Chromate resistance, transport and bioreduction by Exiguobacterium sp. ZM-2 isolated from agricultural soil irrigated with tannery effluent.

Bacterial strain Exiguobacterium sp. ZM-2 isolated from agricultural soil irrigated with tannery effluents, was examined for its resistance to hexavalent chromium. Exiguobacterium sp. ZM-2 could resist 12.37 mM of potassium chromate. The isolate was also found resistant to other heavy metal ions. Exiguobacterium sp. ZM-2 was able to reduce 500 microM hexavalent chromium completely within 56 h under in vitro conditions. Chromate reduction was severely affected in presence of metabolic inhibitors, sodium cyanide and sodium azide. No chromate reduction was observed in presence of 1 mM sodium cyanide while only 17% of 250 microM chromate was reduced when medium contained 1 mM sodium azide. A 10 mM sodium sulphate inhibited hexavalent chromium reduction up to 35%. On the other hand, use of 1 mM 2, 4-dinitrophenol, an uncoupling agent, stimulated the chromate reduction, indicating that the respiratory-chain-linked electron transport to Cr (VI) was limited by the rate of dissipation of the proton motive force. Cell free extract of Exiguobacterium sp. ZM-2 readily reduce Cr (VI) to Cr (III). The kinetics of chromate reductase fit well in the linearized Lineweaver-Burk plot and showed a K(m) of 106.1 microM Cr (VI) and V(max) of 1.24 micromol/min per mg of protein.