166 BIOLOGY AND TECHNIQUE 



subsequent inoculation, the bacteria produce Hj S, this precipitate will 

 turn black. The solution recommended by Pake for this test is prepared 

 as follows: 



1. Weigh out 30 grams of pepton and emulsify in 200 c.c. of tap water at 60° C. 



2. Wash into a liter flask with 80 c.c. tap water. 



3. Add sodium chloride 5 grams and sodium phosphate 3 grams. 



4. Heat at 100° C. for 30 minutes, to dissolve pepton. 



5. Filter through paper. 



6. Fill into tubes, 10 c.c. each, and to each tube add 0.1 c.c. of a one per cent 

 solution of ferric tartrate or lead acetate. These solutions should be neutral. 



7. Sterilize.' 



AccuKATE QUANTITATIVE GAS ANALYSES of bacterial cultures can be 

 made only by the more complicated methods used in chemical labora- 

 tories for quantitative gas analysis. The gas, in such cases, is collected 

 in a bell jar mounted over mercury, and subjected to analysis by the 

 usual method described in works on analytical chemistry. 



Acid and Alkali Formation by Bacteria. — ^Many bacteria produce acid 

 or alkahne reactions in culture media, their activity in this respect 

 depending to a large extent upon the nature of the nutrient material. 

 Many organisms which on carbohydrate media produce acid will give 

 rise to alkali if cultivated upon media containing only proteids. 



Information as to the production of acid or alkali can be obtained 

 by the addition of one of a variety of indicators to neutral media. The 

 indicators most often employed for this purpose are litmus or neutral 

 red. Changes in the color of these indicators show whether acids or 

 alkalis have been produced. 



Great help in differentiation is obtained by adding chemically pure 

 carbohydrates to media to which litmus has been added and then de- 

 termining whether or not acid is formed from the substances by the 

 microorganisms. These tests have been of special importance in the 

 differentiation of the typhoid-colon groups of bacilli. 



Quantitative estimation of the degree of acidity or alkalinity pro- 

 dnced by bacteria may be made by careful titration of definite volumes 

 of the medium before and after bacterial growth has taken place. 



The variety of acid formed by bacteria depends largely upon the 

 nature of the nutrient medium. The acids most commonly resulting 

 from bacterial growth are lactic, acetic, oxalic, formic, and hippuric 

 acids. Qualitative and quantitative estimation of these acids may be 

 made by any of the methods employed by analytical chemists. 



1 Quoted from Eyre, " Bact. Technique," Phila., 1903. 



