128 PATHOGENIC BACTERIA. 



is one color and the agar-agar another. Sometimes the 

 growth is filamentous, sometimes a smooth, shining band. 

 Occasionally the bacterium does not grow upon agar-agar 

 unless glycerin be added (tubercle bacillus) ; sometimes 

 it will not grow even then (gonococcus). 



Still less characteristic are the growths upon potato. 

 Most bacteria produce rather smooth, shining, irregu- 

 larly-extending growths, which often show very beauti- 

 ful colors. 



In milk and litmus milk one must observe the presence 

 or absence of acid-production, the coagulation which may 

 or may not accompany it, and the subsequent gelatiniza- 

 tion or digestion of the coagulum. 



Blood-serum is liquefied by some bacteria. The ma- 

 jority of organisms are not very characteristic in their 

 development upon it. Others, as the bacillus of diph- 

 theria, are, however, characterized by their shape, color, 

 and rapidity of development at given temperatures. 



While most of the saprophytic bacteria will grow well 

 at the ordinary temperature of a well-warmed room, the 

 important pathogenic forms require to be kept at the 

 temperature of the body. To do this accurately an in- 

 cubating oven becomes a necessity. Various forms, of 

 wood and metal, are in the market, the one shown in the 

 illustration (Fig. 30) being one of the newest and best. 



It scarcely need be pointed out that gelatin cultures 

 cannot be grown in the incubating oven, as the medium 

 will not remain solid at temperatures above 20-22 C. 



