Final answer:
Gram-positive bacteria are resistant to complement due to their thick peptidoglycan layer which prevents penetration and lysis by the immune system. This, combined with cross-linked meshwork and the absence of an outer membrane, contributes to their resilience against complement-mediated destruction.
Step-by-step explanation:
Gram-positive bacteria are mostly resistant to complement due to the thickness of their peptidoglycan layer. The cell wall structure of these bacteria is significantly different from their Gram-negative counterparts. While Gram-negative bacteria have a complex cell envelope with a thin peptidoglycan layer and an outer membrane containing lipopolysaccharides (LPS), Gram-positive bacteria possess a relatively simple, yet much thicker peptidoglycan layer, making up to 90% of the cell wall. Embedded within this are teichoic acids, which often covalently link to lipids in the plasma membrane to form lipoteichoic acids, providing rigidity and acting as an anchor between the cell wall and membrane.
Complement resistance in these bacteria is facilitated by this thick layer, which can prevent the complement system from penetrating and causing lysis of the bacterial cells. Additionally, the unique structure of the peptidoglycan, with its strong cross-linked meshwork, contributes to the resistance against immune defense mechanisms. Their relatively simple outer structure, lacking an outer membrane and associated factors like porins that are present in Gram-negative bacteria, makes it more challenging for the complement to take effect.