Stimulatory effects of compounds present in the rhizosphere on natural transformation of Acinetobacter sp. BD413 in soil

The stimulation of natural transformation of Acinetobacter sp. BD413(pFG4) cells by various organic compounds naturally found in the rhizosphere of crop plants was determined in sterile and non-sterile soil. Stimulation of bacterial competence by the organic acids, amino acids, sugars and inorganic fertilizers, was monitored by the transformation frequencies obtained with added cell lysates of Acinetobacter sp. carrying a kanamycin resistance gene (nptII). Clear differences in the capability of the organic compounds to induce gene capture were detected. In sterile soil, most of the organic compounds used stimulated the appearance of transformants. Fewer compounds were able to induce transformation in non-sterile soil, and a high-P salt supplementation was found to be required for detectable gene transfer. The transformation frequencies in non-sterile soil were, however, if found, on average less than 10-fold reduced compared to those obtained under sterile soil conditions. Of the organic acids and amino acids applied in non-sterile soil, acetate, lactate and alanine gave the highest transformation frequencies, i.e. 1 to 2×10−6. Of the sugars used, only glucose produced a few transformants in non-sterile soil (2.5×10−7). Mixtures of the organic and amino acids, or the sugars, and the high-P salts produced the highest transformation frequencies in non-sterile soil, of 4.4 to 6.6×10−6. The addition of fertilizers had a less pronounced effect on the transformation frequencies. We conclude that many of the compounds exuded into the rhizosphere of agriculturally grown plants are able to stimulate natural transformation of Acinetobacter sp. BD413(pFG4) cells in sterile silt loam soil, and that the presence of both carbon sources and inorganic salts is required for detectable gene transfer in non-sterile silt loam.