114 PRINCIPLES OF SOIL MICROBIOLOGY 



Description of species of Azotobacter. Azotobacter is a bacterium, 4 to 

 6^ broad, forming, when young, diplococci or short rods, with hyaline 

 contents, often containing a vacuole and forming a slimy wall. Various 

 involution forms, as threads reaching to 60 to 80m in length, may be 

 formed. Formation of nuclei has been demonstrated by Prazmowski. 

 The younger cells are motile, by means of the 4 to 10 polar flagella, 

 which are about as long as the bacteria. The organisms are aerobic, 

 readily killed on heating. Beijerinck, Lipman and Jones 45 denied the 

 existence of spores in Azotobacter. According to Ashby, Fischer and 

 Krzemieniewsky, spore formation is positive, since the organism can 

 resist drying for a long period of time. Prazmowski 46 demonstrated that 

 spore formation takes place under normal conditions, with sufficient 

 aeration and in the presence of humates. The process of spore forma- 

 tion is later described in detail. 47 - 48 



Azotobacter chroococcum Beij. is universally distributed in soils having a reac- 

 tion above pH 6.0. It produces a crude floating membrane on tap water contain- 

 ing 2 per cent mannite and 0.02 per cent K2HPO4 inoculated with garden soil- 

 Only occasional individuals in young cultures are motile by means of a single 

 flagellum. The size of the cell is differently reported by investigators, varying 

 from 3 to 4 by 5 to 6/* and even 3 to 4 by 9 to 12)u to 2 to 3 by 3 to 4ju (limits 1.5 to 

 7n) and even 1 to 2p for cocci and 1.5 to 2 by 3 to 4/* for rods. 49 Old membranes 

 consist of micrococci of various sizes united to form sarcina-like masses, possessing 

 mucilaginous walls. The older cultures are frequently brown or black. This 

 organism oxidizes numerous carbon compounds to C0 2 and H 2 0. According to 

 Stoklasa, 60 organic acids are also formed; however, Omeliansky suggested that 

 Stoklasa probably worked with contaminated cultures. 



Azotobacter agile Beij. was found universally distributed in canal waters of 

 Delft; the crude and pure cultures are obtained by previous methods. The 



46 Jones, D. H. A morphological and cultural study of some Azotobacter. 

 Centrbl. Bakt., II, 38: 14-21. 1913. Further studies on the growth cycle of 

 Azotobacter. Jour. Bact. 5: 325. 1920. 



46 Prazmowski, A. Die Entwicklungsgeschichte, Morphologie und Cytologie 

 des Azotobacter chroococcum Beij. Centrbl. Bakt. II, 33: 292-305. 1912; 37: 

 299. 1913. Azotobacter-Studien. II. Physiologie und Biologic Ariz. Akad. 

 Krakau. Math.-Naturw. Kl. (B), 1912, 855-950. (Centrbl. Bakt. II, 37: 299-301. 

 1913). 



47 Lohnis and Smith, 1916 (p. 56). 



48 Mulvania. Observations on Azotobacter. Science, 42:463. 1915. 



49 Bonazzi, A. Cytological studies of Azotobacter chroococcum. Jour. Agr. 

 Res., 4: 225-241. 1915. 



60 Stoklasa, J. Beitrag zur Kenntnis der chemischen Vorgiinge bei der Assimi- 

 lation des elementaren Stickstoffs durch Azotobacter und Radiobacter. Centrbl. 

 Bakt. II, 21: 484-509, 620-632. 1908. 



