144 BACTERIOLOGY 



without reduction of the litmus, in a series of tubes of this 

 medium containing the different carbohydrates serves to 

 differentiate between related species of bacteria. For 

 instance, the colon bacillus produces an acid reaction with 

 coagulation and gas-formation with some of the carbohy- 

 drates, while the typhoid bacillus produces a lower degree 

 of acidity with coagulation, but without gas-production. 

 Similarly, the different types of the dysentery bacillus may 

 be differentiated by means of their effects on the different 

 carbohydrates in this medium. 



Guarniari's Gelatin-agar Mixture. For special work, 

 particularly with the organism of pneumonia (bacterium 

 pneumonise) the gelatin-agar mixture recommended by 

 Guarniari is of very great service. It should be exactly 

 neutral in reaction, and should possess the following ingredi- 

 ients : 



Meat infusion 950 c.c. 



Sodium chloride 5 grams 



Peptone . v' . . . . . . . . . 25 to 30 grams 



Gelatin . ..... . 40 to 60 grams 



Agar-agar 3 to 4 grams 



Water 50 c.c. 



The agar-agar should be completely dissolved separately 

 in about 100 c.c. of water in the autoclave while the other 

 ingredients are being prepared. The latter should be filtered 

 and the dissolved agar-agar added to the filtrate. 



A complete list of the special media would be too volu- 

 minous for a book of this size. For their description the 

 reader is referred to the current literature. Those that have 

 been given above will suffice for obtaining a clear under- 

 standing of the principles of the subject. In the chapters 

 upon the Pathogenic Bacteria such special media as have 

 proved of use for purposes of identification and differentiation 

 are described in detail. 



