Growth and nitrogen fixation by cultures of Bacillus sp. S 77 g, Pseudomonas sp. S 44 b, and Klebsiella sp. S 145 m, isolated from Egyptian soils.

Study of the behaviour of representative aerobic freeliving dinitrogen-fixers, other than azotobacters, with respect to different carbon sources, showed that a broad range of compounds is accessible to them. This group of nitrogen-fixers can assimilate molecular nitrogen as a result of utilization of carbohydrates, alcohols, and organic acid salts. The highest nitrogen-fixation productivity, reaching 20.6-27.3 mg of nitrogen per g of carbon utilized, was observed on medium containing glucose as the carbon and energy source. Supplementary growth factors added to the N-deficient medium in the form of small amounts of yeast extract stimulated their growth and fixation of atmospheric nitrogen. This stimulation is particularly evident in mixed cultures. The nature of the mutual influences of micro-organisms in these cultures remains fairly obscure and further inquiry is necessary to elucidate this problem. A medium containing 1% glucose and Difco yeast extract to produce a bound nitrogen concentration of 10 ppm appeared to be the most favourable for the cultivation and N2-fixation of this group of microorganisms.