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I could display before you several other specialists which resemble 

 the sulphur bacteria in being able to grow in purely mineral media. 

 Time limitations prevent me from dwelling on them; I shall merely 

 mention some other examples, such as the bacteria that oxidize nitrite 

 to nitrate, or ammonia to nitrite, and for which glucose is almost as 

 toxic as is sublimate for other organisms. Let me finally also refer to 

 the various types of organisms for which mixtures of hydrogen and 

 oxygen, methane and oxygen, and hydrogen and nitrous oxide con- 

 stitute nutrients. 



If we consider all these data it is well-nigh impossible not to be 

 impressed by the enormous diversity in microbial metabolism. If we 

 further survey the trends in microbiology during the past few decennia 

 it is hard to avoid the conclusion that the attempts to demonstrate 

 this diversity have largely dominated the investigations. It was not so 

 much the microbes themselves that were the starting point for the 

 studies; rather was the mere suspicion that certain chemical trans- 

 formations might occur in nature a sufficient impetus to formulate 

 the hypothesis that there would be microbes to accelerate them. And 

 the correctness of the hypothesis was substantiated in nearly every case 

 by those who had learned to utilize the elective culture method. The 

 very consideration that the cycle of matter on earth is closed, and that 

 consequently the naturally occurring hydrocarbons must eventually 

 be converted into carbon dioxide and water, led Sohngen to the dis- 

 covery and isolation of an important group of bacteria, members of 

 the genus Mycobacterium, that can use these substances as carbon 

 source. 



I can cite an even more striking instance. The studies of the above- 

 mentioned autotrophic bacteria, of which the sulphur-, nitrite-, and 

 ammonia-oxidizing bacteria are the best-known examples, uncon- 

 sciously led Nathansohn to the bold hypothesis that still other reac- 

 tions, proceeding very slowly at ordinary temperatures, might serve 

 as a basis for microbial metabolism even if the oxidizable compound 

 is not known to occur in nature. Thus he could ascertain the fact that 

 the exclusively man-made thiosulphate can serve as the major nutri- 

 ent for particular types of sulphur bacteria. 



These examples may suffice to show how gradually the investiga- 

 tions became subservient to what might be called a 'microbiological 

 imperialism'. It became a contest to open up new, seemingly inacces- 



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