SPECIES OF AZOTOBACTER 179 



become smaller and arranged in short chains, three to eight or 

 more together ; the " sarcina" form appears in abundance when 

 the colonies become brown. In liquid media the pigmentation 

 appears usually in four or five weeks from the time of inoculation 

 where the medium is kept at 20 to 25 C. 



A. agilis, Beijk., originally isolated from canal water, is a large 

 actively motile organism with a bundle of polar cilia. The 

 cells are oval, transparent, with distinct cell-walls, granular 

 protoplasm, and vacuoles. It grows well on an agar medium 

 containing glucose in place of mannite (Ex. 91). On such 

 medium with J per cent, of calcium propionate in place of 

 the sugar, the colonies of A. agilis becomes surrounded by a 

 greenish yellow fluorescent zone which is characteristic of the 

 species. 



A. vinelandii, Lipman, a form isolated from some North 

 American soils, produces a thick white membrane on mannite 

 solutions, which becomes yellowish. Ultimately the liquid 

 becomes yellowish-red in colour. On mannite agar the colonies 

 are semi-transparent at first, rounded, and 2 to 4 mm. in 

 diameter : they produce a yellowish pigment, which diffuses 

 through the medium. 



A. Beijerinckii is another species or form isolated by Lipman 

 from American soils. The organisms are spherical, non-motile, 

 and much larger than the preceding kinds ; in mannite solutions 

 they occur singly or in short chains. 



When grown in mannite solutions a white deposit is produced, 

 and small white colonies appear on the surface of the liquid and 

 on the walls of the flask. 



The colonies ori mannite agar are well developed in four or five 

 days, somewhat rounded, white, and moist, with netted surfaces. 



Ex. 90. Grow and isolate species of Azotobacter from soil. 

 Prepare the following solution : 



Di-potassium phosphate , . . . .2 gr. 



Mannite . "'. . .. . . 20 gr. 



Tap water . . . . . . , i litre 



