What is a Gram positive bacteria?

Bacteria are very small unicellular prokaryotic organisms. Its size is usually below 5 μm, although there are exceptions such as Thiomargarita namibiensis which at 750 μm is the largest known bacterium.

Unlike eukaryotic cells (fungi, animals, plants), bacteria do not have a nucleus or any other membrane-bound cell organelle, but they do have cellular wall.

In function of the cell wall structureBacteria can be classified into two large groups: bacteria Gram positive (or gram positive) and the bacteria Gram negative (or gram negative).

Gram staining is a staining technique for microscopic observation developed by Hans Christian Gram in 1884. Gram-negative bacteria stain fuchsia or pink, and Gram-positive bacteria appear purple or blue.

It is very important to understand that the classification into gram-positive and gram-negative bacteria is a classification based on the response of the bacteria to a staining technique with certain dyes and that it does not correspond to taxonomic groups or phylogenetic relationships between bacterial species.

Next we will see what the structure of the cell wall of Gram-positive bacteria is like and later we will review what Gram staining consists of and why some bacteria are positive and are stained blue-purple and others are negative and are stained pink. - fuchsia.

Gram positive bacteria cell wall

The most important difference between Gram positive and Gram negative bacteria is the composition and structure of the cell wall.

In both, the cell wall consists of the cytoplasmic membrane or cell membrane that surrounds the internal space of the cell. Then there is a layer of peptidoglycanand enters the membrane and peptidoglycan a small space called periplasmic space or periplasm.

In gram positive bacteria, the peptidoglycan layer is thickwhile in gram negative bacteria the peptidoglycan layer is much thinner and is surrounded by yet another membrane, the outer membrane.

In other words, gram positive bacteria have no outer membrane and have a much thicker peptidoglycan layer than ram negative bacteria.

Cytoplasmic membrane

The cytoplasmic membrane is a lipid membrane that surrounds the content or cell cytoplasm.

The broad structure is similar to the structure of the cytoplasmic membrane of eukaryotes: a lipid bilayer with other intercalated molecules with specific functions, especially proteins and other membrane lipids.

The cytoplasmic membrane is present in both gram negative and gram positive without much difference between them.


Surrounding the cytoplasmic membrane is a thick layer of peptidoglycan, but between the two there is a small space known as periplasm or periplasmic space.

The periplasmic space of Gram positive bacteria is Much smaller than Gram negative.

In fact, it is so small that it has been thought for a long time that they did not have it, but through observations with cryoelectron microscopy it was possible to verify that, although very small, gram-positive bacteria do have a periplasmic space.

peptidoglycan layer

peptidoglycan, or murein, is a polymeric substance composed of chains of carbohydrates and amino acids. The peptidoglycan layer is anchored to the cytoplasmic membrane, leaving the periplasmic space between them.

The peptidoglycan layer has a structural function very important. It maintains the shape of the cell, gives it resistance and counteracts the osmotic pressure of the cytoplasm preventing the cell from bursting.

Peptidoglycan is also involved in binary fission during the bacterial division and multiplication phases.

The peptidoglycan layer is present in both gram-positive and gram-negative cells, but in gram-positive it is much thicker and is the outermost layer of the cell wall.

In gram-negative bacteria, the peptidoglycan layer is much thinner and is surrounded by a second lipid membrane, the outer membrane.

In the peptidoglycan layer of Gram positive cells, teichoic acids and lipoteichoic acids, substances that are not present in Gram negative. The lipid part of lipoteichoic acid molecules serves as an anchor for peptidoglycan in the cell membrane.

Because they do not have an outer membrane, gram-positive bacteria do not have lipid A or O antigen, which are characteristic of the outer membrane of gram-negative bacteria.

Also, since gram-positive bacteria only have one membrane, the flagella (in bacteria that have them) are attached to the cell by a two-ring system, compared to the four rings that appear in gram-negative flagella, two for each membrane. .

In some bacteria there is an additional layer above the peptidoglycan which is known as S-layer. The S layer is made up of a crystal structure of proteins and glycoproteins. It can also appear in gram negatives, in this case supported by the outer membrane and not by the peptidoglycan layer.

In short, Gram positive bacteria would have a cell wall with this structure, from the inside out:

  1. Cytoplasmic membrane
  2. Periplasm (very thin)
  3. Peptidoglycan (very thick, with teichoic and lipoteichoic acids)
  4. S layer (in some bacteria)

While Gram negatives would have this structure:

  1. inner membrane
  2. Periplasm
  3. Peptidoglycan (very thin, without teichoic acids)
  4. Periplasm
  5. Outer membrane (lipid A, O antigen)
  6. S layer (in some bacteria)

What is Gram stain and why do Gram positives stain blue-violet?

Gram staining is a staining technique for bacterial cells to increase their contrast so that they can be visualized under an optical microscope. Schematically, it consists of these steps:

  1. A is applied crystal violet solution It stains all bacteria, both Gram positive and Gram negative.
  2. Apply a mixture of iodine and lugol to fix crystal violet.
  3. A mixture of alcohol and acetone is applied to dissolve the unfixed crystal violet. Crystal violet is removed from bacteria in which the dye has not fixed.
  4. Bacteria in which crystal violet remains undissolved appear blue or purple and are called Gram positive. They have fixed the crystal violet dye.
  5. So that the other bacteria in which the crystal violet has dissolved can also be seen under the microscope, another dye is applied, the safranin or fuchsin. These other bacteria will look pink and are called Gram negative. They haven't fixed the violet crystal.

The mechanism by which Gram-positive bacteria retain crystal violet is that the dye is bound to the peptidoglycanmuch more abundant in these bacteria.

In any case, it must be very clear that the Gram stain can give false positives and false negatives with some ease; for example, if too much solvent is applied or applied for too long, all the bacteria would lose the crystal violet.

Also, there are bacteria whose response to the Gram stain does not correspond to the real structure of their cell wall. For example, there are Gram-positive bacteria with thinner-than-usual peptidoglycan layers that are Gram-stained negative.

There are even bacteria, although exceptional, that do not have a cell wall, do not have peptidoglycan and are also negative in the Gram stain. For example, mycoplasmas.

For this and other reasons, the Gram stain is still widely used as a differential approximation methodfor example in the identification of pathogens, but it is not considered a taxonomically relevant classification nor does it correspond to evolutionary groups of bacteria.

As more exact terms to refer to the structure of the bacterial cell wall, we can use monoderma and diderma, depending on whether the bacterium has one or two lipid membranes. These terms were proposed by Radhey S. Gupta in the year 2000.


Among the Gram-positive bacteria there are genera that are typical and common pathogens for humans. Among them:

  • Cocos (round shape): Streptococcus, Staphylococcus.
  • Bacilli (elongated, rod-like shape): Corynebacterium, listeria, Bacillus, Clostridium.
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