Ligands affecting silver antimicrobial efficacy on Listeria monocytogenes and Salmonella enterica

Although silver is being extensively used in food or other applications as the key component to control microbial proliferation, many factors affecting its real potential are still unknown. In the present work, the presence of specific ligands or the contents in organic matter was correlated with silver speciation and its antibacterial performance. Silver was found to be only active in form of free silver ions (FSI). The presence of chloride ions produced an equilibrium of stable silver chloride complexes which were void of antimicrobial efficacy. However, even at relatively high concentrations of chlorides, a small fraction of FSI may still be present, producing a bactericidal effect with concentrations at the nanomolar level under optimum conditions. Low concentrations of thiol groups completely inactivated silver, while methylsulphur groups only affected its efficacy at very high concentrations. Antibacterial performance revealed differences of about 1000-fold between results for environments with high organic matter content and results for aqueous salt buffers. Thiol groups were nonetheless not found directly associated with the decrease in antimicrobial performance in a nutrient rich environment. These results point out the complexity of the antimicrobial systems based on silver and can have relevance in food or other applications of silver as an antimicrobial.