SELECTED PAPERS 



fulfilled as regards the previous history of the cells. Only those cells 

 proved to be active if originated from cultures grown: 



/. on media containing nitrate ; 



2. under anaerobic conditions in a hydrogen atmosphere; 



j. on media poor in organic compounds. 



The first condition is a confirmation of Van Olden's observations on 

 the adaptive character of the nitratase. 



The second condition proves that the same holds for the hydro- 

 genase, the enzyme system activating molecular hydrogen, a result 

 which conforms with the analogous behaviour of 'Knallgasbakterien' 

 as described by Kluyver and Manten [1942]. 



The third condition is the most surprising, but Fig. 1 leaves no 

 doubt that there exists a strong contrast, as far as hydrogen absorp- 

 tion is concerned, between cells cultivated on Grohmann's medium 

 with nitrate and those grown on peptone agar + 1 % KN0 3 both in 

 a hydrogen atmosphere. 



We must, therefore, conclude that production of hydrogenase is 

 suppressed, if the bacterium can rely on organic compounds as sources 

 of active hydrogen. The exactingness of the bacterium as regards hy- 

 drogenase production contrasts strongly with the behaviour of Schatz 

 and Bovell's H. facilis. 



The results obtained opened the possibility that the negative out- 

 come of the hydrogen consumption in the Warburg experiment with 

 the authentic strain of Micrococcus denitrificans might have been due to 

 the fact that the bacteria had been cultivated on a medium con- 

 taining peptone. 



In accordance with this assumption it was, indeed, found that the 

 strain in question was able to bring about a vigorous hydrogen ab- 

 sorption, if inoculated into a Grohmann medium with 2% KN0 3 and 

 0.3% yeast autolysate. 



Herewith the last obstacle against an identification of Koster's bac- 

 terium with Beijerinck's Micrococcus denitrificans has been removed. 



The ability to utilize molecular hydrogen as a hydrogen donator in 

 nitrate reduction suggested an inquiry into the possibility that the 

 bacterium could also use hydrogen gas in aerobic metabolism, i.e., 

 with free oxygen as hydrogen acceptor. This proved, indeed, to be the 

 case, as is shown in Fig. 2, in which curve A gives the gas absorption 



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