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 micrometer. When grown in broth the cells had a single polar flagellum, but when grown on agar at 30 degrees 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. Strians 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 (ATCC29707) is proposed as the neotype strain for A. lipoferum, and strain Sp 7 (ATCC 29145) is proposed as the type strain for A. brasilense.
The morphological, physiological, and nutritional characteristics and the deoxyribonucleic acid (DNA) base compositions of 39 strains of SpirilZum (including the available type strains) were determined. A uniform methodology provided a basis for comparison of the strains. The results, together with previous serological data and with previous results for two strains of Spirillum volutans, indicate that the present genus Spirillum should be divided into three genera, with the original name being restricted to obligately microaerophilic freshwater forms having a large cell diameter and a DNA base composition of 36 to 38 mol % guanine + cytosine (G + C). This genus would contain at present only one species, the type species, S . uolutans. The generic name Aquaspirillum is proposed for the aerobic, freshwater forms having a DNA base composition of 49 to 65 mol % G + C. Although Spirillum Ehrenberg 1832 is one of significance, except for Spirillum minor, the the oldest bacterial generic names and although causative agent of one of the forms of rat-bite spirilla are widespread in freshwater and marine fever in man (3, 8, 29); (ii) difficulty of environments, characterization of this group isolation (7,28,45,49); and (iii) past has not been extensive. This may be attributed difficulties in preservation of cultures, with t o the following factors: (i) lack of medical only continual transfer, the recently developed Contribution no. 3032 of the Woods Hole Oceanographic Institution, Woods Hole, Mass.
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