E. coli were incubated overnight in MHB at 37 ◦C, followed by addition of the stems essential oil to give a final concentration of 1.5×MIC. Then both the treated bacteria and control were incubated at 37 ◦C for 4 h. And the cell suspensions were harvested by centrifugation, then the cells were washed twice with 0.1 M PBS (pH7.2) and fixed with 4% (v/v) glutaraldehyde-3% (v/v) paraformaldehyde in 0.1 M PBS overnight at 4 ◦C. After this, the cells were postfixed with 2% (w/v) OsO4 in 0.1 M PBS for 2 h and washed three times with the same buffer. Next the samples were successively dehydrated using 30%, 50%, 70%, 90% and 100% ethanol. The bacteria were stained with uranyl acetate for 40 min and lead citrate for 15 min to be observed and photographed with a transmission electron microscope (JEM-1400, Japan) operated at 120 kV.