BACTERIA FIXING ATMOSPHERIC NITROGEN 115 



organism is very motile by means of a bundle of polar flagella. The cells are 

 large, transparent, resembling monads, often with a clearly discernible cell wall, 

 protoplasm, nucleus, granules and vacuoles. In the presence of salts of organic 

 acids, it produces a green or red diffusible pigment. 



Azotobacter vinelandii was isolated by Lipman 51 from a New Jersey soil in 1903. 

 In four days it forms on mannite agar colonies 4 mm. in diameter. These are 

 round, raised, concentric and semi-transparent, with denser whitish centers; 

 the deep colonies are white, small, hardly more than 1 mm. in diameter, elliptical 

 to spindle-shaped. A white thick membrane is formed on the mannite solution. 

 When undisturbed, the liquid culture shows the formation of a bright yellow pig- 

 ment, concentrated near the surface and gradually diffusing through the liquid. 

 In older cultures the pigment gradually diffuses throughout the medium and 

 becomes darker, until in old cultures it may become a yellowish red. At the same 

 time the bacterial mass may also become darker. A considerable number of 

 forms, ranging from large rods with rounded ends to spherical organisms, are found 

 in mannite cultures. Most of the organisms are actively motile, showing pro- 

 gressive and at times rotatory motility. As the culture grows older, the number 

 of shorter rods increases, and the cells begin to accumulate and store up fat, 

 which appears in small globules throughout the bacterial body and gives it a 

 granular appearance. Various involution forms are produced in meat extract 

 bouillon. A temperature of 85°C. for five minutes is sufficient to destroy all the 

 cells. The organism stains readily with carbol fuchsin, with aqueous solutions 

 of gentian violet, methyl violet or fuchsin and Loffler's methylene blue. The 

 bright yellow pigment produced in mannite solution is favored by greater surface 

 of medium (oxygen need), is soluble in alcohol and decolorized by weak acids. 

 The organism is very active in the fixation of atmospheric nitrogen. 



Azotobacter beijerinckii was isolated by Lipman in 1904. It forms pure white, 

 moist, soft, irregularly round, netted colonies. In mannite solution, the organ- 

 ism forms a turbidity, with white circular dots on surface and walls, gradually 

 settling to the bottom. The cells are large, almost spherical, occurring singly, 

 or in chains of two or more. This organism is much larger than A. chroococcum 

 and A. vinelandii and does not show any motility. On solid media, a yellowish 

 pigment is formed. Azotobacter vitreum was isolated by Lohnis and Westermann. 82 

 It forms only round cells, is non-motile, grows as transparent, moist colonies on 

 solid media, without any pigment. Prazmowski doubts whether this organism 

 belongs to the genus Azotobacter. 



Distribution of Azotobacter in the soil. Azotobacter is of universal 

 occurrence in the soil, 53 but not to such an extent as Clostridium. Out 

 of one hundred and five soil samples examined, Burri 54 found Azoto- 



51 Lipman, J. G. Experiments on the transformation and fixation of nitrogen 

 by bacteria. N. J. Agr. Exp. Sta. Rpt. 24: 217-285. 1903. 25th Ann. Rpt. 1904, 

 237-289; Azotobacter studies. Ibid. 26: 254. 1905; 29: 137. 1908. 



62 Lohnis, F., and Westermann, T. Uber stickstofffixierende Bakterien. 

 Centrbl. Bakt. II, 22: 234-254. 1908. 



83 Beijerinck and Van Delden. 1902 (p. 105). 



64 Burri, R. Die Nutzbarmachung des LuftstickstofTs durch Bodenbakterien. 

 Schweiz. Ztschr. Forstwesen. 55: 89. 1904. 



