Summary: Very low concentrations of CuSO induced a massive leakage of ü from ATCC 12271 cells suspended in distilled water, Ca/PIPES or Na/PIPES. Cell suspensions in distilled water, Ca/ or Mg/PIPES, treated with Cu, showed appreciable increases in optical density, whereas suspensions in Na/PIPES were unaffected. The addition of monovalent cations to suspensions in distilled water, Ca/ or Mg/PIPES prevented the optical density increase induced by Cu, whereas the addition of Ca or Mg to suspensions in distilled water did not have this effect. Cells suspended in Na/PIPES and treated with Cu showed no major ultrastructural alterations, but cells treated with Cu in distilled water showed pronounced plasmolysis. At all Cu concentrations, two types of cells were observed, normal and heavily plasmolysed. An increase in Cu concentration resulted in an increase in the percentage, but not in the degree of plasmolysis, of the plasmolysed cells. Cells suspended in distilled water or Na/PIPES bound significant amounts of copper. Cu concentrations that induced leakage of most of the unbound K did not saturate the copper-binding sites in the cells. These results indicated that plasmolysis is a direct consequence of the massive K leakage from the cells, in agreement with the notion that K is the main osmolyte in bacteria grown under normal conditions. Monovalent (but not divalent) cations prevented plasmolysis induced by copper by entry into the cells after the release of K ions. Na and Li probably replaced resident K ions in the neutralization of negative charges of cytoplasmic constituents. K efflux and plasmolysis, induced by Cu, appeared to be essentially “all-or-nothing” effects.


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