Methylthiazolyldiphenyl-tetrazolium bromide (MTT) reagent (Sigma-Aldrich, USA) was used for cell viability measurements. E. coli was exposed to 0, 1, 10, 100, or 1000 μg/mL of MSNs in PBS (pH 7.0). Subsequently, the cells were incubated in PBS at 37°C under constant orbital shaking at 220 rpm for 6 h, after which 20 μL PBS solution containing cells, 20 μL MTT solution (1 mg/mL) and 60 μL fresh PBS were transferred into an Eppendorf tube and incubated in a thermostat water bath for 30 min at 37°C. Subsequently, the tube was centrifuged at 10000 g for 5 min and the supernatant removed. 300 μL dimethyl sulfoxide (DMSO) was added to each tube to dissolve the sediment and the tubes were centrifuged at 10000 g for another 5 min. The supernatants were transferred to 96-well plates and the absorbance at 490 nm was measured using a plate-reader (Molecular Devices, SpectraMax M3, USA). The results are expressed as the means ± SD of six parallel measurements.