134 



diagnostic purposes by nearly all investigators who have studied the 

 group linder consideration. 



In later years Beijerinck returned only once to the subject of the 

 acetic acid bacteria. In 1911 he published a paper on pigment 

 formation by acetic acid bacteria in which he described a quite in- 

 teresting species which unaccountably seems to have escaped the 

 attention of all previous workers in this field i). To this species the 

 name of Acetohacter melanogenum was given, because it is character- 

 ized by its property of producing a dark brown or blackish pigment 

 which resembles melanine in many respects. It is noteworthy that this 

 easily distinguishable species, which in Delft can quite frequently be 

 isolated from beer, does not seem ever to have been encountered by 

 investigators working in other parts of the world. 



Although Beijerinck's views regarding the nature of the pigment 

 formed probably need revision it seems likely that a further study of 

 Acetohacter melanogenum and especially of its pigment production 

 will still lead to interesting results. 



1. On sulphate reduction. 



Soon after the paramount importance of microbial activity for the 

 various chemical conversions proceeding in soil and water had been 

 recognized, the process of nitrate reduction — or denitrification as it 

 is of ten called — has been the subject of numerous investigations. 

 From various sides valuable contributions to our knowledge of this 

 process have been made. In contrast thereto, the elucidation of the 

 fundamentals of the corresponding process of sulphate reduction has 

 been mainly the work of one man, Beijerinck. This statement seems 

 to be especially justified if we include in Beijerinck's work the 

 important researches made at the instigation of Beijerinck by van 

 Delden, who was the first to act as an assistant to Beijerinck during 

 the latter's academie career. 



The origin of hydrogen sulphide in nature had since long attracted 

 attention, and it is not astonishing that sulphates had early been con- 

 sidered as a possible source for it. Bet ween 1864 and 1882, several 

 authors had expressed the opinion that microbes might be agents of 

 the conversion of sulphates into sulphides under natural conditions. 

 However, it was pointed out in 1887 by Winogradsky that the 

 greater part of the organisms which the earlier investigators held 

 responsible for the said conversion were in fact organisms which did 

 not produce hydrogen sulphide, but on the contrary consumed it in 

 their metabolism. 



It remained for Beijerinck to give in a preliminary paper in 1894 

 a detailed description of Spirillum desulfuricans — nowadays better 

 known as Vibrio desulfuricans — the causative organism of sulphate 



1) Centralbl, f. Bakt. u. Parasitenk. II, 29, 169, 191 1. 



