4 6 METHODS OF CULTIVATION OF BACTERIA. 



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

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

 100 c.c. take 1.5 grams 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 in a 

 water bath for five hours, till the agar is thoroughly 

 melted. Test with litmus to see that reaction is still 

 slightly alkaline, 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 containing 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). Glycerine Agar. To 3 (a) after filtration add 6 to 

 8 per cent of glycerine and sterilise as above. This is 

 used especially for growing the tubercle bacillus. 



3 (c). Glucose Agar. Prepare as in 3 (a), but add i to 2 

 per cent of grape sugar along with agar. This medium is 

 used for the culture of anaerobic organisms at temperatures 

 above the melting point of gelatine. It is also a superior 

 culture medium for some aerobes, e.g. the B. diphtherise. 



These bouillon, gelatine, and agar preparations constitute 

 the most frequently used media. Growths on 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 gelatine media. These have, however, the dis- 

 advantage 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, however, 

 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 



