Antibiotic resistance is a worldwide public health concern. Conjugative transfer between closely related strains or species of bacteria is an important method for the horizontal transfer of multidrug-resistance genes. The extent to which nanomaterials are able to cause an increase in antibiotic resistance by the regulation of the conjugative transfer of antibiotic-resistance genes in bacteria, especially across genera, is still unknown. Here we show that nanomaterials in water can significantly promote the horizontal conjugative transfer of multidrug-resistance genes mediated by the RP4, RK2, and pCF10 plasmids. Nanoalumina can promote the conjugative transfer of the RP4 plasmid from Escherichia coli to Salmonella spp. by up to 200-fold compared with untreated cells. We also explored the mechanisms behind this phenomenon and demonstrate that nanoalumina is able to induce oxidative stress, damage bacterial cell membranes, enhance the expression of mating pair formation genes and DNA transfer and replication genes, and depress the expression of global regulatory genes that regulate the conjugative transfer of RP4. These findings are important in assessing the risk of nanomaterials to the environment, particularly from water and wastewater treatment systems, and in the estimation of the effect of manufacture and use of nanomaterials on the environment.

The potential risks of nano-materials and the spread of antibiotic resistance genes (ARGs) have become two major global public concerns. Studies have confirmed that nano-alumina can promote the spread of ARGs mediated by plasmids. Nano-titanium dioxide (TiO(2)), an excellent photocatalytic nano-material, has been widely used and is often present in aqueous environments. At various nano-material concentrations, bacterial density, matting time, and matting temperature, nano-TiO(2) can significantly promote the conjugation of RP4 plasmid in Escherichia coli. We developed a mathematical model to quantitatively describe the conjugation process and used this model to evaluate the effects of nano-TiO(2) on the spread of ARGs. We obtained analytical solutions for total and resistant bacteria, which were enumerated by the abundance of genetic loci unique to the plasmid and the chromosome using qPCR. Our results showed that the mathematic model was able to fit the experimental data well and can be used to quantitatively evaluate the effects of nano-TiO(2). According to our model, the presence of nano-TiO(2) decreased the bacterial growth rate from 0.0360 to 0.0323 min(-1) and increased the conjugative transfer rate from 6.69 × 10(-12) to 3.93 × 10(-10 )mL cell(-1) min(-1). These results indicate that nano-TiO(2) inhibited bacterial growth and promoted conjugation simultaneously. The data for morphology and mRNA expression also demonstrated this phenomenon. Our results confirm that environmental nano-TiO(2) may cause the spread of ARGs and thus poses an environmental risk. In addition, we provide a potential method for monitoring changes in ARGs that result from conjugation and evaluating the effects of antimicrobial substances on ARG expression.