458 CONTROL OF CULTURES 



We have used the above method to steriHze a number of flagellates 

 {Euglena, Astasia, Chilomonas) . With a single migration across a Petri 

 dish and the selection of twenty-five organisms at each trial, the per- 

 centage of sterile to contaminated cultures was very high (80 to 90 

 percent) . We used a Plastocoel (transparent) shield over the microscope 

 and always worked in a draft-free room. Tetrahymena gelei'i (Furgason, 

 1940) was also sterilized by a single migration, but with about only 10- 

 percent efficiency. Although the Tetrahymena were more motile than the 

 flagellates, they proved harder to rid of their bacteria, probably because 

 of the tendency of the bacteria to become lodged among the cilia. 



Oehler (1919) states that he was able to free various Protozoa of bac- 

 teria by allowing them to migrate over the surfaces of agar in Petri dishes. 

 This technique may be applicable to a few types which are able to swim 

 in a very thin film of moisture. 



The utilization of large tubes of sterile fluids for migration purposes 

 was first mentioned by Purdy and Butterfield ( 1918) . In their studies on 

 the growth of bacteria in sewage, they state that they obtained on one 

 occasion sterile Paramecium after allowing the ciliate to swim through 

 thirty feet of sterile water. They call this the "marathon bath," but give 

 no details of its construction, use, or (and this appears to be important 

 from the practical standpoint) means of sterilization of so long a tube. 



Glaser and Coria (1930) carried out a rather exhaustive study on 

 methods of sterilization. One of the methods they used with success was 

 the large-tube migration, used with negatively geotropic Protozoa. Their 

 apparatus consisted of a tube fourteen or more inches long, with one- 

 fourth inch bore and a fine tapering point. The large end was plugged 

 with cotton and the whole sterilized in a container. Sterile fluid was 

 drawn up to within two inches of the top by applying suction to the 

 large end through a rubber tube. About 2 ml. of contaminated protozoan 

 culture was then drawn up, and this formed a layer beneath the sterile 

 fluid. The fine end was sealed in the flame and the tube mounted upright 

 in a rack (Fig. 124) . After periods of time varying from five minutes to 

 eighteen hours, depending on the species of Protozoa, samples from the 

 top of the tube contained many organisms which had "washed themselves 

 free of most other microorganisms" (p. 790). It was usually necessary 

 to repeat the migration, and this was accomplished by filling a second 

 tube as before, and then drawing up two inches of fluid from the top 



