The bacterial cell wall is a distinctive and rigid outer layer that encases nearly all bacteria, offering structural support and protection. This external covering sets bacteria apart from other cell types, influencing how they interact with their environment and survive various conditions.
Building Blocks of the Bacterial Cell Wall
The primary component of the bacterial cell wall is a complex macromolecule called peptidoglycan. This substance forms a mesh-like layer that surrounds the bacterial cytoplasmic membrane. Peptidoglycan is a polymer composed of alternating sugar derivatives and short chains of amino acids.
The sugar backbone consists of repeating units of two amino sugars: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). Attached to each N-acetylmuramic acid (NAM) unit is a short peptide chain made of three to five amino acids. These peptide chains can vary slightly between bacterial species.
The strength and rigidity of the peptidoglycan layer come from the cross-linking of these peptide chains between adjacent sugar strands. Enzymes known as penicillin-binding proteins (PBPs) facilitate these cross-links, creating a three-dimensional mesh structure. This continuous linking of subunits is important for maintaining the cell’s form and resisting internal pressure.
Two Main Types of Bacterial Cell Walls
Bacteria are categorized into two main groups, Gram-positive and Gram-negative, based on differences in their cell wall structures, revealed by a staining technique called Gram staining. Gram-positive bacteria possess a thick peptidoglycan layer, ranging from 20 to 80 nanometers, which makes up a portion of their cell wall’s dry weight. This thick peptidoglycan is located between the plasma membrane and a periplasmic space.
A characteristic of Gram-positive cell walls is the presence of teichoic acids. These teichoic acids are woven throughout the peptidoglycan layer, either covalently attached to it or anchored to the cytoplasmic membrane. They contribute to the cell wall’s rigidity and negative charge, influencing ion balance and immune interactions.
In contrast, Gram-negative bacteria have a more complex cell envelope with a much thinner peptidoglycan layer, 7 to 8 nanometers thick, located within a space called the periplasm. This periplasmic space is situated between the inner cytoplasmic membrane and an outer membrane. The outer membrane of Gram-negative bacteria contains lipopolysaccharide (LPS) and outer membrane proteins such as porins. LPS is a toxic molecule classified as an endotoxin, which can trigger a strong immune response in infected animals. Porins are specialized channels within the outer membrane that regulate the passage of hydrophilic substances into the periplasm.
Vital Roles of the Bacterial Cell Wall
The bacterial cell wall performs several functions for bacterial survival. It provides the bacterium with its shape and structural integrity. This rigid enclosure acts as a protective barrier, shielding inner cell components from the external environment.
A primary function of the cell wall is to protect the bacterium from osmotic lysis. Bacteria have a higher concentration of solutes inside their cytoplasm compared to their surroundings, leading to internal osmotic pressure. The rigid cell wall counteracts this pressure, preventing the cell from bursting when water flows inward. Additionally, the cell wall acts as a selective barrier, regulating the movement of substances in and out of the cell, and protecting against harmful compounds. It also provides attachment sites for appendages like flagella and pili, which extend from the cytoplasmic membrane through the cell wall.
Why the Cell Wall Matters for Medicine
The bacterial cell wall is a target for many antibiotics, making it important in medicine. Beta-lactam antibiotics, a large class, work by interfering with peptidoglycan synthesis. This group includes commonly used drugs like penicillins and cephalosporins, which all share a beta-lactam ring structure.
These antibiotics function by binding to penicillin-binding proteins (PBPs), which are bacterial enzymes involved in peptidoglycan synthesis. By inhibiting these enzymes, beta-lactam antibiotics prevent the formation of a strong peptidoglycan mesh. The disruption of cell wall integrity leads to a weakened cell wall structure.
Without a cell wall to counteract internal osmotic pressure, water rushes into the bacterial cell, causing it to swell and ultimately burst, a process known as osmotic lysis. This mechanism is effective against actively growing bacteria because cell wall synthesis is ongoing during bacterial replication. The targeted disruption of this bacterial component highlights the cell wall’s importance in developing effective treatments against bacterial infections.