there are many mechanisms of resistance, as in the multiple-metal-resistant bacterium S. aureus, where Cd2+(and probably Zn2+) effluxis catalysed by the membrane-bound CadA protein and P-typeATPase. CadC protein is required for full resistance and CadR proteinis hypothesized for regulation of the resistance determinant (Nies,1992). The non-toxic metals are an important cofactor for manyenzymes; however, they can show toxic effects at their high con-centrations too. Therefore, bacteria must strictly control the intakeof the metals into the cell for use as a cofactor and, more impor-tantly, must limit free intracellular levels to prevent toxicity (Rouchet al., 1995).