STIMULATING THE GROWTH OF AZOTOBACTER BY 
AERATION 1 
By O. W. Hunter 
Associate Bacteriologist , Kansas Agricultural Experiment Station 
INTRODUCTION 
Numerous theories and methods have been advanced for increasing the 
growth of the Azotobacter cell. Much attention has been directed toward 
the improvement of a medium, the dominating thought being that the 
common media used for azofication purposes are deficient in those 
elements which are necessary for the prompt development of the Azoto¬ 
bacter organism. 
Sohngen (4) 2 states that in Beijerinck's medium only nitrogen and 
oxygen are lacking for the luxuriant growth of Azotobacter. A vigorous 
growth of these organisms could be induced by the addition of his 
(Sohngen’s) colloids to this medium, the colloids causing a direct contact 
between the Azotobacter, oxygen, and nitrogen. 
It is not doubted that the medium commonly employed in the cultiva¬ 
tion of these organisms could be improved; there are few bacteriological 
media which could not be improved. On the other hand, it is not 
assumed that the medium exerts the greatest influence upon the tardy 
development of the Azotobacter. 
The theory of Sohngen that the Beijerinck medium lacks only nitrogen 
and oxygen is here supported. Observations prompt one to believe that' 
both these elements can be supplied by aerating the culture medium and 
that thereby a rapid and vigorous growth of Azotobacter can be promoted. 
The influence of aeration upon nitrogen fixation by Azotobacter is not 
new. Its influence is taken advantage of in studying the azofication 
ability of the soil or pure cultures by inoculating media of shallow depth 
and large surface exposure. This Lipman (1) noticed in the cultivation 
of Azotobacter vinelandi Lipman, for the rate of nitrogen fixation increased 
with the increased surface exposed. 
The relative importance of aeration upon the growth of Azotobacter 
can be surmised from some of its physiological activities. Its obligate 
aerobic characteristics as well as nitrogen requirements for the synthesis 
of cell protoplasm are significant. Inasmuch as nitrogen is lacking in 
the media, the utilization of atmospheric nitrogen is necessitated. The 
energy required for this assimilating process is supplied by the oxidation 
of a carbohydrate. The abundance of air therefore offers two highly 
essential elements, oxygen and nitrogen, both of which are required for 
the efficient metabolism of the cell. 
Observations noted from the growth of Azotobacter cultures on dex¬ 
trose-Ashby agar slants would not lead one to suspect the media as 
1 Accepted for publication Jan. x 6 ( 1923. Contribution No. 38 from the bacteriological laboratories of 
the Kansas Agricultural Experiment Station, reporting in part the results of project No. 138. 
9 Reference is made by number (italic) to * * Literature cited," p. 677. 
Journal of Agricultural Research, 
Washington, D. C. 
abo 
( 66 S ) 
Vol. XXIXI, No. 8 
Feb. 24,1923 
Key No. KMIS.-30 
