Influence of Growth Media on Bacterial Susceptibility to Antimicrobial Metal Ions

Background: Antimicrobial resistance has developed as a significant world health problem, and numerous approaches have been actively studied to address the challenge. Some metals, including silver, copper, and zinc, have emerged as potential alternatives to traditional antibiotics, especially in the context of medical devices and coatings that can be applied to these devices. However, the effective dose and spectrum of activity of these metals have been areas of question for many material scientists, as well as those looking to translate the coatings into clinical trials. Methods: Here, we have evaluated the efficacy of three antimicrobial metals in solution against a panel of clinically relevant bacteria grown in three different growth media types. We used a combination of microbiological approaches, including determining susceptibility by broth microdilution assays, bacterial growth curves, analysis of biofilm mass using crystal violet binding and metabolism using XTT conversion, and analysis of bacterial morphology by scanning electron microscopy. Results: The results demonstrate that not all antimicrobial metals are equally effective in all media types. Across all conditions tested, Ag+ ions were the most effective, followed by Cu2+ and Zn2+. Notably, Cu2+ and Zn2+ showed increased variability in MIC against bacteria grown in nutrient broth media, with both increased susceptibility and decreased growth rates for all bacteria tested. Both E. coli and S. aureus exhibited significantly smaller cell sizes in NB media when treated with metal ions. Bacterial biofilms grown in nutrient broth media exhibited lower metabolic activity and decreased biomass, although not statistically significant in all cases. Importantly, when compared to mammalian cytotoxicity levels, the dramatic effects of media on the MIC values can have significant impact on calculations of effective dosage. Conclusions: These results demonstrate that studies on the antimicrobial efficacy of metal-based antimicrobials, including metal-based coatings, should be viewed through the lens of differential activity depending on the media conditions used.