Effect of field inoculation with Sinorhizobium meliloti L33 on the composition of bacterial communities in rhizospheres of a target plant (Medicago sativa) and a non-target plant (Chenopodium album) - Linking of 16S rRNA gene-based single-strand conformation polymorphism community profiles to the diversity of cultivated bacteria

Fourteen weeks after field release of luciferase gene-tagged Sinorhizobium meliloti L33 in field plots seeded with Medicago sativa, we found that the inoculant also occurred in bulk soil from noninoculated control plots. In r hizospheres of M. sativa plants, S. meliloti L33 could be detected in nonin oculated plots 12 weeks after inoculation, indicating that growth in the rh izosphere preceded spread into bulk soil. To determine whether inoculation affected bacterial diversity, 1,119 bacteria mere isolated from the rhizosp heres of M. sativa and Chenopodium album, which was the dominant weed in th e field plots. Amplified ribosomal DNA restriction analysis (ARDRA) reveale d plant-specific fragment size frequencies. Dominant ARDRA groups were iden tified by 16S rRNA gene nucleotide sequencing. Database comparisons indicat ed that the rhizospheres contained members of the Proteobacteria (alpha, be ta, and gamma subgroups), members of the Cytophaga-Flavobacterium group, an d gram-positive bacteria with high G+C DNA contents. The levels of many gro ups were affected by the plant species and, in the case of M. sativa, by in oculation. The most abundant isolates were related to Variovorax sp,, Arthr obacter ramosus, and Acinetobacter calcoaceticus. In the rhizosphere of M. sativa, inoculation reduced the numbers of cells of A. calcoaceticus and me mbers of the genus Pseudomonas and increased the number of rhizobia, Cultiv ation-independent PCR-single-strand conformation polymorphism (SSCP) profil es of a 16S rRNA gene region confirmed the existence of plant-specific rhiz osphere communities and the effect of the inoculant, AII dominant ARDRA gro ups except Variovorax species could be detected. On the other hand, the SSC P profiles revealed products which could not be assigned to the dominant cu ltured isolates, indicating that the bacterial diversity was greater than t he diversity suggested by cultivation.