PARAMECIUM IN PURE CULTURES OF BACTERIA 449 



From the data recorded in the various tables it seems clear 

 that cultures of mixed bacteria are, as a rule, far superior as a 

 diet for Paramecium to a diet of any one kind of bacteria. Most 

 bacteria in pure cultures, even when isolated from normal, 

 healthy, hay infusions, were quite unfavorable if used alone. 

 This was true even when such bacteria were present in enormous 

 numbers in healthy infusions and were the predominant types. 

 Only a single kind, Bacillus subtilis, approached a mixed diet 

 as a favorable kind of food ; this sometimes seemed to be a better 

 and sometimes a less satisfactory food. 



It would therefore appear, under normal conditions, that Para- 

 mecium thrives by virtue of the use of a diet of different kinds 

 of bacteria. It is possible that Bacillus subtihs is the chief 

 dependence of Paramecium as food, as some have claimed, but 

 the experimental evidence does not show the superiority of this 

 form over a mixture of different kinds. The probabihty that 

 different bacteria are usually eaten is in accord with the struc- 

 ture and habits of Paramecium. This infusorian is one of many 

 which has a mouth constantly open, and apparently there is 

 no cessation in the beating of the cilia. Under such circum- 

 stances it is hardly conceivable that there is any choice of food, 

 rather all bacteria which are not too large are swept into the 

 buccal groove in the ciliary current and taken into the body. 

 Doubtless some of the bacteria which get into the body are but 

 slightly, if at all, digested and assimilated. If such forms of 

 bacteria should come to be the predominant type in an infusion 

 one would expect Paramecium to decrease in vigor and many 

 of them to die. There is httle doubt that some of the 'bad' 

 cultural conditions observed in infusions are due to just this 

 condition of certain bacteria gaining the ascendency and these 

 being so unsatisfactory as food that the animals die. 



It should be possible by using pure cultures of bacteria, mix- 

 ing these known forms in various combinations in sterile infu- 

 sions and growing Paramecium therein, to secure a mixture 

 which would be better than the ordinary mixed culture obtained 

 by a chance infection of the culture fluid from air, hay, or water. 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 22, NO. 2 



