MAKING PLATE AND ESMARCH ROLL CULTURES 29 
EXERCISE VIII 
MAKING PLATE AND ESMARCH ROLL CULTURES 
41. The general principle underlying the separation of bac- 
‘ria by means of plate and roll cultures is to dilute the sub- 
ance containing the bacteria so that the individual organisms 
ill be separated from each other by an appreciable distance 
ad then fixed in a solid medium where each organism can 
wultiply into a growth or colony without coming in contact 
ith any other organism or colony. For this purpose agar 
id gelatin are used. Originally Koch employed a rectangu- 
r piece of glass for holding the layer of medium, and pro- 
icted it from contamination by putting it under a bell jar. 
ater Esmarch introduced the “ roll-culture’”’ method, which 
as extensively followed until the Petri dishes were intro- 
uced. Since that time the latter have been largely used in 
lace of the Koch plate and Esmarch tube. On this account 
ie plate cultures of to-day are usually made in Petri dishes. 
he roll culture is also used. : 
Plate cultures are employed for two distinct purposes: 
t) to isolate bacteria in order to obtain pure cultures from 
ie isolated colonies ; and (2) to determine how many bacteria 
\ere are present in a given quantity of a liquid such as water, 
ilk, or blood. In this exercise the object is to separate the 
icteria to obtain isolated colonies. For quantitative work, 
‘e Exercise LV. 
42. Work for this exercise. Make a series of 3 agar plates, 
ie of 3 gelatin plates, and one of 3 gelatin roll cultures 
Zsmarch rolls) from the bouillon culture of 2. coli communis 
|} 19). Place the agar plates in the incubator and the gelatin 
ates and rolls in a locker for that purpose. 
Reéxamine all the cultures made in previous exercises and 
Id to the laboratory notes a description of any changes in 
