FEEDING PARAMECIUM KNOWN BACTERIA 179 



this technique might prove useful in testing the food value of 

 particular bacteria for Paramecium. 



The question of toxicity is more complex. In the account of 

 the preliminary experiments of this study, 100 per cent of deaths 

 occurred the first twenty-four hours among animals fed with L' 

 and K'L'. A wholesale mortality such as this would lead one to 

 suspect that the bacterium L' was toxic for Paramecium. Yet, 

 when it was combined with J', itself not a satisfactory food, im- 

 mediate death did not result. Phenomena such as these need 

 further investigation. 



The evidence presented in this paper seems to show that mix- 

 tures of bacteria furnish the most satisfactory food for Parame- 

 cium. Of all the mixtures tried, the chance mixture M seems to 

 have been the best, could it have been maintained at an optimum 

 of efiP.ciency. The objection to the use of such a mixture in 

 certain types of experimental work is that its exact content is 

 unknown and is subject to daily variation. A mixture such as 

 A'C, when studied with regard to the division rate and signifi- 

 cance factor, is found to be so nearly the equal of the chance mix- 

 ture during a long period of time, that it may be said to be on 

 the whole as advantageous. It possesses the advantage, more- 

 over, of being known and subject to control. 



Mention has been made of the stimulating action of mixed 

 cultures of bacteria as contrasted with pure cultures. The word 

 stimulating is not used here in the same sense as in the account 

 of the effect of change in medium. In this latter instance, any 

 acceleration of metabolism noted is probably due to the action 

 of some chemical constituent of the medium less complex than a 

 food. Mixing foods may cause an acceleration of metabolism 

 simply because more energy becomes available from an outside 

 source, whereas the effect of a chemical stimulant is to release 

 energy locked up within the organism. The increase in meta- 

 bolic rate as a result of combining pure cultures of bacteria has 

 been the usual experience during this work. Frequent as it has 

 been, however, but two instances, A'C and A'B'C, were ob- 

 served where this acceleration of metabolism continued for any 

 great time. 



