MAKING PLATE AND ESMARCH ROLL CULTURES 29 



EXERCISE VIII 



MAKING PLATE AND ESMARCH ROLL CULTURES 



41. 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 

 wiU 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 rectangu- 

 lar piece of glass for holding the layer of medium, and pro- 

 tected 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 intro- 

 duced. Since that time the latter 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 : 

 (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 Exercise LV. 



42. Work for this exercise. Make a series of 3 agar plates, 

 one of 3 gelatin plates, and one of 3 gelatin roll cultures 

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

 (§ 19). 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 



