Last updated on June 12th, 2021
The cell wall of Gram-negative bacteria is more complex than those of Gram-positive bacteria. Gram-negative bacteria contain an extra layer of cells called outer membrane or lipopolysaccharide (LPS) layer which surrounds the thin peptidoglycan layer. LPS layer is absent in Gram-positive bacteria.
The Gram-Positive Cell Wall
Peptidoglycan layer is the outermost covering of the Gram-positive cell wall. It constitutes as much as 90% of the cell wall of Gram-positive. Many Gram-positive bacteria have several sheets of peptidoglycan stacked one upon another and cross-linked by glycan strands. Many gram-positive bacteria have teichoic acids (polymers of glycerol phosphate or ribitol phosphate) covalently bonded to muramic acid in the wall peptidoglycan or membrane lipids (lipoteichoic acids).
The Gram-Negative Cell Wall
The gram-negative cell wall is chemically complex than gram-positive and consists of at least two layers. Outer membrane (lipopolysaccharide layer, LPS in short) is the outermost covering of the Gram-negative cell wall. Beneath it lies a thin sheet of peptidoglycan which constitutes only 10% of the cell wall of Gram-negative. Outer membrane contains a lipid bilayer bonded with polysaccharides (hence the name lipopolysaccharide).
Major differences between Gram-positive and Gram-negative bacteria
| Properties | Gram Positive Bacteria | Gram Negative Bacteria |
| Thickness of cellwall | Thicker than Gram negative bacteria. around 20 to 25 nm | Generally thinner, 11 to 15 nm |
| Gram reaction | Gram positive bacteria stain a deep blue color (violet/purple) in Gram staining technique | Gram negative bacteria stain pink to red color in Gram staining technique. |
| Lipopolysaccharide (LPS) layer | Absent | Only present in Gram negative bacteria. |
| Peptidoglycan layer | Thick (multilayered) peptidoglycan layer is present in Gram positive bacteria. It accounts 50% or more of the dry weight of the wall of some Gram positive bacteria. | Thin (single-layered). Around 10% weight of the cellwall of Gram negative bacteria. |
| Teichoic acids | Cell wall of gram positive bacteria bacteria contains teichoic acids. | Absent |
| Periplasmic space | Periplasmic space is single and smaller in Gram positive bacteria | There are two periplasmic space in Gram negative bacteria; one between the murein and inner cell membrane and the other between the murein and outer cell membrane. |
| Flagellar structure | 2 rings in basal body | 4 rings in basal body |
| Toxins produced | Primarily exotoxins | Primarily endotoxins, LPS layer has a endotoxic property. |
| Lipid content | Low | High around 11 to 22% of dry weight of the cell wall (because of lipid rich LPS layer). |
| Action of Lysozyme | Cell wall of Gram positive bacteria is easily destroyed by the action of lysozyme. After digestion of Peptidoglycan layer, Gram positive bacteria become protoplast. | Gram negative bacteria are refractory to lysozyme, because large protein molecule cannot penetrate the LPS layer. After digestion of Peptidoglycan layer, Gram negative bacteria become spheroplasts. |
| Examples | Clostridium spp, Bacillus spp, Listeria monocytogenes, Staphylococcus, Streptococcus, etc. | Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Neisseria gonorrhoeae, Vibrio cholerae. |
Spheroplasts: Gram-negative bacteria with the intact cytoplasmic membrane of the protoplast plus the outer membrane (LPS layer) of the cell wall, after peptidoglycan layer is destroyed by lysozyme or its synthesis inhibited by antibiotics.
Protoplasts: Cells whose walls have been completely remove and are incapable of normal growth and division.
