CONTROL OF CULTURES 



465 



Oehler (1924) reports that he was able to obtain sterile Colpoda 

 from cysts which had been heated either at 37° C. for six weeks or at 

 60° -64° C. for several hours. While it is true that certain types of bac- 

 teria are killed by desiccation for considerable lengths of time, it would 

 be nothing short of a miracle if only these delicate forms happened to 

 make up the associated flora from the wild. In this laboratory we have 

 employed a modification of Oehler's method for obtaining large numbers 

 of C. steinii, but a short description of the results will indicate that the 

 method is not to be relied upon for initial sterilization. 



Figure 127. Details of 

 construction of the sponge- 

 and-glass plunger for the 

 collection of cysts. The cen- 

 ter rod is solid, and a piece 

 of tubing holds the sponge 

 down. 



We maintain cultures of C. steinii, sterilized by centrifugation, as de- 

 scribed in a preceding section, in suspensions of the common nonspore- 

 forming coliform bacterium, Aerobacter cloacae. From time to time we 

 have needed large numbers of sterile ciliates for experimental work, 

 and these were obtained from cysts collected as follows. A ring of sponge 

 is placed at the bottom of a glass plunger which will reach well into a 

 culture tube (Fig. 127). These sponge and glass plungers, with the 

 top of the rods wrapped in cotton, are placed in test tubes of water and 

 autoclaved. When cool, the plunger is transferred aseptically to a culture 

 of Colpoda in Aerobacter. After the food organism is largely depleted, 

 the ciliates encyst on and within the sponge. The plunger is then with- 

 drawn and placed in a dry sterile test tube and set aside to dry. At first 



