ESSAYS ON BACTERIOLOGY. 119 



usefulness is widened, since upon this medium will 

 grow well certain bacteria whick grow but indiffer- 

 ently or not at all upon the simpler soils. 



In his researches upon the tubercle bacillus Koch 

 brought forward solidified blood-serum, which has 

 proven the most valuable culture medium for some of 

 the sensitive or the strongly parasitic microbes. It 

 was upon this substance that he was first able to grow 

 the germ of tuberculosis, and, as a result, to work out 

 the perplexing problem of the cause of this disease. 



Certain fluids obtained from the living body are 

 sometimes used alone or in combination as culture 

 media. Such are the liquid exudates in cases of 

 pleurisy, peritonitis, or other inflammations of serous 

 membranes. Finally, the living body is itself made 

 to do service as a laboratory for the growth of bac- 

 teria, thus imitating exactly the natural process. 



Of all of these substances, those most frequently 

 used are the glycerin-agar, and the solidifled blood- 

 serum, either pure or variously modified. The blood- 

 serum and agar are often combined with happy results. 



In all culture work absolute sterilization of ma- 

 terials and instruments is necessary, lest slight con- 

 tamination at the beginning grow into gross and mis- 

 leading contamination at the end. If we are to in- 

 terpret correctly the final findings, we must know with 

 certainty the beginning. There must be in our final 

 cultures no bacteria except those placed there at the 

 start. 



