FEEDING PAEAMECIUM KNOWN BACTERIA 169 



bacteria J, K, L, or their combinations, lived over six days,, 

 whereas many of those in the moneywort with the same food 

 lived seven days. There was very little effect which could be 

 ascribed to the change in medium, but the increased length of life 

 mentioned may indicate a slight tendency of the moneywort to 

 stimulate. However, in case this was so, the effect was not 

 great enough to overcome the unfavorable nature of the bacterial 

 food. 



This experiment indicates that a change of medium in the 

 presence of a food known to be unsatisfactory does not so affect 

 the animals that they are able to utilize such food for any length 

 of time. It would seem, therefore, that whereas the stimulating 

 effect of a changed medium may be an aid in reviving cultures of 

 Paramecium which are not in a thriving condition, the nature of 

 the food is at least as important, and if this be unsuited to th& 

 requirements of the animal, no amount of artificial stimulation 

 by altering the medium will prevent the death of such cultures. 



5. The effect of sterile media. In order to determine whether 

 the results obtained by feeding certain bacteria which did not 

 support active metabolism in Paramecium were due to the toxic 

 action of such bacteria or merely to the fact that they were for 

 some reason not utilizable for food, certain individuals were 

 thoroughly washed and placed in sterile media. The average 

 length of life in hay infusion was 2.15 days, in uncured swamp- 

 hay infusion, 2.54 days, and in moneywort infusion, 1.72 days. 

 The maximum duration of life for animals in the first infusion 

 was 4 days, for those in the second, 5, and for those in the 

 moneywort, 4 days. The minimum duration of life in all three 

 infusions was the same — less than twenty-four hours. Five 

 lines were tested in each case, and controls carried in C during 

 this time showed the normal rate of metabolism for animals fed 

 upon this organism. 



These tests show that Paramecium aurelia will not live for any 

 length of time in sterile media of the type ordinarily used in the 

 laboratory. It is, therefore, to be assumed that the failure of the 

 animals in the preliminary experiments to live in pure or mixed 

 cultures of the bacteria used, was due in most cases to starvation. 



