Transcriptomics and microscopic study of Stenotrophomonas bentonitica BII-R7 in response to uranium

Human activities have increased the amount of uranium (U) in the environment, implying serious environmental and health issues. The use of highly U tolerant microorganisms in the bioremediation of U pollution has been extensively described. However, a lack of knowledge on molecular U resistance mechanisms has become a challenge for the accurate use of these technologies. We report on the transcriptomic and microscopic response of Stenotrophomonas bentonitica BII-R7 to U exposure during lag and exponential growth phases at two concentrations (100 and 250 ?M). Results show that 100 ?M of U exposure displayed up-regulation of 185 and 148 genes during the lag and exponential phases, respectively, whereas 143 and 194 were down-regulated, out of 3786 genes (>1.5-fold change). Cells exposed to 250 ?M of U showed up-regulation of 68 genes and down-regulation of 290 genes during the lag phase of growth. Genes involved in the cell cycle and energy metabolism were down-regulated, especially at higher U concentrations and in the lag phase, while genes involved in cell wall and membrane protein synthesis, efflux systems and acid and alkaline phosphatases were up-regulated under all conditions tested. Microscopic observations evidenced the formation of U-phosphate minerals at membrane and extracellular levels. According to these results, a biphasic process is likely to mediate U tolerance: the increased cell wall size would promote the biosorption of U to the cell surface and its subsequent precipitation as U-phosphate minerals enhanced by the activity of acid and alkaline phosphatases. Furthermore, transport systems and membrane proteins would act preventing U accumulation in the cytoplasm, reducing the U toxicity. These findings help explain the mechanisms through which microbes cope with U toxicity, thus promoting the development of efficient bioremediation strategies by involving highly U tolerant bacterial strains.