Toxicity and accumulation of thallium in bacteria and yeast

Thallium sulphate inhibited microbial growth, with Bacillus megaterium KM more sensitive to the metal than Saccharomyces cerevisiae and Escherichia coli. Inhibition of B. megaterium KM and S. cerevisiae, but not of E. coli, was alleviated by increasing the potassium concentration of the medium; inhibition of respiration of S. cerevisiae, but not of E. coli, was similarly alleviated. Thallium was rapidly bound, presumably to cell surfaces, by S. cerevisiae and E. coli, and was progressively accumulated by energy-dependent transport systems (probably concerned primarily with potassium uptake) with both organisms. Thallium uptake kinetics suggested more than one transport system operated in yeast, possibly reflecting a multiplicity of potassium transport systems. Apparent Km and Ki values for competitive inhibition of thallium uptake by potassium indicated S. cerevisiae to have a higher affinity for thallium uptake than for potassium, while E. coli had a transport system with a higher affinity for potassium than for thallium. The likely systems for thallium transport are discussed. A mutant of E. coli with tenfold decreased sensitivity to thallium was isolated and apparently effected surface binding of thallium in amounts equivalent to the wild type organism, but showed no subsequent uptake and accumulation of the metal from buffer, even though it was able to accumulate potassium to normal intracellular concentrations during growth.