Isolation and characterization of a highly effective bacterium Bacillus cereus b-525k for hexavalent chromium detoxification

The chromate resistant Gram-positive Bacillus cereus strain b-525k was isolated from tannery effluents, demonstrating optimal propagation at 37 °C and pH 8. The minimum inhibitory concentration (MIC) test showed that B. cereus b-525k can tolerate up to 32 mM Cr(6+), and also exhibit the ability to resist other toxic metal ions including Pb(2+) (23 mM), As(3+) (21 mM), Zn(2+) (17 mM), Cd(2+) (5 mM), Cu(2+) (2 mM), and Ni(2+) (3 mM) with the resistance order as Cr (6+) > Pb(2+) > As(3+) >Zn(2+) >Cd(2+) >Ni(2+) >Cu(2+). B. cereus b-525k showed maximum biosorption efficiency (q) of 51 mM Cr(6+)/g after 6 days. Chromate stress elicited pronounced production of antioxidant enzymes such as catalase (CAT) 191%, glutathione transferase (GST) 192%, superoxide dismutase (SOD) 161%, peroxidase (POX) 199%, and ascorbate peroxidase (APOX) (154%). Within B. cereus b-525k, the influence of Cr(6+) stress (2 mM) did stimulate rise in levels of GSH (907%) and non-protein thiols (541%) was measured as compared to the control (without any Cr(6+) stress) which markedly nullifies Cr(6+) generated oxidative stress. The pilot scale experiments utilizing original tannery effluent showed that B. cereus b-525k could remove 99% Cr(6+) in 6 days, thus, it could be a potential candidate to reclaim the chromate contaminated sites.