Bacteria are microscopic, single-celled organisms found almost everywhere. They possess a protective layer surrounding their cell membrane, known as the cell wall. This outer boundary plays an important role in their survival and interaction with their environment. Its composition reveals how bacteria maintain structure and withstand conditions.
The Primary Building Block
Nearly all bacterial cell walls are primarily composed of peptidoglycan, also known as murein. This complex polymer forms a robust, mesh-like sac encasing the bacterial cell.
Peptidoglycan is constructed from repeating disaccharide units: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). These link to form long, linear glycan strands.
Each NAM sugar has a short chain of amino acids, a peptide side chain. These chains extend from the glycan strands, cross-linking adjacent strands.
This cross-linking creates a strong, three-dimensional network, providing the cell wall with tensile strength and rigidity, contributing to its overall integrity.
Variations Across Bacterial Types
While peptidoglycan is a universal component of bacterial cell walls, its quantity and association with other structures vary among different bacterial groups. These differences are evident when comparing Gram-positive and Gram-negative bacteria, a distinction based on a common laboratory staining technique.
Gram-positive bacteria possess a thicker peptidoglycan layer, accounting for up to 90% of their cell wall material. This multi-layered matrix is often interwoven with teichoic acids, which are polymers of glycerol or ribitol phosphate.
In contrast, Gram-negative bacteria feature a thinner peptidoglycan layer, typically comprising only 5% to 10% of their cell wall. This thin peptidoglycan is situated between the inner cytoplasmic membrane and an outer membrane.
The outer membrane of Gram-negative bacteria is a complex structure composed of phospholipids, proteins, and lipopolysaccharides (LPS). LPS molecules contribute to the bacterium’s surface properties and interactions with its environment.
The Cell Wall’s Crucial Roles
The bacterial cell wall performs several important functions related to its composition and integrity. It provides structural support, maintaining the characteristic shape of the bacterial cell, whether rod-like, spherical, or spiral.
This rigid exterior prevents the cell from deforming under various pressures. The cell wall also protects the bacterium from osmotic lysis, a process where the cell bursts due to excessive water intake.
When bacteria are in an environment with a lower solute concentration than their cytoplasm, water tends to rush into the cell. The strong, rigid cell wall acts as a pressure vessel, counteracting this inward osmotic pressure and preventing the cell membrane from rupturing.
It also offers protection against harmful substances and external physical stresses.