Survival of Pseudomonas putida KT2440 in soil and in the rhizosphere of plants under greenhouse and environmental conditions

Pseudomonas putida KT2440 is a root colonizer of potential interest for the rhizoremediation of pollutants and the biological control of pests. The short- and long-term survival of this strain, as well as the possible effects of its introduction on different populations of indigenous soil bacteria, were tested in soil under greenhouse and field conditions. The greenhouse studies showed that inoculated P. putida KT2440 was able to establish itself after 3 d in nonvegetated soils at a density of 8±2×103 CFU g−1 soil. The introduction of this strain had no significant effect on the number of several soil bacteria including those that were resistant to tetracycline; those that utilized p-hydroxyphenylacetic acid as the sole C-source, and total fluorescent pseudomonads. In four independent field assays in nonplanted soils, the numbers of P. putida KT2440 decreased during 50 d from an initial density of 1×106 CFU g−1 soil to approximately 2±1×102 CFU g−1 soil. Thereafter, the number of cells was below detection limits (i.e. <102 CFU g−1 soil), although they were still present because they could be recovered using selective enrichment from the soil for up to 200 d after the beginning of the experiment. This suggested that P. putida was maintained at a low cell density long after inoculation. In contrast, when P. putida KT2440 was introduced in the soil as a coating of corn (Zea mays) or broad bean (Vicia faba) seeds, the bacteria established at high cell densities in the rhizosphere (104–105 CFU g−1 soil in corn; 106–107 CFU g−1 soil in broad beans) during the growth of the crops over 12 to 16 weeks. The numbers of P. putida in the bulk soil after 2 weeks were 1 to 2 orders of magnitude below those in the rhizosphere. During the field assays, the population of p-hydroxyphenylacetic acid users was also monitored in the rhizosphere and the bulk soil. No significant seasonal variations were found.