Special Method of Securing Pure Cultures 205 



In some cases pure cultures may be most satisfactorily 

 secured by animal inoculation. For example, when the 

 tubercle bacillus is to be isolated from milk or urine which 

 contains bacteria that would outgrow the slow-developing 

 tubercle bacillus, it is necessary to inject the fluid into the 

 abdominal cavity of a guinea-pig, await the development of 

 tuberculosis in the animal, and then seek to secure pure cul- 

 tures of the bacillus from the unmixed infectious material in 

 the softened lymphatic glands. 



In many cases, when it is desired to isolate Micrococcus 

 tetragenus, the pneumococcus, and others, it is easier to 

 inoculate the animal most susceptible to the infection and 

 recover it from the blood or organs, than to plate it out and 

 search for the colony among many others similar to it. 



The growth upon agar-agar is in many ways less charac- 

 teristic than in gelatin, but as the medium does not liquefy 

 except at a high temperature (100 C.), it has that advan- 

 tage. The colorless or almost colorless condition of the 

 preparation also aids in the detection of chromogenesis. 



Sometimes the growth is colored ; at times the production 

 of soluble pigment colors the agar-agar as well as the growth ; 

 sometimes the bacterial mass has one color and the agar-agar 

 another. The growth may be filamentous, or simply 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 (gono- 

 coccus). 



Still less characteristic are the growths upon potato. 

 Most bacteria produce smooth, shining, irregularly extending 

 growths, that may show characteristic colors. 



In milk and litmus milk one should observe change in 

 color from the occurrence of acid or alkali production, 

 coagulation, gelatinization, and digestion of the coagulum. 



Blood-serum is liquefied by some bacteria, but the major- 

 ity of organisms have no characteristic reaction upon it. A 

 few, as the bacillus of diphtheria, are, however, characterized 

 by rapid development at given temperatures. 



While most of the saprophytic bacteria grow well at the 

 temperature of a well-warmed room, the important patho- 

 genic forms must be kept at the temperature of the body 

 either to permit growth or to secure typical development. 

 To do this satisfactorily an incubating oven or thermostat be- 

 comes a necessity. Various forms, of wood and metal, are in 

 the market, one being shown in the illustration (Fig. 50). 



