When E. coli was cultured without the addition of MSNs, the cells showed a long and tubular form, and the cell membrane was relatively intact. Some of the cells were found to be shrunken (see Fig 5A and 5B). When E. coli was exposed to low doses of MSNs (1, 10 μg/mL), some cells that changed in cell structure were found (Fig 5C–5F). On the other hand, when E. coli was exposed to high doses of MSNs (100, 1000 μg/mL), some of the cells were also found to be shrunk and their cell membranes appeared to be broken (Fig 5G–5J). These results suggested that MSNs affected the cellular structure of E. coli in a dose-depended manner, which was similar to the observations reported for other nanomaterials . In order to quantify the TEM results, the percentage of shrunken cells was calculated by counting the shrunken E. coli cells in the TEM images. We found that the percentages of shrunken cells were 8%, 12%, 13%, 14% and 16% when the E. coli were treated with 0, 1, 10, 100 and 1000 μg/mL of MSNs, respectively (Fig 6). It was thus found that the percentage of shrunken cells increased slightly when the concentration of MSNs was raised from 0 μg/mL to 1000 μg/mL. This result was in accordance with the results of LDH release and ROS analysis (Fig 4A and 4B). We hypothesized that high doses of MSNs damaged the cellular structure of E. coli, which caused increased LDH release. Moreover, high concentrations of MSNs stimulated the production of high levels of ROS, which would in turn further damage the cellular components.