30 ELEMENTARY BACTERIOLOGY LABORATORY EXERCISES 
EXERCISE 48 
DIFFERENTIAL PLATING METHODS 
In the isolation of bacteria advantage may sometimes be taken of the physi- 
ological characteristics of the organisms in such a way that the colonies of the 
organisms sought may be recognized directly on the plates. For example, bac- 
teria which ferment lactose with the formation of acid may be readily isolated 
by plating with lactose agar which contains some brom cresol purple as an indi- 
cator. Under these conditions the colonies of the lactose fermenting organisms 
would be surrounded by a yellow zone due to the acidity resulting from the 
lactose fermentation. 
By the loop dilution method plate out a sample of milk with brom cresol 
purple lactose agar. Examine the plates for the presence of acid forming colonies 
after incubation at 37° C. for one and two days. 
By the same method plate out a sample of soil, using casein agar. The casein 
.agar may be made by adding a small amount of skimmed milk to ordinary 
nutrient agar. Bacteria which have the power of breaking down proteins will 
digest the casein around their colonies, making a clear zone around the colony 
in the opaque plate. Examine the plates for casein digesting bacteria after two 
and four days’ incubation at 37° C. 
