TRANSFER OF BROAD-HOST-RANGE ANTIBIOTIC-RESISTANCE PLASMIDS IN SOIL MICROCOSMS

Broad-host-range plasmids, belonging to IncP (RP4 and pUPI102) and Inc C (R57.b), were studied for intrageneric and intergeneric gene transfe r in three different soil microcosms. RP4 was transferred intrageneric ally in clay loam, sandy loam, and sandy microcosms at frequencis of 0 .71 x 10(-2), 0.83 x 10(-2), and 0.41 x 10(-2) respectively, optimally at 37 degrees C and at 100% vol/wt moisture content. Under similar co nditions, R57.b was also transferred at frequencies of 0.38 x 10(-2), 0.58 x 10(-2), and 0.80 x 10(-5) respectively at 30 degrees C. Both RP 4 and R57.b were transferred at low frequency at 20 degrees C. Kinetic s of plasmid transfer revealed that 48 h was the optimum time for intr ageneric conjugal gene transfer. Gene transfer frequency was tenfold h igher in all nutrient-amended soil microcosms than in the absence of n utrient amendment. RP4 was transferred to an indigenous soil bacterium Beijerinckia indica in a nonsterile soil microcosm and to other indig enous soil bacteria, viz. Xanthomonas campestris, Azotobacter chroococ cum, Acinetobacter calcoaceticus, Achromobacter agili, and Rhizobium m eliloti in sterile soil microcosms. pUPI102 was transferred fromA. cal coaceticus BD413 to Escherichia coli K12 J53 at a frequency of 0.75 x 10(-6) and 1.1 x 10(-6) in clay loam and sandy loam microcosms respect ively. However, no gene transfer was observed in any soil microcosm wh en strains of A. calcoaceticus BD413 (pUPI102) and E. coli K12 J53.2 ( RP4) were used for conjugal mating. Plasmid RP4 was found to be 100% s table in all the above microorganisms.