[Chap. XLII THE BIOLOGY OF BACTERIA 517 



dealt with in more detail in the following chapter. The energy released 

 by these oxidations is utilized in the synthesis of sugar and other foods. 

 The other autophytes are the green and purple sulfur bacteria, the first 

 of which utilize the energy of light and the second the energy of reduced 

 sulfur in making sugar. ^ 



Most species of bacteria are obligate saprophytes or parasites, but 

 some of them may grow as parasites or as saprophytes. 



External factors and bacteria. The environments in which bacteria and 

 their allied organisms live and reproduce are so different from those of 

 larger plants that some of the effects of environmental factors need to be 

 considered, especially those of light, water, oxygen, and temperature. 



Light. Most bacteria, because of their location in soil, in foods, in 

 decaying matter, and within other living organisms are rarely exposed 

 to the direct rays of light. Most species live only a few hours when 

 exposed to full sunlight. Sunlight, therefore, is a destructive factor in 

 the development of bacteria and it is of great importance in the purifica- 

 tion of rivers polluted with sewage, and in the elimination of bacteria 

 from all exposed surfaces. 



The rapidity with which certain bacteria are killed bv direct light 

 depends upon the intensity and quality of the light. Although the evi- 

 dence is extremely difficult to obtain and the results are often conflicting, 

 it appears that the rays of the blue and violet end of the visible spectrum 

 are most injurious to bacteria, particularly the pathogenic species. The 

 intensity of the light used must always be kept in mind in comparing 

 the destructive effects of various radiations. Near infra-red rays through 

 their heating and drying effects may be germicidal. Ultra-violet rays, if 

 not absorbed too quickly by the medium, are very destructive. Bacteria 

 are destroyed by short rays of light apparently through the coagulation 

 of the protoplasm, or the formation of some toxic substance in the 

 medium. 



Water. Bacteria cannot grow and multiply in the absence of water, 

 although some may remain alive for months or years in an arid environ- 

 ment. 



About 85 per cent of the weight of active bacteria is water. Spores are 



^ In the green sulfur bacteria hydrogen sulfide replaces water in the process of photo- 

 synthesis and sugar, sulfur and water are the end products. The purple sulfur bacteria 

 utilize the energy from oxidation of sulfur to sulfates in the synthesis of sugar. Generalized 

 equations representing these chemical changes are: 



6CO2 + I2H2S » CeHisOe + 6H2O + 12s 



.SH2S + 6CO2 + I2H2O > CeH.sOe + 6H2O + 3H2SO4 



