FEEDING PAEAMECIUM KNOWN BACTERIA 181 



such a medium. For this reason, it is valuable in testing the 

 availability of various bacteria for food. If Paramecia live no 

 longer, or but little longer than in sterile medium, we are justified 

 in assuming that these organisms are unsuitable food; that their 

 energy content is unavailable. Since it has been demonstrated 

 that Paramecium does not utilize the substances dissolved in 

 sterile media of the sort ordinarily used in the laboratory, it would 

 seem that Peters' ('20) contention that Protozoa are capable of 

 saprophytic existence needs further investigation. His experi- 

 ments dealt with the growth of Colpidium in a sterile synthetic 

 medium very different from hay infusion. Colpidium encysts, 

 and so has a means of becoming adjusted to marked environ- 

 mental change. It would seem, therefore, that Paramecium or 

 some other non-encysting protozoan should be tested in this 

 synthetic medium before generalizations can be made regarding 

 the ability of Protozoa to live as saprophytes. 



SUMMARY 



The study of the behavior of Paramecium aurelia when fed 

 with known bacteria shows that it is perfectly feasible to control 

 the bacterial content of a medium and that the technique re- 

 quired is not too laborious. In studying such conditions, it has 

 been found that, in addition to the rate of fission, a consideration 

 of the percentages of high and low divisions and the death rate 

 is of value. The application of the significance factor to data of 

 this type has proved exceedingly useful, making possible decisions 

 with regard to effects of food and media which could not other- 

 wise have been reached. 



The contention of Hargitt and Fray that pure cultures of 

 bacteria are as a rule unsatisfactory food for Paramecium has 

 been sustained in this work. Moreover, in the single instance in 

 which a pure culture could be used over a long period, the meta- 

 bolic rate was consistently lower than that of any mixture em- 

 ployed. Mixtures of bacteria would then appear to be the most 

 satisfactory food for Paramecium. Of all the artificial mixtures 

 tested, but two were found which furnished adequate food over 



