K.LUYVER S CONTRIBUTIONS TO MICROBIOLOGY AND BIOCHEMISTRY 



denitrifying bacterium that Beijerinck had previously and rather cur- 

 sorily described under the name Micrococcus denitrificans ; the latter was 

 not known to be a potential hydrogen bacterium, however. Just as 

 H.flava, the new isolates could oxidize hydrogen as well as organic 

 compounds, and hydrogen was not oxidized by cultures grown in or- 

 ganic media. Furthermore, these strains could use either oxygen or 

 nitrate as oxidant, although nitrate reduction was exhibited only by 

 cultures grown anaerobically in the presence of nitrate, a behaviour 

 that is also characteristic of other denitrifying bacteria. Once these 

 facts had been established, it was possible to test the suspicion that 

 the new isolates were indeed identical with M. denitrificans, and this 

 was confirmed when it was found that Beijerinck's original culture of 

 that organism, maintained for many years in the Delft culture collec- 

 tion on routine organic media, could still be induced to form a nitrate- 

 reducing as well as a hydrogen-oxidizing enzyme system when cultiv- 

 ated under the requisite conditions. In contrast to H.flava, M. denitri- 

 ficans evidently retains its potentiality to live as a hydrogen bacterium 

 even after prolonged growth on organic media. 



In his Harvard lectures Kluyver referred to these studies as exam- 

 ples of microbial variability due to enzymatic adaptation, or, to use 

 the current term, to induced enzyme formation, and stated: 



'. . . it seems warranted to conclude that M. denitrificans offers a 

 striking example of life's flexibility. It can live as a heterotrophe, de- 

 pending on the oxidation of some organic substrate, not only with free 

 oxygen, but also with nitrate as a substitute for the latter ; next it turns 

 out to be a fully autotrophic organism able to thrive on the Knallgas 

 system; and finally it emerges as a chemometatrophic organism dis- 

 playing the seemingly exceptional quality of thriving on the system 

 molecular hydrogen-nitrate. But it has been clearly shown that the 

 ability to use hydrogen as donor and nitrate as acceptor depends on 

 the presence of special enzymes which are produced only in response 

 to well-defined conditions during growth. 



'How strongly the potentiality to produce these enzymes is fixed in 

 the genetic apparatus of the organism is demonstrated in a most con- 

 vincing way by the behaviour of Beijerinck's original culture, which, 

 maintained for forty years - that is, for thousands of generations - on 

 peptone agar, on being transferred to an environment where mole- 

 cular hydrogen is the only energy source available, answers nature's 



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