44 GENERAL PHYSIOLOGY OF BACTERIA 



anabolic requirements. A larger part, however, is dissipated as heat. 

 The heat generated in actively growing cultures of bacteria can be 

 detected with sensitive thermometers, provided losses due to radiation 

 and evaporation are guarded against. The heat production is not 

 great as a rule, although in certain fermentations it may rise as high 

 as 12-15 above the uninoculated controls. The decomposition of 

 protein and protein derivatives (putrefaction) usually gives rise to less 

 heat than the decomposition of carbohydrates (fermentation) under 

 the same conditions. 



I. LIGHT AND ELECTRICITY. 



The vast majority of plants possess a photodynamic pigment, 

 chlorophyll. This pigment can synthesize inorganic substances, as 

 CO 2 and water, together with nitrates, into complex organic compounds 

 through the energy of the sun's rays acting upon it. Plants possessed 

 of this pigment, therefore, are the synthetic agents of nature. Usually 

 this pigment is green; it may, however, be brown or red, the latter 

 pigment being characteristic of certain algae. A group of the higher 

 bacteria, the Rhodobacteriaceae, possess a photodynamic pigment, 

 bacteriopurpurin, which appears to be analogous to chlorophyll of 

 the green plants. These sulphur bacteria prefer light and move toward 

 it. 1 The action of sunlight on this bacteriopurpurin enables them to 

 decompose CO 2 and to utilize the oxygen thus obtained to oxidize 

 H 2 S. 



All other known bacteria have no photodynamic pigment. Light 

 is not a source of energy to them, and they are distinctly harmed by 

 it; they grow best in darkness. Direct daylight kills them rapidly, 

 and even prolonged exposure to diffuse light may be fatal. Bacteria 

 are more rapidly killed by exposure to the sun's rays in June, July 

 and August 2 than exposure of the same time in November, December 

 and January. Expressed differently, many bacteria which are killed 

 after an exposure of from one to two hours' direct sunlight in summer 

 require an exposure of from two to three hours in winter to accomplish 

 the same result. 



Of the spectral rays, the red and infra-red rays, aside from the heating 

 effect, are without noteworthy action on bacteria. The blue, violet, 

 and ultraviolet rays, on the contrary, are distinctly bactericidal. 



1 Yost, Plant Physiology, 223. 



2 In the Northern Hemisphere. 



