Assessing the Impact of Titanium Dioxide and Zinc Oxide Nanoparticles on Bacteria Using a Fluorescent-Based Cell Membrane Integrity Assay

The aim of this research was to determine whether metal oxide nanoparticles (NPs) damage bacterial cell membranes. Bacterial cell membrane integrity was determined after 1 h exposure to titanium dioxide (TiO2) and zinc oxide (ZnO) NPs (0, 10, 50, 100, and 500 mg/L) in ultrapure water and in the presence of natural organic matter. Samples were stained with propidium iodide and counterstained with a nonspecific DNA stain for cell membrane integrity assessment using fluorescence microscopy. Results revealed a measurable impact on the cell membrane integrity of Gram-negative (Escherichia coli) bacteria at 100 and 500mg/L TiO2 even in the presence of natural organic matter (2mg/L dissolved organic carbon). On the other hand, no statistically significant (p = 0.05) change in cell membrane integrity was observed with ZnO that aggregated and precipitated at concentrations at or above 100mg/L. In addition, Gram-positive bacteria (Enterococcus faecium) were not impacted in the presence of ZnO or TiO2. Transmission electron microscopy images show an interaction between TiO2 NPs and E. coli cells. This interaction was not observed for ZnO NPs or Gram-positive cells. The cell membrane integrity assay indicated NP impact on bacterial cells that were not detected by plating techniques. Cell membrane integrity assays show promise for broader application in environmental matrices where traditional growth-based approaches are limited because of their bias toward easily cultured organisms.