Understanding Cadmium Toxicity In Bacteria

Cadmium enters the environment mainly through industrial sources, including burning fossil fuels and recycling electronic waste. Despite cadmium’s accumulation, scientists don’t fully understand the molecular mechanisms behind its toxicity in cells, especially how it leads to oxidative stress. Now researchers in Australia explain that, in a model bacterial organism, the metal causes problems by depleting the cell’s internal stores of manganese and zinc (Nat. Commun. 2015, DOI: 10.1038/ncomms7418). Christopher A. McDevitt of the University of Adelaide and colleagues found that Cd²⁺ sneaks into Streptococcus pneumoniae by competing with Mn²⁺ for a certain transporter protein complex. When Cd²⁺ gets inside, the cell’s metal-sensing machinery mistakes it for Zn²⁺ and triggers the expression of proteins that export Zn²⁺. The team also determined that Cd²⁺ doesn’t directly cause oxidative stress. Instead, the resulting low Mn²⁺ levels lead to decreased expression of an enzyme that removes superoxide radicals. Without this enzyme, the cells become sensitive to reactive oxygen species. Replenishing Mn²⁺ in the cells, even when Cd²⁺ concentrations are high, restores the enzyme’s expression and the cell’s ability to withstand oxidative stress.