REDUCING ACTIONS 99 



in alcohol, or in some of the fat solvents as ether, chloro- 

 form, benzol, etc. These latter are probably closely related 

 to the lipochromes or "fat colors" of higher plants and 

 animals. Attempts have been made to render the produc- 

 tion of pigments a still more reliable means of identification 

 of species of bacteria through a careful examination of the 

 spectra of their solutions, but such study has not as yet led 

 to any valuable practical results. 



The production of pigment depends on the same general 

 factors which determine the growth of the organism but 

 does not necessarily rim parallel with these. It is especially 

 influenced by the oxygen supply (only a very few organisms 

 are known which produce pigment anaerobically— *Spm7/i/m 

 ruhrum is one); by the presence of certain food substances 

 (starch, as in potato, for many bacteria producing yellow and 

 red colors; certain mineral salts, as phosphates and sul- 

 phates, for others); by the temperature (many bacteria 

 cease to produce color at all if grown at body temperature, 

 ^7°—Serratia marcescens— or if grown for a longer time at 

 temperatures a few degrees higher). 



REDUCING ACTIONS. 



Reduction of nitrates to nitrites or to ammonia or even 

 to free nitrogen is brought about by a great many different 

 kinds of bacteria. In many instances this phenomenon is 

 due to a lack of free oxygen, which is obtained by the bac- 

 teria from these easily reducible salts. In other cases a 

 portion of the nitrogen is removed to be used as food material 

 in the building up of new protein in the bacterial cell. This 

 latter use of the nitrogen of nitrates by bacteria might 

 theoretically result in considerable loss of "available nitro- 

 gen" in the soil, as has actually been shown in a few experi- 

 ments. The reduction of nitrates as above mentioned 

 would also diminish this supply, but probably neither of 

 these results has any very great practical effect on soil fer- 

 tility. The building up of protein from these mineral salts 

 by bacteria in the intestines of herbivorous animals has been 



