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The Gram-Positive Cell
As previously mentioned, Gram-positive bacteria are characterized by their
blue-violet color reaction in the Gram-staining procedure. The blue-violet color reaction is caused by crystal-violet,
the primary Gram-stain dye, complexing with the iodine mordant. When the
decolorizer is applied, a slow dehydration of the crystal-violet/iodine complex
is observed due to the closing of pores running through the cell
wall. Because the crystal-violet is still present in the cell,
the counter stain is not incorporated, thus maintaining the cell's blue-violet
color. If you recall, most cell walls contain peptidoglycan, a molecule
made of amino acids and sugar. A distinguishing factor among Gram-positive
bacteria is that roughly 90% of their cell wall is comprised of peptidoglycan
and a Gram-positve bacteria can have more than 20 layers of peptidoglycan
stacked together to form the cell wall. That's pretty thick! Examples of common
Gram-positive cells are Staphylococcus aureus and Streptococcus
cremoris, a bacterium used in dairy production.
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