PARAMECIUM IN PURE CULTURES OF BACTERIA 



437 



TABLE 4 



Shows the reduction in number of the bacteria by washing Paramecium through 5 

 wash fluids in watch crystals and in depression slides, when the crystals and 

 slides were enclosed in sterile Petri-dishes 



remained on the body of the protozoan they would have been 

 the starting point of colonies of bacteria in the agar around it. 

 It is possible therefore to get Paramecium absolutely free from 

 bacteria by washing in sterile water. The success of this method 

 was so marked that it was used throughout the study in pre- 

 paring all animals for growth in pure cultures of bacteria. 



The complete sterilization process as finally modified may be 

 described as follows : A large number of capillary pipettes (40-50) 

 were prepared by drawing out one end of a glass tube into a 

 capillary tube. A part near the opposite end was heated and 

 drawn out slightly so that a narrowing of the tube was pro- 

 duced at this point. Into this constricted portion of the pipette 

 a small roll of cotton was placed and the pipettes were then 

 sterilized in the hot air sterilizer at a temperature of 160-170°C. 

 The rubber bulb of the pipette was not sterilized since heating, 

 even in the steam sterilizer, destroyed its elasticity. But the 

 plug of cotton in the constricted portion of the pipette effec- 

 tively filtered out all bacteria which might have been contained 

 in the bulb, so that none could pass into the pipette beyond 

 the cotton. Consequently none of the bacteria in the non-sterile 

 rubber bulb could get into any of the liquids used in the cul- 

 tures, wash fluids, and the like. Next, depression slides w^ere 

 placed inside Petri-dishes and these were sterilized in the hot 

 air steriUzer for 30 minutes; they were ready for use as soon as 

 cooled without further handling or manipulation. Tap-water 

 was used as the wash fluid and this was sterilized in the auto- 

 clave, removed and cooled. When these preparations had been 



