Transmission electron microscope (TEM).

Preparation of the bacterial samples was conducted in the same manner as described for the SEM treatment. After fixing with 2.5% glutaraldehyde overnight was performed, the bacterial pellets were washed three times with PBS and postfixed with 1% osmium tetroxide–PBS for 2 h. The fixed bacterial cells were washed three times with PBS, followed by dehydration for 15 min in a graded ethanol series (50%, 70%, 90%, and 100%). After being placed in absolute acetone for 20 min, the samples were transferred to 1:1 and 1:3 mixtures of absolute acetone and epoxy resin for 1 h in each mixture, followed by transferring to pure epoxy resin overnight. Ultrathin sections obtained using an ultramicrotome were poststained with uranyl acetate and lead citrate. Specimens were observed by transmission electron microscope (Hitachi H-7650; Hitachi, Japan).