<i>Staphylococcus aureus</i> Biofilms Prevent Macrophage Phagocytosis and Attenuate Inflammation In Vivo

TLDR

Biofilms are complex bacterial communities encased in a polysaccharide, DNA, and protein matrix, and Staphylococcus aureus biofilm infections are chronic, antibiotic‑resistant, yet the immune mechanisms that clear them remain poorly understood. The study aimed to determine whether Toll‑like receptors 2 and 9 contribute to the host immune response during catheter‑associated S. aureus biofilm formation. A mouse catheter model was used to evaluate TLR2 and TLR9 function during biofilm growth, exploiting the presence of their ligands within the biofilm matrix.

Abstract

Biofilms are complex communities of bacteria encased in a matrix composed primarily of polysaccharides, extracellular DNA, and protein. Staphylococcus aureus can form biofilm infections, which are often debilitating due to their chronicity and recalcitrance to antibiotic therapy. Currently, the immune mechanisms elicited during biofilm growth and their impact on bacterial clearance remain to be defined. We used a mouse model of catheter-associated biofilm infection to assess the functional importance of TLR2 and TLR9 in the host immune response during biofilm formation, because ligands for both receptors are present within the biofilm. Interestingly, neither TLR2 nor TLR9 impacted bacterial density or inflammatory mediator secretion during biofilm growth in vivo, suggesting that S. aureus biofilms circumvent these traditional bacterial recognition pathways. Several potential mechanisms were identified to account for biofilm evasion of innate immunity, including significant reductions in IL-1β, TNF-α, CXCL2, and CCL2 expression during biofilm infection compared with the wound healing response elicited by sterile catheters, limited macrophage invasion into biofilms in vivo, and a skewing of the immune response away from a microbicidal phenotype as evidenced by decreases in inducible NO synthase expression concomitant with robust arginase-1 induction. Coculture studies of macrophages with S. aureus biofilms in vitro revealed that macrophages successful at biofilm invasion displayed limited phagocytosis and gene expression patterns reminiscent of alternatively activated M2 macrophages. Collectively, these findings demonstrate that S. aureus biofilms are capable of attenuating traditional host proinflammatory responses, which may explain why biofilm infections persist in an immunocompetent host.