Staphylococcus Aureus Biofilms Interfere With Macrophage Antimicrobial Responses Through Differential Gene Regulation, Toxin Production, and Purine Metabolism

Staphylococcus aureus (S. aureus) is an opportunistic pathogen that is a leading cause of both nosocomial and community-associated infections. Armed with a myriad of virulence factors and the propensity to form a biofilm on native tissues and implanted medical devices alike, S. aureus infections represent a very real public health threat, the treatment of which results in an excessive economic burden. S. aureus biofilm infections are notoriously recalcitrant to antibiotic therapy and adept at evading and neutralizing the host immune antimicrobial response. Previous studies from our laboratory have shown that S. aureus biofilms are able to cause persistent infections, in part, through the reprogramming of the macrophage (MΦ) immune response. While macrophages are readily able to recognize and respond to S. aureus in a planktonic state, their ability to mount a functional antimicrobial attack is thwarted upon encountering S. aureus biofilm. We have observed that MΦs in close proximity to S. aureus biofilms are less phagocytic and skewed towards an anti-inflammatory profile typified by arginase and IL-10 production. We have demonstrated that the ability of S. aureus biofilms to cause chronic infections is due, in part, to TLR2 or TLR9 evasion. However, we have shown that MyD88 signaling does provide some benefit to the host in combating S. aureus biofilm infections, which may be attributed to IL-1 receptor signaling. To better understand how S. aureus biofilms subvert the MΦ antimicrobial response, the work described in this dissertation assessed S. aureus transcriptional activity during co-culture with MΦs, whether S. aureus biofilms inhibit MΦ activity through secreted molecules, and performed a high-throughput screen of the Nebraska Transposon Mutant Library to identify key genes involved in dampening the MΦ NF-κB-regulated proinflammatory response. We found that S. aureus biofilms attenuate their transcriptional activity following MΦ exposure, augment α-hemolysin (Hla) and leukocidin AB (LukAB) secretion to inhibit MΦ phagocytosis and induce cell death, and rely on a functional purine biosynthetic pathway to prevent MΦ invasion and phagocytosis, in part, through controlling the amount of eDNA available for MΦ recognition at the surface of the biofilm extracellular matrix (ECM). Collectively, these studies build upon our previous observations by identifying key mechanisms whereby S. aureus biofilms are able to thwart the MΦ antimicrobial response