INFLUENCE OF EARTHWORM ACTIVITY ON GENE-TRANSFER FROM PSEUDOMONAS-FLUORESCENS TO INDIGENOUS SOIL BACTERIA

We have developed a model system to assess the influence of earthworm activity on the transfer of plasmid pJP4 from an inoculated donor bact erium, Pseudomonas fluorescens C5t (pJP4), to indigenous soil microorg anisms. Three different earthworm species (Lumbricus terrestris, Lumbr icus rubellus, and Aporrectodea trapezoides), each with unique burrowi ng, casting, and feeding behaviors, were evaluated. Soil columns were inoculated on the surface with 10(8) cells per g of soil of the donor bacterium, and after a 2-week incubation period, donor, transconjugant , and total bacteria were enumerated at 5-cm-depth intervals. Transcon jugants were confirmed by use of colony hybridization with a mer gene probe. In situ gene transfer of plasmid pJP4 from P. fluorescens C5t t o indigenous soil bacteria was detected in all inoculated microcosms. In the absence of earthworms, the depth of recovery was limited to the top 5 cm of the column, with approximately 10(3) transconjugants per g of soil. However, the total number of transconjugants recovered from soil was significantly greater in microcosms containing either L. rub ellus or A. trapezoides, with levels reaching about 10(5) CFU/g of soi l. In addition, earthworms distributed donor and transconjugant bacter ia throughout the microcosm columns, with the depth of recovery depend ent on the burrowing behavior of each earthworm species. Donor and tra nsconjugant bacteria were also recovered from earthworm casts and insi de developing cocoons. Transconjugant bacteria from the indigenous soi l microflora were classified as belonging to Acidovorax spp., Acinetob acter spp., Agrobacterium spp., Pasteurella spp., Pseudomonas spp., an d Xanthomonas spp.