PEPTONE GELATIN. 41 



employed for the culture of aerobic bacteria at ordinary tem- 

 peratures. The exact percentage of gelatin used in its prepara- 

 tion depends on the temperature at which growth is to take place. 

 Its firmness is its most valuable characteristic, and to maintain 

 this in suinmer weather, 15 parts per 100 are necessary. A 

 limit is placed on higher percentages by the fact that, if the 

 gelatin be too stiff, it will split on the perforation of its sub- 

 stance by the platinum needle used in inoculating it with a 

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



3 {b). Glucose Gelatin. — The constituents are the same as 

 3 (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 (c). Beerwort Gelatin consists of hopped beer wort, to 

 which has been added 10 or 15 per cent of gelatin. It is a use- 

 ful medium for the cultivation of yeasts and moulds. 



These bouillon, agar, and gelatin 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 gelatin media. 

 These have, however, the disadvantage 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, how- 

 ever, 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 gelatin. It is, for 

 instance, never liquefied, whereas some organisms, by their growth, 

 liquefy gelatin and others do not — a fact of prime importance. 



Litmus Media. — To any of the above media litmus (French, 

 "tournesol") may be added to show change in reaction during 

 bacterial growth. The litmus is added, before sterilisation, as 

 a strong watery solution^ in sufficient quantity to give the 

 medium a distinctly bluish tint. During the development of an 

 acid reaction the colour changes to a pink and may subsequently 

 be discharged. 



Petruschky's Litmus-wliey(as modified by Durham). — "Fresh 

 milk is slightly warmed and clotted by means of essence of 



1 6 grammes of Merck's neutral extract of litmus to 1000 c.c. of distilled water. 



