Phagocytosis of staphylococci biofilms by polymorphonuclear neutrophils: S. aureus and S. epidermidis differ with regard to their susceptibility towards the host defense

Bacteria organized in biofilms are a common cause of relapsing or persistent infections. In patients receiving orthopedic implants, such as endoprostheses or osteosynthesis materials, Staphylococcus aureus and S. epidermidis are prevalent and it is widely assumed that bacteria in biofilms are not only relatively resistant towards antibiotics and biocides, but also towards host defense mechanisms. In that context, we addressed the question how polymorphonuclear neutrophils (PMN), the "first line defense" against bacterial infection, interact with biofilms generated in vitro. By time-lapse video microscopy, we observed migration of PMN towards the biofilms. In the case of S. aureus, the PMN moved across the biofilm and took up bacteria as they moved along. On S. epidermidis, in contrast, the PMN were rather immobile, and phagocytosis was limited to bacteria in the immediate vicinity. By labeling the bacteria within the biofilm with H-thymidine we found that S. aureus biofilms were more sensitive towards the PMN attack than S. epidermidis. Following phagocytosis of either bacteria strain, the PMN underwent apoptosis, in line with the dogma, that phagocytosis induces programmed cell-death in order to prevent spilling of the bactericidal and cytotoxic entities. In conclusion, biofilms are not inherently protected against the attack by phagocytic cells; their sensitivity, however, varies among bacterial strains, presumably due to properties of the extracellular biofilm matrix affecting the motility of PMN on the film.