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 the cells were separated by centrifugation at 4000 rpm for 5 min and the supernatants transferred into 96-well plates. Lactate dehydrogenase (LDH) activity was determined using a lactate dehydrogenase kit (Sigma-Aldrich, USA), according to the manufacturer’s protocol, with final detection of absorption at 450 nm using a Lambda-25 plate reader (Perkin-Elmer, USA). The results are expressed as the means ± SD of six parallel measurements.