Molecular mechanisms of accommodation in Escherichia coli to toxic levels of Cd2+

Abstract

Cells of E. coli strain B develop large intracellular vacuoles and exhibit an abnormally long lag phase when inoculated into a defined medium to glucose and salts containing 3 X 10-6 M Cd2+. Early in this lag, about 95% of the cells fail to form colonies when plated on nutrient broth NaCl agar. Prior to the initiation of proliferation, the morphology of these cells becomes normal. They regain viability in the absence of deoxyribonucleic acid replication. The rate and extent of growth are normal once proliferation begins. This reversible phenomenon of accommodation to a growth inhibiting concentration of Cd2+ does not appear to result from a selection of mutant cells. Cells which are proliferating in the presence of Cd2+ accumulate the ion to a very high concentration. In accommodated cells, 56% of the Cd2+ is associated with the cell wall, 13% in the membranes and 31% in the cytoplasm. In unaccommodated cells, the figures are 2%, 75%, and 23%, respectively. The activity of alkaline phosphatase, a zinc metalloenzyme which is inhibited by cadmium and is located between cell wall and membrane, is not abnormally low in accommodated cells, suggesting that the cadmium is compartmentalized in these cells. Molecular sieve chromatography of cell extracts shows that the Cd2+ is associated with two classes of macromolecules. It appears that accommodation of E. coli to the presence of Cd2+ involves exclusion of the ion from the cell and reversal of damage caused by prior exposure to the ion.