16 CLINICAL BACTERIOLOGY AND 



Gelatin. 



Method. — Take 100 c.c. of broth, add 12£ grammes of 

 finely-cut gelatin, boil thoroughly until all is dissolved, 

 filter into a flask as was done with the broth; but, as 

 gelatin solidifies on exposure to cold, extra heat must be 

 applied during the filtering process. The flame of a spirit 

 lamp placed close to the filter funnel answers the purpose. 

 A double-jacket hot-water funnel can be purchased to serve 

 the same object, or the filter, media, and flask, may be 

 placed in an oven with a temperature registered at 40° C. 



Gelatin is also used to grow bacteria for diagnostic pur- 

 poses, as some microbes produce a digestive ferment and 

 liquefy gelatin, and others do not; again, some liquefy 

 rapidly, and others slowly. Gelatin also may be melted 

 at 25° C, at which temperature bacteria are not destroyed. 

 It is therefore a useful medium for " plating-out " purposes. 



Agar-Agar. 



Method. — Take 100 c.c. broth, cut up 2 grammes of agar 

 into very fine pieces, soak in acetic acid (diluted). 



To make the solution, take 4 c.c. glacial acetic acid, add 

 500 c.c. water, and soak for fifteen minutes ; strain off the 

 acid, wash the agar in water until blue litmus-paper does 

 not become red. Now place the broth in a beaker and boil 

 until all the agar has been dissolved. Neutralize with an 

 alkali, and allow it to cool to about 50° C. To make the 

 media more clear, the white of one egg to each 500 c.c. 

 of media should be added before boiling again. Filter this 

 gently through a double thickness of non-medicated 

 surgical lint which has been previously moistened, and 

 prevent the media from solidifying, as was done in the case 

 of the gelatin. 



Blood-Serum. 



This medium is somewhat difficult to prepare, but, fortu- 

 nately, it can often be dispensed with altogether. More- 

 over, it can be obtained all ready for laboratory work. 



