28 



THE MICROBE AND ITS ACTIVITIES 



that the activities of such large numbers of hving things may 

 continue unnoticed in a small particle of soil and become re- 

 vealed only by a careful search using a highly specialized 

 technique. 



These minute organisms are capable of bringing about very 

 rapid and extensive transformations of the substrates upon which 

 they are growing. A few hours are sufficient for the complete 

 decomposition of a quantity of sugar to carbon dioxide and water, 

 or of protein to ammonia, carbon dioxide, water, and other com- 

 pounds. The extent of the development of these microbes is 

 limited only by the supply of available energy, by the environ- 

 mental conditions favorable or unfavorable to their development, 

 and by the formation of certain products injurious to their own 

 activities. Although modifications in the supply of oxygen, 

 moisture, and of inorganic compounds, or a change in tempera- 

 ture, may affect the develop- 

 ment of the soil population, 

 the greatest response in activ- 

 ity is brought about by the 

 incorporation of organic sub- 

 stances with the soil. 



In general, bacteria are not 

 easily differentiated morpho- 

 logically. The shapes which 

 they may assume are very 

 limited in number, and one 

 must rely largely upon physio- 

 logical distinctions for the 

 identification of their numer- 

 ous species. The soil bacteria 

 are divided into three main 

 groups: (1) The cocci or the 

 spherical forms. (2) The hacilli or the rod-shaped forms, with 

 rounded or square ends, differing both in length and in thickness. 

 Some of them are able to form spores which are more resistant 

 to unfavorable environmental conditions, such as dryness, changes 

 in temperature, and action of antiseptics, than the vegetative cells; 

 those rod-shaped bacteria which are unable to form spores 

 are less resistant to adverse changes in environment. Some of 

 these bacteria depend upon the movements of the fluids in which 



Fig. 15. — Sulfur-oxidizing bacterium, 



Thiohacillus tidooxidans (from Waks- 



man). 



