Host defence against Staphylococcus aureus biofilms infection: phagocytosis of biofilms by polymorphonuclear neutrophils (PMN).

Bacteria organised in biofilms are a common cause of relapsing or persistent infections, particularly in patients receiving medical implants such as ventilation tubes, indwelling catheters, artificial heart valves, endoprostheses, or osteosynthesis materials. Bacteria in biofilms are relatively resistant towards antibiotics and biocides, and--according to the current dogma--towards the host defence mechanisms as well. In that context, we addressed the question, how polymorphonuclear neutrophils (PMN), the "first line defence" against bacterial infection, would interact with Staphylococcus aureus biofilms generated in vitro. By time-lapse video microscopy and confocal laser scan microscopy we observed a migration of PMN towards and into the biofilms, as well as clearance of biofilms by phagocytosis. By labelling the bacteria within the biofilm with (3)H thymidine, and by cytofluorometry we could confirm and quantify clearance and phagocytosis of biofilm as well. Of note, the extent of biofilm clearance depended on its maturation state: developing "young" biofilms were more sensitive towards the attack by PMN compared to mature biofilms. In conclusion, contrary to the current dogma, S. aureus biofilms are not inherently protected against the host defence.