36 METHODS OF CULTIVATION OF BACTERIA 



obviates, to a large extent, the error introduced by increasing 

 the bulk of the medium if a weaker neutralising solution be 

 used. 



1 (b). Glucose Broth. -To the other constituents of 1 (a) 

 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 process, 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 break- 

 ing 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 what first 

 comes through is turbid. If so, replace it in the unfiltered 



FIG. 7. Hot-water funnel. 



