470 CONTROL OF CULTURES 



placed immediately in a medium containing dissolved proteins (tryptone, 

 proteose-peptone, yeast extract, and so forth ) . These types are the true 

 saprozoic forms. Others must have particulate matter, so it is best, espe- 

 cially when dealing with a new type, to inoculate into a wide variety of 

 media. In this laboratory it is the practice to start our newly sterilized 

 Protozoa in five different types of media, usually ten isolations into each. 

 Our standard five types for first tests are 0.1 -percent proteose-peptone; 

 5 -percent yeast autolysate; 0.5 -percent yeast extract; 0.5 -percent malted 

 milk; and 0.5-percent unfiltered Yeast Harris. It is sometimes necessary 

 to have quite a range of pH values within the different media, in order 

 to obtain growth in even one or two of the tubes. 



A number of species of Protozoa appear to be dependent upon living 

 organisms as a source of food. This is true not only of the carnivores, 

 but seems to hold for a number of bacteria-feeders as well. With the 

 carnivores it is usually sufficient to observe their diet in nature to decide 

 upon a suitable food animal. If the food animal can be grown in pure 

 culture, then the chances are good that it will be possible to establish 

 the carnivore in "Zwiegliedrige Kulture." Some carnivores have been 

 found to be very selective, while others are able to feed on any one of a 

 number of organisms. Occasionally a natural bacteria-feeder will turn 

 carnivorous and then can be established without bacteria. 



With the obligatory bacteria-feeders the best that can be done, as far 

 as we now know, is to establish them on a single species of favorable 

 food bacteria. Here again it is absolutely necessary to start with sterile 

 Protozoa, as even in the so-called non-nutritive fluids (salt solutions, 

 distilled water, and so forth) many extraneous bacteria, which are not 

 favorable as food, will multiply and be continually present from trans- 

 plant to transplant. Results may be entirely misleading under these con- 

 ditions, as a number of common bacteria prove to be deleterious to many 

 Protozoa (see Kidder and Stuart, 1939). One method of setting up cul- 

 tures containing a single protozoan species in a suspension of a single 

 species of bacteria is simply to try out a number of known bacteria until 

 one is found which will support growth. However, some ciliates prove 

 to be extremely selective, even as to specific bacteria. If poor growth or 

 no growth results after all the known bacteria have been used, then the 

 investigator must try to isolate from the wild culture the type of bacteria 

 upon which the ciliate was originally feeding. This procedure is tedious, 



