The influence of media components on the antibacterial effect of silver ions on Bacillus subtilis - A different approach to research silver toxicity

Silver is a noble metal which has been used for centuries due to its effective antibacterial, antifungal and antiviral properties. For long, silver and silver salts are used for preserving water or food and preventing infections of wounds. More recently, silver nanoparticles (AgNPs) were developed, resulting in a remarkable increase in silver usage. In order to predict the possible ecological and human toxicity of silver upon release into the environment, as well as to verify the antimicrobial effect of new products and applications, a large number of scientific papers describe toxicity tests of AgNPs, silver ions (Ag+) and colloidal silver towards environmentally and clinically relevant bacteria, both in growth media or biological matrices. However, contradictory results are reported with inhibition concentrations varying by a 100-fold. We investigated whether this variance in results could be attributed to the difference in testing conditions, especially the microbial growth medium. In this study, Bacillus subtilis was exposed to 500 μg/L Ag+ - which is the main active species of silver - in growth media with different concentrations of some commonly used media components. The toxic effect was investigated by flow cytometry, transmission electron microscopy and by analyzing the growth curves obtained by optical density measurements. The results showed a strong influence of media components on the observed toxicity of silver. Therefore, researchers have to be aware of the possible interference of media components with Ag+, and should consider their influence before drawing conclusion.

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