CHAPTER 20 



AZOTOBACTER, RHIZOBIUM, NITROSOMONAS, NITROBACTER, 

 HYDROGENOMONAS, METHANOMONAS, CARBOXYDOMONAS, 

 AND ACETOBACTER 



In this chapter we group together a number of organisms playing an important 

 part in the nitrogen metabolism of the soil. 



THE AZOTIFYING OR NITROGEN-FIXING BACTERIA 



Definition. — Azoiobacter. 



Relatively large rods, or even cocci, sometimes almost yeast-like in appearance, 

 dependent primarily for growth energy upon the oxidation of carbohydrates. 

 Motile or non-motile ; motile forms possess a tuft of polar flagella. Obhgate 

 aerobes, usually growing in a film upon the surface of the culture medium. Capable 

 of fixing atmospheric nitrogen when grown in solutions containing carbohydrates 

 and deficient in combined nitrogen. 



Type species. Azotobacter chroococcum, Beijerinck. 



Isolated by Beijerinck in 1901. He described two species, Az. chroococcum, so 

 called from the brown pigment to 

 which it gives rise, and Az. agilis. 

 The former is widespread in garden /^ 



earth and in fruitful soil of all • 



kinds ; the latter was found in ^ * • - ^ 



canal water in Holland. Since A *W 



then four other species have been 9*^ •• 



described — Az.vinelandii, Az. beije- fO> ^ ta. ** 



rincki, Az. woodstowni, and Az. ^ 



vitreus ; all of them inhabit the * .«»• 



soil. i ^•\ *• 



All species of Azotobacter are j^^^ 



highly pleomorphic ; the cells may 7 



be short, thick and rod-like, ellip- A 



soidal, pyriform, or spindle-shaped ; ^ ^!% ^ 



when occurring in pairs they often "^ 



look like giant diplococci. Their 



size varies from 4-7 fl in length ^m. m.~ Azotobacter chroococcum. 



and 1-5-4 ^ in breadth. Giant From an agar culture, 3 days, 30° C. ( X 1000). 



involution forms, looking like 



amoebse or yeasts, are not uncommon. They are motile by one or more polar, 

 or possibly peritrichate (Hofer 1944), flagella, and are Gram-negative. 



The organism is strictly aerobic. It grows best in tap water containing 2 per 



497 



