Association of Bacillus firmus E3 and Klebsiella terrigena E6 with increased ability for nitrogen fixation

A binary association of Bacillus firmus E3 and Klebsiella terrigena E6 was isolated from the rhizosphere of Dactylus glomerata. This association consisted of a N2-fixing strain (K. terrigena) and a ‘promoter’ strain (B. firmus). Increased nitrogen fixation activity (ca. 110 nmol ethylene mg protein−1 h−1) was detected on the 4th day of growth under aerobic conditions. Living cells of B. firmus, cell-free culture filtrates, and two compounds excreted by B. firmus, phenol and p-hydroxybenzoic acid, increased nitrogenase activity of K. terrigena. This increase was accompanied by the enhancement of K. terrigena respiration (O2-consumption) rate and was probably due to respiration protection of nitrogenase induced by these phenolics.