Scanning Electron Microscopy images of E. coli cells: (a) untreated, (b) after 30 min ultrasound at 500 kHz, (c) after 30 min nisin then 30 min ultrasound at 500 kHz, and (d) after 30 min ultrasound at 500 kHz followed by 30 min nisin. Examples of intact cells (green arrows), ghost cells (red arrows), and cellular debris (blue arrows) are indicated. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)