- The diagram above illustrates the differences in the structure of Gram-positive and Gram-negative bacteria. The two key features that lead to the differing visualization properties of Gram-positive and Gram-negative species are the thickness of the peptidoglycan layer and the presence or absence of the outer lipid membrane.
- This is because the wall structure affects the cell’s ability to retain the crystal violet stain used in the Gram staining procedure which can then be visualized under a light microscope.
- As Gram-positive bacteria lack an outer lipid membrane, when correctly referring to their structure rather than staining properties, are termed monoderms. The outer lipid membrane possessed by Gram-negative bacteria means that, when referring to their physical structure, they are termed diderms.
- The Gram staining technique was developed in 1884 by Danish bacteriologist Hans Christian Gram1. Whilst a Gram stain will not tell you the specific species you are looking at, it can be a quick way to narrow down greatly the list of potential candidates and direct follow-up testing where necessary.
- Most bacteria can be broadly classified as Gram-positive or Gram-negative. Gram-positive bacteria have cell walls composed of thick layers of peptidoglycan. Gram-positive cells stain purple when subjected to a Gram stain procedure. Gram-negative bacteria have cell walls with a thin layer of peptidoglycan.
- The cell wall of gram-positive bacteria is composed of thick layers of peptidoglycan.
- The cell wall of gram-negative bacteria is composed of thin layers of peptidoglycan.
- In the gram staining procedure, gram-positive cells retain the purple-colored stain.
- In the gram staining procedure, gram-negative cells do not retain the purple-colored stain.
- Gram-positive bacteria produce exotoxins.
- Gram-negative bacteria produce endotoxins.
|
Parameter |
Gram-positive bacteria |
Gram-negative bacteria |
|
Cell Wall |
A single-layered,
smooth cell wall |
A
double-layered, wavy cell-wall |
|
Cell Wall thickness |
The thickness of the
cell wall is 20 to 80 nanometres |
The thickness of the
cell wall is 8 to 10 nanometres |
|
Peptidoglycan Layer |
It is a thick layer/
also can be multi-layered. |
It is a thin layer/
often single-layered. |
|
Teichoic acids |
Teichoic acids are
present. |
Teichoic acids are not
present. |
|
Lipopolysaccharide |
Lipopolysaccharide is
not present. |
Lipopolysaccharide is
present. |
|
Outer membrane |
The outer membrane is
not present. |
The outer membrane is
mostly present. |
|
Lipid content |
The Lipid content is
very low. |
The Lipid content is
20% to 30%. |
|
Resistance to
Antibiotic |
These are very
susceptible to antibiotics. |
These are very
resistant to antibiotics. |