308 The Fixation of Nitrogen in Faeces 



was a short rod bacillus enclosed in a mucilaginous capsule. No Azofo- 

 bacfer or Clostridium could be detected, though in subsequent inocula- 

 tions from the faeces in the later fermentation flasks, one or two Azoto- 

 bacter forms were found. After three or four cultivations in dextrose 

 the capsuled organism was obtained in practically pure culture. Young 

 cultures of the first generation fixed 3 to 4 mgms. of nitrogen per gram 

 of dextrose producing considerable butyric acid. None of these remarks 

 refer to pure cultures, as no plates were poured, so that it cannot be 

 said that the fixation was due to a single organism. 



By plating out on dextrose- chalk-agar from raw cultures in mannite 

 inoculated with the garden soil used in all the experiments two organ- 

 isms were isolated, (1) a variety of Azotobacfer, (2) B. lactis aerogenes (?). 



They had the following morphological and cultural characters: 

 (1) Azotobacfer. In cultures 48 hours old; cells oval 2 x 4/>t, motile; 

 colonies on dextrose-chalk-agar are white at first, then yellow with 

 raised centre and finally dark brown. In 14 days the colonies are lobed 

 and may be as large as 10 mm. diameter. (2) B. lactis aerogenes (?). 

 In cultures 48 hours old ; cell rods 1-5 — 2-Oju, long, non-motile; colonies 

 on dextrose-chalk-agar are transparent, moist; capsules 4/x x 2/x. 

 Ferments dextrose, lactose, saccharose; gives Vosges-Proskauer re- 

 action ; gram negative. 



The mixed culture was found to fix considerably more nitrogen 

 when grown in a 2 % starch-phosphate medium than in corresponding 

 dextrose medium. ^ 



Mgms. of Nitrogen fixed 'per gram of carbohydrate in solution. 



Mixed culture Azotohadcr alone 



As it is known that Azotobacter uses its carbohydrate more economic- 

 ally at low concentrations, the gradual action of the enzymes of B. lactis 

 aerogenes on the starch may account for the better fixation of nitrogen 

 given by the mixed culture in that medium. 



Dr H. B. Hutchinson was good enough to isolate the capsuled 

 organism from a dextrose-agar plate and examine the pure culture. 

 He was of the opinion that it was closely related to, or identical with, 

 B. lactis aerogenes, and that the fixation of nitrogen in faeces may be 



