CONTROL OF CULTURES 457 



as a check on sterility. If no turbidity developed, contaminated Tri- 

 chomonas were layered onto the fluid in the large end of the tube. 

 Within forty-eight hours many flagellates had migrated down the tube 

 and could be seen in the last inch or two of the capillary portion. The 

 authors state that the bacteria failed, for the most part, to migrate past 

 the first loop, and never passed the second. The last portion of the capil- 

 lary was cut off and sealed by means of a flame and the cut-off portion 

 was submerged in tincture of iodine (7 percent) for one hour. Then 

 one end was grasped in the fingers and the tube held upright to drain. 

 When dry, pieces of the tube were broken off with sterile forceps and 

 dropped into selected culture media. The authors state that they have 

 repeatedly isolated T. hominis bacteria-free by this method, but have not 

 tested it with other Protozoa. 



The above method appears to be applicable to many types of Protozoa, 

 and should receive serious consideration. The manipulations offer some 

 difficulty, however, and extreme care will have to be exercised to insure 

 against outside contamination, especially during the filling of the tube 

 with the sterile fiuid and again during the breaking of the sections of 

 capillary into nutrient culture media. 



Probably the simplest method which takes advantage of the migration 

 of Protozoa in fluid media is the Petri-dish method. A sterile Petri dish 

 is partially filled with sterile fluid and placed on the stand of the dissect- 

 ing microscope so that one edge is under the objective. After all motion 

 of the fluid has ceased, the Petri dish cover is raised and a drop of con- 

 centrated protozoan culture is placed very near the edge opposite the 

 one under the lens. This manipulation must be done with great care, 

 so that the fluid is disturbed as little as possible. The cover is then gently 

 lowered and sufficient time (five to ten minutes) is allowed for the 

 Protozoa to swim to the opposite edge of the dish. The cover is again 

 raised, and single organisms are picked out with sterile pipettes and 

 transferred to selected media. Minimum time for the migration is im- 

 portant, so that none of the highly motile bacteria will reach the area 

 from which the Protozoa are being taken. Enough Protozoa should be 

 separated singly in this way to allow for the law of averages. The greater 

 the motility of the Protozoa, the smaller their size, and the smoother 

 their bodies, the greater the chance for successful sterilization by this 

 method. 



