Estimation of the acute cesium toxicity by the microbial assay for risk assessment (MARA) test

The microbial assay for risk assessment (MARA) test was used for acute cesium toxicity evaluation in water solutions. The test contained 11 different microorganisms with a wide spectrum of sensitivity. The resistance of microorganisms to cesium was characterized as follows: microbial toxic concentration (MTC), half maximal inhibitory concentration (IC50), maximal inhibitory concentration (IC100). The sensitivity to cesium was characterized by the lowest observed effect level (LOEL). High levels of sensitivity in the range 3.1-6.3 mM were shown by the following microorganisms: Serratia rubidaea > Pseudomonas aurantiaca, Delftia acidovorans, Citrobacter freundii, Staphylococcus warneri. Lower levels of sensitivity (up to 16 mM) were noted for Comamonas testosteroni, Microbacterium species, Kurthia gibsonii, Pichia anomala, whereas that in the range 24-31 mM for Brevundimonas diminuta > Enterococcus casseliflavus. High resistance to Cs+ was found for E. casseliflavus (MTC 86.9 g/l) > the yeast - P. anomala (MTC 19.3 g/l) > K. gibsoni (MTC 17.4 g/l) > B. diminuta (MTC 13.4 g/l). The phenomenon of resistance of enterococcus and yeast strains was discussed.