Graduation Date

Spring 5-4-2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Programs

Pathology & Microbiology

First Advisor

Tammy Kielian

Abstract

Staphylococcus aureus biofilms represent a significant cause of morbidity and economic burden and are often associated with nosocomial infections, including medically implanted devices. In particular, prosthetic joint infections (PJIs) are a growing concern due to the continued increase in orthopedic procedures. Staphylococcal species cause >50% of all PJIs, while S. aureus represents the most invasive and associated with the most morbidity. S. aureus-associated biofilm infections are recalcitrant to antibiotic therapy, due to both the acquisition of genetic elements and metabolic dormancy. Furthermore, S. aureus biofilm infections remain chronic because they cannot be cleared by the immune system. Recent studies from our laboratory have demonstrated that S. aureus biofilms actively polarize the immune response associated with these infections to promote biofilm persistence. Specifically, biofilm infections are characterized by an influx of myeloid derived suppressor cells (MDSCs), paucity of T cells and neutrophils, and the polarization of anti-inflammatory monocytes. Augmentation of monocyte pro-inflammatory activity, either by adoptive transfer or MDSC depletion studies, promotes bacterial clearance, highlighting a critical role for monocyte/macrophage inflammatory polarization in dictating biofilm persistence. The inflammatory properties of leukocytes are linked to their metabolic activity, where anti-inflammatory macrophages primarily rely on oxidative phosphorylation (OxPhos), whereas pro-inflammatory macrophages utilize aerobic glycolysis. This suggests that biofilm-associated monocytes likely shift their metabolism to favor OxPhos over glycolysis. Therefore, we characterized the metabolic state of monocytes during S. aureus orthopedic implant infection and whether the metabolic reprogramming of monocytes would promote pro-inflammatory activity and biofilm clearance. Specifically, nanoparticle targeted delivery of oligomycin to inhibit monocyte ATP-synthase of the electron transport chain significantly altered monocyte metabolism and increased pro-inflammatory gene expression, which resulted in a significant reduction in S. aureus biofilm burdens. In addition, nanoparticle treatment acted synergistically with antibiotics to clear the biofilm infection, suggesting that targeting monocyte metabolic activity may represent a novel therapeutic target during PJIs. Prior work from our laboratory suggested that arginase-1 (Arg-1) expression, a marker of anti-inflammatory polarized monocytes/macrophages, may also contribute to the inability of myeloid cells to clear S. aureus biofilms. Using a conditional knockout mouse model of myeloid Arg-1, we show that while myeloid-derived Arginase-1 does not influence S. aureus biofilm establishment and persistence, it does play a significant role in controlling planktonic S. aureus infection. Collectively, these studies reveal that monocyte metabolism is skewed towards OxPhos during the early stages of biofilm formation that limits pro-inflammatory activity, effectively facilitating biofilm establishment.

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