Composition of Bacterial Cell Wall: Overview, Topics, Structure

Definition of Bacterial Cell Wall

The bacterial cell wall is a firm, protective layer outside the cell membrane. It is very important in the maintenance of a bacterium's shape and defence against environmental stress. It is made mostly of peptidoglycan, a polymer that gives the cell wall strength.

Gram-positive bacteria have thick peptidoglycan layers and teichoic acids. Gram-negative bacteria, in turn, contain a thinner peptidoglycan layer, an outer membrane, and lipopolysaccharides which make them resistant to the effects of antibiotics.

Structure and Composition of Bacterial Cell Wall

The peptidoglycan-a mesh-like polymer of bacterial cell wall provides important rigidity and strength. Teichoic acids are found in Gram-positive bacteria and provide structural stability and function to the cell wall. Lipopolysaccharides are found in the outer membrane of Gram-negative bacteria and are very important in protection from environmental threats and antibiotics.

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This Story also Contains

1. Definition of Bacterial Cell Wall
2. Structure and Composition of Bacterial Cell Wall
3. Functions of the Bacterial Cell Wall
4. Biosynthesis of Bacterial Cell Wall Components
5. Antibiotics Targeting the Bacterial Cell Wall
6. Defective Bacterial Cell Walls
7. Clinical Relevance

Gram-Positive Bacterial Cell Wall

The thick, multilayered nature of peptidoglycan in Gram-positive bacterial cell walls may constitute up to 90% of the cell wall. This vast layer ensures that strength and rigidity are provided to the cell against osmotic pressure and mechanical stress. Embedded in this peptidoglycan matrix are teichoic acids, polymers of glycerol or ribitol phosphate that extend to the cell surface. These are important in maintaining the cell wall, regulating ions, and in pathogenesis.

A diagram illustrating the Gram-positive bacterial cell wall

Gram-Negative Bacterial Cell Wall

The Gram-negative bacterial cell wall has a thin layer of peptidoglycan sandwiched between an inner cytoplasmic membrane and an outer membrane. This outer membrane contains high quantities of lipopolysaccharides, which provide much of the structural integrity of the Gram-negative cell and serve as a barricade to unwanted access to the cell, including access by antibiotics. This compositional uniqueness allows Gram-negative bacteria greater environmental stress resistance and antimicrobial resistance than observed for Gram-positive bacteria.

Functions of the Bacterial Cell Wall

The functions of the cell wall are described below:

Protection and Structural Integrity

The cell wall of bacteria affords critical protection from environmental stress, protecting the cell from osmotic lysis and mechanical damage. Rigid cell wall structure also renders these organisms their shape and enables several bacterial species to survive in a wide range of hostile environments.

Role in Cell Division

In cell division, there occurs large-scale remodelling of the cell wall. Peptidoglycan synthesis is of paramount importance in new cell wall formation and the structural integrity of the newly formed daughter cells.

Interactions with the Environment

The cell wall also reflects the action of antibiotics and many act on peptidoglycan synthesis to inhibit growth. Since components such as lipopolysaccharides contribute much to its pathogenicity and escape from the host immune response, this makes it possible for an infection and development of a disease to progress.

Biosynthesis of Bacterial Cell Wall Components

The biosynthesis process is explained below:

Peptidoglycan Synthesis

It starts with the formation of precursor molecules in the cytoplasm. Thereafter, precursor molecules are transported to the cell membrane where polymerisation occurs. It is the linking together of many long glycan chains. These chains are cross-linked by transpeptidases and carboxypeptidases to form a strong, mesh-like structure. Transpeptidases and carboxypeptidases do such cross-linking to make the bacterial cell wall rigid.

Teichoic Acids and Lipopolysaccharides Synthesis

Teichoic acids in Gram positives are synthesised through pathways involving important enzymes that link glycerol or ribitol phosphate units, attached to the peptidoglycan layer. The typical Gram-negative contains little peptidoglycan and no teichoic acid. In Gram-negatives, the LPS is synthesised in the inner membrane and transported to the outside of the outer membrane. Other enzymes, for example, LPS synthase, are necessary to link the various components of LPS-lipid A, core polysaccharide, and O-antigen-together to form a molecule crucial for the structural and functional integrity of the outer membrane of the bacterium.

Antibiotics Targeting the Bacterial Cell Wall

These antibiotics include penicillin and vancomycin, which act on the bacterial cell wall by inhibiting peptidoglycan synthesis. For instance, penicillin binds and inactivates transpeptidases, hence preventing cross-linking of peptidoglycan strands. On the other hand, vancomycin interacts directly with peptidoglycan precursors in a way that they cannot be bridged into the cell wall.

Resistance Mechanisms

Resistance is developed in bacteria through several strategies, such as producing beta-lactamases that break down beta-lactam antibiotics, including penicillin. Other mechanisms alter target sites, thereby making drug binding to these targets less likely, thus ineffectively disrupting cell wall synthesis.

Defective Bacterial Cell Walls

This cell wall of bacteria can become defective due to interventions like antibiotics, lysozymes, and bacteriophages, which block normal cell wall-forming processes. A bacterial variant without a classical cell wall is called an L-form, which appears after exposure to antibiotics like penicillin. Mycoplasma is a form of naturally occurring cell wall-lacking bacteria. Due to their peculiar characteristics, they are differentiated from bacteria despite their natural defect of cell walls. Spheroplasts and protoplasts are forms of bacterial cells from which part, respectively, the entirety, of the wall has been digested-usually by an enzyme, such as lysozyme or an antibiotic-but can be recovered intact and viable. Spheroplasts are nonrecoverable from Gram positives but can be formed from Gram negatives. Protoplasts are recoverable and viable and are formed from Gram positives. The most important aspect of these forms is that they reflect the ability of bacteria to adapt to and survive, environmental and antimicrobial pressures upon them.

Clinical Relevance

Gram staining is a cornerstone of clinical microbiological diagnosis since it allows for the differentiation of bacteria into two groups, Gram-positive and Gram-negative due to differences in cell wall composition. Gram-staining is a differential procedure in which a bacterial sample is subjected to a sequence of dyes namely; Crystal violet, iodine, alcohol wash, and safranin which generates differential colour reactions. Gram staining therefore facilitates the diagnosis of bacterial infections and the proper selection of antimicrobials for treatment. This is possible because gram-positive and gram-negative bacteria are dissimilarly susceptible to different classes of antibiotics and their pathogenic potential also differs.

Conclusion

In brief, the bacterial cell wall is an important peptidoglycan structure, teichoic acids in Gram-positive bacteria, and lipopolysaccharides in Gram-negative bacteria. It gives protection, structural integrity, and defence mechanisms against environmental stresses. Its composition and functions are significant in microbiology for the identification of bacterial species, guiding antibiotic therapy, and study of bacterial pathogenesis. The understanding of bacterial cell walls is important now more than ever to the advancement of medical treatment and varied biotechnological applications. In essence, research concerning bacterial cell walls is critical to moving the balances in both branches of science—that is, medicine and fundamental and applied sciences.