58 BACTERIOLOGY 



the so-called normal water bacteria, that grow best at about 

 20 C. 



Reaction. The majority of bacteria require an approx- 

 imately neutral medium in which to multiply and function. 

 Certain species may develop in weak acid materials, others 

 in weak alkaline but none can live and grow in media 

 either strongly acid or alkaline. Even those species whose 

 most conspicuous function is the conversion of sugars into 

 acids have their activities checked by the accumulation of 

 free acid beyond a very limited amount. 



Cooperating Bacteria. Under natural conditions it fre- 

 quently occurs that the development of one species or group 

 of species of bacteria is directly dependent upon the func- 

 tional activities of another totally distinct species, the 

 growth of one group resulting in conditions that are of vital 

 importance to the existence of the other. Such interdepen- 

 dence is observed, for instance, in complete nitrification, 

 as already noted; in the course of putrefaction, where, 

 through exhaustion of free oxygen by the actively germinat- 

 ing aerobic varieties, the conditions are supplied that enable 

 the anaerobic species to develop and exercise their biological 

 activities. Again, through the proteolytic activity of 

 enzymes produced by certain species of bacteria, other 

 species are supplied with nutrition that would otherwise be 

 unassimilable or only imperfectly so. Similar cooperative 

 or symbiotic relations between bacteria and higher plants 

 are also noticed, notably that between certain bacteria of 

 the soil and the group of leguminous plants, whereby the 

 latter are enabled, through the assistance of the former, to 

 make up their nitrogen deficit in large part from the free 

 nitrogen of the atmosphere. This latter relationship is 

 probably an example of true symbiosis. 1 



1 See Nitrogen fixing bacteria. 



