MAKING PLATE AND ESMARCH ROLL CULTURES 27 



EXERCISE VIII 

 MAKING PLATE AND ESMARCH ROLL CULTURES 



39. The general principle underlying the separation of bac- 

 teria by means of plate and roll cultures is to dilute the sub- 

 stance containing the bacteria so that the individual organisms 

 will be separated from each other by an appreciable distance 

 and then fixed in a solid medium where each organism can 

 multiply into a growth or colony without coming in contact 

 with any other organism or colony. For this purpose agar and 

 gelatin are used. Originally Koch employed a rectangular 

 piece of glass for holding the layer of medium, and protected 

 it from contamination by putting it under a bell jar. Later 

 Esmarch introduced the " roll culture " method, which was 

 extensively followed, until the Petri dishes were introduced. 

 Since that time they have been largely used in place of the 

 Koch plate and Esmarch tube. On this account the plate 

 cultures of to-day are usually made in Petri dishes. The roll 

 culture is also used. 



Plate cultures are employed for two distinct purposes, viz. 

 (i) to isolate bacteria in order to obtain pure cultures from 

 the isolated colonies, and (2) to determine how many bacteria 

 there are present in a given quantity of a liquid such as water, 

 milk, or blood. In this exercise the object is to separate the 

 bacteria to obtain isolated colonies. For quantitative work, see 

 Exercises LX. and LXI. 



40. Work for this Exercise. — Make a series of 3 agar 

 plates, one of 3 gelatin plates, and a series of 3 gelatin roll 

 cultures (Esmarch rolls) from the bouillon culture of B. colt 

 communis (18). Place the agar plates in the incubator and 

 the gelatin plates and rolls in a locker for that purpose. 



Reexamine all the cultures made in previous exercises and 

 add to the laboratory notes a description of any changes in 



