Mechanism of zinc resistance in a plant growth promoting Pseudomonas fluorescens strain.

Bacterial systems have evolved a number of mechanisms, both active and passive, to manage toxic concentrations of heavy metals in their environment. The present study is aimed at describing the zinc resistance mechanism in a rhizospheric isolate, Pseudomonas fluorescens strain Psd. The strain was able to sustain an external Zn(2+) concentration of up to 5 mM in the medium. The strategy for metal management by the strain was found to be extracellular biosorption with a possible role of exopolysaccharides in metal accumulation. The attainment of equilibrium in biosorption reaction was found to be dependent on initial Zn(2+) concentration, with the reaction reaching equilibrium faster (50 min) at high initial Zn(2+) concentration. Biosorption kinetics of the process was adjusted to pseudo-first order rate equation. With the help of Langmuir and Freundlich adsorption isotherms, it was established that Zn(2+) biosorption by the bacterium is a thermodynamically favourable process.