54 BACTERIA AND SOIL FERTILITY 



tissue. This is one of the reasons why bacteria are of such great 

 importance in nature. 



Why Bacteria Need Food. — Man requires food for two 

 reasons: (i) For the repairing of old tissue and the building of 

 new, and (2) for the furnishing of energy. The energy may be 

 used to perform work, to keep the body warm, or to build com- 

 plex compounds from simple ones. Both the building material 

 and energy are obtained from the food eaten by the animal. 

 Plants also require building material and energy. The first they 

 obtain from the air and soil, the second from the sunlight. The 

 energy of the sunlight they lock up in the sugar, starch, and pro- 

 tein which compose their bodies, from which bacteria and animals 

 get their energy. Hence, we find bacteria require food for two 

 purposes — the construction of the cell and energy. The energy 

 sometimes appears, as we saw in the last chapter, as heat, light, or 

 even electricity. However, the greater quantity of energy which 

 they obtain is used in the building of complex compounds. This 

 invariably comes from the food consumed and not from the sun- 

 light, as is the case with other plants. We can therefore define 

 a food as any substance which bacteria can use in obtaining build- 

 ing material, or energy, for cell construction, or activity. 



Food may be divided into three great classes — organic, ash, and 

 water. The organic compounds consist mainly of carbon, hydro- 

 gen, and oxygen. They comprise the four great groups — carbo- 

 hydrates, fats, proteins, and vitamins. 



Quantity of Food Required. — Bacteria are like men in that 

 the quantity of food actually needed and that used in the presence 

 of an abundant supply is widely different. Bacteria will rapidly 

 multiply in distilled water. At times we find millions in a gal- 

 lon. They have obtained the food for building their body from 

 the very minute impurities in the water. Other microorganisms 

 will grow on the surface of granite, obtaining their food from 

 the small amount brought to them by the rain water. The maxi- 

 mum amount of food used by bacteria is often enormous. They 

 quickly decompose the body of an ox after death. Tons of mate- 

 rial are carried into the septic tanks of large cities, all of which 



