UNITY AND DIVERSITY IN THE METABOLISM OF MICRO-ORGANISMS 



This idea has been used in an eminent manner by Orla-Jensen [1909] 

 in his sketch of a general system of classification of bacteria. By as- 

 signing every microbe to one of the natural groups, whose delineations 

 become progressively clearer, it will frequently be possible to predict 

 its properties on the basis of its relationships, and it will also become 

 feasible to indicate more rational culture conditions for it. For example, 

 one frequently hears complaints concerning the difficulty of cultiva- 

 ting numerous streptococci; this must in the first place be ascribed to 

 the fact that the investigators fail to realize that they are working 

 with organisms belonging to the group of genuine lactic acid bacteria. 

 Table II indicates the distribution of dissimilatory processes among 

 some important groups of microbes. It shows that, as a rule, oxidative 

 processes do not exhibit any great specificity as far as the nature of the 

 oxidizable substrate is concerned. Not infrequently the oxidizing ca- 

 pacity extends to nitrogen-containing compounds, to carbohydrates 

 and sugar alcohols, and to organic acids, although in the series, acetic 

 acid bacteria, moulds, aerobic sporeformers, and the Pseudomonas 

 group,* a decreasing tendency towards carbohydrate oxidation and 

 an increasing one towards the oxidation of nitrogen-containing com- 

 pounds may be detected. In contrast, the fermentative processes are 

 generally more dependent on a specific substrate, even though in 

 some groups several potentialities may co-exist. 



When further considering the meaning of the energetic aspects of 

 metabolism we are led to still other notions. That man or a higher 

 animal requires energy is a notion that we immediately seem to rec- 

 ognize as befitting. The maintenance of the body temperature, the 

 performance of internal and external work, these are inconceivable 

 without energy supply. But if one raises the question what function is 

 served by the continuous flow of energy which, according to experi- 

 ence, is requisite for the perpetuation of life even of a non-motile mi- 

 crobe, maintained at constant temperature, and not displaying any 

 internal movements, there appear to be only two rational answers. On 

 the one hand we may assume that the energy conversion is simply a 

 necessary condition for the living substance as such; on the other hand 

 it is tempting to postulate that there exists a close connexion between 



* Although the organisms assembled in this group exhibit physiological similarities, 

 they do not form a 'natural group' ; see the previous footnote. 



T 99 



