Bacteria are microscopic, single-celled organisms abundant in nearly every environment. They possess various outer structures important for their survival. Among these, the cell wall serves as a protective layer. This outer boundary helps maintain the bacterium’s shape and shields it from external pressures.
The Building Block of Bacterial Walls
Peptidoglycan is a polymer primarily found in bacterial cell walls. It is a complex mesh-like layer composed of sugar derivatives and amino acids. The sugar component consists of alternating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked in long chains.
Short chains of amino acids, known as peptides, attach to the N-acetylmuramic acid units. These peptide chains then cross-link with peptide chains from other sugar strands, creating a strong, three-dimensional network. This structure provides the bacterial cell wall with rigidity, protecting it from internal osmotic pressure, which could otherwise cause the cell to burst.
Peptidoglycan in Eubacteria
Eubacteria possess peptidoglycan in their cell walls. The arrangement and thickness of this layer classify eubacteria into two major groups: Gram-positive and Gram-negative bacteria. This distinction is visible through Gram staining.
Gram-positive bacteria have a thick peptidoglycan layer, constituting a significant portion of their cell wall, up to 90% of its dry weight. This layer can be 20 to 80 nanometers thick and is composed of interconnected sheets. Gram-positive cell walls also contain teichoic acids interwoven within the peptidoglycan, contributing to its strength.
In contrast, Gram-negative bacteria possess a much thinner peptidoglycan layer, 5 to 10 nanometers thick, representing a smaller percentage (5-10%) of their cell wall’s dry weight. This thin layer is located in the periplasmic space, between an inner and an outer membrane. The outer membrane of Gram-negative bacteria is complex, containing lipopolysaccharides (LPS), proteins, and phospholipids, which contribute to its protective functions. These structural differences are important for bacterial identification.
Beyond Eubacteria: Where Peptidoglycan Isn’t Found
While peptidoglycan is a defining characteristic of eubacteria, it is absent in other domains of life. Archaea, which are also single-celled microorganisms, possess cell walls that differ significantly in their chemical composition. Instead of peptidoglycan, archaeal cell walls may be composed of various materials, such as pseudopeptidoglycan (also known as pseudomurein), glycoproteins, or protein-based S-layers. Pseudopeptidoglycan, while structurally similar to peptidoglycan in some ways, lacks N-acetylmuramic acid and specific amino acids found in true peptidoglycan.
Eukaryotic cells, which include organisms like plants, fungi, and animals, also exhibit diverse cell wall structures, or in some cases, a complete absence of a cell wall. Plant cells have cell walls primarily made of cellulose, a complex carbohydrate that provides structural support. Fungal cell walls are composed mainly of chitin, another polysaccharide. Animal cells, however, do not possess cell walls, relying instead on their cell membranes and an extracellular matrix for support and protection. This distinction highlights the unique nature of peptidoglycan as a component specific to the cell walls of eubacteria.