38 METHODS OF CULTIVATION OF BACTEEIA 



is a proteid, agar is a carbohydrate. It is derived from the 

 stems of various seaweeds growing in the Chinese seas, com- 

 mercially classed together as " Ceylon Moss." For bacteriological 

 purposes the dried stems of the seaweed may be used, but there 

 is in the market a purified product in the form of a powder, 

 which is preferable. 



3 (a). " Ordinary " Agar. — This has the following composi- 

 tion : — 



Meat extract ..... 1000 c.c. 



Sodium chloride . . . 5 grms. 



Peptone albumin . . . . 10 „ 



Agar 15 „ 



Cut up the agar into very fine fragments (in fact till it is as 

 nearly as possible dust), add to the meat extract with the other 

 ingredients, and preferably allow to stand all night. Then boil 

 gently in a " Koch " for two or three hours, till the agar is 

 thoroughly melted. The process of melting may be hastened 

 by boiling the medium in a sand bath and passing through it a 

 stream of steam generated in another flask ; the steam is led 

 from the second flask by a bent glass tube passing from just 

 beneath the cork to beneath the surface of the medium (Eyre). 

 Eender slightly alkaline with sodium hydrate solution, and if 

 necessary make up to original volume with distilled water, and 

 filter. Filtration here is a very slow process, and is best carried 

 out in a tall Koch's steriliser. In doing this, it is well to put a 

 glass plate over the filter funnel to prevent condensation water 

 from dropping off the lid of the steriliser into the medium. If 

 a slight degree of turbidity may be tolerated, it is sufficient to 

 filter through a felt bag or jelly strainer. Plug the flask con- 

 taining the filtrate, and sterilise either in autoclave for fifteen 

 minutes or in Koch's steriliser for one and a half hours. Agar 

 melts just below 100° O, and on cooling solidifies about 39° C. 



3 (b). Glycerin Agar.— To 3 (a) after filtration add 6 to 8 

 per cent, of glycerin and sterilise as above. This is used 

 especially for growing the tubercle bacillus. 



3 (c). Glucose Agar. — Prepare as in 3 (a), but add 1 to 2 

 per cent, of glucose, or, better still, a corresponding amount of 

 a 10 per cent, sterile solution of glucose after filtration. This 

 medium is used for the culture of anaerobic organisms at 

 temperatures above the melting-point of gelatin. For the 

 growth of the tetanus bacillus a specially suitable medium is 

 composed of meat extract with 2 per cent, agar, 2 per cent, 

 peptone, and '5 per cent, alkaline sodium phosphate added, and 



