Sub-inhibitory concentrations of heavy metals facilitate the horizontal transfer of plasmid-mediated antibiotic resistance genes in water environment

Abstract

Although widespread antibiotic resistance has been mostly attributed to the selective pressure generated by overuse and misuse of antibiotics, recent growing evidence suggests that chemicals other than antibiotics, such as certain metals, can also select and stimulate antibiotic resistance via both co-resistance and cross-resistance mechanisms. For instance, tetL, merE, and oprD genes are resistant to both antibiotics and metals. However, the potential de novo resistance induced by heavy metals at environmentally-relevant low concentrations (much below theminimum inhibitory concentrations [MICs], also referred as sub-inhibitory) has hardly been explored. This study investigated and revealed that heavy metals, namely Cu(II), Ag(I), Cr(VI), and Zn(II), at environmentally-relevant and sub-inhibitory concentrations, promoted conjugative transfer of antibiotic resistance genes (ARGs) between E. coli strains. The mechanisms of this phenomenon were further explored, which involved intracellular reactive oxygen species (ROS) formation, SOS response, increased cell membrane permeability, and altered expression of conjugation-relevant genes. These findings suggest that sub-inhibitory levels of heavy metals that widely present in various environments contribute to the resistance phenomena via facilitating horizontal transfer of ARGs. This study provides evidence from multiple aspects implicating the ecological effect of low levels of heavy metals on antibiotic resistance dissemination and highlights the urgency of strengthening efficacious policy and technology to control metal pollutants in the environments.

Zinc and Copper Reduce Conjugative Transfer of Resistance Plasmids from Extended-Spectrum Beta-Lactamase-Producing Escherichia coli

“…A previous study has shown that the two plasmids included in this study are inter-and intra-species transferable at different conditions, indicating that they may contribute to the maintenance of antibiotic-resistant genes in the environment. 29 In contrast to earlier studies, which show that Zn and Cu are associated with increased conjugation, [50][51][52][53] we found that Zn and Cu reduced the conjugation frequency between E. coli strains in a concentrationdependent manner. There was also no difference in the number of colonies on the donor-and recipient-selective plates from the mating cultures, which confirms that Zn and Cu does not affect growth of donor-or recipient strains.…”

“…59 To understand the effect of Zn and Cu on conjugation in our study, we performed a real-time transcriptional analysis that showed a reduction in expression of conjugation-associated genes in response to subinhibitory concentrations of Zn and Cu. However, our findings contrast the report by Zhang et al, 52 who found that different concentrations of heavy metals increased conjugation and upregulated the expression of genes involved in conjugation of plasmids. Strain background, concentrations of the metals, and experimental conditions might explain the contradictory results.…”

Urbanization and Waterborne Pathogen Emergence in Low-Income Countries: Where and How to Conduct Surveys?

“…Heavy metals persist within sediment, where they increase the half-life of antibiotics [66]. Even at low concentrations, heavy metals can contribute to the emergence and spread of antimicrobial-resistant strains through coselection of genetic elements encoding both heavy metal and antibiotic resistance genes [67][68][69][70][71]. This was recently confirmed by the selection of antibiotic-resistant enterococci by very low heavy-metal concentrations [72].…”

Metallic nanoparticles and ions accelerate the uptake of extracellular antibiotic resistance genes through transformation

“…For instance, heavy metal ions (e.g., Cu 2+ , Cd 2+ , Hg 2+ and Zn 2+ ) and nanoparticles (e.g., ZnO NPs, Al 2 O 3 NPs and Ag NPs) have been reported to induce antibiotic resistance via co-selection or direct mutation [12,13,15,16]. In addition, heavy metal ions (including Cu 2+ , Ag + , Cr 6+ , Ti 4+ and Zn 2+ ) [17][18][19] and nanoparticles (e.g., Al 2 O 3 NPs, CuO NPs, Ag NPs and TiO 2 NPs) [18][19][20][21] could promote the transfer of ARGs via conjugation. However, it is unclear whether heavy metal-based NPs and ions could enhance the dissemination of ARGs via natural transformation.…”