Cellular response of Shewanella oneidensis to strontium stress

The physiology and transcriptome dynamics of the metal ion-reducing bacterium Shewanella oneidensis strain MR-1 in response to nonradioactive strontium (Sr) exposure were investigated. Studies indicated that MR-1 was able to grow aerobically in complex medium in the presence of 180 mM SrCl2, but showed severe growth inhibition at levels above that concentration. Temporal gene expression profiles were generated from aerobically grown, mid-exponential-phase MR-1 cells shocked with 180 mM SrCl2, and analyzed for significant differences in mRNA abundance with reference to data for nonstressed MR-1 cells. Genes with annotated functions in siderophore biosynthesis and iron transport were among the most highly induced (> 100-fold [P < 0.05]) open reading frames in response to acute Sr stress, and a mutant (SO3032::pKNOCK) defective in siderophore production was found to be hypersensitive to SrCl2 exposure, compared to parental and wild-type strains. Transcripts encoding multidrug and heavy metal efflux pumps, proteins involved in osmotic adaptation, sulfate ABC transporters, and assimilative sulfur metabolism enzymes also were differentially expressed following Sr exposure but at levels that were several orders of magnitude lower than those for iron transport genes. Precipitate formation was observed during aerobic growth of MR-1 in broth cultures amended with 50, 100, or 150 mM SrCl2, but not in cultures of the SO3032::pKNOCK mutant or in the abiotic control. Chemical analysis of this precipitate using laser-induced breakdown spectroscopy and static secondary ion mass spectrometry indicated extracellular solid-phase sequestration of Sr, with at least a portion of the heavy metal associated with carbonate phases.