Paenibacillus brasilensis sp nov., a novel nitrogen-fixing species isolated from the maize rhizosphere in Brazil

Sixteen nitrogen-fixing strains isolated from the rhizosphere of maize planted in Cerrado soil, Brazil, which showed morphological and biochemical characteristics similar to the gas-forming Paenibacillus spp., were phenotypically and genetically characterized. Their identification as members of the genus Paenibacillus was confirmed by using specific primers based on the 16S rRNA gene. SDS-PAGE of whole-cell proteins, API 50CH, morphological and biochemical tests, amplified rDNA-restriction analysis (ARDRA), DNA-relatedness analyses, denaturing-gradient gel electrophoresis (DGGE) and 16S rRNA gene sequence determinations were performed to characterize the novel isolates and to compare them to strains of other nitrogen-fixing Paenibacillus spp. Phenotypic analyses showed that the 16 strains were very homogeneous and shared a high level of relatedness with Paenibacillus polymyxa and Paenibacillus peoriae. However, none of the novel isolates was able to ferment glycerol (positive test for P. polymyxa), L-arabinose or D-xylose (positive tests for P. polymyxa and P. peoriae) or utilize succinate (positive test for P. peoriae). Genetic approaches also indicated a high level of similarity among the novel isolates and P. polymyxa and P. peoriae, but the novel strains clearly could not be assigned to either of these two recognized species. On the basis of the features presented in this study, the 16 novel isolates were considered to represent members of a novel species within the genus Paenibacillus, for which the name Paenibacillus brasilensis is proposed. The type strain is PB172(T) (=ATCC BAA-413(T)=DSM 14914(T)

Cultivation of hitherto-uncultured bacteria belonging to the Verrucomicrobia subdivision 1 from the potato (Solanum tuberosum L.) rhizosphere

The role of dominant bacterial groups in the plant rhizosphere, e.g., those belonging to the phyla Acidobacteria and Verrucomicrobia, has, so far, not been elucidated, and this is mainly due to the lack of culturable representatives. This study aimed to isolate hitherto-uncultured bacteria from the potato rhizosphere by a combination of cultivation approaches. An agar medium low in carbon availability (oligotrophic agar medium) and either amended with potato root exudates or catalase or left unamended was used with the aim to improve the culturability of bacteria from the potato rhizosphere. The colony forming unit numbers based on colonies and microcolonies were compared with microscopically determined fluorescence-stained cell numbers. Taxonomical diversity of the colonies was compared with that of library clones made from rhizosphere DNA, on the basis of 16S rRNA gene comparisons. The oligotrophic media amended or not with catalase or rhizosphere extract recovered up to 33.6% of the total bacterial numbers, at least seven times more than the recovery observed on R2A. Four hitherto-uncultured Verrucomicrobia subdivision 1 representatives were recovered on agar, but representatives of this group were not found in the clone library. The use of oligotrophic medium and its modifications enabled the growth of colony numbers, exceeding those on classical agar media. Also, it led to the isolation of hitherto-uncultured bacteria from the potato rhizosphere. Further improvement in cultivation will certainly result in the recovery of other as-yet-unexplored bacteria from the rhizosphere, making these groups accessible for further investigation, e.g., with respect to their possible interactions with plants

Gene mobility in microbiomes of the mycosphere and mycorrhizosphere - role of plasmids and bacteriophages

Microbial activity in soil, including horizontal gene transfer (HGT), occurs in soil hot spots and at "hot moments". Given their capacities to explore soil for nutrients, soil fungi (associated or not with plant roots) can act as (1) selectors of myco(rrhizo) sphere-adapted organisms and (2) accelerators of HGT processes across the cell populations that are locally present. This minireview critically examines our current understanding of the drivers of gene mobility in the myco(rrhizo) sphere. We place a special focus on the role of two major groups of gene mobility agents, i.e. plasmids and bacteriophages. With respect to plasmids, there is mounting evidence that broad-host-range (IncP-1 beta and PromA group) plasmids are prominent drivers of gene mobility across mycosphere inhabitants. A role of IncP-1 beta plasmids in Fe uptake processes has been revealed. Moreover, a screening of typical mycosphere-inhabiting Paraburkholderia spp. revealed carriage of integrated plasmids, next to prophages, that presumably confer fitness enhancements. In particular, functions involved in biofilm formation and nutrient uptake were thus identified. The potential of the respective gene mobility agents to promote the movement of such genes is critically examined