INTRODUCTION. IS 



blood containing anthrax bacilli. But complete proof that these 

 bacilli were the cause of the disease required that they should 

 produce it when injected alone and when freed from the smallest 

 trace of material derived from the first diseased animal. Unless 

 these conditions were complied with, some other rnaterial, for 

 example an enzyme or ferment, might be supposed to be carried 

 from the first to the second animal and to be the real cause of 

 the disease. For this purpose it was necessary to cultivate the 

 bacilH in nutrient fluids, such as meat broth, as was done by 

 Pasteur. It then became possible to demonstrate that their 

 properties could remain vmaltered after being grown in success- 

 ive generations on different lots of broth. As bacteria of two 

 or three species were often encountered in mixtures, it became 

 most important to secure a method by which the different species 

 could be separated from one another and be propagated as 

 separate "pure cultures." This was done successfully by 

 diluting such mixtures greatly, so that a drop planted in a new 

 tube of broth should contain only a single organism. The 

 growth ensuing would of course consist of the same kind of 

 organism exclusively. Such procedures were uncertain and 

 very laborious. 



Koch introduced in 1881 his method of separating bacteria 

 by "plating," described below (Part I., Chapter V.), and this is 

 probably the most important single contribution to bacteriologi- 

 cal technique which has ever been made. Koch also brought 

 solid culture-media into general use by employing gelatin. 

 Other important technical improvements of the same period 

 were the adoption of the illuminating apparatus of Abb6 and 

 immersion objectives, and of aniline dyes for staining bacteria 

 and making them visible (Weigert and Ehrlich). Beginning 

 with the bacillus tuberculosis described by Koch in 1882, a 

 number of pathogenic bacteria were discovered during the 

 ensuing years in rapid succession. 



The apphcation of the newly-gained knowledge concerning 



