When E. coli was used as donor, no transfer of pKJK10 was detected to any of the individual 15 soil isolates, but P. putida was observed to transfer pKJK10 to Stenotrophomonas rhizophila. The plasmid transfer frequency from P. putida ABT-199 ic50 to S. rhizophila

was higher when the filters were placed on TSA medium (1.07 ± 3.05 × 10−1) compared with R2A medium (0.33 ± 2.32 × 10−2, Table 2), supporting the fact that the metabolic state of the cells may in some cases influence conjugation frequencies (van Elsas & Bailey, 2002). These results reflect the fact that the host range of plasmids depends on the identity of the donor strain (De Gelder et al., 2005). 1.07 ± 3.05 × 10−1 0.33 ± 2.32 × 10−2c In contrast to the results observed when transferring pKJK10 to individual isolates, no plasmid transfer events were observed from P. putida to the mixed community consisting of the same 15 strains applied individually above. Transconjugants were, however, obtained when applying E. coli as donor of pKJK10. The green fluorescent transconjugant cells were sorted by FACS and cultured on TSA agar plates. By sequence analysis of the 16S rRNA gene from four colonies from each replicate, the selected transconjugants were shown all to be identical and identified

as Ochrobactrum rhizosphaerae. selleck chemical This does not exclude the possibility that other isolates may also have received the plasmid, but it does show that O. rhizosphaerae

in fact did so and that it was the most dominant strain among the 3-mercaptopyruvate sulfurtransferase plasmid recipients. Interestingly, O. rhizosphaerae was not able to receive the plasmid in the individual mating experiment, indicating that the plasmid permissibility does not only depend on the abilities of the plasmid, host and recipient strains, but also on the surrounding microbial community, which may reduce or enhance plasmid transfer. Both of these scenarios were observed in this study; transfer of pKJK10 from P. putida to S. rhizophila was observed in diparental mating experiments, but not in a mixed community, possibly caused by reduced survival/competition ability of the strains or by the fact that the donor and this specific recipient populations had less opportunity for interaction in the mixed community. In contrast, the presence of a mixed community induced pKJK10 transfer from E. coli to O. rhizosphaerae, which may be due to altered physical cell–cell interaction or the presence of one or several intermediate plasmid host(s). These ‘plasmid step-stones’ may facilitate plasmid transfer from E. coli to O. rhizosphaerae, but are unable to establish and stabilize the plasmid in their own population. Because it was not possible to isolate the strains individually after growth in the community, the fraction of O. rhizosphaerae herein could not be determined; It is possible that O.