148 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



Sunlight as a Source of Energy to Microorganisms. It 



is somewhat doubtful whether any group of bacteria, 

 yeasts, or molds, are capable of utilizing the energy of 

 light for manufacture of foods from inorganic compounds. 

 The possible exceptions are to be found in that group of 

 bacteria termed the Thiobacteriales (sulphur bacteria). 

 Certain of these species contain two coloring materials, 

 bacteriopurpurin, a purple coloring substance, and bac- 

 teriochlorin, a green coloring substance. The organisms 

 possessing these pigments apparently prefer to live in the 

 presence of light and are attracted toward the more bril- 

 liantly illuminated side of the vessel in which they are 

 growing, that is, they show positive phototaxis. That 

 they actually make use of the sunlight, however, as a 

 source of energy for the manufacture of foods from simple 

 inorganic substances has not been definitely proved. 



Oxidation as a Source of Energy for Microorganisms. 

 All bacteria, yeasts, and molds secure energy by the 

 oxidation of compounds, either organic or inorganic. For 

 convenience in discussion they may be divided into three 

 groups. 1. The prototrophic bacteria secure energy by the 

 oxidation of inorganic elements and compounds and 

 utilize the energy thus secured in building up their own 

 foods. Such organisms are totally independent of ex- 

 traneous organic food substances. 2. The metatrophic 

 organisms apparently require organic sources of carbon 

 and secure growth energy usually by oxidation. They 

 may, however, utilize inorganic compounds of nitrogen. 

 3. The paratropkic bacteria usually require both nitrogen 

 and carbon in the form of organic compounds. For the 

 most part they are parasitic bacteria and many of them 

 are disease producers. 



The Prototrophic Bacteria. These are the organisms 

 which by means of the oxidation of purely inorganic sub- 

 stances are able to make use of the same or other inor- 



