The metabolism and morphology mutation response of probiotic Bacillus coagulans for lead stress

Lead is among the most common toxic heavy metals and its contamination is of great public concern. Bacillus coagulans is the probiotic which can be considered as the lead absorption sorbent to apply in the lead contaminant water directly or indirectly. A better understanding of the lead resistance and tolerance mechanisms of B. coagulans would help further its development and utilization. Wild-type Bacillus coagulans strain R11 isolated from a lead mine, was acclimated to lead-containing culture media over 85 passages, producing two lead-adapted strains, and the two strains shown higher lead intracellular accumulation ability (38.56-fold and 19.36-fold) and reducing ability (6.94-fold and 7.44-fold) than that of wild type. Whole genome sequencing, genome resequencing, and comparative transcriptomics identified lead resistance and tolerance process significantly involved in these genes which regulated glutathione and sulfur metabolism, flagellar formation and metal ion transport pathways in the lead-adapted strains, elucidating the relationships among the mechanisms regulating lead deposition, deoxidation, and motility and the evolved tolerance to lead. In addition, the B. coagulans mutants tended to form flagellar and chemotaxis systems to avoid lead ions rather than export it, suggesting a new resistance strategy. Based on the present results, the optimum lead concentration in environment should be considered when employed B. coagulans as the lead sorbent, due to the bacteria growth ability decreased in high lead concentration and physiology morphology changed could reduce the lead removal effectiveness. The identified deoxidization and compound secretion genes and pathways in B. coagulans R11 also are potential genetic engineering candidates for synthesizing glutathione, cysteine, methionine, and selenocompounds.