Are silver nanoparticles better than triclosan as a daily antimicrobial? Answers from the perspectives of gut microbiome disruption and pathogenicity
Graphical abstract
Introduction
Triclosan [5-chloro-2-(2,4-dichlorophenoxy)phenol, TCS] is a broad-spectrum antimicrobial ingredient that is added to a wide range of personal care products, such as toothpaste, soap and body wash (Oliver et al., 2020). Nevertheless, various toxic effects of TCS have been reported, including disruption of the gut microbiota (Yee and Gilbert, 2016), endocrine disruption (Arya et al., 2020), weakening of the immune system (Chen et al., 2018), and potential carcinogenicity (Sanidad et al., 2019). Its effect on the spread of antibiotic resistance particularly raises many concerns (Jutkina et al., 2018; Lu et al., 2020b). Thus, since 2016 the FDA has banned the use of TCS in hand and body washes. As an alternative, silver nanoparticles (AgNPs) have been applied in many consumer products, ranging from textiles, toys, and cosmetics to biomedical supplies, due to their effective biocidal activities (Roszak et al., 2020). However, the question remains of whether AgNPs are substantially better than TCS from the perspective of health effects.
Interestingly, TCS and AgNPs share certain similarities in terms of the microbiome disruption, implications in antibiotic resistance and effects on opportunistic pathogens. Both TCS and AgNPs have been detected inside the body after oral exposure (Gan et al., 2020; Yee and Gilbert, 2016), and disturbances in the gut microbiota have been observed. For instance, the α-diversity of the gut microbiome of infants who received breast milk containing detectable levels of TCS (1–13.6 μg/L) for 8–13 weeks was significantly lower than infants without TCS exposure (Bever et al., 2018). During 28 days of oral exposure to food supplemented with AgNPs (0, 46, 460 or 4600 μg/L), the evenness (α-diversity) and populations (β-diversity) of the gut microbiota in mice were noticeably decreased (van den Brule et al., 2016). Furthermore, TCS affects the spread of antibiotic resistance by inducing cross resistance to drugs, increased numbers of mutations, the selection of multidrug resistance genes and acceleration of horizontal gene transfer (Lu et al., 2018). Recently, AgNPs have also been suggested to facilitate the plasmid-mediated horizontal gene transfer of antibiotic resistance genes (ARGs) by inducing reactive oxygen species (ROS) production and increases in membrane permeability. For example, at levels as low as 0.1 μg/L, AgNPs increased the conjugative transfer level up to 1.8-fold compared to the level measured in the absence of nanoparticles during a 6-h mating period (Lu et al., 2020a). AgNPs administered at a concentration of 100 mg/L altered the resistome of the soil microbiome after 60 days of exposure, increasing the relative abundances of efflux pump genes (Chen et al., 2019). Finally, TCS exposure enriched opportunistic pathogens in the gut. As an illustration, both mothers and infants exposed to higher TCS levels (through products used daily) showed an enrichment in Proteobacteria, which are involved in dysbiosis, in feces after 4 months of exposure (Ribado et al., 2017). Similarly, AgNPs (after 2 h of exposure at 3 mg/L) are more toxic to common commensal microbes (i.e., Lactobacillus bulgaricus and Lactobacillus casei) but less toxic to pathogenic microbes (i.e., Escherichia coli and Staphylococcus aureus), thus potentially resulting in the selective blooming of opportunistic pathogens (Tian et al., 2018).
However, AgNPs differ considerably from TCS in many ways, including their structural properties, toxicity mechanisms, and bioavailability. Compared with TCS, the antimicrobial effects of AgNPs are determined by many structural factors, such as size, shape, coating and ion release (Li et al., 2019a; van den Brule et al., 2016). Regarding the toxic mechanism, TCS interferes with bacterial fatty acid synthesis by inhibiting the enzyme activity of the enoyl-acyl reductase fabI (Heath et al., 1999), whereas both silver ions (Ag+) and particles possess multiple antibacterial properties. For instance, Ag+ inhibits enzyme functions by complexing with thiol groups (Matsumura et al., 2003), and AgNPs penetrate the cell membrane and infiltrate the intracellular compartment (Geng et al., 2017). The bioavailability of AgNPs is modulated by transformation (i.e., sulfidation inside the body), aggregation, and corona formation, whereas the biocidal effect of TCS is simply dose- or concentration-dependent. Thus, a comparison of how AgNPs and TCS disturb the gut microbiota in terms of the degree, composition shift, functional network and restoration ability is interesting. We also particularly wondered whether the resistome is similar in the presence of the same concentrations of TCS and AgNPs.
The main objectives of this study are 1) to compare how the intestinal microbiota is disrupted by AgNPs and TCS using a simulator of the human intestinal microbial ecosystem (SHIME), which is a simple in vitro model allowing the exclusion of potentially confounding factors present in in vivo models and underscoring the effect of target compound; 2) to explore changes in the resistance profile upon TCS or AgNP exposure, including effects on silver resistance-related genes; and 3) to identify the different effects of TCS and AgNPs on opportunistic pathogens in the gut microbiota and disease-related pathways. The results provide fundamental information to determine how TCS and AgNPs govern the extent and intensity of microbiota-modulating effects.
Section snippets
Characterization of AgNPs
AgNPs were obtained from Shanghai Huzheng Nanotechnology (China) and citrate buffer (2 mM) was used as a capping and reducing agent. The morphology and elemental composition were characterized using transmission electron microscopy (TEM, Talos F200X, FEI, USA) coupled with energy dispersive X-ray spectroscopy (EDS, superX, FEI). X-ray diffraction (XRD) patterns were obtained using a D/Max-2500 diffractometer (Rigaku, Japan) with Cu Kα radiation. The absorption spectrum in deionized water was
Different effects on SCFA production and gut homeostasis
Fig. S3 shows spherical AgNPs with diameters of approximately 14.03 ± 3.71 nm, and the characteristic peaks in the X-ray diffraction (XRD) pattern and the UV–vis spectrum of AgNPs in water were consistent with those of particulate Ag (Li et al., 2015; Liu et al., 2012). Purity was also verified from the energy-dispersive X-ray spectroscopy (EDS) spectra and X-ray elemental map. Dissolution rates in water were 2.34% (w/w) and 3.89% (w/w), respectively, after 5 and 24 h; nevertheless, a rate of
Conclusions
Due to the changing properties of AgNPs in the gut environment, they produced less disruption than the same concentration of traditional antimicrobial TCS, as evidenced by gut dysfunction, the imbalance of the bacterial community, dissemination of antibiotic and Ag resistance genes, and overabundances of opportunistic pathogens. Nevertheless, the low level of stress caused by AgNPs, the enrichment of Ag resistance determinants and the potential prevalence and spread of AgNPs microbial
CRediT authorship contribution statement
Mingzhu Li: Investigation, Data curation, Visualization, Writing - original draft. Chengdong Zhang: Conceptualization, Supervision, Writing - review & editing, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This project was supported by the National Natural Science Foundation of China (Grants 21777077 and 21976018). Partial support was also provided by the Fundamental Research Funds for the Central Universities and the Interdisciplinary Research Funds of Beijing Normal University.
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