AGAR MEDIA 37 



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 water bath 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). 

 After melting render slightly alkaline with sodium hydrate 

 solution, 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 roof 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 C., and on cooling solidifies about 39 C. 



3 (6). 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 grape sugar along with agar. This medium is used 

 for the culture of anaerobic organisms at temperatures above the 

 melting-point of gelatin. It is also a superior culture medium 

 for some aerobes, e.g. the b. diphtherise. 



These bouillon, gelatin, and agar preparations constitute 

 the most frequently used media. Growths in bouillon do not 

 usually show any characteristic appearances which facilitate 

 classification, but- such a medium is of great use in investigating 

 the soluble toxic products of bacteria. The most characteristic 

 developments of organisms take place on the gelatin media. 

 These have, however, the disadvantage of not being available 

 when growth is to take place at any temperature above 24 C. 

 For higher temperatures agar must be employed. Agar is, how- 

 ever, never so transparent. Though quite clear when fluid, on 

 solidifying it always becomes slightly opaque. Further, growths 

 upon it are never so characteristic as those on gelatin. It is, 

 for instance, never liquefied, whereas some organisms, by their 

 growth, liquefy gelatin and others do not a fact of prime 

 importance. 



Litmus Media. To any of the above media litmus (French, 



