28 THE NATURE OF BACTERIA. 







unlimited power which is constantly curbed by the lack of 

 proper conditions. Bacteria may thus be looked upon as 

 possessing this wonderful possibility of reproduction, a force 

 of inconceivable magnitude, held more or less in check by 

 adverse conditions, but ever ready to exert its influence when 

 the conditions are favorable. It is this reserve force, pos- 

 sessed in greater or less degree by all bacteria, which makes 

 them such .wonderful and powerful agents in producing the 

 great changes in nature which we are now forced to attribute 

 to them. 



GENERAL PROPERTIES OF BACTERIA. 



Relation to Food. A few general facts concerning the 

 conditions of life of bacteria must be mentioned as necessary 

 to an understanding of their activities. They are colorless 

 organisms and consequently are commonly unable to make 

 use of the energy of sunlight, but must, like animals, de- 

 pend upon the decomposition of organic foods as a source 

 of life energy. Hence organic material is commonly needed 

 for food. Some of them obtain this organic food from the 

 dead bodies of animals and plants existing in abundance 

 throughout nature. Such bacteria are called saprophytes, 

 and they find their sustinence in the lifeless organic matter 

 in water and soil. Other species have the habit of feeding 

 upon living organic matter, a habit which makes them para- 

 sites: These live upon the living tissues of either animals 

 or plants. It must be noted here that a few bacteria are 

 exceptions to this need for organic food and can subsist upon 

 mineral food. 



Relation to Oxygen. Most bacteria demand oxygen to 

 enable them to carry on their life processes, thus agreeing 

 with animals and plants in general. But there are some 

 species, quite numerous and abundant as we now know, that 

 can live without a supply of oxygen, and some indeed, that 

 can grow and multiply only when in an atmosphere devoid 

 of oxygen. This property of living without oxygen makes 



