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As we studied the bacteria cell wall, the chemical nature of the wall is made up of muco-complex, which consists of proteins, polysaccharides, and lipids.
That’s why due to this complexity, the wall is divided into two types i.e., gram positive and gram negative cell wall as below. This chemical composition is an essential element in the cell wall and that’s why the bacteria overall holds immense importance.
Gram Positive Vs Gram Negative Cell Wall
Cell Wall Structure Of Gram Positive Bacteria
The gram-positive cell wall has a peptidoglycan layer that is relatively thick (ca.40nm) and comprises approximately 90% of the cell wall. This thick peptidoglycan layer, which is considerably hydrated, accounts for the staining reaction observed in the gram stain procedure.
The primary stain (crystal violet) passes across the wall freely and is firmly attached to the cell structure by the addition of the mordant (gram’s iodine).
The decolorizing agent (ethanol or acetone) dehydrates the wall, causing it to shrink and trap the primary stain iodine complex. Thus, gram-positive bacterial cells retain the primary stain and appear blue-purple following gram-staining.
The cell walls of most gram-positive bacteria also have teichoic acids, which are acidic anionic polysaccharides. Teichoic acids contain carbohydrates such as glucose, phosphate, and alcohol (either glycerol or ribitol).
The teichoic acid is bonded to the peptidoglycan, making them an integral part of the gram-positive cell wall structure. Teichoic acids can bind protons, thereby maintaining the cell wall at a relatively low pH.
This low pH prevents autolysins from degrading the cell wall. Teichoic acids also bind cations such as Ca2+ and Mg2+ and act as a receptor site for some viruses.
When phosphate concentration is low, gram-positive bacteria replace the phosphate-rich teichoic acids of the cell wall with teichuronic acids. This enables them to conserve phosphate that is essential for ATP, DNA, and other cellular components.
Teichuronic acids are polysaccharides chains of uronic acids and N-acetylglucosamine, which fulfill the cell requirements for the acidic, anionic polysaccharides in the cell wall.
Gram Negative Bacteria Cell Wall
The gram-negative cell wall is far more complex than the gram-positive cell wall. The peptidoglycan layer of the gram-negative bacteria is very thin (2nm). An often comprises only 10% or less of the cell wall.
The gram-negative staining occurs because the wall is too thin to retain the crystal violet iodine complex when treated with the decoloring agent. Teichoic acid does not occur in gram-negative bacteria cell walls.
Rather lipoprotein is bonded to the peptidoglycan, forming an integral part of the gram-negative bacteria wall, additionally, there are layers of lipopolysaccharide, phospholipids, and protein outside the peptidoglycan layer.
Although these layers are sometimes considered part of the cell wall, it is now more common to view the peptidoglycan layer as the wall component of a large, more complex structure, called the cell envelope of the gram-negative bacterial cell.
The cell wall structure is divided into two descriptive portions: Outer Membrane and Periplasm.
The cell envelope of gram-negative bacterial cell extends, outwards from the cytoplasmic membrane – the outer membrane.
The outer membrane is a lipid bilayer containing phospholipids, proteins, lipoprotein, and lipopolysaccharides; unlike the cytoplasmic membrane, it is relatively permeable to most small molecules.
An electron microscope revealed that cytoplasmic membrane in some gram-positive bacteria may be joined or fused at many points around the cell.
Whereas the inner surface of gram-negative bacteria’s outer membrane is bridged to the peptidoglycan layer through lipoprotein
The outer membrane contains lipopolysaccharides (LPS), which are not found in the cytoplasmic membranes. LPS is a complex molecule composed of distinct regions and is able to cause toxicity to the host cells.
Functionally, the outer membrane of the gram-negative bacterial cell is a course molecular sieve. The permeability of the outer membrane to the nutrients is due to the outer membrane proteins (Omp), collectively called porins.
The porins usually occur in aggregates of three and form cross-membrane channels through which some molecules can diffuse.
Hydrophilic and hydrophobic molecules can diffuse through the outer membrane but the cytoplasmic membrane excludes almost all hydrophilic substances except water.
The region between the cytoplasm and outer membrane is known as periplasm (also called periplasmic space or periplasmic gel).
This is an important region in gram-negative bacteria where diverse reactions occur, including oxidation-reduction reaction, osmotic regulation, solute transport, protein secretion, and hydrolytic activities.
Moreover, many proteins are found in this region, which include binding proteins, chemoreceptors, and enzymes including those involved in the oxidation-reduction process.
The binding proteins facilitate the transport of substances to carriers into the cell by delivering substances to carriers that are bound to the cytoplasm. The chemoreceptors bind with substances and direct the cell movement towards or away from these substances.
What Are Hydrolytic Enzymes?
Hydrolytic (Hydro- water, lytic- break down) enzymes are generally catabolic enzymes that break the chemical bond between atoms of large molecules in the presence of water
These hydrolytic enzymes function in the periplasm and break down large molecules into smaller ones on their way to be used to produce ATP and cellular constituents.
List Of Gram Positive And Gram Negative Bacteria
Here we are discussing some of the gram-positive and gram-negative bacteria examples as follows:
Gram Positive Bacteria Examples
- Staphylococcus aureus
List Of Gram Positive Bacteria
Below are a few of the gram-negative bacteria example species are as follows:
- Campylobacter helicobacter
Bacterial Capsules, Slime Layer, and S Layers
What is Glycocalyx?
Various external layers may surround the bacterial cell wall, playing various roles, including protection of the cell. Collectively these structures are called the glycocalyx.
This general term includes the complex polysaccharides or protein and includes capsules, slime layer, and S layers.
Some bacteria form a protective layer called a capsule. The capsule surrounds the cell wall. The chemical composition of the capsule varies among the species of the bacteria.
It often is composed of polysaccharides attached externally to the cell wall. In contrast, some bacteria have a cell wall composed of glutamic acid.
The capsule protects the bacteria against phagocytosis (phagocytosis definition – the process by which certain cells i.e. Phagocytes engulfs a large particle to form an internal part known as a phagosome) by various protozoa and human white blood cells.
Thus, having a capsule is the major factor in determining the pathogenicity of a bacterium. Although capsules and slime layer are similar in composition a distinction can be made between them.
Although both capsule and most slimy layers are composed of polysaccharides slimy layers are not as tightly bound to the cell as a capsule. These external layers may protect the bacterium against dehydration and loss of nutrition.
In some cases, they act as traps for nutrients. In some cases, they act as traps for restricting the flow of substrates away from the cell. In addition to these layers, some bacteria have a crystalline protein layer surrounding the bacteria cell. It is known as the S layer.
This layer occurs outside the wall of gram-positive bacteria and is external to the outer membrane of gram-negative bacteria. It is also the only layer observed surrounding the cytoplasmic membrane. The function of the S layer is not yet known.