The bacterial cell wall is a unique and robust outer layer that encases most bacterial cells, providing essential structural support and protection. This external barrier distinguishes bacteria from other cell types, including human cells, which lack such a structure. The cell wall helps bacteria maintain their shape and withstand various environmental stresses, allowing them to thrive in diverse conditions.
The Core Building Block: Peptidoglycan
The primary component of the bacterial cell wall is a complex molecule called peptidoglycan, also known as murein. This macromolecule forms a mesh-like layer that surrounds the bacterial cytoplasmic membrane. Peptidoglycan is constructed from repeating disaccharide units, specifically N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), linked together in long glycan strands.
Short peptide chains are attached to the N-acetylmuramic acid units, and these peptides cross-link the glycan strands, creating an intricate, three-dimensional network. This cross-linking process provides the peptidoglycan layer with its rigidity and strength. The resulting robust structure is capable of counteracting the internal osmotic pressure within the bacterial cell, which can be comparable to the pressure inside a fully inflated car tire. The continuous synthesis and remodeling of this layer are necessary for bacterial growth and cell division.
Gram-Positive vs. Gram-Negative Cell Walls
While peptidoglycan is a universal component, its arrangement and association with other molecules differ significantly between Gram-positive and Gram-negative bacteria. These differences are the basis for the Gram stain, a laboratory technique used to classify bacteria.
Gram-positive bacteria possess a thick peptidoglycan layer, which can constitute up to 90% of the cell wall. Embedded within this dense peptidoglycan matrix are teichoic acids and lipoteichoic acids, which are polymers that extend through the peptidoglycan layer or are anchored to the cytoplasmic membrane.
In contrast, Gram-negative bacteria have a more complex cell wall structure with a much thinner peptidoglycan layer, representing only 5-10% of the total cell wall. This thin peptidoglycan layer resides in the periplasmic space, a gel-like region situated between the inner cytoplasmic membrane and an outer membrane. The outer membrane of Gram-negative bacteria contains lipopolysaccharides (LPS), phospholipids, and porin proteins that form channels for molecular transport.
Why the Bacterial Cell Wall Matters
The bacterial cell wall serves several functions for bacterial survival. One primary function is to protect the bacterium from osmotic lysis, a process where the cell bursts due to the influx of water caused by higher internal solute concentrations. The rigid cell wall withstands this internal pressure, maintaining the cell’s integrity.
Beyond protection, the cell wall also helps maintain the characteristic shape of bacterial cells, whether they are spherical, rod-like, or spiral. This shape is relevant for how bacteria grow, move, and interact with their environment.
The bacterial cell wall is also a frequent target for many antibiotics, such as penicillin, because it is crucial for bacterial survival and is absent in human cells. Penicillin, for example, interferes with the synthesis of peptidoglycan, specifically inhibiting the cross-linking of peptide chains, which weakens the cell wall and leads to bacterial death. Furthermore, certain components of the cell wall, like the lipopolysaccharides (LPS) found in Gram-negative bacteria, can contribute to the bacterium’s ability to cause disease by modulating host immune responses.