Differences between Gram-negative Bacteria and Gram-positive Bacteria
Contents
Gram-positive vs. Gram-negative bacteria
Gram staining is a laboratory method used to differentiate bacterial species into two large groups based on the physical and chemical properties of their cell walls.[1][2] The technique is named after the Danish bacteriologist Hans Christian Gram, who developed the procedure in 1884.[3] The staining process separates bacteria into Gram-positive and Gram-negative groups by identifying differences in the thickness of the peptidoglycan layer in their cell walls.[3][2] Gram-positive bacteria retain the initial crystal violet stain and appear purple, while Gram-negative bacteria lose the initial stain and take up the safranin counterstain, appearing pink or red.[4][5][1]
Comparison Table
| Feature | Gram-positive Bacteria | Gram-negative Bacteria |
|---|---|---|
| Gram stain result | Retains crystal violet dye; appears purple/blue.[4][5][1] | Loses crystal violet; takes up safranin counterstain and appears pink/red.[4][5][1] |
| Peptidoglycan layer | Thick (many layers).[5] | Thin (single layer).[5] |
| Outer membrane | Absent. | Present, containing lipopolysaccharides (LPS) and porins.[4][5] |
| Lipopolysaccharide (LPS) | Absent. | Present in the outer membrane; the lipid A component acts as an endotoxin. |
| Teichoic acids | Present (includes lipoteichoic and wall teichoic acids). | Absent.[5] |
| Periplasmic space | Very small or absent. | Present, located between the inner and outer membranes.[5] |
| General antibiotic resistance | Generally more susceptible to beta-lactam antibiotics. | More intrinsically resistant due to the outer membrane acting as a permeability barrier.[5] |
Cell wall structure
The primary structural difference between these two groups of bacteria is the composition of their cell envelope.
A Gram-positive cell wall is characterized by a thick peptidoglycan layer, which can be up to 30 layers thick and constitutes up to 60% of the cell wall mass. This thick mesh is what retains the crystal violet-iodine complex during the decolorization step of the Gram stain.[3][2] Embedded within the peptidoglycan layer are polymers called teichoic acids, which are unique to Gram-positive bacteria. These include wall teichoic acids, which are covalently linked to the peptidoglycan, and lipoteichoic acids, which are anchored to the cytoplasmic membrane.
In contrast, the Gram-negative cell wall is more complex but has a much thinner peptidoglycan layer, often only a single layer thick.[4][5] This thin layer is located within the periplasmic space, an area between the inner cytoplasmic membrane and a distinct outer membrane.[5] Because of this thin peptidoglycan layer, the crystal violet-iodine complex is easily washed out by the decolorizer.[3] The outer membrane is a defining feature, composed of a lipid bilayer where the outer leaflet contains lipopolysaccharide (LPS). The lipid A portion of LPS is an endotoxin, which can trigger a strong immune response in humans.
Clinical significance
The differences in cell wall structure have significant implications for bacterial pathogenicity and treatment. The outer membrane of Gram-negative bacteria provides an additional protective barrier, making them inherently more resistant to many antibiotics, detergents, and lysozyme.[5] This membrane prevents certain drugs from reaching their target within the bacterial cell.
Gram-positive bacteria lack this outer membrane and are typically more susceptible to antibiotics that target the peptidoglycan cell wall, such as penicillin and other β-lactams. However, many Gram-positive pathogens have acquired resistance through other mechanisms, such as altering the antibiotic's target site. The LPS in Gram-negative bacteria is a potent endotoxin that can be released when the bacterial cell is destroyed, leading to symptoms like fever, inflammation, and, in severe cases, septic shock.
References
- ↑ 1.0 1.1 1.2 1.3 "clevelandclinic.org". Retrieved December 02, 2025.
- ↑ 2.0 2.1 2.2 "carleton.edu". Retrieved December 02, 2025.
- ↑ 3.0 3.1 3.2 3.3 "ksu.edu.sa". Retrieved December 02, 2025.
- ↑ 4.0 4.1 4.2 4.3 4.4 "medicalnewstoday.com". Retrieved December 02, 2025.
- ↑ 5.00 5.01 5.02 5.03 5.04 5.05 5.06 5.07 5.08 5.09 5.10 5.11 "byjus.com". Retrieved December 02, 2025.
