Response of Cereal and Forage Grasses to Inoculation with N2-Fixing Bacteria

Sixty-one strains of the root-associated nitrogen fixer Spirillum lipoferum exhibited a similar morphology in peptone–succinate salts medium: vibrioid cells having a diameter of 1.0 μm. When grown in broth the cells had a single polar flagellum, but when grown on agar at 30 °C lateral flagella of shorter wavelength were also formed. The DNA base composition was 69–71 mol % guanine + cytosine when determined by thermal denaturation. DNA homology experiments indicated the occurrence of two distinct but related homology groups: 46 strains were in group I and 15 strains were in group II. Group II strains were distinguished by their ability to use glucose as a sole carbon source for growth in nitrogen-free medium, by their production of an acidic reaction in a peptone-based glucose medium, by their requirement for biotin, and by their formation of wider, longer, S-shaped or helical cells in semisolid nitrogen-free malate medium. The results indicate that two species exist, and on the basis of their characteristics it is proposed that they be assigned to a new genus, Azospirillum. Strains belonging to group II are named A.lipoferum (Beijerinck) comb, nov., while those belonging to group I are named A.brasilense sp. nov. Strain Sp 59b (ATCC 29707) is proposed as the neotype strain for A. lipoferum, and strain Sp 7 (ATCC 29145) is proposed as the type strain for A. brasilense.