The cell envelope is made of the cell wall. In some bacteria structures like slim and capsule are also present.

(a) Cell Wall

The cell wall is present beneath the extra-cellular substances. It is present outside the cytoplasmic membranes. It determines the shape of bacteria. The cell wall also protects the cells from osmotic lysis. The cell wall is absent in Mycoplasm. The cell wall is necessary for the survival of some bacteria. The antibiotic penicillin can kill bacteria by inhibiting the synthesis of peptidoglycan.

Composition of Cell wall

1. The cell wall of most bacteria has a unique macromolecule called peptidoglycan.  It is composed of a long framework of glycan (glucose) chains. These glycan chains are linked with peptide fragments. 
2. The intact cell wall also contains some molecules of sugar, teichoic acid, lipoproteins, and Lipopolysaccharides. These molecules are linked to peptidoglycan.
3. The cell wall structure is present in gram-positive and gram-negative bacteria is absent in some bacteria. some bacteria have no cell wall at all. Archaeobacteria do not contain peptidoglycan. Their cell wall is composed of proteins, glycoprotein, and polysaccharides. 

(b) Capsule

Some bacteria produce capsule cells outside the cell wall. it is made up of repeating units of polysaccharides or protein or both. it has a thick and gummy nature. It makes the encapsulated bacteria sticky. There are 14 different types of capsule in bacteria.

1. Bacteria use their capsule to attach themselves with other substrates and with each other to form colonies.
2.  It prevents phagocytosis and hence it enhances the ability of bacteria to cause disease.
3. Capsules also contain water. It protects the bacteria against desiccation.
4. The capsule also excludes bacterial viruses and most hydrophobic toxic materials like detergents.

(c) Slime

Some bacteria are covered with a loose, soluble shield of macromolecules called slime. It is an unorganized layer of extracellular material that surrounds bacteria cells. It mostly consists of exopolysaccharides, glycoproteins, and glycolipids. The Slime layer in bacteria is easily removed.

1. Slime provides greater pathogenicity to bacteria. 
2. It protects bacteria cells from environmental dangers like antibiotics.
3. The slime layer also allows bacteria to adhere to the smooth surface.
4. It protects the bacteria from chemicals like chlorine and iodine etc.

(d) Fimbriae

Fimbriae are fine protein. These are just 2-10 nanometers in diameter and up to several micrometers in length. Fimbriae are involved in attachments to the solid surface or other cells. These are also essential for the virulence of some bacterial pathogens.

(e) Pili

Pili are hollow, non-helical and filamentous appendages on a cell wall. Pili are smaller than flagella. Ture pili are present only in gram-positive bacteria. They are involved in conjugation which is used in the mating process.

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