Monitoring the spread of broad host and narrow host range plasmids in soil microcosms

Escherichia coli recipient and E. coli donor strains carrying streptothricin‐resistance genes were inoculated together into different soil microcosms. These genes were localized on the narrow host range plasmids of incompatibility (Inc) groups FII, Il, and on the broad host range plasmids of IncP1, IncN, IncW3, and IncQ. The experiments were intended to study the transfer of these plasmids in sterile and non‐sterile soil with and without antibiotic selective pressure and in planted soil microcosms. Transfer of all broad host range plasmids from the introduced E. coli donor into the recipient was observed in all microcosm experiments. These results indicate that broad host range plasmids encoding short and rigid pili might spread in soil environments by conjugative transfer. In contrast, transfer of the narrow host range plasmids of IncFII and IncI1, into E. coli recipients was not found in sterile or non‐sterile soil. These plasmids encoded flexible pili or flexible and rigid pili, respectively. In all experiments highest numbers of transconjugants were detected for the IncP1‐plasmid (pTH16). There was evidence with plasmids belonging to IncP group transferred by conjugation into a variety of indigenous soil bacteria at detectable frequencies. Significantly higher numbers of indigenous transconjugants were obtained for the IncP‐plasmid under antibiotic selection pressure, and a greater diversity of transconjugants was detected. Availability of nutrients and rhizosphere exudates stimulated transfer in soil. Furthermore, transfer of the IncN‐plasmid (pIE1037) into indigenous bacteria of the rhizosphere community could be detected. The transconjugants were determined by BIOLOG as Serratia liquefaciens . Despite the known broad host range of IncW3 and IncQ‐plasmids, transfer into indigenous soil bacteria could not be detected.