Cultures upon Potatoes 



213 



fortably heated room, not subject to excessive variations, such as 

 are caused by steam heat and the burning of gas, etc., is about the 

 most appropriate. Like the colonies, the cultures must be care- 

 fully examined from day to day, as it not infrequently happens that 

 a growth which shows no signs of liquefaction to-day may begin 

 to liquefy to-morrow or a week hence, or even as late as two weeks 

 hence. 



The Stroke Culture. — In most cases, the culture is planted by 

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

 agar blood serum, or other solid medium 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 obUque surface to the bottom of the tube. This 

 enables us to tell whether the bacteria 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 par- 



Fig. S3. — 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). 



ticular 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. * 



Stroke cultures upon agar-agar have the advantage that the 

 cultures may be kept in the incubating oven. The colorless or 

 almost colorless condition 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. 



