CHAPTER XL 



CULTURE MEDIA IN RELATION TO METABOLIC PRODUCTS- 

 TESTS FOR THE LATTER. 



A CONSIDERABLE number of the culture media described in the 

 preceding chapter are especially devised in order to demonstrate the 

 formation of certain metabolic products of bacterial growth, which 

 in some instances are so characteristic that they assist materially in 

 the identification of certain species of bacteria. 



Proteolytic Ferments. As already pointed out, it is often important 

 to determine whether bacteria produce a peptonizing, proteolytic 

 ferment. This can be ascertained by growing the culture on the 

 boiled white of an egg, blood serum, or gelatin. Since the liquefaction 

 of gelatin is the most complete, it is the preferable medium for the 

 identification of peptonizing species. The presence of other ferments, 

 such as diastase, invertin, etc, sometimes has to be determined. 



Diastase. The enzyme diastase can be demonstrated by the addi- 

 tion of starch to a fluid culture medium which has been made abso- 

 lutely sugar-free. To remove the traces of sugar contained in both 

 meat and meat extract the former after having been finely chopped 

 is kept for two days at 10 to 15 C. At this temperature the muscle 

 sugar is decomposed into lactic acid. According to Theobald Smith, 

 the meat of poorly nourished tubercular cattle is also free from sugar. 

 A sugar-free bouillon can also be prepared by inoculating nutrient 

 bouillon with colon bacilli (which decompose the sugar) and subse- 

 quent sterilization and filtration of the medium. 



After a sugar-free bouillon has been obtained, from 1 to 2 per cent, 

 of the best acid-free starch is added and gelatinized. The process 

 is as follows: Shake together 200 c.c. sugar-free bouillon and 10 to 

 20 grams of starch; heat 800 c.c. of bouillon to the boiling point and 

 add gradually the 200 c.c. of starch emulsion. Stir continually to 

 insure a uniform gelatinizing of the starch, distribute to test-tubes and 

 flasks and sterilize by the fractional method. Long-continued sterili- 

 zation might form sugar from the starch. After sterilization some 

 of the tubes and flasks are tested for sugar, and if free from it the 

 media may be inoculated. If a diastase or amylolytic ferment is liber- 

 ated in the bacterial growth some of the starch will be converted into 

 amylodextrin, achroodextrin, and maltose. The presence of these 

 bodies is indicated (1) by a liquefaction of the gelatinized starch, 

 (2) by the iodin test, (3) by testing the reducing power of the 

 fluid with Fehling's solution, the well-known reagent determining 



