PREPARATION OF CULTURE MEDIA. 15 



requisite quantity of broth (enough to fill the tube to 

 the depth of an inch and a half or two inches) is 

 allowed to run in ; the clip is then re-applied 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 sterilised. The 

 vessels are placed in the steam steriliser and exposed to 

 steam for half an hour on three successive days; this 

 process is called intermittent sterilisation, and its 

 rationale is very simple. The first steaming destroys 

 all developed bacteria, and would sterilise the fluid 

 entirely if no spores were present. In the interval 

 between the first and second sterilisation most of the 

 spores which may be present will develop into mature 

 bacteria, and these will be killed by the second steam- 

 ing. The third sterilisation 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 sterilised flask or into 

 test-tubes and sterilise in the steam steriliser for half an 

 hour on three successive days. 



NUTRIENT GELATIN is broth which has been solidified 

 by the addition of from ten to fifteen per cent, of gelatin ; 



