Characterization of cadmium uptake in Lactobacillus plantarum and isolation of cadmium and manganese uptake mutants

Abstract

Two different Cd2+ uptake systems were identified in Lactobacillus plantarum. One is a high-affinity, high-velocity Mn2+ uptake system which also takes up Cd2+ and is induced by Mn2+ starvation. The calculated K(m) and V(max) are 0.26 μM and 3.6 μmol g of dry cell-1 min-1, respectively. Unlike Mn2+ uptake, which is facilitated by citrate and related tricarboxylic acids, Cd2+ uptake is weakly inhibited by citrate. Cd2+ and Mn2+ are competitive inhibitors of each other, and the affinity of the system for Cd2+ is higher than that for Mn2+. The other Cd2+ uptake system is expressed in Mn2+-sufficient cells, and no K(m) can be calculated for it because uptake is nonsaturable. Mn2+ does not compete for transport through this system, nor does any other tested cation, i.e., Zn2+, Cu2+, Co2+, Mg2+, Ca2+, Fe2+, or Ni2+. Both systems require energy, since uncouplers completely inhibit their activities. Two Mn2+-dependent L. plantarum mutants were isolated by chemical mutagenesis and ampicillin enrichment. They required more than 5,000 times as much Mn2+ for growth as the parental strain. Mn2+ starvation-induced Cd2+ uptake in both mutants was less than 5% the wild-type rate. The low level of long-term Mn2+ or Cd2+ accumulation by the mutant strains also shows that the mutations eliminate the high-affinity Mn2+ and Cd2+ uptake system.