Purpose: To recognize the three basic shapes of bacterial cells.
Principle: In order to observe most bacterial cells using bright field microscopy the cells must be dark enough to see, that is they must have contrast to the light. To create contrast a simple stain can be used. Simple stains use basic dyes which are positively charged. These positive dyes interact with the slightly negatively charged bacterial cell wall thus lending the color of the dye to the cell wall.
The slide to the right shows a cocci that has been stained with crystal violet.