1 6 MANUAL OF BACTERIOLOGY. 



technic which has ever been made. Koch also pointed 

 out the advantages of solid media for the propagation of 

 pure cultures. 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 described 

 by Koch in 1882, a number of pathogenic bacteria were dis- 

 covered during the ensuing years in rapid succession. 



The application of the newly-gained knowledge concerning 

 the bacteria causing infectious diseases to the prevention and 

 cure of these diseases was begun almost immediately by Pasteur. 

 A few facts existed to guide the direction 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 from a disease 

 would probably be secured by producing a mild attack of the 

 disease. Such a mild attack might be expected to follow if a 

 susceptible individual were inoculated with microbes of lowered 

 virulence. Various methods were employed to reduce the 

 virulence of bacteria, notably cultivation at high temperatures 

 (43C.). 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 has been employed 

 with some success. A somewhat similar principle has led to 

 the preparation of a vaccine for the disease of cattle called 

 "black leg," and such vaccine is now distributed gratuitously 

 to farmers by the United States government. Inoculation of 

 human subjects with the attenuated virus is used for hydro- 

 phobia. This method also was devised by Pasteur. 



