Staphylococcus aureus Tissue Infection During Sepsis Is Supported by Differential Use of Bacterial or Host-Derived Lipoic Acid

Abstract

To thrive in diverse environments, bacteria must shift their metabolic output in response to nutrient bioavailability. In many bacterial species, such changes in metabolic flux depend upon lipoic acid, a cofactor required for the activity of enzyme complexes involved in glycolysis, the citric acid cycle, glycine catabolism, and branched chain fatty acid biosynthesis. The requirement of lipoic acid for metabolic enzyme activity necessitates that bacteria synthesize the cofactor and/or scavenge it from environmental sources. Although use of lipoic acid is a conserved phenomenon, the mechanisms behind its biosynthesis and salvage can differ considerably between bacterial species. Furthermore, low levels of circulating free lipoic acid in mammals underscore the importance of lipoic acid acquisition for pathogenic microbes during infection. In this study, we used a genetic approach to characterize the mechanisms of lipoic acid biosynthesis and salvage in the bacterial pathogen Staphylococcus aureus and evaluated the requirements for both pathways during murine sepsis. We determined that S. aureus lipoic acid biosynthesis and salvage genes exist in an arrangement that directly links redox stress response and acetate biosynthesis genes. In addition, we found that lipoic acid salvage is dictated by two ligases that facilitate growth and lipoylation in distinct environmental conditions in vitro, but that are fully compensatory for survival in vivo. Upon infection of mice, we found that de novo biosynthesis or salvage promotes S. aureus survival in a manner that depends upon the infectious site. In addition, when both lipoic acid biosynthesis and salvage are blocked S. aureus is rendered avirulent, implying an inability to induce lipoic acid-independent metabolic programs to promote survival. Together, our results define the major pathways of lipoic acid biosynthesis and salvage in S. aureus and support the notion that bacterial nutrient acquisition schemes are instrumental in dictating pathogen proclivity for an infectious niche.

A Lipoylated Metabolic Protein Released by Staphylococcus aureus Suppresses Macrophage Activation

“…Lipoic acid is an organosulfur molecule that is covalently attached to conserved lysines within E2 subunits of metabolic complexes including: pyruvate dehydrogenase (PDH), 2-oxoglutarate dehydrogenase (OGDH), and branched-chain 2-oxoacid dehydrogenase (BCODH) complexes, as well as to the H protein of the glycine cleavage system (Gcs) (Cronan, 2016; Spalding and Prigge, 2010). Prior studies indicate that lipoic acid synthesis and acquisition pathways are crucial for S. aureus infection (Zorzoli et al, 2016). Therefore, we sought to determine the mechanism behind the hyper-inflammatory response elicited by NE264.…”

“…A Δ lipA mutant grew identically to WT in tryptic soy broth (TSB), but not Roswell Park Memorial Institute (RPMI) medium lacking free lipoic acid (Figure 1C and 1D). When free lipoic acid was supplemented into RPMI, a Δ lipA mutant grew identically to the WT strain (Figure 1D) (Zorzoli et al, 2016). The growth defect of a Δ lipA mutant was rescued in RPMI bypass medium containing branched chain carboxylic acids and sodium acetate (BCFA), leading to equivalent optical density after 7 hours and identical exoprotein profiles (Figure 1E and S1B).…”

“…Lipoic acid acquisition by S. aureus occurs through either synthesis or salvage (Zorzoli et al, 2016 and Figure 2A). LipA, LipM, and LipL are required for synthesis, while LplA1 and LplA2 are involved in salvage.…”

Targeting fundamental pathways to disrupt Staphylococcus aureus survival: clinical implications of recent discoveries

“…Disruption of this pathway likely would not affect colonization or superficial infection but could limit the organism's ability to persist during tissue hemoinvasion. Similarly, S. aureus produces an enzyme, Lp1A2, that is not critical for in vitro growth but is required for invasion in vivo (33). This enzyme allows scavenging of lipoic acid, an enzyme complex cofactor critical for intermediary metabolism, in conditions in which free lipoic acid is limited.…”

“…S. aureus produces a glutamate dehydrogenase (GudB) and an acetate kinase that can catabolize free amino acids from the host (32). Additionally, S. aureus produces a pyruvate kinase and glucose transporters that facilitate glycolysis (27)(28)(29)(30)(31) and is able to scavenge free lipoic acid (33). (B) Numerous critical steps have been identified for the acquisition of host metals by S. aureus.…”