Cultures upon Potato 



211 



signs of liquefaction to-day may begin to liquefy to-morrow or a week 

 hence, or even as late as two weeks hence. 



The Agar-agar Culture. In most cases, the culture is planted by 

 a simple stroke made from the bottom of the tube in which the agar- 

 agar has been obliquely solidified, and where it is fresh and moist, 

 to the upper part, where it is thin and dry. In addition to this, it is 

 advisable to make a puncture from the center of the oblique surface 

 to the bottom of the tube. This enables us to tell whether the bacte- 

 ria can grow as readily below the surface as above. Some workers 

 always make a zigzag stroke upon the surface of the agar-agar. This 

 does not seem to have any particular advantage except in cases where 

 it is desired to scatter the transplanted organisms as much as possible, 

 in order that a large bacterial mass may be secured. 



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

 than in gelatin, but as the medium does not liquefy except at a high 

 temperature (iooC.), it has the advantage that cultures may be 



Fig. 60. Types of streak cultures: i, Filiform (B. coli); 2, echinulate (Bact. 

 acidi-lactici) ; 3, beaded (Str. pyogenes); 4, effuse (B. vulgaris); 5, arborescent 

 (B. mycoides) (Frost). 



kept in the incubating oven. The colorless or almost colorless con- 

 dition of the preparation also aids in the detection of chromogenesis. 



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 (gonococcus). 



Cultures upon Blood-serum. Bacteria are planted upon coagu- 

 lated blood serum and blood-serum preparations as upon agar-agar. 



Blood-serum is liquefied by some bacteria, but the majority of 

 organisms have no characteristic reaction upon it. A few, as the 

 bacillus of diphtheria, are, however, characterized by rapid develop- 

 ment at given temperatures. 



Cultures upon Potato. These are made by simply stroking the 

 surface of the culture-medium, the density and opacity of the 

 potato making it impracticable to puncture it. 



Most bacteria produce smooth, shining, irregularly extending 

 growths upon potato, that may show characteristic colors. 



