AZOTOBACTER CHROOCOCCUM 499 



Broth. — 2 days at 30° C. Moderate growth, with moderate, finely granular turbidity, 

 and slight, finely granular sediment, not disintegrating on shaking. After 6 days, 

 surface ring growth. 

 Loejfler's Blood Serum. — 6 days at 30° C. Moderate, confluent, creamy growth, with 



slight yellowish colour ; smooth, mirror-like surface ; no liquefaction. 

 Potato. — 10 days at 30° C. Abundant, raised, opaque brown growth, with wrinkled 

 honeycombed surface. During first few days, growth is yellowish, but later it 

 becomes brown. Potato unchanged. 

 Resistance.- — Killed by 55° C. in 30 minutes. 



Metabolism. — Aerobic. No growth anaerobically. Opt. temp. 28-30° C. Limits of 

 pH 6-5-8-6 in pure culture. Pigment ; grown in mannitol and other suitable 

 media, a surface scum is formed, which becomes brown, and later black. Pigment 

 is insoluble in water, alcohol, CHCI3, ether, CSj, but shghtly soluble in alkalies, 

 which destroy it. 



Nutritional. Grows best in 2-10 per cent, mannitol solutions, and in 0-5 per 

 cent, solutions of K, Ca, or Na propionate. Growth on agar improved by 1 per 

 cent, mannitol. 

 Biochemical. — Fixes atmospheric nitrogen, converting it into NH3, nitrite and nitrate. 

 Capable of growing in media free from combined nitrogen but containing carbo- 

 hydrates. Can utilize nitrates for its nitrogen supply ; NH3 and peptone can be 

 used only shghtly. 



Utihzes dextrose, maltose, mannitol, lactose, dextrin, starch, glycerol, alcohol, 



propionate, acetate, butyrate, citrate, lactate, malate, and succinate ; gives off 



COj. Indole—. M.R. — . V.P. — . Nitrates—. M.B. reduction —. Catalase+. 



NH3 very slight. L.M. partly decolorized, and rendered clearer and more fluid. 



Pathogenicity. — Nil. 



Az. agilis is a large, oval organism containing granules and vacuoles, and provided 

 with a bundle of polar flagella. The other four species of Azotobacter that have been 

 described differ in minor respects from the type species Az. chroococcum. 



Numerous non-sporing rod-shaped organisms, capable of fixing nitrogen, have been 

 found in horse and cow dung by Fulmer and Fred (1917). The chief of these, which 

 they call B. azophile, is a rod-shaped organism, 1-6 X OS fi, motile and Gram-positive. 

 It gives a light-orange, wrinkled growth on agar, a heavy membranous surface growth 

 in broth, and a brownish growth on potato. Gelatin is liquefied ; milk is peptonized. 

 Nitrates are reduced to nitrites and to gaseous nitrogen. Indole -(- . Strict aerobe. Grows 

 well in mannitol solution, in which it fixes nitrogen in considerable quantities. 



RHIZOBIUM 



Definition. — Rhizobium. 



Minute rods, motile when young. Speciahzed forms abundant and charac- 

 teristic when grown under suitable conditions. Obhgate aerobes,^ capable of 

 fixing atmospheric nitrogen when grown in the presence of carbohydrates and in 

 the absence of compounds of nitrogen. Produce nodules on the roots of leguminous 

 plants. 



Type species. Rhizobium legum,inosarum, Frank. 



The first member of this group was isolated by Beijerinck in 1888, who named 

 it Bacillus radicicola. He found this organism in the root-nodules of legu- 

 minous plants, and in numerous specimens of soil and water of different origin ; 

 he noted its variable morphology ; he described its cultural and biochemical 

 reactions ; and he showed how the bacilli in the " swarmer " stage penetrated 



^ This is not absolutely certain ; we have observed growth of one strain in broth, though 

 Dot on agar, under strictly anaerobic conditions. 



