92 VETERINARY BACTERIOLOGY 



bacteria will have used up all the sugar present. The broth is 

 then prepared as above. 



Sugar Broth. Sugar-free broth is generally modified by the 

 addition of carbohydrates, such as dextrose, saccharose, and 

 lactose, making 1 per cent, solutions. Such media must be 

 subjected to intermittent sterilization in flowing steam and not 

 in the autoclave, as the carbohydrates readily decompose. 



Glycerin Broth. Five or 6 per cent, of glycerin added to 

 broth makes it a much more favorable medium for many organisms. 



Serum Broth. Blood-serum secured under strict aseptic pre- 

 cautions may be added to sterile broth in various proportions. 

 Tubes prepared in this manner should be incubated for a few 

 days to determine whether or not the medium is sterile. Steril- 

 ization can be effected only by filtration, as heating would coagulate 

 the serum. 



Dunham's Solution. This is a solution containing 1 per cent, 

 peptone and 0.5 per cent, sodium chlorid in water. It is used in 

 growing organisms for the determination of indol. 



Beerwort. Unhopped beerwort is frequently used for the 

 growth of yeasts and molds. 



Milk. Fresh separated milk is tubed and subjected to intermit- 

 tent sterilization. Commonly, litmus is added in sufficient quan- 

 tities to make the milk a distinct blue. 



Synthetic Media. It is sometimes desirable to prepare a 

 medium in which the exact chemical composition of every ingre- 

 dient is known. The nature of all the changes brought about by 

 bacteria can be studied chemically in such a medium, and the 

 food requirements ^ determined by changes in the composition. 

 Most synthetic media contain as a basis an aqueous solution 

 of certain salts, among them potassium phosphate and sodium 

 chlorid. Special media of this kind are used extensively in the 

 study of the soil bacteria; it is only occasionally that such a 

 medium proves serviceable in the study of pathogenic forms. The 

 most commonly used of the synthetic media is Uschinsky's solution. 



Water, distilled 1000 c.c. 



Asparagin 4 gin . ; 



Ammonium lactate 6 gm.; 



NajHPO, 2gm.; 



NaCl 



