Multidrug-resistant Staphylococcus aureus robustly activates the complement system but resists downstream effector mechanisms.

The complement system is a serum-borne set of over 30 inactive liver-derived proteins, components that activate in a proteolytic cascade when pathogens invade. Activation of complement system promotes phagocytosis, inflammation, and direct microbial lysis. Complement targets pathogens through three main pathways: classical, lectin, and alternative. Together, these innate defenses mediate pathogen recognition and elimination. This study investigated complement activation against American Type Culture Collection (ATCC) 9144 and Multidrug-Resistant (MDR) Staphylococcus aureus, focusing on whether complement proteins play a key role in combating S. aureus. All experiments were performed in vitro using sera collected from healthy individuals. Enzyme-linked immunosorbent assay (ELISA) was employed to detect complement proteins (C1q, MBL, Ficolin-L, Ficolin-H, Ficolin-M, CL-11), and complement activation assays (C3, C4, C5, factor Bb, and MAC deposition assays). Findings showed that lectin, classical, and alternative complement pathways collectively drove C3 activation on S. aureus surface. However, C5b deposition onto the bacterial surface was weaker, and C9 failed to integrate, preventing Membrane Attack Complex (MAC) mediated lysis. Additional serum bactericidal and phagocytosis assays revealed that S. aureus resisted complement-mediated killing under the in vitro conditions used. Notably, complement recognition and activation profiles of MDR isolate were similar to or stronger than non-resistant strains, highlighting complement’s importance in targeting MDR S. aureus.