After 12 h of incubation at 37°C under shaking at 220 rpm, the cells were collected by centrifugation at 8000 rpm and 5°C for 5 min. The pellets were re-suspended in 25% glutaraldehyde and fixed for 12 h, after which the cells were washed with PBS three times. 2% Osmium tetroxide (OsO4) was added to the pellets and contacted for 1 h on a sample rotator. The cells were dewatered by subsequent rinses with solutions comprising 35%, 50%, 70%, 90% and 95% ethanol, followed by a final rinse in absolute ethanol. After contacting with propylene oxide for 30 min, propylene oxide/resin (Meryer, China) (1:1) was added and allowed to react for 12 h at 45°C. Finally, the cells were embedded in 100% resin and polymerization was conducted in an oven at 70°C for 24 h, after which the samples were prepared as 90 nm sections using an EM UC6 Ultramicrotome (Leica, Germany). After staining with 3% uranyl acetate and lead citrate (Beijing chemical works, China) for double staining, the sections were examined under a JEM-1011 transmission electron microscope (JEOL, Japan).