[Molecular effects of a microbicidal substance on relevant microorganisms: electron microscopic and biochemical...

The microbicidal activity of the broad spectrum antimicrobial agent povidone-iodine is due to the strong oxidizing effects of free iodine on functional groups of amino acids, nucleotides and double bonds of unsaturated fatty acids. While the chemical mechanism of action of PVP-iodine is well understood, the actual sequence of events on the cellular and molecular level that causes rapid cell death has not been fully understood. The aim of this study was to elucidate effects of povidone-iodine on cell ultrastructure by electron microscopy and to monitor changes in enzyme activity and nucleotide efflux. Staphylococcus aureus, E. coli and C. albicans, medically relevant gram-positive, gram-negative and yeast micro-organisms, served as models. In the presence of povidone-iodine, rapid partitioning of the cytoplasm and pronounced coagulation of nuclear material was noted. Especially C. albicans exhibited a rapid, dose-dependent "loosening" of the cell wall; cells remained intact without lysis, rupture or wall breakage. Changes in beta-galactosidase and nucleotide concentrations were measured in E. coli. A rapid and dose-dependent loss of cellular beta-galactosidase activity was found, with no increase in the supernatant; loss of cellular nucleotides corresponded with an increase in the supernatant. Electron microscopy and biochemical observations support the conclusion that povidone-iodine interacts with cell walls of micro-organisms causing pore formation or generating solid-liquid interfaces at the lipid membrane level which lead to loss of cytosol material, in addition to enzyme denaturation. The chemical mechanism of action explains the fact that povidone-iodine does never generate resistance in micro-organisms.