Background: The manganese pollution is very serious surrounding the mine area, which could be enriched and harmful to animal, plant and human. Manganese oxidation bacteria (MOB) can completely remove the toxicity of Mn(II) with diverse mechanisms.
Results: To seek a resource and disclose the oxidation mechanism of MOB, we isolated the Bacillus safensis strain ST7 from the soil of Songtao manganese mine in Guizhou province, China. Strain ST7 could survive in media containing 2200 mg/L Mn(II) with the Mn(II) removal efficiency of 82% after seven days cultivation. The rate was 7.75 μmol/L of Mn(II) each day detected by LBB method. The manganese oxides appeared after stationary growth phase and lots of irregular precipitates were observed on the surface of bacteria by scanning electron microscopy (SEM). We further constructed eight cDNA libraries at two growth stages of strain ST7, at which the first stage is the mid-exponential growth phase (stage1) and the second one at the onset of stationary phase (stage2). The gene expression patterns were analyzed across the entire transcriptome under 250 mg/L Mn(II) stress by using Illumina Hiseq platform. After mapping to the reference B. safensis genome, we detected 3574 expressed genes from the eight libraries. At the first stage, 1040 differently expressed genes (DEGs) were determined with 502 genes up-regulated in Mn(II) dealt group. For the second stage, 760 genes were increased and 702 genes down-regulated under Mn(II) stress. Of those, the expressed trend of seventeen random selective genes were confirmed by RT-qPCR method. Only nine high expressed DEGs were screened out and all of them were up-regulated in the manganese dealt group at stage1. The great changes at stage 1 were focused on the genes related with siderophore synthesis to help Mn(II) uptake and oxidation and gene cheA to elevate the chemotaxis and the motility of bacteria. It was observed that the motility of strain ST7 was much active in the media with Mn(II) supply. And the expression level of gene601, coded for a multicopper oxidase (MCO) enzyme-like protein, raised about 3.66 times than its control group at stage 1. By using homologous recombination technology, it was demonstrated that the Mn(II) oxidase ability decreased obviously when the gene601 of B. safensis strain ST7 was knocked out. For stage 2 of strain ST7 dealt with Mn(II), there were nineteen genes related with sporulation and most of flagellum genes were inhibited. However, lots of transporters genes were augmented to function as pumps to extrude manganese outside of the bacterium cell.
Conclusions: In a brief, the isolated B. sanfensis took two strategies against Mn stress including manganese oxidation at exponential growth stage and transformation of Mn(II) at stationary phase. The strain could be used to treat the environmental manganese pollution to minimize the use of chemical oxidants as a cost-effective technology.