36 METHODS OF CULTIVATION OF BACTERIA 



there is added 1 or 2 per cent, of grape sugar. The steps in the 

 preparation are the same. Glucose being a reducing agent, no 

 free oxygen can exist in a medium containing it, and therefore 

 glucose broth is used as a culture fluid for anaerobic organisms. 



1 (c). Glycerin Broth. The initial steps are the same as in 

 1 (a), but after filtration 6 to 8 per cent, of glycerin (sp. grav. 

 1'25) is added. This medium is especially used for growing the 

 tubercle bacillus when the products of the growth of the latter 

 are required. 



2. Gelatin Media. These are simply the above broths, with 

 gelatin added as a solidifying body. 



2 (a). Peptone Gelatin : 



Meat extract 

 Sodium chloride . 

 Peptone albumin . 

 Gelatin 



. 1000 c.c. 



5 grms. 

 . 10 

 100-150 



(The "gold label" gelatin of Coignet et Cie, Paris, is the best.) 

 The gelatin is cut into small pieces, and added with the other 

 constituents to the extract ; they are then thoroughly melted on a 

 sand bath, or in the " Koch." The fluid medium is then rendered 

 slightly alkaline, as in 1 (a), and filtered through filter paper. 

 As the medium must not be allowed to solidify during the pro- 

 cess, it must be kept warm. This is 

 effected by putting the flask and 

 funnel into a tall Koch's steriliser, 

 in which case the funnel must be 

 supported on a tripod or diaphragm, 

 as there is great danger of the neck 

 of the flask breaking if it has to 

 support the funnel and its contents. 

 The filtration may also be carried out 

 in a funnel with water-jacket which is 

 heated, as shown in Fig. 7. Whichever 

 instrument be used, before filtering 

 shake up the melted medium, as it is 

 apt while melting to have settled into 

 layers of different density. Sometimes 

 the first portion of filtrate is turbid. 



If so, replace it in the unfiltered part : often the subsequent 

 filtrate in such circumstances is quite clear. A litre flask of 

 the finished product ought to be quite transparent. If, however, 

 it is partially opaque, add the white of an egg, shake up well, 

 and boil thoroughly over the sand bath. The consequent coagula- 



FIG. 7. Hot-water funnel. 



