CULTURAL TECHMQUK 9 



if more nearly full, the cotton plug is likely to be wet by the 

 medium during sterilization. 



Plugging tubes. The test tubes filled with media are to 

 be plugged with cotton at once. Ordinary roll cotton (not 

 absorbent) is used for this purpose. A piece of cotton should 

 be pulled from the roll, folded upon itself twice, the cotton 

 compressed with the thumb and fingers and gradually forced 

 into the tube. The plug should extend into the tube at least 

 1.5 cm. and about the same distance above it, in order to 

 protect the lip of the tube from dust and to enable one to 

 remove the plug easily. 



The tubes may also be plugged rapidly by pulling off a 

 sufficient amount of cotton with a pair of dissecting forceps 

 and thrusting it into the tube by means of the forceps. 



Sterilization of media. Media in tubes or flasks is steril- 

 ized by an exposure to streaming steam at a temperature of 

 100 C. from fifteen to sixty minutes, depending on the size 

 of the containers, on each of three consecutive days, or by 

 a single exposure to a considerably higher temperature, as in 

 steam under pressure in a closed chamber (autoclave). 



Various types of sterilizers are used. The Arnold is most 

 convenient for ordinary laboratory purposes, as it heats rapidly 

 and does not allow large quantities of steam to escape into 

 the room. The media in tubes should be heated for twenty 

 minutes on three consecutive days. With flaaks the period of 

 exposure should be 1 extended in proportion to the size of the 

 containers. The time should be computed from the moment 

 when steam is freely generated in the sterilizer. During the 

 interval between heatings, the media should be kept at room 

 temperature, in order to allow the spores present to germi- 

 nate. This system of sterilization is known as the intermit- 

 tent or discontinuous method. 



