PREPARATION OF CULTURE MEDIA II 



is better than the white absorbent wool, as it is less easily- 

 wetted. 



The broth (and other culture medium after being melted) may 

 be poured into the tubes in the following way : A sterilized funnel 

 is united by a short length of indiarubber tubing to a piece of glass 

 tubing drawn out to a point ; the rubber tube is clipped by a 

 spring clip or a pair of pressure forceps. The funnel is now 

 mounted on a retort-stand, filled with the medium, and covered 

 over with a piece of glass. The cotton-wool plug is removed from 

 a test-tube, and the latter placed so that the glass tube attached to 

 the funnel reaches nearly to the bottom. The clip is released, and 

 the requisite quantity of broth (enough to fill the tube to the depth 

 of ij or 2 inches) is allowed to run in ; the clip is then reapplied 

 and the tube removed and plugged. This process is repeated 

 until enough tubes have been filled. 



The tubes and the broth which remains over (after having been 

 poured back into the flask and the latter plugged with cotton-wool) 

 are now sterilized. The vessels are placed in the steam sterilizer 

 and exposed to steam for half an hour on three successive days ; this 

 process is called intermittent sterilization, and its rationale is very 

 simple. The first steaming destroys all developed bacteria, and 

 would sterilize the fluid entirely if no spores were present. In 

 the interval between the first and second sterilization most of the 

 spores which may be present will develop into mature bacteria, 

 and these will be killed by the second steaming. The third 

 sterilization is to kill off any bacteria which may not have 

 developed from spores in the first interval. A very similar 

 process is adopted by the gardener in freeing soil from weeds ; 

 the application of chemical weed-destroyers or a thorough hoeing 

 will destroy developed plants, but will not injure seeds which may 

 be contained in the soil, and these processes are repeated, intervals 

 being allowed to permit the development of the plants, until they 

 reach the stage in which they are vulnerable. 



To recapitulate : Mix the ingredients, and heat until they are 

 dissolved, render slightly alkaline, boil half an hour, filter. Then 

 place in sterilized flask or into test-tubes, and sterilize in the steam 

 sterilizer for half an hour on three successive days. 



Nutrient Gelatin is broth which has been solidified by the 

 addition of from lo to 15 per cent, of gelatin ; the former amount 

 is used in the winter, the latter in the summer. For general 

 purposes I2| per cent, may be used in all cases. 



