394 BACTERIA 



They are not all necessarily harmful. Certain bacterial parasites 

 are found in the intestinal tracts of animals, but they do not 

 seem to produce any serious effects. The pathogenic bacteria 

 are all parasites. All of these are harmful. 



Bacteria that inhabit and obtain their food from nonliving 

 organic material are saprophytes. The bacteria that decay dead 

 organisms are saprophytes. The bacteria that change alcohol 

 to vinegar, sour milk, and ripen cheese are saprophytes. Practi- 

 cally all of these are useful to man. 



When a parasite and its host both flourish, and each one pro- 

 motes the growth of the other, their relation is one of mutual help- 

 fulness and is known as symbiosis. Each member, the bacterium 

 and its host, is known as a symbiont. For example, certain bac- 

 teria, known as nitrogen-fixing bacteria, live in little nodules or 

 swellings on the roots of clover plants. These bacteria take free 

 nitrogen from the air and build it into nitrates which the plant 

 can use. The clover plant uses some of these nitrates for making 

 protein. At the same time the bacteria absorb sugar from the 

 clover plant and use it for food. Neither organism suffers from 

 this relation and each one benefits from it. 



Nutrition. All types of bacteria give off digestive juices or 

 enzymes which digest the food upon which they are living. This 

 food may be mineral nutrients, dead plant and animal tissue, or 

 even living tissue. Digestion is external to the cell, not within 

 the cell, as it is in the amoeba. For example, the tuberculosis 

 bacilli digest certain cells of the body, then absorb the digested 

 material. Practically all bacteria absorb protein from other 

 organisms. The absorbed protein is either assimilated into new 

 protoplasm or oxidized for the release of energy. As a result of 

 assimilation, the bacteria grow, divide, and form groups or colonies. 



Reproduction. After the bacterium reaches its maximum size, 

 it builds a cell wall across the middle of the cell and thus divides 

 in half by fission. If conditions are favorable, certain bacteria 



