Which characteristic makes S. aureus effective at resisting opsonization?

Staphylococcus aureus is effective at resisting opsonization primarily due to the presence of a capsule. The capsule acts as a physical barrier that helps to prevent the binding of opsonins, such as antibodies and complement proteins, which would typically tag the bacterium for destruction by phagocytic immune cells. The capsule interferes with the immune system's ability to recognize and effectively engulf the pathogen, thereby enhancing the bacterium's virulence.

In contrast, while a thick peptidoglycan layer offers some protection against physical disruption and lysis, it does not specifically impede opsonization. Endotoxin production is related to a different class of bacteria (Gram-negative) and its role is more associated with eliciting immune responses rather than resisting opsonization. Lastly, flagella motility does not provide any advantage regarding opsonization resistance; instead, it may help with mobility and colonization but does not play a direct role in evading opsonization by the immune system. Thus, the capsule is the key feature that enables S. aureus to effectively resist opsonization.