164 GENERAL PHYSIOLOGY 



Association. Bacteria are undoubtedly greatly affected 

 by their associates. This association may be with other 

 bacteria or with higher plants and animals. In perhaps the 

 majority of cases no influence is exerted by one organism 

 upon another, but very frequently one species retards the 

 growth of another ; that is, there is antagonism. This is very 

 frequently seen in cases where a culture medium is inoculated 

 with several different species. One species gets the upper 

 hand and grows for a period to the practical exclusion of all 

 others; then conditions change and another form appears, 

 and so on. That there is a very marked antagonism by some 

 bacteria for others is shown by a number of experiments that 

 have been made which clearly demonstrate that one germ may 

 completely destroy another, probably through the by-products 

 which it produces. Examples of this have been instanced by 

 a number of investigators. The association of organisms at 

 times is advantageous to one or both. The association of the 

 root-nodule bacteria with the leguminous plant is a classical 

 example of symbiosis, and it is thought that many bacteria 

 may live in symbiotic relationship with others, especially 

 certain anaerobes and aerobes. Our knowledge of this is very 

 incomplete and fragmentary. 



