Determining the Role of Vitronectin in Bacterial Resistance to Bactericidal Activity of Human Serum

To investigate the impact of vitronectin — a human serum glycoprotein, on bacteria, begin with tubes containing Haemophilus influenzae suspension.

Treat one tube with normal human serum containing vitronectin and the other with vitronectin-depleted serum. Both serum variants contain antibodies and various complement proteins — essential for bactericidal activity.

During incubation, antibodies interact with bacterial surface proteins, activating complement proteins to form C5 convertase, which cleaves C5. C5b associates with C6, followed by C7, forming the C5b6-7 complex. This complex binds to the bacterial cell membrane and recruits C8, forming C5b67-8.

This process facilitates the attachment of C9 monomers and their polymerization, generating the membrane attack complex — a pore-forming structure. These pores cause the influx of ions and water, leading to bacterial lysis.

In contrast, in the tube with normal serum, vitronectin binds to a bacterial surface protein that interacts with the C5b6-7 complex and C9 monomers. These interactions prevent the recruitment of C8 and the polymerization of C9, inhibiting membrane attack complex formation and leading to bacterial survival.

Take the bacterial suspension from both the tubes and culture on individual agar plates.

The higher bacterial colonies in serum containing vitronectin confirm its role in bacterial resistance against the bactericidal activity of human serum.