Escherichia coli on blood agar (top), on a agar with pH
indicator (center) showing a red zone around the colonies,
and on a CPSII agar (bottom) together with enterococci
that grow as blue spots
Even the most famous bacterium, the intestinal bacterium Escherichia coli can become pathogenic under certain conditions. One cannot simply recognize pathogenic properties under the microscope. Helpful are selective cultivation conditions and appropriate indicators.