REDUCING ACTIONS 101 



which they grow. Others are not soluble in water but are 

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

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

 to the li'pochromes 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 run parallel with these. It is especially influ- 

 enced by the oxygen supply (only a very few organisms 

 are known which produce pigment anaerobically — Spirillum 

 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 sulphates, 

 for others); by the temperature (many bacteria cease to 

 produce color at all if grown at body temperature, 37° — 

 Bacillus prodigiosu^ — or if grown for a longer time at tem- 

 peratures 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 

 bacteria 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 fertility. 

 The building up of protein from these mineral salts by bac- 

 teria in the intestines of herbivorous animals has been sug- 



