How does a bacterial cell wall form?

Formation of a bacterial cell wall

As the cell wall is a little bit different in Gram-positive and Gram-negative bacteria, as a broad term, the cell envelops can be used. It consists of a cytoplasmic member plus cell wall, additionally, an outer membrane in Gram-negative bacteria. Cell envelop is a protective layer that surrounds the bacteria and protect it from diverse and extreme environments. The cell wall is part of this cell envelops.
In Gram-positive bacteria, this cell wall is tough, rigid and thick. And there is no outer membrane outside of the cell wall. On the other hand, in Gram-negative bacteria cell wall is thin but supplemented externally by a second lipid membrane named the outer membrane. This lipid membrane contains large amounts of lipopolysaccharide (LPS), a molecule that is very toxic to human. In Gram-negative bacteria, the space between the cytoplasmic membrane and the outer membrane is called periplasmic space or periplasm.

The cell wall is composed of multiple layers of peptidoglycans, which consists of a polymer of sugar and amino acids. The polymers are repeats of two sugars, N-acetylglucosamine (NAGA) and N-acetylmuramic acid (NAMA). A peptide chain of 3-5 amino acids is linked to N-acetylmuramic acid.

To make the cell wall complex, rigid and tough there is cross-linkage between peptidoglycans layers, converting it into a 3D mess layer with great strength, enough to protect the bacteria from hostile environments. This cross-linking is mediated by bacterial enzymes called penicillin-binding proteins (PBP). These enzymes recognize the terminal two amino acids of the peptide side chains, which are usually d-alanine–d-alanine and either directly, cross-link them to a second peptide side-chain or indirectly cross-link them by forming a bridge of glycine residues between the two peptide side chains.

Note:  In Gram staining, the colours of gram-positive bacteria are blue or purple, and gram-negative bacteria is pink.

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