Conjugal gene transfer to aquatic bacteria detected by the generation of a new phenotype

An experimental approach based on the assembly of genes of a catabolic pathway was used to detect transconjugants in aquatic communities. Resistance to phenylmercury acetate was established in transconjugants when wide-host- range conjugal plasmids containing merB, the gene encoding organomercurial lyase, were transferred to strains from aquatic communities that had been acclimated to inorganic mercury and thus enriched for populations containing merA, the gene encoding mercuric reductase (T. Barkay, Appl. Environ. Microbiol. 53:2725-2732, 1987). Conjugation was confirmed by using the plasmids' encoded antibiotic resistance patterns and by hybridization with a eukaryotic gene. Three merB-conjugal plasmids, belonging to incompatibility groups W (pGTE16), P1 (pGTE26), and N (pGTE25), were prepared. Transfers by filter matings of pGTE16 and pGTE26 from Pseudomonas aeruginosa PAO1 to indigenous strains were at efficiencies of 4.5 x 10^-2 and 4.8 x 10^-3 transconjugant per potential recipient, respectively. These efficiencies were from 1 to 2 orders of magnitude below those observed for intraspecies matings with genetically marked recipients. The third plasmid, pGTE25, was not stably maintained in P. aeruginosa donors, and its transfer from Escherichia coli donors was below the level of detection. Characterized transconjugant strains were shown to be Pseudomonas spp. Potential applications of the described experimental approach in the creation of bacterial populations with new catabolic capabilities in hazardous waste sites and in the detection of transfer of recombinant DNA from engineered microorganisms to indigenous bacteria are discussed.