Quaternary ammonium esters as soft antimicrobial agents

Abstract

Introduction: Amphiphilic quaternary ammonium compounds (QACs) have been used for many years for rapid disinfection of the surfaces of the human body and for items related to it. They are cationic surfactants, bind to all kinds of biological membranes, and disrupt the membrane barrier causing leakage of intracellular material and cell death. The use of ìsoftî QACs, which degrade after they have achieved their killing effects, would minimize the damage to mammalian cells and the environment. ìSoftî analogues to monoalkyltrimethylammonium halides have been prepared as esters of betaine and choline which are liable to alkaline or enzymatic hydrolysis, respectively, yielding non-toxic metabolites in a predictable manner. The aim of this thesis has been to investigate the binding of ìsoftî QACs to different cell surfaces, their microbicidal effects, and the hydrolysis under different conditions. Results and conclusions: The binding process and microbicidal mechanisms of series of betaine and choline esters with hydrocarbon chain lengths of 10, 12 and 14 carbon atoms are analogues to those of stable compounds with 12, 14 and 16 carbon atoms. They bind to cell surfaces, particularly to membranes by charge and hydrophobic interaction, and kill the cells rapidly by disorganising the cell membranes. These effects are influenced by temperature, pH, competing material etc. to different extents depending on the length and structure of the hydrocarbon chain.Reduction of temperature from 308C to 08C does not change the binding of QAC to Salmonella typhimurium 395 MS but reduces the killing effect very much, particularly so for the QACs with longer chains. Similarly, the presence of bovine serum albumin reduces the bactericidal effect of the longer QACs more. In contrast, the lowering of pH reduces the effect of shorter QACs more. Thus the former effects seem to be governed by hydrophobic interaction, whereas the binding of the shorter compounds are more influenced by charge. This also implies that the disorganisation of the membrane with loss of its barrier effect is greatly dependent on the fluidity of the membrane.Tetradecylbetainate binds to the outer membrane (OM) of Escherichia coli and may release the periplasmic marker enzyme (-lactamase without release of the cytoplasmic marker enzyme (-galactosidase.Hydrolysis of tetradecylbetainate bound to S. typhimurium 395 occurs but is retarded.The kinetics of binding of QACs to Candida albicans is different from that to gram-negative bacteria. The QAC molecules assemble as submicellar complexes on the yeast cell surface and initiate membrane damage appearing as release of UV-absorbing material, increased uptake of a DNA binding probe, and fungicidal effect. The microbicidal activity of choline esters towards bacteria and yeasts is similar to that of betaine esters. Esterases, active towards the choline esters is present in certain microorganisms such as Salmonella infantis and Candida glabrata. The choline esters may kill these microorganisms rapidly as well as the esters become degraded.Summary: Our results confirm that ìsoftî antimicrobial substances are degraded on site into non-toxic substances after achieving killing effect. They may become preferable alternatives to the stable QACs used nowadays.