ANIMAL TISSUES AND FLUIDS 35 



[The use of carbohydrate media has been considerably extended in recent 

 years in connexion with the identification and differentiation of the various 

 members of the same group of micro-organisms, e.g. the differentiation of 

 members of the typhoid-colon group, the differentiation of the streptococci, 

 etc. (Gordon, Andrewes and Horder and others). For this purpose a medium 

 differing somewhat from that given above, and having the following com- 

 position, is in general use. 



Peptone. - 1-2 grams. 



Water, - - 100 c.c. 



Test substance, - 1 gram. 



Kahlbaum's litmus solution. - - Q.8. 



[The test substance may be either a sugar, e.g. glucose, lactose, Isovulose, 

 saccharose, etc. an alcohol, e.g. mannite, dulcite or a glucoside, e.g. salicin, 

 coniferin, etc. 



[Care must be taken to obtain guaranteed pure chemicals from reliable firms, and 

 an equal amount of care must be bestowed upon the sterilization of the media,, 

 since it is well known that in the presence of water and under the influence of heat 

 many of these highly complex compounds undergo decomposition, often of the 

 nature of an hydrolysis. Filtration through a porcelain filter would seem to 

 be the best method of sterilization. After distribution into sterile tubes the latter 

 must be incubated for a few days and those showing any change rejected.] 



Glycerin broth. 



Add 5 per cent, or 50 grams per litre of pure glycerin to peptone-beef- 

 broth before distribution into tubes (Stage 7, p. 31). 



Glycerin in the same proportion may also be added to the carbohydrate 

 broths prepared as above. 



Carbonated broth. 



Add calcium carbonate (2 per cent.) x to lactose-, mannite-, glucose-, etc., 

 broth before distributing into tubes (Stage 7, p 31). 



Calcium carbonate is most frequently added to lactose- broth. When an organism 

 which ferments a given sugar is grown in a carbonated broth containing that sugar, 

 the acids formed by decomposition of the carbohydrate act on the chalk with the 

 formation of CO 2 and the evolution of a considerable quantity of gas. 2 



Milk. 



Milk is used as a culture medium in several ways. 



(A) Fresh milk, alkaline in reaction, is distributed in tubes (15-20 c.c. 

 per tube). 



The tubes are plugged with wool, and sterilized at 115 C. for 20 minutes. 



This is the most simple method of preparation and suffices in the great 

 majority of cases ; it is the method ordinarily employed. 



[In England it is usual to add sufficient litmus solution to tint the milk 

 blue (p. 57), and to sterilize by steaming at 100 C. (Chap. I.). In our 

 experience it is a very difficult matter to sterilize milk by steam at 100 C. 

 in bulk ; it is much safer to tube the milk and then sterilize it.] 



(B) Since a temperature of 115 C. alters to some extent the properties of 

 milk, it may be desirable for some purposes to sterilize at a lower tempera- 

 ture. 



In that case, after washing the cow's udder with an antiseptic, the milker 



[ l In our experience 0'5 per cent., or even 0'25 per cent, of calcium carbonate is 

 sufficient.] 



[ 2 It sometimes happens, however, that when an organism is grown in a litmus-sugar- 

 carbonate-broth, acid is formed as shown by the change in colour of the litmus but no 

 gas is evolved.] 



