Natural genetic transformation of Pseudomonas stutzeri in a non-sterile soil

Natural transformation of the soil bacterium Pseudomonas stutzeri JM300 in a non-sterile brown earth microcosm was studied. For this purpose, the microcosm was loaded with purified DNA (plasmid or chromosomal DNA, both containing a high-frequency-transformation marker, his+, of the P. stutzeri genome), the non-adsorbed DNA was washed out with soil extract and then the soil was charged with competent cells (his-1). Both chromosomal and plasmid transformants were found among the P. stutzeri cells recovered from the soil. The number of plasmid transformants increased in a linear fashion with the amount of DNA added [10-600 ng (0.7 g soil)-1]. The observed efficiency of transformation, the time course of transformant formation and the complete inhibition of transformation by DNase I, when added to the soil, were similar to that seen in optimized transformations in nutrient broth. Addition of cells as late as 3 d after loading the soil with plasmid DNA still yielded 3% of the initial transforming activity. This suggests that nucleases indigenous to the soil destroyed the transforming DNA, but at a rate allowing considerable DNA persistence. Transformants were also obtained when intact P. stutzeri cells were introduced into the soil to serve as plasmid DNA donors. Apparently, DNA was released from the cells, adsorbed to the soil material and subsequently taken up by recipient cells. The results indicate that competent cells of P. stutzeri were able to find access to and take up DNA bound on soil particles in the presence of micro-organisms and DNases indigenous to the soil.