Abstract
Gold nanoparticles (AuNPs) and gold ion complexes have been investigated for their antibacterial activities. However, the majority of the reports failed to disclose the concentration of free Au(I) or Au(III) present in solutions of AuNPs or gold ion complexes. The inconsistency of antibacterial activity of AuNPs may be due to the effect of the presence of Au(III). Here we report the antibacterial activity of Au(I) and Au(III) to four different bacteria: one nonpathogenic bacterium: E. coli and three multidrug-resistant bacteria: E. coli, S. typhimurium DT104, and S. aureus. Au(I) and Au(III) as chloride are highly toxic to all the four bacteria, with IC50 of 0.35 – 0.49 µM for Au(III) and 0.27–0.52 µM for Au(I).The bacterial growth inhibition by both Au(I) and Au(III) increases with exposure time and is strongly affected by the use of buffers. The IC50 values for Au(I) and Au(III) in different buffers are HEPES (0.48 and 1.55 µM) > Trizma (0.41 and 0.57 µM) > PBS (0.14 and 0.06 µM). Bacterial growth inhibition by AuNPs is gradually reduced by centrifugation-resuspension to remove residual Au(III) ion present in the crude synthetic AuNPs. After 4 centrifugations-resuspensions, AuNPs become non-toxic. In addition, both Au(I) and Au(III) are cytotoxic to skin keratinocyte and blood lymphocyte cells. These results suggest that Au(I) and Au(III) in pure or complex forms may be explored as a method to treat drug-resistant bacteria, and the test of AuNPs toxicity must consider residual Au(III), exposure time, and the use of buffers.
Highlights
Gold nanoparticles (AuNPs) have attracted considerable interests for fundamental and applied research
Bacteria and cell lines used in this study include non-pathogenic E. coli (BAA-1431), multidrugresistant E. coli (BAA-1161), multidrug-resistant S. typhimurium DT-104 (ATCC 700408), multidrug-resistant S. aureus (MRSA, BAA-44), and the blood lymphocyte cell line, TIB-152, were purchased from American Type Culture Collection (ATCC) (Manassas, VA)
This work clearly demonstrates that both Au(I) and Au(III) ions are strongly antibacterial against all four tested bacteria: one nonpathogenic E. coli and three multidrug resistant bacteria: E. coli, S. typhimurium DT104, and S. aureus (MRSA)
Summary
Gold nanoparticles (AuNPs) have attracted considerable interests for fundamental and applied research. As AuNP applications continue to increase, growing human safety concerns are gaining attention [1,2,3,4,5,6,7,8,9]. It was pointed out that the toxic effects of AuNPs are complex due to co-existing chemicals, such as the presence of citrate and Au(III) ions during the photomutagenecity test of AuNPs [10]. Gold nanorods are toxic to human skin cells due to the surface coating chemical CTAB (cetyltrimethylammonium bromide), but not the gold nanorods [11,12,13]. CTAB alone is toxic to cells at sub-micromolar concentrations.
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