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 cr op plants was determined in sterile and non-sterile soil. Stimulation of ba cterial 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 gen e (nptII). Clear differences in the capability of the organic compounds to induce gene capture were detected. In sterile soil, most of the organic com pounds used stimulated the appearance of transformants. Fewer compounds wer e able to induce transformation in non-sterile soil, and a high-P salt supp lementation was found to be required for detectable gene transfer. The tran sformation frequencies in non-sterile soil were, however, if found, on aver age less than 10-fold reduced compared to those obtained under sterile soil conditions. Of the organic acids and amino acids applied in non-sterile so il, acetate, lactate and alanine gave the highest transformation frequencie s, i.e. 1 to 2 x 10(-6). Of the sugars used, only glucose produced a few tr ansformants in non-sterile soil (2.5 x 10(-7)). Mixtures of the organic and amino acids, or the sugars, and the high-P salts produced the highest tran sformation frequencies in nonsterile soil, of 4.4 to 6.6 x 10(-6). The addi tion of fertilizers had a less pronounced effect on the transformation freq uencies. We conclude that many of the compounds exuded into the rhizosphere of agriculturally grown plants are able to stimulate natural transformatio n of Acinetobacter sp. BD413(pFG4) cells in sterile silt loam soil, and tha t the presence of both carbon sources and inorganic salts is required for d etectable gene transfer in non-sterile silt loam. (C) 2001 Elsevier Science Ltd. All rights reserved.