Gram-positive bacteria are generally resistant to complement-mediated ...
Explanation:
Gram-positive bacteria are generally resistant to complement-mediated lysis because of the presence of a thick peptidoglycan layer in their cell wall. The complement system is a part of the innate immune response and plays a crucial role in defense against bacterial infections. It consists of a group of proteins that work together to recognize and destroy foreign invaders, including bacteria.
Mechanism of complement-mediated lysis:
1. Recognition: The complement system recognizes the foreign bacteria by binding to specific molecules on their surface, such as polysaccharides or proteins.
2. Activation: This recognition triggers a cascade of reactions that leads to the activation of complement proteins, including C3 and C5.
3. Assembly of the membrane attack complex (MAC): Activated complement proteins form a complex called the membrane attack complex (MAC), which is composed of C5b, C6, C7, C8, and multiple copies of C9. The MAC inserts into the bacterial cell membrane, creating pores that disrupt the integrity of the membrane.
4. Lysis: The pores formed by the MAC allow for the uncontrolled influx of ions and molecules, leading to cell lysis and death of the bacterium.
Role of peptidoglycan layer:
Gram-positive bacteria have a thick peptidoglycan layer in their cell wall, which acts as a physical barrier that prevents the insertion of the MAC into the inner membrane. The peptidoglycan layer provides rigidity and strength to the cell wall and is composed of a mesh-like structure of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) residues, cross-linked by peptide chains.
The MAC, being a large protein complex, cannot penetrate the peptidoglycan layer to reach the inner membrane of Gram-positive bacteria. As a result, the MAC is unable to form pores and cause lysis of the bacterial cell.
Summary:
Gram-positive bacteria are resistant to complement-mediated lysis because their thick peptidoglycan layer prevents the insertion of the MAC into the inner membrane. This physical barrier provides protection against the bactericidal effects of the complement system.