286 APPLIED BIOLOGY 



areas of such immense size that bacteria could not be carried 

 on dust from other regions). In fact, bacteria are distrib- 

 uted wherever they can get food and proper temperature for 

 growth and multiplication. Unlike other organisms, cer- 

 tain bacteria do not require oxygen from the air, for they can 

 get it by decomposing the organic substances on which they 

 live. Such bacteria are called anaerobic (living without air). 

 Some of them can live deep in the soil ; but at depths below 

 where roots of plants and earthworms penetrate, the soil is 

 usually without bacteria. Hence water from deep wells is 

 usually pure, unless there are openings between strata which 

 somewhere communicate with lakes or other surface water. 



The wide distribution of bacteria, especially on or in every- 

 thing connected with the inhabited surface of the earth, 

 makes it extremely difficult to eliminate them. In fact, 

 only in closed bottles, etc., in which all bacteria can be killed 

 by heat or chemicals, is it possible to keep any substance 

 free from bacteria (i.e., sterile). This is the reason for the 

 careful work necessary in preserving many foods. 



258. Useful Bacteria. The popular impression is that all 

 bacteria are harmful in that they decompose our foods or 

 cause dangerous diseases of man or of useful animals. This 

 is far from true, for very many bacteria are directly useful. 



(a) Bacteria in Soils. Most important of the useful bac- 

 teria are those which deal with the nitrogen compounds in the 

 soil. We have already noted that the nitrogen excretions of 

 animals are in turn used by plants ( 117) ; but these excre- 

 tions must first be prepared by bacteria. These organisms 

 first decompose the rather complex animal excretions like urea, 

 and then unite the contained nitrogen into simple compounds 

 (nitrites and nitrates) which the roots of plants can absorb 

 and use. The guano from Peru, which has long been a valu- 

 able agricultural fertilizer, was formed by bacterial action on 

 the excreta of countless thousands of sea-birds which lived 

 in prehistoric times. It contains concentrated nitrogen com- 



