38 METHODS OF CULTIVATION OF BACTERIA 



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). 

 Render 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 C., 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 

 made faintly alkaline to phenol- phthalein ; 1 per cent, of glucose 

 is added as above. 



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. 



