Strontium-induced genomic responses of Cupriavidus metallidurans and strontium bioprecipitation as strontium carbonate

The physiological and global transcriptomic response of the metal-resistant bacterium Cupriavidus metallidurans CH34 to stable non-radioactive strontium (Sr) was investigated. C. metallidurans CH34 was able to survive and proliferate in the presence of relatively high concentrations of SrCl2 (concentration of Sr needed to inactivate 90 % of the cells = 70 mM; minimum inhibitory concentration = 120 mM). Gene expression in cells was analyzed after short-term (30 min) exposure to low (5 mM) and high (60 mM) concentrations of SrCl2. The transcription of the gene clusters annotated as hmyFCBA and czcCBADRS, coding for ion efflux pumps, was significantly induced following exposure to Sr relative to that in non-stressed CH34 cells. Sr precipitation was observed during aerobic growth of CH34 in the presence of 60 mM SrCl2. Transmission electron microscopy images of cells grown with Sr also showed very small solid-phase precipitates located in the vicinity of cells, but mainly extracellular. Environmental scanning electron microscopy with energy-dispersive X-ray spectrometry indicated that the sequestered extracellular solid-phase Sr was present in the form of strontium carbonate.