Impact of cyanogen iodide in killing of Escherichia coli by the lactoperoxidase-hydrogen peroxide-(pseudo)halide system

In the presence of hydrogen peroxide, the heme protein lactoperoxidase is able to oxidize thiocyanate and iodide to hypothiocyanite, reactive iodine species, and the inter(pseudo)halogen cyanogen iodide. The killing efficiency of these oxidants and of the lactoperoxidase-H₂O₂-SCN^?/I^? system was investigated on the bioluminescent Escherichia coli K12 strain that allows time-resolved determination of cell viability. Among the tested oxidants, cyanogen iodide was most efficient in killing E. coli, followed by reactive iodine species and hypothiocyanite. Thereby, the killing activity of the LPO-H₂O₂-SCN^?/I^? system was greatly enhanced in comparison to the sole application of iodide when I^? was applied in two- to twenty-fold excess over SCN^?. Further evidence for the contribution of cyanogen iodide in killing of E. coli was obtained by applying methionine. This amino acid disturbed the killing of E. coli mediated by reactive iodine species (partial inhibition) and cyanogen iodide (total inhibition), but not by hypothiocyanite. Changes in luminescence of E. coli cells correlate with measurements of colony forming units after incubation of cells with the LPO-H₂O₂-SCN^?/I^? system or with cyanogen iodide. Taken together, these results are important for the future optimization of the use of lactoperoxidase in biotechnological applications.