Contribution of the earthworm Lumbricus rubellus (Annelida, Oligochaeta) to the establishment of plasmids in soil bacterial communities

The contribution of the earthworm Lumbricus rubellus in spreading plasmids from a nonindigenous bacterial species to the soil microbial community was studied with Escherichia coli strains as donor organisms. The selected dono r strains harbored marker-gene tagged plasmids with different transfer prop erties and host ranges. Prototrophic benzoate degrading indigenous bacteria were analyzed as potential recipients. In filter-mating experiments, donor strains were mixed with bacterial cell consortia extracted from earthworm casts (feces) and incubated on nutrient agar at 28 degreesC. Transfer was d etected with the broad host range IncP plasmid pRP4luc; with the IncQ plasm id, pSUP104luc, but only when it was present in a mobilizing donor strain; and with the transposon delivery vector pUTlux. No transfer was detected wi th the nonmobilizable pUCluc and the mobilizable pSUP202luc, both of narrow host range. In microcosm studies with E. coli inoculated soil incubated at 12 degreesC, transconjugants were only detected in casts of L, rubellus bu t not in bulk soil, indicating that the gut passage was a precondition for plasmid transfer. Plasmid pRP4luc was transferred at higher frequencies tha n detected in filter mating. Results of the filter matings were confirmed e xcept that transfer of pUTlux could not be detected. The majority of transc onjugants isolated in this study lost their acquired plasmid upon further c ultivation. Stable transconjugants, however, were obtained and identified a t the 16S rRNA gene level as members of the beta- and gamma -subgroups of P roteobacteria. Incubation of E. coli and selected transconjugants in soil m icrocosms with L. rubellus demonstrated that the gut passage resulted in a slight but significant reduction of ingested cells. In contrast to the dono r strains, however, the population sizes of transconjugants in bulk soil an d in casts did not decrease over time. This demonstrated that the transferr ed plasmids had established themselves in the soil microbial community.