129 



Beijerinck's treatment of the group was merely a by-product of his 

 studies on the formation of hydrogen sulphide in nature. A scrutiny of 

 the chapter entitled "Creation of the genus Aerobacter" leaves no 

 doubt that Beijerinck had made himself quite generally acquainted 

 with the various species belonging to the new genus. 



As will be seen in one of the following sections, Beijerinck had at 

 that time already made important observations regarding the process 

 of sulphate reduction, the chief biological source of hydrogen sulphide 

 in nature. However, Beijerinck emphasized from the very beginning 

 that there are also minor biological sources of hydrogen sulphide, and 

 he early indicated the bacterial decomposition of sulphur-containing 

 proteins as one of these. 



The regular production of bad smells by the water of the canals in 

 Delft during the hot summer-time made Beijerinck decide to 

 investigate whether a decomposition of sulphate, accompanied by 

 formation of hydrogen sulphide, also took place under the semi- 

 aerobic conditions prevailing in the canal-water. By dispersing finely- 

 divided lead carbonate in ordinary nutriënt peptone agar he was able 

 to obtain white media on which many bacteria present in the water 

 appeared to develop quite satisfactorily. The great advantage of this 

 medium is that all bacteria, producing hydrogen sulphide from the 

 peptone, can be recognized at a glance because of the brown to 

 black colour conferred on the colonies by the formation of lead 

 sulphide. 



Beijerinck found that the majority of the sulphide-forming colo- 

 nies could be identified with one of the two bacterial species then 

 called Bacterium coli commune and Bacterium lactis aerogenes. 



Numerous publications dealing chiefly with the hygienic signific- 

 ance of these bacteria had already appeared, and from these it had 

 become clear that there exist many varieties and intermediate forms 

 of these two species. On studying the group, Beijerinck almost 

 completely ignored the hygienic questions. His first impulse was 

 characteristically scientific: namely, to stress the desirability of 

 separating the two species and their related forms from the many 

 other forms which at the time were designated by the generic name 

 Bacterium. He, therefore, proposed the new genus Aerobacter to 

 contain the species under consideration. From the diagnostic cha- 

 racters of the genus laid down by Beijerinck, the following may be 

 mentioned: non-spore-forming rods, either motile or non-motile, 

 which ferment various sugars and poly-alcohols with production of 

 lactic acid and gas, the latter always consisting of a mixture of hy- 

 drogen and carbon dioxide ; nitrates are easily reduced with formation 

 of nitrites, but sulphates arenot attacked. 



Beijerinck added some interesting remarks on the relation of his 

 new genus to other natural groups, especially to the aerobic spore- 

 forming bacteria (for which, incidentally, he* introduced the two new 



M. W. Beijerinck, His life and his work. 9 



