Bacteria can be stained with an iodine-based chemical dye called Gram’s stain (after the scientist who discovered the technique). Different bacteria react diﬀerently to exposure to the stain. Broadly, the bacterial specimens are stained ﬁrst with gentian violet, then with Gram’s stain, and ﬁnally counterstained with a red dye after a decolorising process. Bacteria that retain the gentian stain are called gram-positive; those that lose it but absorb the red stain are called gram-negative. Some species of staphylococcus, streptococcus and clostridium are gram-positive, whereas salmonella and Vibrio cholerae are gram-negative.
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