Composition of Bacterial Cell Wall- Overview, Topics, Structure

Composition of Bacterial Cell Wall: Overview, Topics, Structure

Edited By Irshad Anwar | Updated on Jul 02, 2025 05:56 PM IST

The bacterial cell wall is made up of peptidoglycan, a complex molecule of sugars and amino acids. The composition of the bacterial cell wall varies between bacteria, with gram-positive bacteria having a thick peptidoglycan layer, while gram-negative bacteria have a thinner layer and an outer membrane. The bacterial cell wall structure is essential for maintaining cell shape and protecting the cell. A bacterial cell wall diagram typically shows the differences between gram-positive and gram-negative bacteria. This is an important topic of biology as it links some of the major chapters.

This Story also Contains

Composition of Bacterial Cell Wall
Gram-Positive Bacterial Cell Wall
Gram-Negative Bacterial Cell Wall
Defective Bacterial Cell Walls

Composition of Bacterial Cell Wall

The bacterial cell wall is a firm, protective layer outside the cell wall, essential for maintaining the bacterium's shape and providing defence against environmental stress. The composition of the bacterial cell wall is primarily made up of peptidoglycan, a polymer that provides strength to the structure. Gram-positive bacteria have a thick peptidoglycan layer and teichoic acids, while gram-negative bacteria feature a thinner peptidoglycan layer, an outer membrane, and lipopolysaccharides that contribute to antibiotic resistance.

Composition of Bacterial Cell Wall

The peptidoglycan, a mesh-like polymer of the 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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Gram-Positive Bacterial Cell Wall

The bacterial cell wall of gram-positive bacteria is made up of a thick, multilayered peptidoglycan layer, which may constitute up to 90% of the composition of the bacterial cell wall. This extensive peptidoglycan layer provides strength and rigidity to the cell, protecting it against osmotic pressure and mechanical stress. Some of the basics are discussed below:

Gram-positive bacterial cell walls consist of a thick, multilayer peptidoglycan coat, as much as 90% of the cell wall.
The structural rigidity and strength for the cell are provided by the thick peptidoglycan layer, shielding the cell against osmotic pressure and mechanical stress.
Teichoic acids are embedded in the cell wall and have significant functions in wall stability, ion homeostasis, and bacterial virulence.
The peptidoglycan and teichoic acids combination guarantees stability and survival of Gram-positive bacteria.
The structure is crucial for bacterial survival and is usually represented in bacterial cell wall diagrams for simplicity.

A diagram of the Gram-positive bacterial cell wall

Gram-positive bacterial cell wall

Gram-Negative Bacterial Cell Wall

The Gram-negative bacterial cell wall is made up of a thin layer of peptidoglycan, which is sandwiched between an inner cytoplasmic membrane and an outer membrane. This outer membrane contains high quantities of lipopolysaccharides, which contribute significantly to the structural integrity of the Gram-negative cell wall and act as a barrier against unwanted access, including that of antibiotics. Some of the basics are discussed below:

Gram-negative bacterial cell walls consist of a thin peptidoglycan layer positioned between the inner cytoplasmic membrane and the outer membrane.
The outer membrane is lipopolysaccharide (LPS) rich, adding extra structural strength and serving as a barrier against toxic substances such as antibiotics.
The structure confers more resistance to environmental stress and antimicrobial agents than Gram-positive bacteria.
The composition of the Gram-negative cell wall makes the bacteria stronger and more resistant to hostile conditions.
The components of the cell wall peptidoglycan, lipopolysaccharides, and other molecules collaborate to stabilise and protect the bacterial cell.

Defective Bacterial Cell Walls

The bacterial cell wall is required for shape maintenance, rigidity, and osmotic stress protection. Nevertheless, under some environmental or chemical challenges, like the presence of antibiotics (e.g., penicillin), lysozymes, or bacteriophage infection, the bacterial cell wall integrity can be disrupted. The disruption results in cell wall-deficient structures, which are temporary (L-forms) or permanent (as in Mycoplasma). These adaptations allow bacteria to thrive in extreme environments and be resistant to antimicrobial agents by altering or dropping their cell walls. Some basic points are discussed below:

L-forms are transient, cell wall-deficient bacterial forms provoked by stress, particularly antibiotics, and are capable of reverting to their normal form.
Mycoplasma is a naturally occurring cell wall-deficient bacterial genus and is inherently resistant to beta-lactam antibiotics.
Spheroplasts are incomplete cell wall-deficient forms usually derived from Gram-negative bacteria and are usually non-viable.
Protoplasts are full-wall-deficient forms derived from Gram-positive bacteria and are usually viable and can be regenerated.
Bacterial wall deficiency is a reflection of adaptive survival under antimicrobial or environmental stress.

Other useful Resources:

Fungus Life Cycle	Classification of Virus
Basis of Biological Classification	Mycoplasma
Phylum Ctenophora	Phylum Mollusca

Frequently Asked Questions (FAQs)

1. What are the main differences between Gram-positive and Gram-negative bacterial cell walls?

Gram-positive bacteria contain thick peptidoglycan and teichoic acids, while Gram-negative bacteria have thin peptidoglycan sandwiched between an inner and outer membrane; most of the latter contain lipopolysaccharides.

2. How does penicillin target the bacterial cell wall?

These penicillins are acting on the transpeptidase enzyme, which normally cross-links the peptidoglycan strands. These ultimately result in the weakening of the cell wall, and the foremost consequence is the lysis of bacterial cells.

3. What is the role of peptidoglycan in bacterial cells?

This provides the bacterial cell wall with rigidity and structural support that protects the cell from osmotic pressure and helps in the shape maintenance of the cell.

4. Why are Gram-negative bacteria more resistant to antibiotics?

Gram-negative bacteria have an outer membrane barrier that prevents many antibiotics from reaching their target sites within the cell wall or membrane.

5. How is the bacterial cell wall involved in pathogenicity?

Bacterial adhesion, colonisation, and evasion of host immune responses are facilitated by a component such as lipopolysaccharides and teichoic acids, thereby enhancing the ability to cause disease.

6. How does the composition of Gram-positive bacterial cell walls differ from Gram-negative ones?

Gram-positive bacterial cell walls have a thick layer of peptidoglycan (20-80 nm) and teichoic acids, while Gram-negative cell walls have a thin peptidoglycan layer (2-7 nm) sandwiched between an inner and outer membrane, with lipopolysaccharides on the outer membrane.

7. Why do Gram-positive bacteria retain crystal violet stain during the Gram staining process?

Gram-positive bacteria retain crystal violet stain due to their thick peptidoglycan layer, which traps the dye-iodine complex. The alcohol wash in the staining process cannot penetrate this thick layer, keeping the cells purple.

8. How do lipopolysaccharides (LPS) contribute to the function of Gram-negative bacterial cell walls?

Lipopolysaccharides in Gram-negative bacteria contribute to the structural integrity of the outer membrane, protect against certain antibiotics, and act as endotoxins that can trigger immune responses in host organisms.

9. What role do teichoic acids play in Gram-positive bacterial cell walls?

Teichoic acids in Gram-positive bacteria help regulate cell division, maintain cell shape, and facilitate the binding of divalent cations like magnesium and calcium. They also contribute to the cell's negative charge and can act as attachment sites for bacteriophages.

10. What is the role of porins in the bacterial cell wall?

Porins are protein channels in the outer membrane of Gram-negative bacteria. They allow the passive diffusion of small, hydrophilic molecules across the membrane, facilitating nutrient uptake and waste excretion while maintaining a barrier against larger molecules.

11. What is peptidoglycan and why is it important in bacterial cell walls?

Peptidoglycan is a mesh-like polymer composed of sugar and amino acid chains. It's crucial for maintaining cell shape and integrity, providing protection against osmotic pressure, and serving as an attachment point for other cell wall components.

12. What is the significance of the periplasmic space in Gram-negative bacteria?

The periplasmic space in Gram-negative bacteria is the area between the inner and outer membranes. It contains enzymes for nutrient breakdown, proteins for cell envelope biogenesis, and components of transport systems, playing a crucial role in cellular processes and antibiotic resistance.

13. What is the primary function of the bacterial cell wall?

The bacterial cell wall's primary function is to provide structural support and protection to the cell. It maintains the cell's shape, prevents osmotic lysis in hypotonic environments, and acts as a barrier against harmful substances.

14. How does the structure of the bacterial cell wall contribute to antibiotic resistance?

The cell wall structure can contribute to antibiotic resistance by acting as a physical barrier, modifying antibiotic targets, or harboring enzymes that degrade antibiotics. For example, the outer membrane of Gram-negative bacteria can prevent certain antibiotics from entering the cell.

15. How does osmotic pressure affect bacteria with different cell wall structures?

Bacteria with thick cell walls (like Gram-positive bacteria) are more resistant to osmotic pressure changes. In contrast, Gram-negative bacteria, with thinner cell walls, are more susceptible to osmotic shock but are protected by their outer membrane.

16. How does the composition of archaeal cell walls differ from bacterial cell walls?

Archaeal cell walls differ significantly from bacterial cell walls. They lack peptidoglycan and instead may contain pseudopeptidoglycan, S-layers, or other unique polymers. This difference reflects the evolutionary distance between Archaea and Bacteria.

17. How do mycolic acids contribute to the unique cell wall structure of Mycobacteria?

Mycolic acids are long-chain fatty acids found in the cell walls of Mycobacteria. They form a waxy, hydrophobic layer that contributes to the bacteria's resistance to many antibiotics and disinfectants, and helps them survive in harsh environments.

18. What is the difference between peptidoglycan and murein?

Peptidoglycan and murein are actually the same thing. Murein is another name for peptidoglycan, which is the mesh-like polymer that forms the cell wall of most bacteria. The terms are often used interchangeably in scientific literature.

19. How does the bacterial cell wall synthesis process (involving penicillin-binding proteins) make it a good target for antibiotics?

Bacterial cell wall synthesis involves unique enzymes like penicillin-binding proteins (PBPs) that are not found in human cells. This makes the process an excellent target for antibiotics, as drugs can disrupt bacterial cell wall formation without directly affecting human cells, reducing side effects.

20. What is the function of D-amino acids in bacterial cell walls?

D-amino acids, which are rare in nature, are crucial components of the peptidoglycan in bacterial cell walls. They contribute to the strength and rigidity of the cell wall by forming cross-links between peptidoglycan strands and help protect against enzymatic degradation by most peptidases.

21. What is the role of N-acetylmuramic acid (NAM) in bacterial cell walls?

N-acetylmuramic acid (NAM) is a modified sugar molecule that forms the backbone of peptidoglycan along with N-acetylglucosamine (NAG). NAM provides attachment points for the peptide chains that cross-link the glycan strands, contributing to the overall strength and structure of the cell wall.

22. How do bacteria regulate cell wall thickness during growth and division?

Bacteria regulate cell wall thickness through a balance of synthesis and degradation. They use sensor proteins to detect cell wall stress and adjust the activity of cell wall synthesis enzymes. During division, they localize cell wall synthesis machinery to create the septum between daughter cells.

23. What is the function of the Braun's lipoprotein in Gram-negative bacterial cell walls?

Braun's lipoprotein, also known as murein lipoprotein, connects the outer membrane to the peptidoglycan layer in Gram-negative bacteria. This helps maintain the structural integrity of the cell envelope and contributes to the overall stability of the cell wall.

24. How does the bacterial cell wall respond to changes in osmotic pressure?

When faced with changes in osmotic pressure, the bacterial cell wall can stretch or compress to some extent. In hypotonic environments, it prevents the cell from bursting by resisting internal turgor pressure. In hypertonic environments, it helps maintain cell shape as the cell loses water.

25. What is the significance of D-alanine in bacterial cell walls?

D-alanine is a crucial component in the peptide chains of peptidoglycan. It's involved in the cross-linking of adjacent peptidoglycan strands, contributing to the strength and rigidity of the cell wall. Its unique D-configuration also provides resistance against many peptidases.

26. How do L-forms of bacteria survive without a cell wall?

L-forms are bacteria that have lost their cell wall, either naturally or due to antibiotic treatment. They survive by adapting their metabolism and cellular processes, relying on osmotic protection from their environment. They often revert to walled forms when conditions allow.

27. What is the role of penicillin-binding proteins (PBPs) in bacterial cell wall synthesis?

Penicillin-binding proteins (PBPs) are enzymes that catalyze the final stages of peptidoglycan synthesis. They perform transglycosylation (linking glycan strands) and transpeptidation (cross-linking peptide chains), which are crucial for cell wall formation and integrity.

28. How does the bacterial cell wall contribute to pathogenicity?

The bacterial cell wall contributes to pathogenicity in several ways: it helps bacteria survive in host environments, its components (like LPS in Gram-negative bacteria) can trigger immune responses, and it can provide attachment sites for virulence factors or resist host defense mechanisms.

29. What is the function of anionic polymers in bacterial cell walls?

Anionic polymers, such as teichoic acids in Gram-positive bacteria and lipopolysaccharides in Gram-negative bacteria, contribute to the overall negative charge of the cell surface. This charge helps regulate ion flow, affects cell adhesion, and can influence interactions with host immune systems.

30. How do bacteria modify their cell walls to resist antibiotics?

Bacteria can modify their cell walls to resist antibiotics by altering the composition or cross-linking of peptidoglycan, changing the expression of porins, or producing enzymes that modify or degrade antibiotics. Some bacteria can also thicken their cell walls to reduce antibiotic penetration.

31. What is the role of autolysins in bacterial cell wall dynamics?

Autolysins are enzymes that break down peptidoglycan. They play crucial roles in cell growth, division, and peptidoglycan remodeling. By selectively degrading parts of the cell wall, autolysins allow for the insertion of new material during growth and the separation of daughter cells during division.

32. How does the bacterial cell wall influence bacterial shape?

The bacterial cell wall, particularly its peptidoglycan component, is a major determinant of bacterial shape. The arrangement and cross-linking of peptidoglycan strands, along with the action of shape-determining proteins, dictate whether a bacterium will be spherical, rod-shaped, or have other morphologies.

33. What is the significance of the periplasmic space in antibiotic resistance?

The periplasmic space in Gram-negative bacteria can harbor enzymes that degrade antibiotics before they reach their targets. For example, beta-lactamases in the periplasm can break down penicillin-like antibiotics, contributing to antibiotic resistance.

34. How do mycoplasmas survive without a cell wall?

Mycoplasmas are a group of bacteria that naturally lack cell walls. They survive by having a cholesterol-containing plasma membrane that provides some structural support. They are also osmotically sensitive and require specific environmental conditions or host cells to survive.

35. What is the role of lipoteichoic acids in Gram-positive bacterial cell walls?

Lipoteichoic acids are amphipathic molecules anchored in the cell membrane of Gram-positive bacteria. They extend through the peptidoglycan layer and contribute to cell wall integrity, regulate autolysins, and play a role in adhesion to host cells and in triggering immune responses.

36. How does the composition of the bacterial cell wall affect its interaction with bacteriophages?

The bacterial cell wall composition influences bacteriophage interactions by providing specific receptors for phage attachment. For example, teichoic acids in Gram-positive bacteria and lipopolysaccharides in Gram-negative bacteria can serve as phage receptors. The cell wall structure also affects the ability of phages to penetrate and lyse the cell.

37. What is the function of diaminopimelic acid (DAP) in bacterial cell walls?

Diaminopimelic acid (DAP) is an amino acid found in the peptide chains of many bacterial peptidoglycans, especially in Gram-negative bacteria. It plays a crucial role in cross-linking peptidoglycan strands, contributing to the strength and rigidity of the cell wall. DAP is also an important nutrient for some bacteria and can be used as a bacterial growth factor.

38. How does the bacterial cell wall change during sporulation?

During sporulation, the bacterial cell wall undergoes significant changes. The developing spore forms a thick, multilayered cell wall that includes a cortex (modified peptidoglycan) and a spore coat. This specialized structure provides extreme resistance to environmental stresses, allowing the spore to survive harsh conditions.

39. What is the role of carboxypeptidases in bacterial cell wall metabolism?

Carboxypeptidases are enzymes that remove amino acids from the peptide chains in peptidoglycan. They play a role in cell wall remodeling and recycling, and can affect the degree of cross-linking in the peptidoglycan. This activity is important for cell growth, division, and maintaining cell wall integrity.

40. How does the bacterial cell wall contribute to biofilm formation?

The bacterial cell wall contributes to biofilm formation in several ways: it provides attachment sites for adhesins that allow bacteria to stick to surfaces and to each other, it can produce extracellular polymeric substances (EPS) that form the biofilm matrix, and its structural properties can influence the overall architecture of the biofilm.

41. What is the significance of the O-antigen in Gram-negative bacterial cell walls?

The O-antigen is the outermost part of the lipopolysaccharide (LPS) in Gram-negative bacterial cell walls. It contributes to bacterial virulence by helping the cell evade the host immune system, provides protection against certain environmental stresses, and can serve as a receptor for bacteriophages. The variability of O-antigens also forms the basis for serotyping many Gram-negative bacteria.

42. How do bacteria incorporate unusual amino acids into their cell walls?

Bacteria incorporate unusual amino acids, such as D-amino acids, into their cell walls through specialized enzymes. For example, racemases convert L-amino acids to D-amino acids, and specific ligases incorporate these into peptidoglycan precursors. This process is crucial for creating the unique structure and properties of bacterial cell walls.

43. What is the role of peptidoglycan hydrolases in bacterial cell wall dynamics?

Peptidoglycan hydrolases are enzymes that cleave specific bonds in the peptidoglycan structure. They play crucial roles in cell wall remodeling during growth and division, cell wall turnover, and the release of cell wall fragments that can act as signaling molecules. Their activity must be carefully regulated to maintain cell wall integrity.

44. How does the bacterial cell wall composition affect antibiotic penetration?

The composition of the bacterial cell wall significantly affects antibiotic penetration. In Gram-negative bacteria, the outer membrane acts as a barrier to many antibiotics. The thickness of the peptidoglycan layer in Gram-positive bacteria can slow antibiotic penetration. Modifications to cell wall components, such as changes in porin expression or LPS structure, can also alter antibiotic permeability.

45. What is the function of meso-diaminopimelic acid in bacterial cell walls?

Meso-diaminopimelic acid is an isomer of diaminopimelic acid found in the peptidoglycan of many bacteria, particularly Gram-negative species. It plays a crucial role in cross-linking peptidoglycan strands, contributing to the strength and rigidity of the cell wall. It's also an important precursor for lysine biosynthesis in some bacteria.

46. How do bacteria regulate the degree of cross-linking in their cell walls?

Bacteria regulate the degree of cross-linking in their cell walls through several mechanisms: controlling the expression and activity of penicillin-binding proteins (PBPs) that perform cross-linking, modulating the activity of carboxypeptidases that can remove potential cross-linking sites, and adjusting the production of cell wall precursors. This regulation is crucial for maintaining cell wall integrity while allowing for cell growth and division.

47. What is the role of D-glutamic acid in bacterial cell walls?

D-glutamic acid is an important component of the peptide chains in bacterial peptidoglycan. It contributes to the unique structure of the cell wall and provides resistance against many peptidases. The presence of D-glutamic acid also affects the overall charge of the cell wall, influencing its interactions with the environment and host immune systems.

48. How does the bacterial cell wall change in response to antibiotic exposure?

When exposed to antibiotics, bacteria can modify their cell walls in several ways: they may increase cell wall thickness, alter the degree of cross-linking, change the expression of porins, or produce enzymes that modify the cell wall structure. These changes can contribute to antibiotic resistance by reducing drug penetration or altering antibiotic targets.

49. What is the significance of N-acetylglucosamine (NAG) in bacterial cell walls?

N-acetylglucosamine (NAG) is one of the two sugar components that form the backbone of peptidoglycan, alternating with N-acetylmuramic acid (NAM). It plays a crucial role in maintaining the structural integrity of the cell wall. NAG is also important in other cellular processes and can serve as a nutrient source for some bacteria.

50. How do bacteria synthesize the precursors for cell wall components?

Bacteria synthesize cell wall precursors through complex pathways involving multiple enzymes. For peptidoglycan, they first produce UDP-N-acetylglucosamine, then convert it to UDP-N-acetylmuramic acid. Amino acids are added to form the peptide chain. These precursors are then transported across the cell membrane for incorporation into the growing cell wall.