| Abstract |
The survival and transfer of chromosomal genes coding for the synthesis of amino acids (threonine, tryptophan, histidine, leucine, methionine) and of plasmid-borne genes coding for resistance to antibiotics (chloramphenicol, kanamycin, erythromycin) by transformation in sterile and nonsterile soil (the soil was amended to 12% vol/vol with the clay mineral, montmorillonite) was studied. In pure culture, the numbers of vegetative cells of the Bacillus subtilis strains decreased by 1 to 1.5 orders of magnitude within one week, but spores of each strain showed lesser decreases. In sterile soil, the populations of vegetative cells and spores decreased by 1.5 to 3 orders of magnitude within 2 to 4 days and then showed little additional decreased. The transformation frequencies (number of transformants/numbers of donors and recipients) of individual amino acid-genes invitro ranged from 1.3 ± 0.6×10^-6 to 6.0 ± 2.3×10^-6, of two amino acid-genes from 8.5± 0.7×10^-8 to 3.1 ± 0.6×10^-7, and of the antibiotic-resistance genes from 1.5 ± 0.2×10^-7 to 1.4 ± 0.4×10^-5. In sterile soil, the frequencies of transfer of individual amino acid-genes ranged from 2.0×10^-7 to 2.0×10^-5 and of the antibiotic-resistance genes from 2.0×10^-7 to 9.4 ± 4.7×10^-6. The transfer of two amino acid-genes in sterile soil was detected at a frequency of 2.0×10^-6 to 4.5×10^-6, but only in three instances. The transformation frequencies of antibiotic-resistance genes in nonsterile soil were essentially similar to those in sterile soil. However, to detect transformants in nonsterile soil, higher concentrations of antibiotics were needed, as the result of the large numbers of indigenous soil bacteria resistant to the concentration of antibiotics used in the sterile soil and in vitro studies. The results of these studies show that genes can be transferred by transformation in soil and that this mechanism of transfer must be considered in risk assessment of the release of genetically engineered microorganisms to the environment. |
