Difference in toxicity of Pd (II) and mechanism of action before and after reduction by Bacillus wiedmannii MSM

Abstract

At present, there are many studies on microbial reduction of Pd (II), although few of these studies assess the bacterial toxicity of Pd (II) before and after reduction by microorganisms. In this study, the toxicity of Pd (II) to Bacillus wiedmannii MSM (B. wiedmannii MSM) was assessed before and after reduction by live B. wiedmannii MSM cells (referred to as "Pd (0)-loaded cells") and after biosorption by dead B. wiedmannii MSM cells (referred to as "Pd (II)-loaded cells"). B. wiedmannii MSM is a widely occurring, nonpathogenic aerobic bacterium. Compared with Pd (II), the EC₂₀, EC₅₀, and EC₈₀ of Pd (0)-loaded cells increased by 77.73-, 112.75-, and 163.91-fold, respectively, while the EC₂₀, EC₅₀, and EC₈₀ of Pd (II)-loaded cells increased by only 11.55-, 9.77-, and 8.29-fold, respectively. The sterilization contribution rates of the mechanisms of action of Pd (0)-loaded cells were ranked in the order of: remaining Pd (II) > oxidative stress > physical puncture. Pd (II) was found to increase cell membrane permeability, glutathione S-transferase (GST) enzyme activity, and reactive oxygen species levels in cells and decrease the cell membrane potential. XPS results indicated that Pd (II) increased the content of polysaccharides and peptides and decreased the content of hydrocarbons in cells. These findings reveal the bactericidal mechanism of toxicity of Pd (II) and Pd (0)-loaded cells on Bacillus wiedmannii MSM and provide an environmentally friendly and inexpensive method for Pd (II) detoxification.

Keywords: Bacillus wiedmannii MSM; Bioreduction; Detoxification; Palladium; Toxicity mechanism.