Date of Award

2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Microbiology and Immunology

Abstract

Upon entry into the host, pathogens must overcome innate immunity in order to cause disease. The innate immune system represents a fast-acting initial line of defense to prevent infection. In order to withstand innate defenses, bacterial pathogens like the Gram-positive bacterium Staphylococcus aureus, produce a wide array of virulence factors that can inhibit innate immune cell recruitment and antimicrobial activity, or directly target and kill phagocytic leukocytes thereby facilitating pathogenesis. Infection with S. aureus can cause disease in virtually any tissue site and is a significant burden to human health. In this dissertation, we sought to understand how S. aureus counters the host innate immune system to cause disease. Macrophages are professional phagocytic leukocytes that are central to innate defenses. As such, we hypothesized that S. aureus must be able to overcome macrophage inflammatory responses to aid in its pathogenesis.Data from a forward genetic screen using S. aureus cell free supernatants derived from a transposon mutant library, uncovered that a mutation in the gene encoding the lipoic acid synthetase (LipA), which is required for the de novo synthesis of the cofactor lipoic acid, resulted in enhanced TLR2-dependent activation of macrophages. We found that the hyper-inflammatory response elicited by a lipA mutant correlated with the absence of lipoylation on the E2 subunit of the pyruvate dehydrogenase complex (E2-PDH). In wild type cells, the release of lipoyl-E2-PDH occurred during exponential growth and required the major staphylococcal autolysin Atl. Purified S. aureus lipoyl-E2-PDH prevented TLR1/2 activation by triacylated lipopeptides. Moreover, the absence of lipoyl-protein production in vivo resulted in the recruitment of activated inflammatory macrophages that are better able to restrict S. aureus growth through production of bactericidal reactive oxygen and nitrogen species.Overall, data in this dissertation indicates that S. aureus lipoylated E2-PDH moonlights as a novel immune evasion protein by suppressing TLR-mediated macrophage activation. Our data also suggest that lipoic acid synthesis in S. aureus promotes bacterial persistence during infection through limitation of reactive oxygen and nitrogen species generation by macrophages. Broadly, this work furthers our understanding of the intersections between bacterial metabolism and the immune response to infection.

Creative Commons License

Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License.

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