Cadmium resistance (0.1 to 1.0 mM) was studied in four pure and one mixed culture of sulfate-reducing bacteria (SRB). The growth of the bacteria was monitored with respect to carbon source (lactate) oxidation and sulfate reduction in the presence of various concentrations of cadmium chloride. Two strains Desulfovibrio desulfuricans DSM 1926 and Desulfococcus multivorans DSM 2059 showed the highest resistance to cadmium (0.5 mM). Transmission electron microscopy of the two strains showed intracellular and periplasmic accumulation of cadmium. Dot blot DNA hybridization using the probes for the smtAB, cadAC, and cadD genes indicated the presence of similar genetic determinants of heavy metal resistance in the SRB tested. DNA sequencing of the amplified DNA showed strong nucleotide homology in all the SRB strains with the known smtAB genes encoding synechococcal metallothioneins. Protein homology with the known heavy metal-translocating ATPases was also detected in the cloned amplified DNA of Desulfomicrobium norvegicum I1 and Desulfovibrio desulfuricans DSM 1926, suggesting the presence of multiple genetic mechanisms of metal resistance in the two strains.Many genetic systems are known in bacteria for maintaining intracellular homeostasis of essential metal ions and for acquiring resistance against toxic metals (44). Two well-studied genetic mechanisms of metal resistance in bacteria include heavy metal efflux systems (30) and the presence of metal binding proteins (32,39).Many operons of the efflux system are known, for example, the cadA operon (26), in which a P-type ATPase is involved in metal ion transport across the cell membranes (3). The cadA operon is composed of two genes designated cadA and cadC (44). cadA acts as a P-type ATPase, while cadC acts as a regulatory gene of cadA. The cadA operon has been reported to provide cadmium resistance in Bacillus subtilis (48), Staphylococcus aureus (31), Stenotrophomonas maltophilia (1), Pseudomonas putida (28), Listeria monocytogenes (26), and Helicobacter pylori (17). The cadA homolog zntA has been reported in Escherichia coli (5, 38), which acts as a Zn, Cd, and Pb translocating P-type ATPase pump. Another cadA homologue, ziaA, has been shown to confer Zn and Cd tolerance in Synechococcus sp. strain PCC 6803 (47).A plasmid-mediated metal resistance mechanism in Staphylococcus aureus is governed by the cadB operon, with two genes designated cadB and cadX (35). It has been suggested that cadB provides protection by enabling cells to bind cadmium in their cell membranes (35). Chromosomal DNA mediated cadmium resistance gene cadD in Staphylococcus aureus (9) has shown sequence similarity with the cadB-like gene from Staphylococcus lugdunensis (7).Another mechanism of metal detoxification and homeostasis that involves metal-binding proteins is mediated by metallothionein-encoding genes (39). Metallothioneins are small, cysteine-rich proteins (15), synthesized under heavy metal stress conditions that have been found in both prokaryotes (32, 39) and eukaryotes (33). The ...