PARAMECIUM IN PURE CULTURES OF BACTERIA 451 



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time. If these fluids are sterilized at a temperature of 100°C. 

 they are not so modified and are satisfactory media for the 

 growth of Paramecium. 



5. It was found possible to get the body of Paramecium abso- 

 lutely sterile by washing in sterile water. This washing was 

 done in sterile tap-water in depression slides enclosed within 

 sterile Petri-dishes. A bacteriological examination of the wash 

 waters showed no bacteria present after the third washing, but 

 five wash waters were used in the preparation of every Para- 

 mecium for further study. The Paramecium itself when tested 

 by being placed on an agar plate developed no colonies of bac- 

 teria showing positively that its body had been entirely freed 

 from these by the washing. Paramecium so sterilized was not 

 injured in any way and suffered no loss of vigor. 



6. Sterile Petri-dishes, large enough to contain a single depres- 

 sion shde, were used as moist chambers and it was demonstrated 

 that they are as satisfactory as large moist chambers. Further- 

 more the Petri-dishes have certain marked advantages as moist 

 chambers over larger receptacles. 



7. Pure cultures of bacteria, with Paramecium growing in 

 them, usually remained uncontaminated by foreign bacteria for 

 periods of at least two weeks (they were not tested out for longer 

 periods). The most extreme case of contamination of such a 

 pure culture was at the rate of 1 foreign bacterium to 350 of the 

 pure culture. It is believed that such a contamination is with- 

 out significance as having effect on the food supply of Para- 

 mecium when the cultures are not carried for long periods. 



8. Paramecia were grown in pure cultures of bacteria isolated 

 from normal and abnormal hay infusions. In no case was a 

 single kind of bacteria as satisfactory for food as a mixture of 

 many kinds. This was true whether the bacteria came from 

 normal or from abnormal infusions. The bacteria isolated from 

 abnormal infusions were so unfavorable as to cause death of 

 the protozoa in a few days; probably toxic excretions were set 

 free by these bacteria. The only bacteria in pure culture which 

 approached the control mixed cultures as satisfactory food for 

 Paramecium was Bacillus subtilis; this sometimes seemed better 



