190 Microbes and You 



SYMBIOSIS. This relationship can hardly be expected to be a fifty- 

 fifty proposition. In time one or the other of the organisms will 

 predominate; bacteria are no exceptions to the law of competitive 

 spirit. 



In nature microbes not only work together with other microbes, 

 but they also cooperate with animal life and with some of the 

 higher plants. One of the better known examples of a symbiotic 

 relationship between bacteria and plants has practical application 

 in the cultivation of some crops that you and I depend upon for 

 food. Legumes (legere, Latin, to gather) derive their name from 

 the fact that the peas and beans may be picked without cutting 

 the plants. Nodules develop on the roots of legumes, and in these 

 nodules are formed specialized bacteria which are capable of ex- 

 tracting gaseous nitrogen from the atmosphere which contains 

 about 78% of nitrogen, and converting it into a form available to 

 the plant. Nitrogen in this free state is unavailable to plants unless 

 nitrogen-fixing bacteria are able to convert it into a utilizable form. 

 The leguminous plants, in turn, provide the bacteria with materials 

 and conditions favorable for their growth. In this way both the 

 plant and the bacteria benefit through symbiosis. Nitrogen econ- 

 omy will be discussed in more detail later in this book. 



The word commensalism (literally, eating at the same table- 

 messmates) is applied when an organism lives as a parasite but 

 does not harm or help the host. The bacteria which make up the 

 normal flora of the intestines of warm-blooded animals are repre- 

 sentative of this group. 



There are some reactions of organisms that depend upon the 

 combined efforts of two or more organisms. Neither organism is 

 capable of causing the reaction by itself, but when combined with 

 another living agent, positive reactions are produced. For example, 

 lactose may not be fermented by bacterium "A" or bacterium "B," 

 but when "A" and "B" are both inoculated into lactose broth, fer- 

 mentation may occur. Certain multiple microbial infections are 

 examples of this relationship, which is termed synergism. 



Metabiosis is a condition in which an organism produces in its 

 growth substances or conditions that can be used by another 



