Cell envelope

Gram-positive cellwall-schematic

Gram negative cell wall

Cell envelope refers to the multi-layered structure enclosing the cytoplasm of a bacterial cell. This complex structure is crucial for maintaining the integrity of the cell, facilitating communication with the environment, and determining the cell's shape. The cell envelope varies significantly between the two main classes of bacteria, Gram-positive and Gram-negative, primarily in the composition and thickness of its layers.

Structure

The cell envelope consists of up to three basic layers: the cell membrane (also known as the plasma membrane), the cell wall, and, in Gram-negative bacteria, an outer membrane. Each layer has distinct components and functions.

Cell Membrane

The innermost layer of the cell envelope, the cell membrane, is a phospholipid bilayer embedded with proteins. It controls the passage of substances in and out of the cell, hosts various biochemical reactions, and plays a role in energy generation.

Cell Wall

The cell wall lies outside the cell membrane and is primarily composed of peptidoglycan, a polymer that provides structural strength to resist osmotic pressure and maintain cell shape. In Gram-positive bacteria, the cell wall is thick and contains teichoic acids, which are absent in Gram-negative bacteria. In contrast, Gram-negative bacteria have a thinner peptidoglycan layer and an additional outer membrane.

Outer Membrane

Exclusive to Gram-negative bacteria, the outer membrane is located outside the peptidoglycan layer. It consists of a lipid bilayer containing lipopolysaccharides (LPS), which contribute to the virulence of many bacteria and protect against certain antibiotics. The outer membrane also contains porins, proteins that allow the passage of small molecules.

Function

The cell envelope serves several critical functions:

• It acts as a barrier against harmful substances while allowing the passage of nutrients and waste products.
• It maintains the cell's osmotic balance, preventing lysis due to osmotic pressure.
• In Gram-negative bacteria, the outer membrane's LPS can act as an endotoxin, playing a role in pathogenicity.
• The cell envelope's structure contributes to the cell's shape and rigidity.
• It is involved in cell division and the secretion of substances outside the cell.

Clinical Significance

The cell envelope is a target for many antibiotics, which can disrupt its integrity or inhibit its synthesis, leading to cell death. For example, penicillin and other beta-lactam antibiotics inhibit peptidoglycan synthesis, affecting mainly Gram-positive bacteria. Understanding the cell envelope's structure and function is crucial for developing new antimicrobial strategies.

See Also

• Bacterial cell structure
• Gram staining
• Antibiotic resistance