Another study has called nanoparticle safety into question (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.1107254109). Researchers in China led by Jun-Wen Li of the Institute of Health & Environmental Science, in Tianjin, have demonstrated that alumina nanoparticles enhance the transfer of antibiotic-resistant genes from one bacterial cell to another by as much as a factor of 200. This transfer is of major concern, Li says, because more and more nanomaterials are being added to commercial products, and super-resistant bacteria pose a threat to human health. A small amount of resistant DNA moves between bacteria via cell-to-cell contact without the aid of nanoparticles, the researchers confirmed. But the tiny alumina particles—used commercially as sorbents and catalysts—boost the transfer rate, particularly when they are present at an optimal concentration of 5 mmol/L. On the basis of transmission electron microscopy imaging and other analysis, Li and coworkers suggest that the resistant genes hitch a ride on the nanoparticles, which likely damage and then penetrate the cells’ membranes via oxidative stress. Catherine J. Murphy, a chemist at the University of Illinois, Urbana-Champaign, says that the transfer mechanism needs to be more fully worked out but that the study results are provocative.