222 



Cultures, and their Study 



liquefying tendency of the micro-organisms. Various types 

 of gelatin cultures are shown in the accompanying diagrams, 

 and it is rather important that the student should familiarize 

 himself with the terms by which these different growths are 

 described, in order that uniformity of description may be 

 maintained. Gelatin cultures may not be kept in the incu- 

 bating oven, as the medium liquefies at such temperatures. 

 On the other hand, it must not be kept where the tempera- 

 ture is too low, else the bacterial growth may be retarded. 

 The temperature of a comfortably 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 carefully examined 

 from day to day, as it not infrequently happens that a growth 



Fig. 50. Types of streak cultures : 1, Filiform (B. coli) ; 2, echinulate 

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

 5, arborescent (B. mycoides) (Frost). 



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 Agar=agar Culture. Different operations vary in 

 the exact method adopted in transplanting to agar-agar. In 

 most cases, a simple stroke is 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 bot- 

 tom 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 particular 



