SOME ASPECTS OF NITRATE REDUCTION 



INTRODUCTION 



It seems a somewhat risky enterprise to make bacterial nitrate reduc- 

 tion the subject of a contribution to a modern symposium of bacterial 

 metabolism. Most bacteriologists will consider the subject distinctly 

 demoded, and they are fully satisfied with their knowledge of the proc- 

 ess. Does not the answer to the question 'nitrates reduced, or not?' 

 yield a diagnostic character in quite common use, and is there not a 

 very convenient routine procedure for the establishment of this char- 

 acteristic? 



Annually thousands of cultures will be grown on broth agar to 

 which 0.1% KN0 3 has been added, and tested for the presence of 

 nitrite in the medium after a few days of cultivation. It is clear that if 

 this test yields a positive result the question 'nitrates reduced' may be 

 answered in the affirmative sense, at least if due precautions have been 

 taken to exclude the possibility that the positive result of the so extrem- 

 ely sensitive nitrite reactions is solely caused by the traces of nitrous 

 acid not seldom present in the laboratory air. Furthermore it is com- 

 mon practice to answer the question regarding the nitrate reducing 

 ability of the organism in the negative, as soon as nitrite can not be 

 detected under the conditions of the experiment. Usually it is not 

 realized that formation of nitrite out of nitrate is only a first step in 

 nitrate reduction, and that there are numerous examples in the litera- 

 ture which testify to the ability of certain micro-organisms to reduce 

 nitrate to further reduction stages, like nitrous oxide and nitrogen. Yet 

 such vigorous nitrate-reducers are not seldom listed as unable to reduce 

 nitrate according to the standard procedure. 



The foregoing may only serve as an illustration that the way in 

 which living cells react on the presence of nitrate in their medium may 

 differ considerably, and this suggests at once the desirability of some 

 further analysis of the situation. 



In the first place we then encounter the case in which the nitrate 

 taken up from the medium is completely used for the building-up 



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