BACTERIAL CELL WALL: CHARACTERISTICS, BIOSYNTHESIS, FUNCTIONS - BIOLOGY - 2025

The bacterial cell wall is a complex and semi-rigid structure, responsible for providing protection and shape to bacteria. Structurally, it is made up of a molecule called peptidoglycan. In addition to protection against pressure changes, the bacterial wall provides an anchoring site for structures such as flagella or pilis and defines various properties related to virulence and cell motility.

A widely used methodology to classify bacteria according to their cell wall structure is the Gram stain. This consists of a systematic application of purple and pink dyes, where bacteria with a thick wall and rich in peptidoglycan stain purple (gram positive) and those with a thin wall surrounded by lipopolysaccharides stain pink (gram negative).

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Although other organic beings such as archaea, algae, fungi and plants have cell walls, their structure and composition differ profoundly from the bacterial cell wall.

Characteristics and structure

The bacterial wall: a peptidoglycan network

In biology we usually define the boundaries between the living and the non-living using the plasma membrane. However, there are many organisms that are surrounded by an additional barrier: the cell wall.

In bacteria, the cell wall is made up of an intricate and complex network of a macromolecule called peptidoglycan, also known as murein.

In addition, we can find other types of substances in the wall that are combined with peptidoglycan, such as carbohydrates and polypeptides that vary in length and structure.

Chemically, peptidoglycan is a disaccharide whose monomeric units are N-acetylglucosamine and N-acetylmuramic (from the murus root, which means wall).

We always find a chain made up of tetrapeptides, consisting of four amino acid residues linked to N-acetylmuramic.

The structure of the bacterial cell wall follows two schemes or two general patterns, known as gram positive and gram negative. In the next section we will develop this idea in depth.

Structures outside the cell wall

Usually the cell wall of bacteria is surrounded by some external structures, such as the glycocalyx, flagella, axial filaments, fimbriae and pilis.

The glycocalyx consists of a gelatinous matrix that surrounds the wall, and is of variable composition (polysaccharides, polypeptides, etc.). In some bacterial strains the composition of this capsule contributes to virulence. It is also a crucial component in the formation of biofilms.

Flagella are filamentous structures, whose shape resembles a whip and contributes to the mobility of the organism. The rest of the aforementioned filaments contribute to cell anchoring, motility and the exchange of genetic material.

Atypical bacterial cell walls

Although the aforementioned structure can be generalized to the vast majority of bacterial organisms, there are very specific exceptions that do not conform to this cell wall scheme, since they lack it or have very little material.

Members of the genus Mycoplasma and phylogenetically related organisms are among the smallest bacteria ever recorded. Due to their small size, they do not have a cell wall. In fact, at first they were considered viruses and not bacteria.

However, there has to be some way that these little bacteria get protection. They do this thanks to the presence of special lipids called sterols, which contribute to protection against cell lysis.

Features

-Biological functions of the bacterial cell wall

Protection

The main function of the cell wall in bacteria is to provide protection to the cell, functioning as a kind of exoskeleton (like that of arthropods).

Bacteria contain a significant amount of dissolved solutes inside. Due to the phenomenon of osmosis, the water that surrounds them will try to enter the cell creating an osmotic pressure, which if not controlled can lead to the lysis of the cell.

If the bacterial wall did not exist, the only protective barrier inside the cell would be the fragile plasma membrane of a lipid nature, which would quickly yield to the pressure caused by the osmosis phenomenon.

The bacterial cell wall forms a protective barricade against pressure fluctuations that may occur, which prevents cell lysis.

Stiffness and shape

Thanks to its stiffening properties, the wall helps shape the bacteria. That is why we can differentiate between various forms of bacteria according to this element, and we can use this characteristic to establish a classification based on the most common morphologies (cocci or bacilli, among others).

Anchor site

Finally, the cell wall serves as an anchoring site for other structures related to motility and anchoring, such as flagella.

-Cell wall applications

In addition to these biological functions, the bacterial wall also has clinical and taxonomic applications. As we will see later, the wall is used to discriminate between various types of bacteria. Furthermore, the structure makes it possible to understand the virulence of the bacterium and what kind of antibiotic it may be susceptible to.

As the chemical components of the cell wall are unique to bacteria (lacking in the human host), this element is a potential target for the development of antibiotics.

Classification according to Gram stain

In microbiology, stains are widely used procedures. Some of them are simple and their purpose is to clearly show the presence of an organism. However, other stains are of the differential type, where the dyes used react depending on the type of bacteria.

One of the most widely used differential stains in microbiology is the Gram stain, a technique developed in 1884 by the bacteriologist Hans Christian Gram. The technique allows the bacteria to be classified into large groups: gram positive and gram negative.

Today it is considered a technique of great medical utility, although some bacteria do not react properly to the coloration. It is usually applied when the bacteria are young and growing.

Gram stain protocol

(i) Application of the primary dye: a heat-fixed sample is covered with a basic purple dye, usually crystal violet is used for this. This stain permeates all the cells found in the sample.

(ii) Application of iodine: after a short period of time, the purple dye is removed from the sample and iodine, a mordant agent, is applied. At this stage, both gram-positive and negative bacteria are stained a deep purple.

(iii) Washing: the third step involves washing the colorant with an alcohol solution or with an alcohol-acetone mixture. These solutions have the ability to remove color, but only from some samples.

(iv) Application of safranin: finally, the solution applied in the previous step is removed and another dye, safranin, is applied. This is a basic red coloring. This dye is washed and the sample is ready to be observed under the light of the optical microscope.

Gram positive bacterial cell wall

In step (iii) of the staining only some bacteria retain the purple dye, and these are known as gram positive bacteria. The color of safranin does not affect them, and at the end of the coloration those that belong to this type are observed purple.

The theoretical principle of staining is based on the structure of the bacterial cell wall, since it depends on whether or not the purple dye escapes, which forms a complex together with iodine.

The basic difference between gram negative and positive bacteria is the amount of peptidoglycan they present. Gram positives have a thick layer of this compound that allows them to retain the purple coloration, despite subsequent washing.

The violet crystal that enters the cell in the first step forms a complex with the iodine, which makes it difficult to escape with the alcohol wash, thanks to the thick layer of peptidoglycan that surrounds them.

The space between the peptidoglycan layer and the cell membrane is known as the plasmic space and it consists of a granular layer composed of lipoteichoic acid. Additionally, gram positive bacteria are characterized by having a series of teichoic acids anchored to the wall.

An example of this type of bacteria is the Staphylococcus aureus species, which is a pathogen for humans.

Gram negative bacterial cell wall

Bacteria that do not retain the staining of step (iii) are, by rule, gram negative. This is the reason why a second dye (safranin) is applied to visualize this group of prokaryotes. Thus, gram negative bacteria appear pink in color.

Unlike the thick peptidoglycan layer that gram positive bacteria have, negative bacteria have a much thinner layer. Additionally, they present a layer of lipopolysaccharides that is part of their cell wall.

We can use the analogy of a sandwich: the bread represents two lipid membranes and the interior or the filling would be the peptidoglycan.

The lipopolysaccharide layer is made up of three main components: (1) lipid A, (2) a core of polysaccharides, and (3) polysaccharides O, which function as an antigen.

When such a bacterium dies, it releases lipid A, which functions as an endotoxin. The lipid is related to the symptoms caused by infections of gram-negative bacteria, such as fever or dilation of blood vessels, among others.

This thin layer does not retain the purple dye applied in the first step, as the alcohol wash removes the lipopolysaccharide layer (and along with it the dye). They do not contain the teichoic acids mentioned in the gram positives.

An example of this pattern of organization of the bacterial cell wall is the famous E. coli bacteria.

Medical consequences of Gram stain

From a medical perspective it is important to know the structure of the bacterial wall, since gram positive bacteria are usually easily eliminated by the application of antibiotics such as penicillin and cephalosporin.

In contrast, gram-negative bacteria are usually resistant to the application of antibiotics that fail to penetrate the lipopolysaccharide barrier.

Other colorations

Although the Gram stain is widely known and applied in the laboratory, there are also other methodologies that make it possible to differentiate bacteria according to structural aspects of the cell wall. One of them is acid staining that binds strongly to bacteria that have wax-like materials attached to the wall.

This is used specifically to differentiate Mycobacterium species from other species of bacteria.

Biosynthesis

The synthesis of the bacterial cell wall can occur in the cytoplasm of the cell or in the inner membrane. Once the structural units have been synthesized, the assembly of the wall proceeds outside the bacteria.

The synthesis of peptidoglycan occurs in the cytoplasm, where nucleotides are formed that will serve as precursors for this macromolecule that makes up the wall.

Synthesis proceeds at the plasma membrane, where the generation of membrane lipid compounds takes place. Inside the plasma membrane, polymerization of the units that make up the peptidoglycan occurs. The whole process is assisted by different bacterial enzymes.

Degradation

The cell wall can be degraded thanks to the enzymatic action of lysozyme, an enzyme that is naturally found in fluids such as tears, mucus and saliva.

This enzyme acts more efficiently on the walls of gram positive bacteria, the latter being more vulnerable to lysis.

The mechanism of this enzyme consists of the hydrolysis of the bonds that hold together the monomeric blocks of the peptidoglycan.

Cell wall in Arqueas

Life is divided into three main domains: bacteria, eukaryotes, and archaea. Although the latter are superficially reminiscent of bacteria, the nature of their cell wall is different.

In archaea there may or may not be a cell wall. If the chemical composition exists, it varies, including a series of polysaccharides and proteins, but so far no species with a wall composed of peptidoglycan has been reported.

However, they can contain a substance known as pseudomurein. If the Gram stain is applied, they will all be gram negative. Therefore, staining is not useful in archaea.

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