Extracellular DNA and polysaccharide intercellular adhesin protect Staphylococcus epidermidis biofilms from phagocytosis by polymorphonuclear neutrophils

Staphylococcus epidermidis is the leading cause of implant-associated infections, where it forms biofilms that are highly effective at evading the immune system. Here we investigate to what extent the biofilm extracellular matrix components extracellular DNA (eDNA) and polysaccharide intercellular adhesin (PIA) protect S. epidermidis from phagocytosis by polymorphonuclear neutrophils (PMN). We visualised phagocytosis using time-lapse confocal laser scanning microscopy of PMN interacting with planktonic S. epidermidis and 24 h old biofilms formed by the wildtype strain or mutant strains lacking either eDNA or PIA. We also compared phagocytosis of 24 h vs. 6 h old biofilms. PMN easily moved around and phagocytised S. epidermidis that were adhered to a surface from a planktonic culture. In contrast, PMN quickly became immobilised when interacting with biofilms. Very few PMN were able to phagocytise young (6 h) and mature (24 h) biofilms, suggesting that the accumulation of matrix components quickly provides a protective effect. Biofilms lacking either eDNA or PIA were much less dense, and many more PMN were able to phagocytise bacteria in these biofilms. Our findings suggest that both eDNA and PIA contribute to the ability of S. epidermidis biofilms to resist phagocytosis.