Hexavalent chromium (Cr(VI)) is a toxic environmental contaminant which detoxification consists in reduction to Cr(III). In this work, the Cr(VI)-resistant and reducing Ochrobactrum tritici 5bvl1 produced phosphate nanoparticles upon exposure to Cr(VI) and Fe(III), effectively removing chromium from solution. Under Cr(VI) stress, higher siderophore production by strain 5bvl1 was observed. Cr(VI) toxicity was decreased in presence of Fe(III), increasing the growth and Cr(VI)-reduction rates in cell cultures, lowering the amount of morphologically compromised cells and promoting chromium immobilization as insoluble extracellular phosphate complexes. The formation of phosphate nanoparticles increased with Cr(VI) and Fe(III) concentrations and was also stimulated by Ni(II). Under these experimental conditions, nanoparticle formation occurred together with enhanced inorganic phosphate consumption by cells and increased polyphosphate kinase (PPK) activity. NMR analysis of the particles showed the presence of both polyphosphate and phosphonate together with orthophosphate, and FT-IR supported these results. also showing evidences of Cr(III) coordination. This work demonstrated that O. tritici 5bvl1 possesses protection mechanisms against chromium toxicity other than the presence of the Cr(VI) pump and SOD related enzymes previously described. Future assessment of the molecular regulation of production of these nanoparticles will open new perspectives for remediation of metal contaminated environments. (C) 2011 Elsevier B.V. All rights reserved.