Clean leaf of Iceberg lettuce showing two stomata (respiration orifices). On leaves infected with bacteria such as Escherichia coli, the bacteria congregate and propagate at the stomata. Infection may be caused by irrigation water contaminated with fecal material. Infected lettuce (or spinach) represent a severe health hazard. Enhanced SEM, image width: 85 micrometers; magnification: 1175x if the image is printed 10 cm wide.