METHODS OF STUDYING PHYSIOLOGIC CH^IjACTERS 101 



of gas until the material is subjected t 



As the result of such a chemical test, the ratio of carbon 



dioxide to hydrogen may be determined. 



Determination of Reduction Changes. It has already 

 been noted that certain facultative and other bacteria 

 growing in the absence of free oxygen, may take oxygen 

 from certain compounds, reducing nitrates to nitrites, sul- 

 phates to sulphides, and decolorizing certain pigments, such 

 as litmus or methylene blue. The facts are sometimes of 

 advantage in the differentation of bacteria. 



Reduction of Nitrates to Nitrites The transformation of 

 nitrates into nitrites may take place in a number of differ- 

 ent ways as the result of growth of microorganisms. The 

 test is, therefore, not particularly reliable and is not empha- 

 sized by bacteriologists as it was formerly. A peptone 

 broth containing usually two-tenths per cent of potassium 

 nitrate may be inoculated with the organism to be studied. 

 After standing for four days the nitrite test may be per- 

 formed by adding two cubic centimeters of each of the 

 following solutions: 



(a) 5N Acetic acid, 1000 cc. 



Sulphanilic acid 8 grams 



(b) 5N Acetic acid 1000 cc. 



Alpha amido napthalene 5 grams 



If nitrite is present a reddish or rose color will develop. It 

 is always necessary to test an uninoculated tube at the same 

 time as a check to be sure that no nitrites were initially 

 present in the medium used. 



Reduction of Sulphates to Sulphides Some organisms 

 reduce sulphates to sulphides under anaerobic conditions. 

 Media may be prepared containing either lead acetate or 

 iron chloride. The production of sulphides from sulphates 

 will be evident by the blackening of the medium. It should 

 be recognized, however, that sulphides may also originate 

 during the decomposition of organic sulphur compounds. 



