| Heavy metal cadmium(Cd) has been released into the environment from both natural and anthropogenic sources which contaminated foods and agricultural products. Oral ingestion of Cd would lead to adverse health effects in organs and body systems. A novel research trend for decontamination of Cd from food and human body has been introduced by using GRAS lactic acid bacteria(LAB). The aim of this work was to evaluate the applicability of LAB as a tool for Cd removal by analyzing their characteristics and mechanism.A total of 104 strains of Cd-resistant bacteria were isolated from soil and human feces. Among them, 11 bacterial strains showed high resistant to Cd(MIC≥4.0 g/L). 7 LAB strains were then selected by using MIC test and 16 S r RNA sequencing. Moreover, 4 out of 17 Lactobacillus standard strains stored in our lab were also selected by MIC test(MIC≥4.0 g/L).The above selected bacteria were assessed in vitro for their ability to remove Cd. The removal efficiency of these 11 LAB strains were among 21.0% and 31.4% in 100 mg/L Cd2+ solution. The effects of p H, time, temperature, initial concentration of Cd2+ ion, concentration of bacterial strains and pre-treatment of biomass on removal ability were investigated using 4 strains, MT 2-4(Enterococcus faecium), MT 3-5(E. faecium), RW 2-4(E. faecalis) and LAB-5(Lactobacillus rhamnosus). The results showed: increased initial concentration of Cd2+ would increase removal capacity and decrease removal efficiency; increased bacterial concentration would decrease removal capacity and increase efficiency; extended time would increase both removal capacity and efficiency; p H 7.0 was the optimal p H for strain MT 2-4, MT 3-5 and RW 2-4, p H 4.0~6.0 were the optimal p H for strain LAB-5; temperature has little influence on Cd removal; pre-treatment of biomass increased Cd removal. In all, the optimal conditions for Cd removal were as following: 100 mg/L initial Cd2+ concentration, 5 g/L bacterial concentration, p H 6.0~7.0, 37 °C and 60~120 min.To clarify the removal mechanism of RW 2-4, MT 3-5 and LAB-5, fourier transform infrared spectroscopy(FTIR), scanning electron microscopy(SEM) and transmission electron microscopy(TEM) were used to structurally and morphologically characterize the interaction of bacteria and Cd. LAB-54, a Cd-sensitive strain, was used as control. Results indicated that hydroxyl(O-H), carboxyl(C=O), phosphoryl(P=O), amide(N-H) and hydrocarbonyl(C-H) groups were involved in biosorption process. Cd removal mechanisms of LAB might include complexation, ion exchange, physical adsorption(electrostatic interaction), precipitation and diffusion. Cell microstructure was heavily damaged by treatment with Cd. Distorted, sunken, rough, crumpled, cracked and perforated cells were observed.Two strains, LAB-5 and LAB-54(as a control) were acclimated to high concentrations of Cd by successive inoculation with increasing Cd. Growth curve measurement during acclimation revealed the toxicity of Cd to bacteria. As Cd concentration was increased successively, time for bacteria to adapt to the new environment was prolonged. It was inferred that LAB activated its defense system and lower its metabolic rate under Cd stress. PCR and pulsed field gel electrophoresis(PFGE) were used to analyze the genomic stability of wild and adapted bacterial strains. A Cd-resistant gene, cad A was found in LAB-5 but not in LAB-54. No mutation was detected in cad A sequencing after acclimation. Furthermore, LAB-5 and LAB-54 owned their exclusive and unchanged PFGE fingerprints after acclimation, which proved the stability and safety of acclimation.In summary, LAB strains used in this study were isolated from gastrointestinal tract of human body, therefore their safety are proved for human consumption. This study provides preliminary theoretical foundation to develop these LAB strains as Cd biosorbents and detoxicants. |
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