Research
Contents
1. Lipoic acid as a nutrient and immunomodulatory agent in vivo
2. Modification of the bacterial lipid microenvironment to promote immune evasion.
3. Host lipids as a nutrient source in vivo
4. Protease-dependent regulation of immunomodulatory virulence factors.
1. Lipoic acid as a nutrient and immunomodulatory agent in vivo
S. aureus uses multiple adaptive traits such as nutrient acquisition to survive in disparate infectious environments. An essential nutrient for S. aureus replication is lipoic acid, a cofactor found on a range of metabolic enzyme complexes. We have been working to define the lipoic acid biosynthesis and salvage pathways of S. aureus using genetic and biochemical
approaches and have identified unique requirements for de novo synthesis and salvage of lipoic acid in distinct tissue sites in vivo. In conjunction with this work, we have identified a series of novel features of the S. aureus lipoic acid biosynthesis and salvage pathways that are a source of continued investigation.
In addition, we determined that the lipoylated E2 subunit of pyruvate dehydrogenase is released by S. aureus where it dampens macrophage Toll-like receptor 1/2-mediated recognition of S. aureus lipoproteins. Moreover, the absence of protein lipoylation during infection correlates with increased proportions of activated macrophages that have improved capacity to limit bacterial growth ex vivo via the production of ROS and RNS. We continue to explore how S. aureus synthesis of lipoic acid blunts production of ROS and RNS by innate cells to promote bacterial survival.
2. Modification of the bacterial lipid microenvironment to promote immune evasion.
Commensal and pathogenic bacteria hydrolyze host lipid substrates with secreted lipases and phospholipases for nutrient acquisition, colonization, and infection. Bacterial lipase activity on mammalian lipids and phospholipids can promote release of free fatty acids from lipid stores, detoxify antimicrobial lipids, and facilitate membrane dissolution. We made the unexpected observation that a secreted lipase, Geh, inhibits activation of innate immune cells by hydrolyzing S. aureus lipoproteins, a major pathogen-associated molecular pattern. We continue to investigate the effects of S. aureus lipases on immune signaling and pathogenesis and the impact of the S. aureus lipases on host lipid stores.
In addition, we are examining how changes in S. aureus fatty acid composition alter innate immune responses and infection dynamics. Our results indicate that a shift in fatty acid repertoire
away from branched fatty acids and toward unsaturated fatty acids results in tissue-specific disruption of infection dynamics that correlate with lipoprotein-mediated induction of innate immunity. de novo branched chain fatty acid synthesis by S. aureus shifts the balance away from host unsaturated fatty acid-induced inflammation and stabilizes bacterial burden during infection. Thus, acyl chain composition represents a facet of the bacterial cell envelope that calibrates the immune response and bacterial infection outcome. We are now exploring how diet-induced changes in the circulating lipid repertoire alter bacterial survival and host response to infection.
3. Host lipids as a nutrient source in vivo
During infection, pathogenic microbes adapt to the nutritional milieu of the host through metabolic reprogramming and nutrient scavenging. For S. aureus, virulence in diverse infection sites is driven by the ability to scavenge myriad host nutrients. We found that S. aureus can overcome a requirement for branched-chain fatty acids, an essential component of staphylococcal membrane lipids and the agent of membrane fluidity, during skin infection by scavenging host-derived unsaturated fatty acids from the skin. Our findings are providing insight into an adaptive trait that bypasses de novo lipid synthesis to facilitate S. aureus persistence during superficial infection. We are working to understand the extent of lipid accumulation and the impacts of this adaptation on survival during infection.
4. Protease-dependent regulation of immunomodulatory virulence factors.
S. aureus gene regulation is critical to the production of pathogenic enzymes, toxins, and immunomodulatory virulence factors. We identified a gene in S. aureus known as mroQ An mroQ mutant was found
to strongly activate macrophages and aid in bacterial clearance and resolution of infection suggesting a role in immune evasion. We also determined that an mroQ mutant phenocopies a strain of S. aureus that lacks the accessory gene regulatory quorum sensing
system, Agr. The Agr system of S. aureus plays a key role in regulating virulence factor gene expression and secretion, an effect that is mediated by the production of a small signaling peptide (AIP). Our data suggest that MroQ acts in some way to promote the processing and/or secretion of AIP and therefore facilitates proper regulation of secreted virulence factors. We are working to explore how MroQ interfaces with the Agr system to regulate pathogenesis.