Gram-positive and gram-negative, respectively, refers to the behavior of cells during Gram-stain. It is used for the taxonomical classification of bacteria into two groups.
The first step of Gram-stain is a primary staining, in which all bacteria behave the same. This is followed by a decolorization. Here, the dye is trapped in Gram-positive bacteria because of their thick multilayered coat, whereas it is washed from Gram-negative bacteria. Counterstaining makes them again visible. Gram-stain is a useful feature to distinguish bacteria especially in the case of infectious diseases as it allows a rapid decision on effective antibiotic medication.