147 



aid of the energy derived from the oxidation of ammonia or of nitrite. 



But in BEIJERINCK'S particular experiment, the nitrogen had been 

 added to the medium in its highest stage of oxidation - - as nitrate - 

 and for this reason at first sight no energy source could be traced. 

 Nevertheless there remained the undeniable fact that the media in 

 question after inoculation with some soil were fairly soon covered 

 with a thin, white or feebly rose-coloured very dry film consisting of 

 minute bacteria stuck together by a slimy substance. This organism, 

 to which the name of Bacillus oligocarbophilus was given, could with- 

 out any difficulty be transferred into fresh culture media, and the 

 cultures so obtained could be kept going indefinitely. By chemical 

 analysis it was shown convincingly that in such cultures very con- 

 siderable amounts of carbon accumulated in the media, and since these 

 could not be derived from the carbon dioxide of the air, the conclusion 

 was inevitable that unknown organic compounds present in the pol- 

 luted air of the laboratory - - and in general in the air of all inhabited 

 dwellings were directly responsible for the proliferation of 

 Bacillus oligocarbophilus. In agreement herewith it was shown that 

 practically no development took place in the much purer air of a 

 greenhouse. Apart from acting as carbon food the said impurities 

 must also serve as a substrate for the respiration of the bacterium and 

 thus partly be converted into carbon dioxide. It is clear that all this 

 means that the organism in question acts as a powerful agent of air 

 purification, a process which forms an interesting counterpart to the 

 well-known processes of water purification. 



The interest of these findings is manifold. In the first place, the 

 mode of discovery of Bacillus oligocarbophilus is a very fine example 

 of what may be called "a perfect accumulation experiment", i.e., a 

 case in which enrichment experiments in the highly elective medium 

 led after a very few transfers to an almost pure culture ' ) . Secondly, 

 it shows that it is possible to demonstrate in our everyday atmosphe- 

 re the presence of not-negligible amounts of organic substances which 

 are usually overlooked. This implied that the surrounding air is a po- 

 tential source of microbial life which may manifest itself where it has 

 not been expected. This may lead to erroneous conclusions with regard 

 to the nature of a microflora present under special conditions. It is 

 easily understood that if one finds an abundant development of a 

 certain microbe in a fully inorganic medium containing nitrite one 

 will be inclined to consider this compound as the energy source of the 

 vegetation. It is nevertheless possible that the development is due to 

 the organic energy sources present in impure air. It seems probable 

 that even in recent studies on nitratation this point of view has been 



i) Some reserve seems indicated here, since KINGMA BOLTJES recently found in 

 Hyphomicrobium vulgare a second agent of air purification with closely related physiolo- 

 gical properties. Cf. T. Y. KINGMA BOLTJES, Archiv f. Mikrobiol. 7, 188, 1936. 



