Question

List several functions of the outer membrane in gram-negative Bacteria. What is the chemical composition of the outer membrane? (Section 2.11)

Short answer

The outer membrane functions as a barrier and transport system, and its chemical composition includes lipopolysaccharides, phospholipids, and proteins.

Step-by-step solution

- Identify the Functions of the Outer Membrane

The outer membrane of gram-negative bacteria functions as a protective barrier. It prevents the entry of harmful substances, including antibiotics and detergents. It also contains proteins that act as channels (porins) facilitating the transport of nutrients and waste products.

- Explain the Protective Nature

The outer membrane is a part of the cell's defense mechanism, protecting the bacteria from toxic substances in the environment. It is particularly resistant to bile salts, lysozyme, and various antibiotics, contributing to the virulence of certain pathogens.

- Discuss its Role in Transport

Porins, which are proteins in the outer membrane, allow the passage of small hydrophilic molecules while blocking larger molecules. This selective permeability is crucial for the survival of the bacteria in various environments.

- Identify the Chemical Composition

The chemical composition of the outer membrane includes lipopolysaccharides (LPS), phospholipids, and proteins. LPS consists of three parts: lipid A (an endotoxin), a core polysaccharide, and an O-antigen polysaccharide. These components play key roles in the membrane's function and the immune response in hosts.

Key concepts

Protective Barrier Function

The outer membrane of gram-negative bacteria acts as a crucial protective barrier. This membrane prevents harmful substances, including toxins and antibiotics, from entering the cell. This protection is essential for the bacteria's survival. The outer membrane also provides resistance against bile salts and lysozyme, which attack bacterial walls.
Additionally, some bacteria are pathogenic because this barrier makes it hard for the host's immune system to kill them.

Selective Permeability

Selective permeability is a vital function of the outer membrane. This trait allows the bacteria to control what enters and exits the cell. The outer membrane contains proteins known as porins that act as channels.
These channels facilitate the entrance of small hydrophilic molecules while blocking larger, potentially harmful molecules.
This ability to regulate transport is essential for the bacteria to thrive in different environments.

• Utilizes porins
• Allows small hydrophilic molecules
• Blocks larger harmful molecules

Lipopolysaccharides Composition

The outer membrane is rich in lipopolysaccharides (LPS). LPS consists of three components: lipid A, core polysaccharide, and O-antigen polysaccharide. Lipid A acts as an endotoxin. This component can trigger strong immune responses in the host. The core polysaccharide provides structural support, while the O-antigen polysaccharide varies among different bacteria.
This variance helps bacteria evade the host's immune system by appearing as multiple strains.
Key points:

• Lipid A (endotoxin)
• Core polysaccharide (structural support)
• O-antigen polysaccharide (immune evasion)

Porins

Porins are proteins embedded in the outer membrane. They form channels that allow the passage of small molecules like nutrients and waste products. These channels are highly selective, letting in beneficial substances while blocking harmful ones.
Each porin can regulate what passes through based on size, charge, and other properties.
This selectivity aids in the bacteria's adaptability and survival.

Resistance to Antibiotics

One of the main functions of the outer membrane is its role in antibiotic resistance. The outer membrane acts as an effective barrier that prevents many antibiotics from penetrating the bacterial cell.
This resistance is accomplished through:

• Porins regulating entry
• Lipopolysaccharides blocking harmful molecules

This barrier is why gram-negative bacteria can sometimes be challenging to treat. It prevents antibiotics from reaching their target sites within the bacterium, making infections harder to eradicate.