Synergistic effect of lactate and Fe²⁺ on cell killing in E. coli and other bacteria

Since we previously found that lactate enhances OH radical generation in the Fenton reaction, the bactericidal effect of lactate modified Fenton reaction was studied on E. coli JM109 by the colony counting method. However, the lactate addition did not significantly enhance the cytotoxicity of pure Fenton system. The colony forming ability of E. coli was seriously damaged by lactate/Fe²⁺ coupled without H₂O₂. The viable cell number decreased with increasing concentrations of lactate in the presence of a fixed amount of Fe²⁺. Likewise, Fe²⁺ cytotoxicity was markedly enhanced in the presence of lactate, although Fe³⁺ was essentially non-toxic up to 1 mM in spite of lactate presence. Since Fe²⁺ chelating agent, deferoxamine inhibited the lethal effect of lactate/Fe²⁺ couple, an increased cellular uptake of Fe²⁺ was expected to be the cause of cytotoxicity induced by the lactate/Fe²⁺ couple. However, the cytotoxicity was not manipulated in the presence of membrane permeable OH radical scavengers, indicating intracellular Fenton reaction mediated by the Fe²⁺ may not be the only cause of cell death. The lethal effect of lactate/Fe²⁺ couple was also observed in other bacterial strains such as Bacillus subtilis ATCC 6633, Pseudomonas aeruginosa ATCC 27853 and Klebsiella pneumoniae ATCC 13883. No relationship was determined between their gram staining natures and the sensitivities toward lactate/Fe²⁺.