Principle of bacterial differentiation
The separation of microorganisms by the Gram method directly depends on the architecture of their cell wall.
- Gram-positive bacteria (G+) are characterized by the presence of a very thick cell wall. Due to its structure, specific staining gives them a deep blue-purple color.
- Gram-negative bacteria (G-), by contrast, have a thin cell wall. After completing all staining steps, such cells appear red (or pink-red).
Physicochemical principles of the method
The difference in final color is explained by the reaction of peptidoglycan to sequential treatment with chemical reagents, primarily alcohol.
- In Gram-positive bacteria, the wall is based on a robust, multi-layered peptidoglycan. When treated with alcohol, this dense framework tightly retains the crystal violet-iodine complex inside the cell. Therefore, the cells retain their primary blue-purple hue.
- In Gram-negative bacteria, the peptidoglycan content is critically low—accounting for only 5–10% of the wall mass. Treatment with alcohol rapidly washes out the primary dye, and the cell is completely decolorized. To visualize these bacteria, a counterstain (fuchsin or safranin) is applied, which gives them a red color.
Peptidoglycan biochemistry
Peptidoglycan is a complex polymer that provides structural strength to the cell wall. Its molecular framework is formed by glycan chains consisting of alternating molecules of:
- N-acetylglucosamine
- N-acetylmuramic acid
To make the wall monolithic, these glycan chains are interconnected by robust peptide cross-links.
Action of lysozyme and immune response
The body has a natural mechanism for destroying the bacterial wall using the enzyme lysozyme. Lysozyme acts by specifically cleaving the chemical bonds within the peptidoglycan glycan chains.
The process of bacterial wall degradation plays an important role in immune activation. The main product of this cleavage—muramyl dipeptide—is a potent immunomodulator that stimulates the host's defense reactions.
Briefly: answers to topical questions
What dyes are used as primary and counterstains in the Gram stain?
Crystal violet is used as the primary stain in the Gram stain, and fuchsin or safranin is used as the counterstain.
- Primary stain — crystal violet, which together with iodine forms a complex tightly retained by the peptidoglycan of Gram-positive bacteria.
- Counterstain — fuchsin or safranin, which are taken up by Gram-negative bacteria after they are decolorized with alcohol.
What steps are included in the standard Gram staining procedure for bacteria?
The Gram staining procedure includes treatment with a primary stain and iodine, decolorization with alcohol, and counterstaining.
- Primary staining — application of crystal violet and iodine to form a stained complex.
- Decolorization — treatment with alcohol, which washes the dye out of the Gram-negative bacterial cell wall.
- Counterstaining — using fuchsin or safranin to color the decolorized cells red.
Gram-POSITIVE — Piled UP (laid down) a lot of peptidoglycan (a thick wall firmly holds the purple dye).
Class and exam questions
Why do Gram-positive bacteria not decolorize with alcohol?
They have a robust multi-layered peptidoglycan that, after alcohol treatment, tightly holds the crystal violet-iodine complex inside the cell.
Which dye gives Gram-negative bacteria a red color?
The red color is provided by a counterstain—fuchsin or safranin—which the cells absorb after complete decolorization with alcohol.
What components make up the glycan chains?
They consist of alternating residues of N-acetylglucosamine and N-acetylmuramic acid.
How does lysozyme affect the bacterium?
The enzyme lysozyme cleaves chemical bonds directly within the glycan chains of peptidoglycan, leading to degradation of the cell wall.
What else to study on this topic
- Steps of crystal violet-iodine complex formation
- Mechanism of the immunomodulatory action of muramyl dipeptide
- Functions of peptide cross-links in the cell wall
- Structure of the bacterial capsule and flagella
- Differences between the cytoplasmic membranes of G+ and G- bacteria
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