The positive and negative controls are bacterial smears we use to test if the Gram stain was performed properly.
If the positive and negative controls are not as expected, you can not trust that the stain was performed properly.


As a Positive Control we use a known Gram-positive bacteria. In this simulation, Staphylococcus aureus is used.
If the Gram stain is performed properly, the positive control should appear purple (Gram-positive).


As a Negative Control we use a known Gram-negative bacteria. In this simulation, Escherichia coli is used.
If the Gram stain is performed properly, the negative control should appear pink (Gram-negative).


Horizontally aligned glass slide with three circles, one next to each other. First circle on the left is coloured in purple and called positive control. The circle in the middle is transparent and called an unknown sample. The last circle on the right is coloured in pink and called negative control.

Figure 1: Color difference between positive and negative control