TRANSPORT OF A GENETICALLY-MODIFIED PSEUDOMONAS-FLUORESCENS AND ITS PARENT STRAIN THROUGH UNDISTURBED TROPICAL SOIL CORES

The transport of a genetically modified strain of Pseudonronas fluores cens, BR12, and its parent, BR5, was studied after irrigation of undis turbed clayey and sandy soil cores, simulating heavy tropical rainfall (56.6 mm/h). Vertical transport of both inoculant strains was detecte d in all soil cores. Transport was negatively affected by the organic matter content of soil, whereas soil texture did not affect the degree of transport. Both factors influenced the survival of the introduced bacteria, in particular that of the modified strain. The initial moist ure and structural status of the clayey soil, which was related to sea sonal influences, also influenced the degree of bacterial translocatio n. In cores with two clay soil types, the genetically modified strain was transported to a greater extent than its parent. However, the modi fied strain showed similar or lower survival rates than its parent in topsoil layers of the different clay soil type and the sandy soil core s. Counts of culturable cells of both inoculant strains in the percola tion water remained roughly stable or decreased during the experiment, and the inoculant survival was clearly affected by the soil from whic h the percolation water was obtained. Both strains showed good surviva l in the percolation water from microcosms-containing clay soil enrich ed with manure to about 6% organic matter, whereas their population si zes declined in percolation water from non-manured clay or sandy soil cores. The results suggested that the release of derivatives of P. flu orescens strain BR5 into soils in tropical climates should be carefull y planned and monitored in order to guarantee their efficacy as biocon trol agents and to assess the potential contamination of ground and su rface waters. (C) 1997 Elsevier Science B.V.