2O MANUAL OF BACTERIOLOGY. 



Koch introduced in 1881 his method of separating bacteria 

 by "plating" (described in Part I.), probably the most 

 important single contribution to bacteriological technique. 

 He also brought solid culture-media into general use by em- 

 ploying gelatin. Other important technical improvements 

 of the same period were the adoption of the illuminating 

 apparatus of Abbe and immersion objectives, and of aniline 

 dyes for staining bacteria and making them visible ( Weigert 

 and Ehrlich). Beginning with the bacillus tuberculosis de- 

 scribed by Koch in 1882, a large number of pathogenic bac- 

 teria were discovered during the ensuing years in rapid suc- 

 cession. 



The application of the newly-gained knowledge concern- 

 ing the bacteria causing infectious diseases to the preven- 

 tion and cure of these diseases was begun almost imme- 

 diately by Pasteur. A few facts existed to guide the direc- 

 tion of the research. It had been known even in ancient 

 times that one attack of an infectious disease, such as scarlet 

 fever, may confer immunity from subsequent attacks. 



The protection against small-pox which was furnished by 

 vaccination also was suggestive, although the mechanism by 

 which this protection came about was not understood. 



Pasteur worked on the theory that immunity to a disease 

 might be secured by producing a mild attack of the disease. 

 Such a mild attack might be expected to follow if a sus- 

 ceptible individual were inoculated with microbes of lowered 

 virulence. Various methods were employed to reduce the 

 virulence of bacteria, notably cultivation at high tempera- 

 tures (43C.). In this manner Pasteur was able to produce 

 immunity against a number of the diseases of the lower 

 animals. His method of inoculating sheep and cattle against 

 anthrax is widely and successfully used. A similar prin- 

 ciple has led to the preparation of a vaccine for the disease 

 of cattle called " black leg," and such vaccine is now dis- 



