Cobalt can fully recover the phenotypes related to zinc deficiency in Salmonella Typhimurium

Cobalt is an essential element for living systems, which, however, make very limited use of this metal, using it mainly in cobalamin-containing enzymes. The reduced use of cobalt compared to other transition metals is generally attributed to the potential toxicity of this element. In this work, we demonstrate that cobalt not only does not have an obvious toxic effect on Salmonella Typhimurium, but that it can efficiently compensate for zinc deficiency in a znuABC deleted strain. In fact, cobalt, but not cobalamin supplementation, rescued all major phenotypic defects of the znuABC strain, including the reduced ability to grow and swim in zinc-deficient media and the high susceptibility to hydrogen peroxide stress. Growth in a cobalt-supplemented defined medium led to the accumulation of large amounts of cobalt both in the wild type and in the znuABC strain. These data suggest that atoms of cobalt may be incorporated in bacterial proteins in place of zinc, ensuring their functionality. In support of this hypothesis we have shown that, in vivo, cobalt can accumulate in ribosomes and replace zinc in a periplasmic Cu,Zn superoxide dismutase (SodCII). Finally, we provide evidence of the ability of cobalt to modulate the intracellular concentration of zinc-regulated proteins (ZnuA, ZinT, and SodCII). Although some observations suggest that in some proteins the replacement of zinc with cobalt can lead to subtle structural changes, the data reported in this study indicate that Salmonella has the ability to use cobalt instead of zinc, without evident harmful effects for cell physiology.