Agglutination of pseudomonad cells by plant products

Effect of complexing ligands on the surface adsorption, internalization, and bioresponse of copper and cadmium in a soil bacterium, Pseudomonas putida

Environmental quality criteria for metals toxic to soil and water organisms, using the free ion activity model or the biotic ligand model, are based on the concept that the major form of the metal available to the organism is the free metal ion, yet various metal complexes are bioavailable to a variety of soil and water organisms. We test here whether neutral copper or cadmium sulfates, negatively-charged copper or cadmium citrates and positively-charged copper acetate and cadmium chloride are bioavailable to a soil bacterium, Pseudomonas putida. Adsorption onto the cell surface and uptake into the periplasm and cytoplasm of this Gram-negative root colonizing bacterium was studied by adding a single concentration of Cu or Cd and varying the concentration of the ligands to complex 10-100% of the metal. Metal association from the complexes on and within the cell was defined using selective extraction procedures and compared with free ion controls using the Langmuir isotherm. Cellular responses also were assessed using a P. putida biosensor. Both uptake and bioresponse methodologies showed that P. putida was sensitive to the metal complexes. In particular, the bioresponse to Cu and Cd supplied as a citrate complex occurred with activities of free metal ions two orders of magnitude lower than for the control. We concluded that the tested metal complexes for Cu and Cd are taken up into the cell, where they trigger a bioresponse. We also discuss the implications of these findings on interactions between soil and water organisms and nanoparticles that release metal ions.

Defining the surface adsorption and internalization of copper and cadmium in a soil bacterium, Pseudomonas putida

The distribution of cadmium (Cd) and copper (Cu) ions onto and within two soil pseudomonads, Pseudomonas putida strains KT2440 and Corvallis, was investigated using selective extraction procedures and modeled using Langmuir isotherms. Cadmium and Cu associated differently with the surface, periplasm and cytoplasm of the two strains. Both of these pseudomonad cells allowed more Cu to pass into the periplasmic space and to the cytoplasm than Cd. The distribution of Cu among the cellular spaces was solution concentration dependent, with limited amounts of Cu entering the cell at higher exposure concentrations. The Langmuir isotherm with a single binding site fit well to the observed data for Cu cell association. Cadmium was mainly found on the surface of the cells. The capacity of surface exchange sites for Cd increased with solution concentration, possibly indicating a modification of surface functional groups with ion concentration. This surface sorption behavior of Cd was best described using a two-site Langmuir model, whereas all other Cu and Cd associations were described using a one-site model. Although potentiometric titration identified differences in site densities for proton binding to the two strains, these differences were not consistently displayed with Cu and Cd surface interactions.

Fusarium Wilt Suppression and Agglutinability of Pseudomonas putida

Mutants of Pseudomonas putida (Agg − ) that lack the ability to agglutinate with components present in washes of bean and cucumber roots showed limited potential to protect cucumber plants against Fusarium oxysporum f. sp. cucumerinum . However, a higher level of protection was observed against Fusarium wilt in cucumber plants coinoculated with the parental bacterium (Agg + ), which was agglutinable. The Agg − mutants did not colonize the roots of cucumber plants as extensively as the Agg + parental isolate did. In competition experiments involving bean roots inoculated with a mixture of Agg + and Agg − bacteria, the Agg + strains colonized roots to a greater extent than the Agg − cells did. These data suggest that the Agg + phenotype provides additional interactions that aid in the beneficial character of P. putida.