OCTOBEK 1, 1915] 



SCIENCE 



463 



OBSERVATIONS ON AZOTOBACTER 



The group of bacteria having the capacity 

 of using free atmospheric nitrogen in their 

 metabolic processes consists of three general 

 types: (1) Those associated with the nodule 

 formation of legumes (Ps. radicicola) ; (2) 

 large bacilli which produce spores located in 

 the center of the cell, causing an increased 

 diameter of the cell at that point {Clostridium 

 pasteraneum) ; and (3) a form displaying con- 

 siderable variation in size and shape, which, 

 according to original descriptions, is without 

 endospores (Azotohacter sp.)- 



The discovery of the extraordinary ability 

 of these organisms to secure a supply of nitro- 

 gen from the air brought them into immediate 

 prominence as objects of systematic study. 

 The nitrogen-assimilating property was first 

 detected in the radicicola^ and hence the earlier 

 studies were concentrated upon this organism. 

 After the isolation of the other two forms, 

 however, they assumed quite as much impor- 

 tance as had attached to the legume organism. 

 In fact, recently more attention has been 

 given to the former than to the latter. This 

 has been especially true of the Azotohacter. 

 The Clostridium has never been so inviting as 

 either of the others; perhaps because of its 

 morphological uniformity. Then, too, it ap- 

 pears to have less nitrogen-gathering power. 



At various times, while working with other 

 phases of the problem of soil bacteriology, I 

 have attempted to isolate the Azotohacter from 

 our local soils and study it rather intensively. 

 On several occasions these efforts have re- 

 sulted in the securing of pure cultures of the 

 bacillus. So far as the observations went. 



however, no peculiarities were presented which 

 had not previously been noted by other inves- 

 tigators. During the year 1913 a culture was 

 secured which showed some striking qualities. 

 These attracted immediate attention and led to 

 an extensive study. 



Soon after the isolation of Azotohacter 

 croococcum by Beyerinck, other investigators 

 gave descriptions of five or six types of Azoto- 

 hacter which they regarded as distinct species. 

 One of these forms was given the name Azoto- 

 hacter vinelandii by Dr. J. G. Lipman, who 

 isolated and described it. The chief basis upon 

 which this species was established is the qual- 

 ity of pigment production. In this case the 

 color of the pigment is a distinct yellow, as 

 contrasted with the heavy brown pigment of 

 A. croococcum. 



From the outset the bacillus under consid- 

 eration here has been regarded as a variety of 

 A. vinelandii. In many respects, however, our 

 type differs so markedly from the original 

 description of A. vinelandii as to create con- 

 siderable doubt as to the identity of the species. 

 Nevertheless, I am still disposed to regard it as 

 a mere variety within this group. As a matter 

 of fact, the recent work of Prazmowski and 

 others tends to eliminate the various species 

 of this organism and regard them all as one 

 species consisting of several varieties. My ovm 

 observations lead me to favor this contention. 

 The pigment color of the culture in my labo- 

 ratory shows considerable variation, ranging 

 from yellow to brown. This seems to depend 

 largely upon the medium used. 



The following comparison will show the chief 

 points in which our type varies from the spe- 

 cies description: 



Features Considered Original Description 



Mannite agar plates Colonies 4 mm. in 4 day 



Mannite agar plates Colonies whitish. 



Glucose solution Surface film. 



Mannite solution Yellow pigment. 



Glucose agar stab White. 



Mannite agar stab White. 



Potato Dirty white. 



Spore formation Absent. 



Thermal death point 80°-85° C, 5 min. 



Observation Made in the Present 

 InTestigation 



Colonies 6-8 mm. in 4 days. 



Colonies yellow. 



No surface film. 



No pigment. 



Yellow. 



Yellow. 



Pink, young; yellow, old. 



Present. 



90°-95° C, 5 min. 



