Abstract: Objectives To investigate the binding of natural IgM to Staphylococcus aureus and its role in the phagocytosis of S. aureus by phagocytes, and to pave way for further study on the role and mechanism of natural IgM in defense of bacteria. Methods The binding of natural antikeratin IgM 3B4 to S. aureus was analyzed by ELISA and indirect immunoiluorescence. The role of 3B4 in the phagocytosis was analyzed by colony forming assay and flow cytometry (FCM). Results Both ELISA and indirect immunofluorescence proved the binding of natural IgM 3B4 to S. aureus. Colony forming assay found that the amount of colony forming units decreased significantly when 3B4 was added. The analysis of FCM showed that 3B4 augmented phagocytosis of 5. aureus by phagocytes. Conclusions Natural antikeratin IgM 3B4 can bind to S. aureus and regulate the phagocytosis of it, indicating that natural IgM may play some role in the defense against bacterial infection.

Key words: Immunoglobulin M, Phagocytosis, Slaphylococcus aureus