of cultivation. A drop of the 

 liquid containing the organism that is desired to be grown 

 is largely diluted with sterile nutrient fluid favourable to 

 its growth. Drops of this diluted culture are then in- 

 oculated into separate test-tubes containing nutrient fluid. 

 By the extension of this process pure cultures are eventu- 

 ally obtained. It is of interest to note that Pasteur's first 

 experiments, which have led to such far-reaching results, 

 were made to disprove the spontaneous generation theory. 

 The great controversy which started during the latter 

 part of the last century in connection with this subject 

 was briefiy this. Those on one hand regarded bacteria. 

 as produced from organic matter by the process of putre- 

 faction, while those on the other hand believed they were 

 derived from living germs already present. The first 

 theory is that of ' abiogenesis,' or ' spontaneous genera- 

 tion ' ; the second that of ' biogenesis,' or ' life from life.' 



