Diluent Mitigates the Inhibitory Effect of Zinc Oxide Nanoparticles on Escherichia coli and Staphylococcus aureus

The effects of zinc oxide nanoparticles on Escherichia coli and Staphylococcus aureus in deionized water, normal saline and phosphate buffered solution were investigated based on culture-dependent growth response. After 6 h incubation in phosphate buffered solution, the 10, 100 and 1000 µg mL⁻ˡ treatment resulted in a range of 1.0 to 1.5 log reduction in growth of E. coli and S. aureus compared with the control. In the normal saline, 1.1 to 3.3 and 1.1 to 3.6 log reduction whereas 1.2 to 4.1 and 1.2 to 3.8 log reduction in deionized water was observed for E. coli and S. aureus respectively. Inhibitory effect of ZnO nanoparticles on the organisms in the diluents was concentration-dependent with 1.8 to 2.6 times higher viability of E. coliand S. aureus in PBS compared to the normal saline and deionized water, but the difference was significant (P = .05). The evidence suggests differential susceptibility of E. coli and S. aureus to ZnO nanoparticles in diluents routinely used in microbiological assay and raises concern on the challenges of interpreting the inhibitory effect in culture-dependent microbiological analysis. Compared with deionized water and normal saline, phosphate buffered solution mitigated the inhibitory effect of ZnO nanoparticles on the growth dynamics and population density of E. coli and S. aureus.