THE METABOLISM OF THE SULPHUR BACTERIA 39 



The assimilation, as distinct from the toleration of, organic 

 matter is more of a disputed matter. Nadson inferred the 

 assimilation of organic matter because he was able to keep 

 sulphur bacteria in a living condition for a long time in the 

 absence of sulphuretted hydrogen. It is difficult to see the 

 validity of the inference. Molisch (4) showed that Chroma- 

 tium Okenii thrives particularly well in a medium contain- 

 ing I per cent, peptone and i per cent, dextrin. Skene 

 emphasizes the unimportance of organic matter, and regards 

 it as a negligible factor in the metabolism of the sulphur 

 bacteria. This writer does not do full justice to the facts 

 which were discovered by him, for whilst the best growth 

 was obtained when ammonium sulphate was the source of 

 nitrogen he obtained appreciable growths when such sources 

 of nitrogen as peptone, asparagin, urea, glycocoll, ammonium 

 nitrate or calcium nitrate were used. He, however, appears 

 to have placed a doubtful construction on these facts. Growth 

 in presence of organic matter was regarded as due to ammon- 

 ium sulphate, produced by the decomposition of these sub- 

 stances by saprophytic bacteria. The cultures (which were 

 raw) were found at the close of the experiment to give 

 almost as strong a reaction with Nessler's reagent as am- 

 monium sulphate itself. It must be pointed out, however, 

 that ammonium sulphate is not invariably produced by the 

 saprophytic decomposition of these organic substances. 



The two investigators who succeeded in obtaining pure 

 cultures of the sulphur bacteria, namely, Keil (the colourless 

 bacteria) and Bavendamm (the coloured bacteria), regarded 

 organic matter as unessential to their development. Both 

 considered the sulphur bacteria to be obligate autotrophs, that 

 is, that under no circumstances is organic matter used as a 

 source of nitrogen. 



The following facts, however, show that organic matter 

 may be occasionally assimilated, that is, that the organisms 

 Site facultative not obligate autotrophs : — 



(i) Skene's results are not consistent with his interpreta- 

 tion of them, since the organic substances that were found 

 to favour growth do not all produce ammonium sulphate 



