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Gram Negative Bacteria

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  • Last Updated : 21 Dec, 2022
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Bacteria are typical, primarily free-living creatures with a rare single biological cell. They make up a sizable portion of the prokaryotic microbial world. Bacteria, which are typically a few micrometers long and were among the first life forms to appear on Earth, are found in the majority of its habitats. In addition to soil, water, acidic hot springs, radioactive waste, and the deep biosphere of the Earth’s crust, bacteria can also be found there. By recycling nutrients from the atmosphere, such as nitrogen, bacteria play a significant part in numerous stages of the nutrition cycle. 

The decomposition of dead bodies is a part of the nutrient cycle; microbes are in charge of the putrefaction phase of this process. Extremophile bacteria transform dissolved substances like hydrogen sulfide and methane into energy to support life in the biological communities that surround hydrothermal vents and cold seeps. Additionally, bacteria coexist with plants and animals in parasitic and symbiotic ways. Numerous kinds of bacteria cannot be produced in a lab, and the majority have not yet been fully characterized. Bacteriology, a subfield of microbiology, is the study of bacteria.

Gram-negative Bacteria

Gram-negative Bacteria


In terms of the differential staining method, these bacteria represent the antithesis of gram-positive bacteria. Following a wash with alcohol and gram staining, gram-negative bacteria will lose their crystal violet dye hue and take on the pinkish-red color of the counterstain, safranin.

Due to differences in the composition of their cell walls, Gram-positive and Gram-negative bacteria can be distinguished from one another using gram staining. Gram-positive bacteria have a thick layer of lipids and lack or have a very thin layer of peptidoglycan, whereas Gram-negative bacteria have a thick layer of lipids. Since they lack peptidoglycan, their cell walls are weaker and the primary stain can be removed with alcohol and water.

Gram-negative bacteria, both benign flora like Escherichia coli and pathogenic bacteria like Klebsiella pneumonia and Chlamydia trachomatis, can be found in a variety of habitats, especially those that support life.

Characteristics of Gram-negative Bacteria

Traditional gram-negative (LPS-diderm) bacteria have the following traits:

  • There is an inner cell membrane (cytoplasmic)
  • There is a thin peptidoglycan coating (this is much thicker in gram-positive bacteria)
  • It has an outer membrane that contains phospholipids in the inner leaflet and lipopolysaccharides (LPS, which is made up of lipid A, core polysaccharide, and O antigen) in the outer leaflet.
  • The outer membrane contains porins that function as pores for specific chemicals.
  • There is a gap filled with a concentrated gel-like substance called periplasm between the cytoplasmic and outer membranes.
  • Instead of the peptidoglycan, the S-layer is directly linked to the outer membrane.
  • Teichoic acids or lipoteichoic acids are lacking, and flagella, if present, contain four supporting rings rather than two.
  • Linked to the polysaccharide backbone are lipoproteins.
  • Some contain Braun’s lipoprotein, which forms a covalent bond between the peptidoglycan chain and the outer membrane.
  • With few exceptions, the majority do not produce spores.

Structure of Gram-negative bacteria

The periplasmic space, a single layer of peptidoglycan sandwiched between the cytoplasmic membrane and the outer membrane, is a characteristic of gram-negative bacteria. Murein, another name for peptidoglycan, is a polymer made of amino acids with a carbohydrate backbone.

In contrast to Gram-positive bacteria, which have extensively cross-linked peptide chains, Gram-negative bacteria have partially cross-linked peptide chains within the peptidoglycan structure.

Lipopolysaccharide, a large molecule that is poisonous to animals, is present in the outer membrane. The alcohol applied to the sample during Gram staining damages Gram-negative bacteria’s outer membrane, making it impossible for the crystal violet stain to be retained by the peptidoglycan’s thin layer.

By staining the decolorized Gram-negative bacteria through the thin peptidoglycan layer and being light enough to not interfere with the crystal violet staining on the Gram-positive bacteria, the counterstain is applied to offer contrast.

Periplasmic Space

In Gram-negative bacteria, the periplasmic space is made up of a number of proteins that aid in acquiring nutrients, including binding proteins and hydrolytic enzymes that actively aid in the transfer of resources into the bacterial cell and attack nucleic acids and phosphorylated compounds. Enzymes that produce peptidoglycan and alter potentially harmful toxic substances are also present in the periplasm.


Above the plasma membrane, the Gram-negative bacterial cell wall has a thin peptidoglycan layer that accounts for around 5% of the dry weight of the cell. E. coli is one bacteria that has peptidoglycans that are 2 nm thick (2-3 sheets of peptidoglycan).

Outer Membrane and the Lipopolysaccharides

  1. The thin peptidoglycan layer is located above the outer membrane. Braun’s lipoprotein, which is covalently attached to the peptidoglycan and imbedded to the outer membrane via hydrophobic ends, makes up the membrane. The peptidoglycan and the outer membrane are tightly connected by the Braun’s lipoprotein.
  2. The adhesion sites on the outer membrane, which are important for enabling cell contact and membrane fusion, also contribute to the strengthening of the gram-negative cell wall. These adhesion sites allow substances to enter cells.
  3. Lipopolysaccharides (LPSs), which are big, complex molecules consisting of lipids and carbohydrates, make up the majority of the outer membrane. Lipid A, the core polysaccharides, and the O side chain are the three components that make up a lipopolysaccharide. Salmonella Typhimurium produces the gram-negative bacterial lipopolysaccharide that has received the most research. The lipopolysaccharide A unit is made up of two glucosamine sugar derivatives, each of which contains three fatty acids and pyrophosphate. Any remaining pieces of the lipopolysaccharide protrude from the membrane’s surface.
  4. The O side chain, which is often referred to as the O antigen, is a chain that radiates from the core. It is composed of carbohydrates, which result in differences between bacterial strains. These O antigens are also in charge of how bacteria avoid immune system reactions.
  5. The cell wall’s defense against outside threats is another duty of the lipopolysaccahrides.
  6. The negative charge of the LPSs causes a negative charge on the cell surface. As a result, the membrane structure is stabilized.
  7. Lipid works as an endotoxin because it adds to the poisonous component of lipopolysacchaides.
  8. Antibiotics, bile salts, and other hazardous substances are substantially hindered from entering and disturbing the cell by the gram-negative bacteria’s outer membrane and lipopolysaccharide.
  9. As a result, porin proteins make up the outer membrane, making it permeable and permitting the passage of tiny molecules like glucose.
  10. Specific carriers are required to pass the outer membrane with larger molecules like vitamin B12.
  11. Additionally, the outer membrane aids in preventing component loss, particularly from the periplasmic region.

Gram-negative Bacteria as Pathogens

Gram-negative bacteria are frequently harmful and include Vibrio cholera, a waterborne pathogen that causes cholera outbreaks, and Escherichia coli, a major cause of food poisoning. The constituent membrane elements of Gram-negative bacteria are what give them their harmful potential.

The lipopolysaccharide endotoxin found in the outer membrane can harm the host animal or strongly stimulate its immune system. When gram-negative bacteria enter the bloodstream, they may release lipopolysaccharides in sufficient quantities to start an immunological reaction that harms the host’s organs and tissues.

The presence of circulating lipopolysaccharides in the blood of sepsis patients suggests that endotoxins are a major therapeutic target for the treatment and prevention of septic shock.

Gram-negative Bacteria and Resistance to Antibiotics

Because of their outer membrane, gram-negative bacteria are less vulnerable to antibiotics. This is so that treatments that would typically harm the inner membrane are protected against by the outer membrane. It has been observed that bacteria strains resistant to antibiotics exhibit changes in the lipid or protein content of the outer membrane.

Porins, which are protein channels that offer a passageway through the outer membrane, are the focus of small hydrophilic antibiotics. Through alterations to the outer membrane profile or decreased permeability brought on by certain mutations, antibiotic resistance has been related to a decrease in the rate of entry for antibiotics through porins.

FAQs on Gram-negative Bacteria

Question 1: What kills Gram-negative?


It has been discovered that a mixture of hydrogen peroxide and ascorbic acid produces an antibacterial mechanism that is effective against gram-negative bacteria. Bacterial mortality occurs as a result, and the organism becomes vulnerable to lysozyme lysis.

Question 2: Where are gram-negative bacteria mostly found?


There are gram-negative bacteria in almost every place on Earth that supports life. Escherichia coli, a model organism, and other pathogenic bacteria, including Pseudomonas aeruginosa, Chlamydia trachomatis, and Yersinia pestis, are all examples of gram-negative bacteria.

Question 3: What protects gram-negative?


Gram-negative bacteria have a double membrane surrounding them that functions as an extremely effective barrier of defense and increases the cell’s resistance to drugs.

Question 4: What color is Gram-negative?


The hue of a Gram stain is purple. The bacteria in a sample will either stay purple or change pink or red when the stain and bacteria interact. The bacteria are Gram-positive if they continue to be purple. The bacteria are Gram-negative if they turn pink or crimson.

Question 5: What antibiotics are used for Gram-negative bacteria?


Gram-negative bacteria may develop resistance to one or more significant groups of antibiotics, despite the fact that these drugs are typically effective against them, including:

  1. Ureidopenicillins (piperacillin)
  2. Cephalosporins from the third or fourth generation (cefotaxime, ceftazidime)
  3. Carbapenems (imipenem, meropenem) (imipenem, meropenem)
  4. Fluroquinolones (ciprofloxacin) 

Question 6: Why do we use Gram staining?


Gram staining is a method frequently used to distinguish between two sizable groups of bacteria based on the distinct components of their cell walls. By staining these cells red or violet, the Gram stain method distinguishes between Gram-positive and Gram-negative groupings.

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