CHAPTER VIII 



BIOCHEMICAL TESTS 



THE physiological characteristics of bacteria are of considerable 

 importance hi the differentiation of species. A knowledge of 

 such characteristics is of assistance in the isolation and recognition 

 of certain species, as in the detection of sewage bacteria in water. 



Acid Production. Acids are most frequently and readily pro- 

 duced by bacteria in the presence of suitable carbohydrates. 

 Litmus may be added to a sugar medium for the detection of acid. 

 A quantitative determination of the acids produced in a liquid 

 medium may be made by titrating against decinormal alkali. 

 The ability of organisms to produce acid from various carbohy- 

 drates is used in the separation of the members of the intestinal 

 group of bacteria from each other. A record of the changes in reac- 

 tion from time to time has been found valuable in the differentiation 

 of the organisms causing bovine and human tuberculosis. 



Alkali Production. The alkali most commonly produced by 

 bacteria is free ammonia. It may be detected qualitatively by 

 means of filter paper dipped in Nessler's solution and exposed above 

 the medium. The ammonia gas turns the paper brown or black. 

 The amounts of alkali produced may be determined by titration 

 against normal acid. 



Gas Production. A few pathogeniobacteria produce gas from 

 proteins, but most gas-producing species require the presence of 

 a carbohydrate. The gases most commonly formed are carbon 

 dioxid and hydrogen. One qr more species of cellulose-fermenting 

 organisms can also produce methane, and some of the denitrifiers 

 free nitrogen. 



The ability to produce gas may be determined by inoculation 

 of the organism into a dextrose agar or gelatin tube. Gas-bubbles 

 will appear in the medium if the organisms can ferment dextrose. 

 This sugar is generally used, as it is more easily fermented than 

 most other carbohydrates. The fermentation tube is commonly 

 used for the study of gas production. The closed arm is entirely 



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