BACTERIA 93 



they spring. A single cell, as a rule, gives rise to but one spore, so 

 that spore formation can not be looked upon as a process of mulii- 

 pUcation. It is generally considered that the bacterial spore is a 

 resting stage, physiologically similar to an encysted amoeba and 

 serving to tide the species over a period of hard times. Not all 

 bacteria are spore producing; in fact the number known to form 

 spores is rather limited. 



Great adaptabihty is shown by bacteria to extremes of tempera- 

 ture. Some species have been found multiplying in the water of 

 polar seas at or near the freezing point, while others have been 

 found living in the water of hot springs at a temperature of 79° C. 

 Most of the ordinary bacteria found in pond or river water multiply 

 abundantly at a temperature of about 20° C. When water is 

 frozen, most of the bacteria that it contains are killed at once. A 

 small proportion survive, but in gradually diminishing numbers, 

 so that at the end of a few weeks clear ice is practically sterile. 

 Bacteria contained in masses of organic matter, however, may 

 have their life in ice considerably prolonged. 



Bacteria not only adapt themselves to great extremes of tem- 

 perature, but to varied sources of food supply. Many species can 

 content themselves with relatively simple chemical compounds, 

 such as the ammonium salts of the organic acids. Others require 

 for their development complex nitrogenous substances. The nitri- 

 fying bacteria, so abundant in most soils and waters, obtain the 

 energy necessary for their development altogether from inorganic 

 compounds. On the other hand, certain bacteria are entirely 

 dependent upon particular organic compounds present in the bodies 

 of the higher animals, and can thrive only in the presence of blood 

 serum or similar fluids. 



Fundamental differences exist among bacteria in respect to their 

 relative need for oxygen. Some, the obligatory aerobes, require free 

 oxygen for the maintenance of their life activities, while others, the 

 obligatory anaerobes, do not grow except in the almost complete 

 absence of free oxygen. There are also some, the facultative anaer- 

 obes, that can multiply either in the presence or absence of free 

 oxygen. The anaerobic bacteria, as a class, thrive best in the pres- 

 ence of substances capable of undergoing reduction or fermentation. 



