Abstract: BACKGROUND: Totally 60%-70% of hospital-acquired infections are related to medical devices of different materials, and bacterial biofilm cells play an important role in chronic infections. Bacterial adhesion and biofilm formation are affected by the characteristics of biological materials, such as surface charge, hydrophobicity, hardness, and surface chemistry.
OBJECTIVE: To study the phagocytosis of macrophages on the early biofilm cells of E. coli on polydimethylsiloxane, a biomaterial with different surface hardness.
METHODS: Hard (5:1), medium (20:1) and soft (40:1) polydimethylsiloxane with three surface hardness was prepared by controlling the mass ratio of base material and curing agent. (1) E. coli were inoculated on the surface of three kinds of hard materials to incubate bacterial biofilm cells. Activated macrophages were dropped onto the surface of each material. CFU cell counting, flow cytometry and inverted fluorescence microscope were used to detect the difference in macrophages Phagocytosis of E. coli biofilm cells on surface hardness materials. (2) Macrophages were inoculated on the surface of three hard materials, and then E. coli was added. CFU cell count was applied to detect macrophages phagocytosis of E. coli biofilm cells on materials with different surface hardness.
RESULTS AND CONCLUSION: (1) Bacteria were inoculated first and then macrophages were added to the experiment: CFU cell count, flow cytometry technology and inverted fluorescence microscope detection showed that as the surface hardness of the material increased, the phagocytosis of macrophages increased and the number of E. coli biofilm cells decreased. The difference in the phagocytic rate of bacterial biofilm cells among the three groups was significant (P < 0.001). (2) Macrophages were inoculated first and then bacteria were added to the experiment: CFU cell count test showed that as the surface hardness of the material increased, the phagocytosis of macrophages increased and the number of bacterial cells decreased. It could be seen under the microscope that the macrophages were on the hard surface material more spread, with higher activity, and visible extension of pseudopods. (3) The results show that with the increase of the hardness of polydimethylsiloxane, the efficiency of macrophages phagocytosis of E. coli biofilm cells is higher. It is speculated that by optimizing the hardness of biological materials, endophyte-related infections can be better controlled.

Key words: materials, hardness, polydimethylsiloxane, macrophages, phagocytosis, bacteria, biofilm