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.
Highlights
The survival of pathogenic microbes within host tissues depends upon the ability to adapt to the physical and nutritional restrictions imposed within that tissue
Based upon amino acid sequence identity comparisons to B. subtilis and L. monocytogenes, we identified five S. aureus open reading frames within the sequenced genome of USA300 isolate FPR3757 that encode proteins with similarities to both lipoic acid biosynthesis and salvage enzymes (Table 1 and S1 Fig)
Recent studies have determined both SAUSA300_0930 and SAUSA300_0328 function as lipoic acid ligases that catalyze the addition of free lipoic acid to two glycine cleavage H (GcvH) proteins (GcvH and GcvH-like protein (GcvH-L)) in S. aureus [42]. Based upon these amino acid sequence identity comparisons to lipoic acid biosynthesis and salvage enzymes in B. subtilis and L. monocytogenes, as well as naming conventions implemented by Rack et al, we designate the S. aureus lipoic acid biosynthesis and salvage proteins as follows: SAUSA300_0829 –LipA, SAUSA300_0571 –LipL, SAUSA300_1494 –LipM, SAUSA300_0930 –LplA1, and SAUSA300_0328 –LplA2 (Table 1 and Fig 1B) [42]
Summary
The survival of pathogenic microbes within host tissues depends upon the ability to adapt to the physical and nutritional restrictions imposed within that tissue. Strains of S. aureus exhibit considerable genetic diversity such that infectious strains can harbor unique virulence factors and/or exhibit divergent gene regulatory schemes that preclude the development of universal therapeutic targets against all disease-causing strains [14,15,16,17]. These characteristics make the identification of universally effective antimicrobials against S. aureus a challenging pursuit
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