PREPARATION OF CULTURE MEDIA 9 



for bacteriological use it is best to choose one that will 

 accommodate a litre flask holding a funnel, as it is often a 

 great convenience in the filtration of fluids which become 

 solid on cooling to carry out the process in an atmosphere of 

 steam. 



Exact details of the way it is used will be given subsequently. 



PREPARATION OF CULTURE MEDIA 



Bacteria are grown in the same way as other plants. A 

 gardener who wishes to grow a plot of a particular plant will first 

 prepare a soil suitable for the growth of that plant, and free it as 

 far as possible of all seeds, roots, etc. He will then sow it with 

 the seeds of the plant in question, and do what he can to expose 

 them to a suitable temperature. An exactly similar process is 

 adopted when we wish to cultivate the smallest of all plants. The 

 soil which we prepare is called the culture medium, and differs in 

 the case of different bacteria ; the process of freeing this soil from 

 bacteria and their spores is called sterilization, and we insure a 

 suitable temperature by means of an incubator, the heat of which 

 is kept constant. 



The culture media which are used for special purposes are 

 almost innurtierable, but in the daily routine of the laboratory 

 and for diagnostic purposes, broth, gelatin, agar-agar, and 

 blood-serum, are all that are really necessary in the vast 

 majority of cases. These media may all be bought from any 

 firm of manufacturing chemists, or from any bacteriological 

 laboratory ; and their purchase saves a great deal of work, 

 and is to be recommended for those who only wish to use 

 them occasionally. They are sold in test-tubes, which are kept 

 sterile by being plugged fairly firmly with cotton-wool ; this sub- 

 stance prevents the passage of bacteria as long as it is kept dry. 

 The tubes are best stored in jars provided with tightly fitting lids, 

 and it is an advantage to place a shallow layer of a solution of 

 perchloride of mercury (or other non-volatile antiseptic) in the 

 bottom of each jar to prevent the medium from drying up. It is 

 scarcely necessary to add that not the smallest trace of the lotion 

 should be allowed to come into contact with the cotton-wool 

 plug of the tubes. Or the medium may be kept from drying 

 up by covering the tubes with indiarubber caps sold for the 

 purpose. 



