An in vivo model to study the pathobiology of infectious biofilms on biomaterial surfaces

This study examines the morphology, ultrastructure, and microbiology of the intact biofilm developing on an implant surface. Silastic subdermal implant material was colonized with P. aeruginosa and surgically inserted into the peritoneal cavity of adult rabbits. After 4, 8, 28, and 42 days implants were recovered and the intact biofilms examined. P. aeruginosa colonized the implant throughout the entire experimental time. Microcolonies of glycocalyx-coated bacteria were observed within the biofilm. However, the bulk of the biofilm was host-generated and typically contained phagocytes trapped within a thick mesh of fibrin. Polymorphonuclear neutrophils were the predominant cell type. Isolated erythrocytes, macrophages, and fibroblasts were also observed. By day 28, the biofilm was enclosed in a fibrous capsule of vascularized connective tissue. The low numbers of neutrophils seen in biofilms from sterile Silastic sheets implanted into control animals suggested that neutrophilia may represent a specific cellular response to the bacterial colonization. The results indicate that the cell-mediated immune response provides for most of the biofilm mass on colonized implant surfaces. Inactivated phagocytes trapped in fibrin may "wall-off" the embedded bacterial microcolonies and thus shield them from live phagocytic leucocytes. Such a mechanism may play an important role in the pathogenesis of prosthetic device infections.