Physiological importance and role of Mg²⁺ in improving bacterial resistance to cesium

Cesium (Cs) is an alkali metal with radioactive isotopes such as ¹³⁷Cs and ¹³⁴Cs. ¹³⁷Cs, a product of uranium fission, has garnered attention as a radioactive contaminant. Radioactive contamination remediation using microorganisms has been the focus of numerous studies. We investigated the mechanism underlying Cs⁺ resistance in Microbacterium sp. TS-1 and other representative microorganisms, including Bacillus subtilis. The addition of Mg²⁺ effectively improved the Cs⁺ resistance of these microorganisms. When exposed to high concentrations of Cs⁺, the ribosomes of Cs⁺-sensitive mutants of TS-1 collapsed. Growth inhibition of B. subtilis in a high-concentration Cs⁺ environment was because of a drastic decrease in the intracellular potassium ion concentration and not the destabilization of the ribosomal complex. This is the first study demonstrating that the toxic effect of Cs⁺ on bacterial cells differs based on the presence of a Cs⁺ efflux mechanism. These results will aid in utilizing high-concentration Cs⁺-resistant microorganisms for radioactive contamination remediation in the future.