208 SCIENCE PROGRESS 



fluid was inoculated with mud from an aquarium which was 

 rich in nitrates and nitrites. A rich bacterial flora developed, 

 and in a few days the solution was free from nitrates. 



Baur then inoculated a solution containing fish-broth, 

 peptone, and potassium nitrite with the mixed culture from 

 the former solution, and by suitable methods two new bacteria 

 were isolated from this medium. These he called Bacterium 

 actinopelte and B. lobatum. The growth of these organisms 

 in pure culture is indicated by the formation of a froth and 

 by the later evolution of gas bubbles which are nitrogen, and 

 finally nitrogen with some nitric oxide. The nitrate and nitrite 

 are broken up into free nitrogen, so that finally the solution 

 becomes free from these compounds, and the metaphenylene- 

 diamine sulphuric acid reaction becomes negative. Both bacteria 

 are aerobes, but they can exercise their denitrifying activities 

 in the absence of oxygen, the latter element being obtained 

 from the nitrite or nitrate decomposed. The moderate access 

 of oxygen, however, stimulates the decomposition of the nitrogen 

 compounds. Both require carbohydrates, and can live apparently 

 in decaying vegetable refuse. 1 Lowered salinity of the sea- 

 water does not seem to affect them, and one of them can live 

 in fresh water, though whether the denitrifying activity can 

 be carried on in the presence of the numerous bacteria of the 

 latter is not clear. 



The relation of the denitrifying activity of both of these 

 micro-organisms to temperature is of the very greatest interest. 

 Baur made a series of twenty cultures at different temperatures. 

 At 25 C. froth formation (the evolution of free nitrogen) began 

 about twenty-four hours after the commencement of the experi- 

 ment, and in a period of seven to eleven days all the nitrite 

 in the solution had been decomposed, the frothing ceased, 

 and the metaphenylene-diamine sulphuric acid reaction was 

 negative. At 15 C. the gas formation began after three or 

 four days, and the nitrite reaction and frothing did not disappear 

 till about twenty-eight days after the commencement of the 

 experiment. At 4/5 C. frothing did not begin until the 

 eighteenth to twenty-second day after the establishment of 



1 Baur found that proteid matter had to be supplied in the form of broth or 

 peptone. Carbohydrates were also necessary. Gran found that denitrifying 

 bacteria might obtain their nitrogen from nitrite and their carbon from organic 

 acids. 



