You perform a simple stain and when viewed under a microscope you see tiny and large bacillus bacteria. you can conclude this is a pure culture because all species on the slide are clearly bacillus.

You perform a simple stain and when viewed under a microscope you see tiny and large bacillus bacteria. Can you conclude this is a pure culture because all species on the slide are clearly bacillus? No! The reason being there are two types of bacteria, gram positive and gram negative. The gram positive bacteria have a thick outer cell wall. The simple staining will only stain the gram positive bacteria as their cell wall retains the dye (crystal violet dye). The gram negative bacteria do not have a cell wall. Thus do not get stained with crystal violet dye. Hence just by performing a one-step simple staining of the culture plate, one can't conclude that it has only one kind of bacteria.