In gram-positive bacteria, the outer membrane is missing but they are often covered by layers of peptidoglycan, which is usually thicker than the gram-negative bacteria. However, a thin cell wall, made out of peptidoglycan is present around gram-negative bacteria, which is surrounded by an outer membrane containing lipopolysaccharide. A complex multilayered structure protecting organisms from unpredictable and often hostile environments is known as the bacterial cell envelope. The cell envelopes of most bacteria come under one of two major groups. The composition and organisation of these envelope layers and recent insights into the mechanisms of cell envelope assembly are discussed below.

Cell envelope structure

The cell envelope comprises the cell membrane, cell wall, and outer membrane. A cell wall, cytoplasmic membrane, and capsule comprise the cell envelope of gram-positive bacteria and on the other hand, a cell wall, cytoplasmic membrane, outer membrane, periplasmic space, and capsule make the cell envelope of gram-negative bacteria.

Functions of cell envelope

Following are the essential functions of the cell envelope:

• The cell shape is maintained by the bacterial cell wall and it also prevents it from bursting or collapsing but, as observed in the case of motile bacteria, it forms filamentous extensions called flagella.
• By extending the plasma membrane into the cell wall, a unique structure known as a  mesosome is formed. These extensions are usually in the form of vesicles, tubules, and lamellae. Their main function is to assist the cell with various functions like cell wall synthesis, DNA replication, daughter cell distribution, respiration, secretions, etc.
• The transport of substances through the cell is also regulated by the cell membrane.

The gram-positive cell wall

The main characteristic of the gram-positive cell wall is a very thick peptidoglycan layer, which is responsible for crystal violet dyes retention during the gram staining procedure.

 Several gram-positive bacteria have a layer of protein called an S-layer, outside the cell wall. This S-layer helps in attaching and in biofilm formation. Outside the S-layer there is often a capsule of polysaccharides. The capsule helps the bacteria evade host phagocytosis. Due to reductive evolution, the S-layer and capsule are often lost. In laboratory culture, it is called the loss of a trait in the absence of positive selection.

Significance of gram-positive cell wall components to the initiation of body defences

The cell envelope of a gram-positive bacteria is made up of a cell wall with a thick peptidoglycan layer. This helps in retaining the crystal violet dyes during gram-staining. The cell wall is embedded with teichoic acids and lipoteichoic acids.

Our body has two immune systems which are the innate immune system and the adaptive immune system.

1. Immediately or within several hours after exposure to almost any microbe, a host uses an antigen-nonspecific defence mechanism called Innate immunity. It is the initial or the very first response by the body to eliminate microbes and prevent infection. Humans are born with this immunity.
2. Adaptive or acquired immunity is an antigen-specific defence mechanism that takes many days to become protective and it is made to remove and react with a specific antigen. This kind of immunity is developed throughout life.

Conclusion

The cell envelope is a combination of these three – the cell membrane, cell wall, and outer membrane if it is present. Usually, this envelope is one of the characteristics of prokaryotes like bacteria. It consists of the inner cell wall and the bacterium cell. The structural integrity is provided to the cell by the cell envelope. In the case of prokaryotes, because of this, the cell is protected from the internal turgor pressure which is caused due to a high concentration of macromolecules inside the cell.The two kinds of bacterial cell envelopes are the gram-positive cell wall and 1 gram-negative cell wall. Gram-positive bacteria stain purple after gram staining due to the nature of their cell wall. The gram-positive cell wall lacks an outer membrane and consists of many interconnected layers of peptidoglycan.