SAPROPHYTES 413 



to have the decay go forward at a rate sufficiently rapid to 

 supply our growing crops with an abundance of mineral 

 salts. So it is with many things that are useless to us; we are 

 quite willing that they shall be disintegrated and not allowed 

 to encumber the earth, but there are many things such as our 

 food, clothing, and lumber which we wish to preserve and 

 which the bacteria and other fungi stand ready to consume. 



CONDITIONS WHICH FAVOR BACTERIAL GROWTH 



471. Warding off Attacks of Bacteria. We can ward off the 

 attacks of bacteria by so conditioning the material which we 

 wish to preserve that bacteria can not grow in it. In doing 

 this, we need to know the conditions which favor bacterial 

 growth. These briefly stated are as follows: a suitable food, 

 an abundance of water, a favorable temperature, suitable 

 chemical conditions, absence of bright light for most of them, 

 presence of free oxygen for some, and absence of free oxygen for 

 others. We shall consider these briefly in separate paragraphs. 



472. Food for Bacteria. Bacteria can feed on a wide 

 diversity of substances. Almost all organic compounds are 

 food for one or more different kinds. Almost all species of 

 bacteria can feed on protein substances and yet some can 

 get along with no protein at all in their diet. Some, even, can 

 nourish themselves on inorganic salts alone. Since the organic 

 materials which we wish to preserve are almost always a 

 mixture of many different kinds of compounds, we may be 

 quite sure that any of these things of plant or animal origin 

 are subject to the attacks of bacteria. 



473. Water Necessary for Bacterial Growth. A mass of food 

 material, in order to support an active growth of bacteria, 

 must contain from 25 per cent, to 30 per cent, of water. This 

 is considerably more water than is required to support a growth 

 of mold, and so many materials will be found to mold when they 

 are entirely free from bacterial action. The high percentage 

 of water required for bacterial growth enables us to preserve 



