“Gram-positive” and “gram negative” are two terms used to classify two distinct kinds of bacteria. The distinction is in the form cells’ walls and the way they react with Gram staining.

Gram-positive bacteria’s cell walls are composed by a thick layer peptidoglycan. The cell walls of Gram-negative bacteria are made up of a small amount of peptidoglycan however, they also contain an outer membrane that is not present in gram-positive bacteria.

Gram staining is an approach that employs violet dye to differentiate between gram positive and gram negative bacteria. In the case of bacteria that are gram positive the thick proteinoglycan layer inside the cell walls will hold the dye, and they’ll stain with violet. If the bacteria are not gram-positive, the dye will escape out of the peptidoglycan layer and the bacteria will be stained red.

What is Bacteria?

Bacteria (single bacteria) are a kind of prokaryotic, unicellular organism that is found in almost all the environments on Earth. Bacteria are extremely diverse and are found in the ocean, soil hot springs, in our homes as well as inside our bodies. A lot of them are harmless, or beneficial. For instance, the bacteria which reside in the human digestive tract (known as the microbiota of the gut) assist in digestion, and can even help stop or treat certain diseases. Other (called pathogens) are not as welcoming and have the potential to cause illnesses.

There are believed to be millions of bacteria that live on Earth However, they could be broadly classified into two types with a common characteristic which is the cell wall structure.

What is Gram Positive Bacteria?

Bacteria that retain that crystal violet color after staining of gram, giving positive color in tests, are referred to as Gram positive bacteria. They are visible in a purple hue under the microscope because they stain. The thick peptidoglycan layer in Gram positive bacteria plays a role to keep the stain after color change. One of the distinctive characteristics of Gram positive bacteria is that they are more vulnerable to antibiotics due to the absence the outer layer.

Example: Gram Positive Cocci

Gram positive cocci are Gram positive bacteria with a spherical shape. Two Gram positive genera of cocci recognized for their roles in human disease are Staphylococcus as well as Streptococcus. Staphylococcus are spherical and their cells form clusters after division. Streptococcus cells are characterized by large chains following division. The Gram positive types of cocci which infect the skin comprise Staphylococcus epidermidis Staphylococcus aureus and Streptococcus pyogenes.

Cell Wall Structure in Gram Positive Bacteria

Gram positive bacteria possess the cell wall that is continuous, called the sacculus. It’s 20 to 80 nanometers thick. Cell walls are made up of peptidoglycan , also known as murein. Peptidoglycans have a glycan backbone that is composed of both N-acetylated Muramic acid and the glucosamine. In Gram positive bacteria, this glycan’s backbone is cross-linked by the oligopeptides. The b-lactam antibiotics attack the enzyme transpeptidase that is responsible for cross-linking. In gram positive bacteria, the teichoic acids is present, which is covalently connected to the peptidoglycan backbone. Teichoic acid has a powerful negative charge and is very antigenic.

What is Gram Negative Bacteria?

Bacteria that do not keep the stain of crystal violet when the gram staining process are referred to as Gram negative bacteria. The peptidoglycan layerthat is responsible for keeping the stain of crystal violet is very thin in gram negative bacteria. It is sandwiched between both the inner membrane of the cytoplasm as well as an outer layer of bacteria. Thus bacteria that are gram negative can be stained with this counterstain which is safranin, during the gram staining process that gives the red-to- pink hue. Escherichia Coli is gram-negative and is an example organism for many research studies on bacterial. Gram negative bacteria tend to be pathogenic because of their lower resistance to the effects of antibiotics. Resistance to antibiotics in Gram negative bacteria is provided by the outer membrane that is present within these bacteria. Neisseria gonorrhoeae Pseudomonas Aeruginosa, and Yersinia pestis, which are Gram negative bacteria can cause disease.

Example: Gram Negative Cocci

Gram negative cocci are Gram negative bacteria with a spherical shape. Bacteria belonging to this genus Neisseria can be examples of Gram positive cocci that can cause diseases in humans. Neisseria meningitidis is a diplococcus meaning that its spherical cell remain in pairs following cell division. Neisseria meningitidis is a cause of meningitis bacterial and can cause shock and septicemia.

Cell Wall and Cell Envelope Structure of Gram Negative Bacteria

The wall inside the cell of Gram negative bacteria is 5-10 nanometers thick and contains an amorphous layer of peptidoglycan. The peptidoglycan backbone can be partially cross-linked in Gram negative bacteria. Teichoic acid isn’t present inside the cell walls of Gram negative bacteria. Gram negative bacteria are made up of a cell envelope on the cell’s outer wall, referred to as the outer membrane. It is 7.5-10 Nm thick. Within the membrane that surrounds Gram negative bacteriaare lipopolysaccharides that serve as endotoxins are present. The membrane’s outer layer is non-covalently linked to lipoproteins. These are known as Braun’s lipoproteins. They are bound by covalent bonds with the peptidoglycan layer. The outer and inner membranes are joined through many Bayer patches.

Toxins produced by Gram-positive and Gram-Negative Bacteria

Endotoxins and exotoxins are two types of toxins created by bacteria. Endotoxins are also referred to as lipopolysaccharides. They are found in the membranes that surround Gram-negative bacteria. Since gram-positive bacteria do not have an outer membrane they are unable to make endotoxins.

Exotoxins are toxic substances that are released by bacteria. They are produced by both gram-negative as well as Gram-positive bacteria.

What exactly is Gram Staining? and how does it work?

Gram staining is a popular method used to differentiate the difference between Gram negative bacteria and positive bacteria. The name was coined in honor of Hans Christian Gram, a Danish scientist who developed the technique in 1882.

Gram staining involves staining sample of bacterial cells using the crystal violet dye, followed by a solution of Gram’s iodine (containing Iodine and potassium iodide). Then an decolorizer (such as acetone or ethyl alcohol) can be added to the specimen. It dehydrates the peptidoglycan lining inside the cell walls and causes that it shrinks and to tighten. In gram-positive bacteria, crystal violet dye gets trapped within the peptidoglycan layer. In gram-negative organisms, the peptidoglycan layer, which is thin, cannot hold the dye and it is released from cells’ walls.

Then, the bacteria can be examined under microscopes. The color of the cells following staining is a sign that they are gram-positive or negative.

Gram Staining Results for Gram-positive and Gram-Negative Bacteria

If the bacteria are Gram-positive, the Gram staining process can turn them into violet. This is because the mesh-like, thick layer of peptidoglycan that is in the cell walls holds on to that crystal violet color. If the bacteria found in the sample are not gram negative, the dye will stain them in red or pink because their peptidoglycan walls are thin and do not contain coloration with crystal violet throughout the decolorizing process.

What are the similarities between Gram Positive and Gram Negative Cell Wall?

• Cell walls of Gram negative and positive are found in bacterial cells.
• Both contain peptidoglycan layer.
• They provide structural support for bacteria.

The difference between Gram-Negative and Gram Positive Bacteria

Character	Gram-Positive Bacteria	Gram-Negative Bacteria
Gram Reaction	Retain crystal violet dye and stain blue or purple on Gram’s staining.	Accept safranin after decolorization and stain pink or red on Gram’s staining.
Cell wall thickness	Thick (20-80 nm)	Thin (8-10 nm)
Peptidoglycan Layer	Thick (multilayered)	Thin (single-layered)
Rigidity and Elasticity	Rigid and less elastic	Less rigid and more elastic
Outer Membrane	Absent	Present
Variety of amino acid in cell wall	Few	Several
Aromatic and Sulfur-containing amino acid in cell wall	Absent	Present
Periplasmic Space	Absent	Present
Teichoic Acids	Mostly present	Absent
Porins	Absent	Present
Lipopolysaccharide (LPS) Content	Virtually None	High
Lipid and Lipoprotein Content	Low (acid-fast bacteria have lipids linked to peptidoglycan)	High (because of the presence of outer membrane
Ratio of RNA:DNA	8:1	Almost 1
Mesosomes	Quite Prominent	Less Prominent
Flagellar Structure	2 rings in basal body	4 rings in basal body
Magnetosomes	Usually absent.	Sometimes present.
Morphology	Usually cocci or spore-forming rods (exception: Lactobacillus and Corynebacterium)	Usually non-spore-forming rods (Exception: Neisseria)
Endospore formation	Some produce endospores during unfavorable conditions.	Usually not found to produce endospores.
Toxin Produced	Exotoxins	Endotoxins or Exotoxins
Pathogens	Few pathogenic bacteria belong to the Gram-positive group.	Most pathogens are Gram-negative.
Nutritional Requirements	Relatively Complex	Relatively Simple
Resistance to Physical Disruption	High	Low
Cell Wall Disruption by Lysozyme	High	Low (requires pretreatment to destabilize outer membrane)
Susceptibility to Penicillin and Sulfonamide	High	Low
Susceptibility to Streptomycin, Chloramphenicol, and Tetracycline	Low	High
Inhibition by Basic Dyes	High	Low
Susceptibility to Anionic Detergents	High	Low
Resistance to Sodium Azide	High	Low
Resistance to Drying	High	Low
Rendering	They can render Gram -ve by increasing acidity	They can render Gram +ve by increasing alkalinity
Examples	Staphylococcus Streptococcus Bacillus Clostridium Enterococcus Listeria	Escherichia Salmonella Klebsiella Proteus Helicobacter Pseudomonas

References

• https://pediaa.com/difference-between-gram-positive-and-gram-negative-bacteria/
• https://microbenotes.com/differences-between-gram-positive-and-gram-negative-bacteria/
• https://www.thoughtco.com/gram-positive-gram-negative-bacteria-4174239
• https://biologydictionary.net/gram-positive-vs-gram-negative/
• https://www.technologynetworks.com/immunology/articles/gram-positive-vs-gram-negative-323007