Influence of earthworm activity on gene transfer from Pseudomonas fluorescens to indigenous soil bacteria

Abstract

We have developed a model system to assess the influence of earthworm activity on the transfer of plasmid pJP4 from an inoculated donor bacterium, Pseudomonas fluorescens C5t (pJP4), to indigenous soil microorganisms. Three different earthworm species (Lumbricus terrestris, Lumbricus rubellus, and Aporrectodea trapezoides), each with unique burrowing, 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. Transconjugants were confirmed by use of colony hybridization with a mer gene probe. In situ gene transfer of plasmid pJP4 from P. fluorescens C5t to 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. rubellus or A. trapezoides, with levels reaching about 10 5 CFU/g of soil. In addition, earthworms distributed donor and transconjugant bacteria throughout the microcosm columns, with the depth of recovery dependent on the burrowing behavior of each earthworm species. Donor and transconjugant bacteria were also recovered from earthworm casts and inside developing cocoons. Transconjugant bacteria from the indigenous soil microflora were classified as belonging to Acidovorax spp., Acinetobacter spp., Agrobacterium spp., Pasteurella spp., Pseudomonas spp., and Xanthomonas spp.

Interactions of earthworms with Atrazine-degrading bacteria in an agricultural soil

“…ADP inoculation was not detectable on the surface area of the T-RF of 80 bp, reinforcing the possibility of a decline of the inoculated population. This is in agreement with reports in the literature of a 2-log decline observed for Pseudomonas fluorescens (Daane et al, 1996) and Pseudomonas aeruginosa (Glew et al, 1993) 15 days after their inoculation to a soil microcosm. Moreover, we showed that the amount of Atrazine mineralized after Pseudomonas sp.…”

The ecology of transfer of mobile genetic elements

“…The role of soil animals, including Collembola and earthworms, as providers of hot spots, has been indicated in the above [28,29]. Speci¢cally, work by Thim et al [28], following pioneering work of Daane et al [44], con¢rmed this hot spot for HGT in soil, by providing evidence that earthworms, in particular Lumbricus rubellus, can facilitate the spread of plasmids from Escherichia coli to soil bacteria. Marker-tagged plasmids with di¡erent transfer properties, i.e.…”

Micro-scale determinants of bacterial diversity in soil

“…Nonmotile bacteria must rely on passive dispersal, for example, by passing invertebrates. In a very original study, it was demonstrated that earthworm movement in nonsterile microcosms not only resulted in the dispersal of bacteria, but also promoted horizontal gene transfer (HGT) by bringing cells into contact with each other (Daane et al, 1996). In general, habitats where different populations are brought into close proximity or contact will promote HGT (Sengeløv et al, 2000).…”