The difference between Gram-positive and Gram-negative bacteria

The bacteria are divided into two different groups: Gram-negative and Gram-positive bacteria. It is based on whether they can be stained by Gram staining and these two type of bacteria have some common difference.

Staining: When stained with crystalline violet and iodine solution, a purple complex was formed in the bacteria cell wall. Since the cell wall of Gram-positive bacteria is rich in peptidoglycan, which is thick and cross-linked tightly, the peptidoglycan will form a dense network to lock the purple complex after dehydration by ethanol.

Gram-negative bacteria have a thinner layer of peptidoglycan and lipopolysaccharides outer membrane that can be dissolved by alcohol, causing them to lose the purple stain. They are stained by safranin to appear pink or red under the microscope.

Cell wall structure: Gram-negative bacteria have a thinner peptidoglycan layer in their cell walls compared to Gram-positive bacteria. In addition to peptidoglycan, Gram-negative bacteria also have an outer membrane made of lipopolysaccharides (LPS) that surrounds the peptidoglycan layer, which is absent in Gram-positive bacteria.

Sensitivity to antibiotics: Gram-negative bacteria are generally more resistant to antibiotics than Gram-positive bacteria due to the presence of the outer membrane that acts as a barrier and reduces the permeability of the cell wall to many drugs. Penicillin is effective in inhibiting Gram-positive bacteria, but is poorly effective against Gram-negative bacteria. Macrolide antibiotics interfere with the synthesis of proteins in ribosome, and they are effective against some Gram-negative bacteria.

This division is usually acted as an antibiotic treatment guide, as Gram-positive and Gram-negative bacteria may respond differently to different antibiotics.

Exotoxin vs endotoxins: Gram-negative bacteria have endotoxins to make the host ill. When they died and cell walls break down, their outer membrane lipopolysaccharides (LPS) are released into blood to cause inflammatory response of host. Gram-positive bacteria usually secrete exotoxins which are biochemicals to disturb metabolism of host.

Frequently Asked Questions

The Gram stain process involves several steps

Gram staining was developed by Danish physician Hans Christian Gram in 1884 and is widely used in the classification of bacterial species.

Preparation of the slide: A thin smear of the bacterial sample is made on a clean microscope slide and allowed to air dry.

Crystal violet staining: Drip the crystal violet to stain the sample for about 1 minute. Then wash away the stain by water.

Iodine treatment: Iodine solution is applied to the slide and allowed to sit for about 1 minute. Iodine forms a complex with crystal violet and help to fix the dye in the cell walls. Then wash away the iodine solution by water.

Decolorization: The slide is washed with a decolorizing agent, usually alcohol or acetone, for 20 to 30 seconds. This step is critical as it differentiates the Gram-positive and Gram-negative bacteria based on their cell wall structure.

Counterstaining: The slide is then colored with a contrasting stain, such as safranin, for about 2 or 3 minutes. The safranin stains the decolorized cells to allow them to be visualized under a microscope.

Rinse and dry: Excess stain is washed off the slide with water, and the slide is allowed to air dry.

Gram-positive bacteria will appear purple or violet, while Gram-negative bacteria will appear pink or red.