Biofilm formation associated with calcium phosphate coating on implant metals

Three clinically relevant implant metals, i.e., stainless steel (SS316L), pure titanium (grade 4), and the titanium alloy Ti6Al4V (grade 5), were coated with octacalcium phosphate from supersaturated aqueous calcium phosphate solution. Calcium phosphate coatings are frequently applied to enhance the osteoconductivity of metal implants. However, this leads to a higher surface roughness that increases the risk for bacterial adhesion and biofilm formation. The bacterial species Escherichia coli (Gram-negative rods) and Staphylococcus xylosus (Gram-positive cocci) were seeded on the bare metals and the calcium phosphate-coated metals, respectively, and cultivated for up to 72 h to assess the biofilm formation. The efficiency of biofilm production by bacteria was evaluated by the crystal violet assay, scanning electron microscopy, and confocal microscopy. The growth of S. xylosus was always strong, with and without calcium phosphate coating, whereas E. coli proliferated better on calcium phosphate-coated metals. Both bacterial species colonized cavities within the porous calcium phosphate coating as indicated by scanning electron microscopy. The metabolic activity of S. xylosus caused a pH drop to 5.5 that led to corrosion of the calcium phosphate layer by acidic dissolution. In contrast, E. coli led to an increase in pH to about 8.9 that did not affect the coating. Osteoblast-like MG-63 cells adhered and proliferated well on both coated and uncoated metals, underscoring the good osteocompatibility before and after coating.