48 METHODS OF CULTIVATION OF BACTERIA. 



platinum needle used in inoculating it with a bacterial 

 growth; 15 per cent gelatine melts at about 24 C. 



2 (b\ Glucose Gelatine. The constituents are the 

 same as 2 (a), with the addition of i to 2 per cent of 

 grape sugar. The method of preparation is identical. 

 This medium is used for growing anaerobic organisms at 

 the ordinary temperatures. 



3. Agar Media (French, "ge*lose"). The disadvantage 

 of gelatine is that at the blood temperature (38 C.), at 

 which most pathogenic organisms grow best, it is liquid. 

 To get a medium which will be solid at this temperature, 

 agar is used as the stiffening agent instead of gelatine. 

 Unlike the latter, which is a proteid, agar is a carbohydrate. 

 It is derived from the stems of various sea-weeds grow- 

 ing in the Chinese seas, popularly classed together as 

 "Ceylon Moss." The best for bacteriological purposes is 

 that consisting of the thin dried stem of the sea -weed 

 itself. 



3 (a). "Ordinary" Agar. Prepare peptone bouillon, 

 medium i (a), up to the stage of sterilisation. For every 

 100 c.c. take 1.5 grammes agar. Cut it up into very fine 

 fragments (in fact till it is as nearly as possible dust), add 

 to the bouillon and allow to stand all night. Then boil 

 gently in a water bath for two or three hours, till the agar is 

 thoroughly melted. Test with litmus to see that reaction 

 is still slightly alkaline, make up to original volume with 

 distilled water, and filter. Filtration here is a very slow 

 process and must be 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 (/>). Glycerine Agar. To 3 (a) after filtration add 



