Plasmid dependent impermeability barrier to cadmium ions in Staphylococcus aureus

The nature of plasmid born resistance of Staphylococcus aureus to cadmium ions has been investigated. Plasmid harbouring strain was significantly more resistant than its plasmid negative variant, both in growth tests and in manometric experiments. Radioisotopic studies have revealed that the resistant organism did not take up 115mCdCl2 at concentrations 10-4M--10-6M, which were also harmless to its respiration. In contrast to this, plasmid negative variant was permeable to all concentrations used (10-3M--10-6M), and the uptake of 115mCd2+ ions always resulted in inhibition of respiration. These findings suggest that the cadmium resistance could be due to the impermeability barrier controlled by plasmid genes. This barrier, however, could be broken through, since at 10-3M some marked uptake of radioactive cadmium by resistant strain was observed, which was accompanied by partial inhibition of respiration. Binding of Cd2+ ions to the sensitive cells was irreversible and temperature dependent (took place at 37 degrees, but did not occur at 2-4 degrees). The bound molecules could not be removed either by extensive washing with water or by cysteine treatment, although cells pretreated with this amino acid were protected against the uptake of 115mCdCl2.