SEM of E. coli bacteria on Lettuce. Escherichia coli bacteria on a lettuce leaf congregate at the stomata (respiration openings in the leaves) Their presence may constitute a health hazard and cause severe food poisoning, particularly if the bacteria are toxigenic. They may get onto the vegetables if the water used for irrigation is contaminated with fecal material. Although thorough washing is recommended, electron microscopy shows that the bacteria enter the leaves through the stomata and would resist being removed. Similar bacterial contamination was also found with spinach and green onions. Enhanced SEM. Image width: 51.2 micrometers; magnification: 2740x if the image is printed 10 cm wide.