EXOGENOUS ISOLATION OF MOBILIZING PLASMIDS FROM POLLUTED SOILS AND SLUDGES

Exogenous plasmid isolation was used to assess the presence of mobiliz ing plasmids in several soils and activated sludges. Triparental matin gs were performed with Escherichia coli (a member of the gamma subgrou p of the Proteobacteria) as the donor of an IncQ plasmid (pMOL155, con taining the heavy metal resistance genes czc: Co-r, Zn-r, and Cd-r), A lcaligenes eutroplius (a member of the beta subgroup of the Proteobact eria) as the recipient, and indigenous microorganisms from soil and sl udge samples as helper strains. We developed an assay to assess the pl asmid mobilization potential of a soil ecosystem on the basis of the n umber of transconjugants obtained after exogenous isolations. After in oculation into soil of several concentrations of a helper strain (E. c oli CM120 harboring IncP [IncP1] mobilizing plasmid RP4), the log numb ers of transconjugants obtained from exogenous isolations with differe nt soil samples were a linear function of the log numbers of helper st rain CM120(RP4) present in the soils. Four soils were analyzed for the presence of mobilizing elements, and mobilizing plasmids mere isolate d from two of these soils. Several sludge samples from different waste water treatment plants yielded much higher numbers of transconjugants than the soil samples, indicating that higher numbers of mobilizing st rains were present. The mobilizing plasmids isolated from Gent-O sludg e and one plasmid isolated from Eislingen soil hybridized to the repP probe, whereas the plasmids isolated from Essen soil did not hybridize to a large number of rep probes (repFIC, repHI1, repHI2, repL/M, repN , repP, repT, repU, repW, repX). This indicates that in Essen soil, br oad-host-range mobilizing plasmids belonging to other incompatibility groups may be present.