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For these reasons the various denitrifying bacteria tested in the 

 'resting cell' experiments were all grown in a hydrogen atmos- 

 phere on an agar medium containing i % peptone and 2 % potassium 

 nitrate. 



The results of these experiments, in which Pseudomonas aeruginosa, 

 Ps. denitrofuorescens, Ps. stutzeri, Micrococcus denitrificans and Bacterium 

 vulpinum were tested, were, however, entirely negative. Pressure 

 changes were practically the same whether or not nitrate had been 

 added to the bacterial suspension, and whether the atmosphere con- 

 sisted of hydrogen or of nitrogen. 



The negative outcome of these experiments strongly suggested that, 

 if bacteria exist which can bring about a denitrification with molecular 

 hydrogen, they are of a specific type, and do not belong to the group 

 of better known denitrifiers. This has induced Mr. Koster to start 

 enrichment cultures in a medium containing nitrate and other essen- 

 tial mineral salts under conditions in which molecular hydrogen was 

 the sole hydrogen donator available. This implied the absence of all 

 organic compounds so that in other words only autotrophic bacteria 

 could develop. 



Some garden soil was inoculated into a somewhat modified Eld- 

 redge tube containing a mineral medium as prescribed by Grohmann 

 [1924] for the cultivation of his 'Knallgasbakterien' with the addition 

 of 2% KNO3. The air in the vessel was then removed by a mixture of 

 15% C0 2 and 85% H 2 . The culture vessel was continuously shaken at 

 a temperature of 30 °C. 



After two to three days a considerable reduction of gas pressure 

 could be established. However, attempts to isolate a hydrogen absorb- 

 ing bacterium by streaking on an agar medium containing the mineral 

 constituents used by Grohmann with addition of 2% KN0 3 - replac- 

 ing the distilled water by tap water - and cultivating in the C0 2 /H 2 

 mixture were unsuccessful. No growth occurred. Moreover, transfers 

 directly from the enrichment culture into fresh liquid media of the 

 same composition also quickly lost their hydrogen absorbing capacity. 



The successful start and the so disappointing continuation of the 

 experiment seemed to indicate that the hydrogen consuming organism 

 in the enrichment experiment was not a strict autotroph. This consid- 

 eration led to the attempt to isolate the organism on the medium ear- 

 lier applied, however, with addition of 0.3% yeast autolysate. This 



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