The role of exopolymers in protection of Ralstonia sp., a cadmium-resistant bacterium, from cadmium toxicity

Abstract

Production of exopolymers is one of heavy metal resistance mechanisms in bacteria. Ralstonia sp. TAK1, a cadmiumresistant bacterium, was isolated from a high cadmium (Cd) contaminated soil at the zinc mine, Tak province, Thailand. The bacterium was cultivated in LB broth and its growth was monitored. The yields of exopolymers were measured by the phenol-sulfuric method at different growth phases. The levels of Cd resistance were quantitatively determined by survival cell assay. The highest amount of exopolymers (0.69 mg glucose equivalent/ mg dry weight) was found at the stationary phase and sharply decreased at the late-stationary phase. In addition to high production of exopolymers at the stationary phase, Ralstonia sp. TAK1 was more resistant to Cd than that of exponential phase cells. These results suggested that the resistance to Cd toxicity in Ralstonia sp. TAK1 at the stationary phase is mediated by exopolymer production. Contradictorily, there was no correlation between Cd resistance level and exopolymer production of cells at exponential phase indicating that other mechanism(s) is responsible for Cd resistance of exponential phase cells. In addition, 0.4 mM CdCl2 was able to induce the increasing of exopolymers at the mid-exponential phases compared to uninduced cells. Exopolymer production of Cd-induced cells was constant from the mid-stationary to late-stationary phase. However, the highest exopolymers was found in uninduced cells at the stationary phase.