Plays an essential role in immune evasion by helping bacteria to resist complement-mediated killing by neutrophils. Inhibits the deposition of host C3b on bacterial surfaces and the release of the chemoattractant C5a by cleaving the central complement protein C3. The cleavage site renders the C3b molecule vulnerable to proteolytic degradation by host regulators. Cleaves and inactivates host SERPINA1, which is an endogenous protease inhibitor essential for controlling neutrophil serine protease elastase. Plays also an essential role in the cleavage and subsequent activation of the serine protease SspA which is involved in colonization and infection of human tissues.