Modulation by surroundings of the antibacterial efficiency of silver in water environments

Silver is a usual antibacterial agent acting in water solutions. However, quantitative data fail to anticipate the silver availability for interactions with bacterial cells and the resulting fluctuations of the inhibitory and bactericidal concentrations according to the silver amount and chemical composition of the liquid environment. We investigated whether and how three different biological media modify silver ions (Ag+) and nanoparticles (AgNPs) and alter their antibacterial performances against Escherichia coli K12. The predominant silver species highly differed in terms of nature and quantity according to the medium. Bioavailability was reduced to a few percent of the initial Ag+ amount in physiological serum (NaCl) and lysogeny broth (LB), while it was higher than 67% in minimal phosphate medium (M63G). To reach similar antibacterial performances, Ag+ concentrations were 100 times lower in M63G than in the other media. Even with a silver concentration of 10−1 M, 53% of the bacterial cells living after 3 h in LB were undamaged and revealed normal metabolic activity, while the cells were almost completely eliminated in M63G. Effects of AgNPs were also strongly but differently modulated by the medium’s components. AgNPs revealed higher antibacterial effect than Ag+ in NaCl and M63G with similar silver content, thus providing an argument to assert that AgNPs act through a combination of the release of Ag+ and intake of AgNPs. Our results finally provide guidelines about the Ag+ dose that must be released from coatings to reach efficient performances according to the surrounding medium and the framework of the study.