Branched chain fatty acid synthesis drives tissue-specific innate immune response and  infection dynamics of  Staphylococcus aureus.

Fatty acid-derived acyl chains of phospholipids and lipoproteins are central to bacterial membrane fluidity and lipoprotein function. Though it can incorporate exogenous unsaturated fatty acids (UFA), Staphylococcus aureus synthesizes branched chain fatty acids (BCFA), not UFA, to modulate or increase membrane fluidity. However, both endogenous BCFA and exogenous UFA can be attached to bacterial lipoproteins. Furthermore, S. aureus membrane lipid content varies based upon the amount of exogenous lipid in the environment. Thus far, the relevance of acyl chain diversity within the S. aureus cell envelope is limited to the observation that attachment of UFA to lipoproteins enhances cytokine secretion by cell lines in a TLR2-dependent manner. Here, we leveraged a BCFA auxotroph of S. aureus and determined that driving UFA incorporation disrupted infection dynamics and increased cytokine production in the liver during systemic infection of mice. In contrast, infection of TLR2-deficient mice restored inflammatory cytokines and bacterial burden to wildtype levels, linking the shift in acyl chain composition toward UFA to detrimental immune activation in vivo. In in vitro studies, bacterial lipoproteins isolated from UFA-supplemented cultures were resistant to lipase-mediated ester hydrolysis and exhibited heightened TLR2-dependent innate cell activation, whereas lipoproteins with BCFA esters were completely inactivated after lipase treatment. These results suggest that de novo synthesis of BCFA reduces lipoprotein-mediated TLR2 activation and improves lipase-mediated hydrolysis making it an important determinant of innate immunity. Overall, this study highlights the potential relevance of cell envelope acyl chain repertoire in infection dynamics of bacterial pathogens. Author summary: Fatty acids present in the bacterial cell envelope are diverse and represent key components of membrane phospholipids and membrane-anchored lipoproteins, the latter of which are potent inducers of antibacterial innate immunity. While S. aureus exclusively synthesizes saturated straight and branched chain fatty acids, it can also acquire host unsaturated fatty acids from the host environment. In this study, we examined how changes in S. aureus fatty acid composition alter innate immune responses and infection dynamics. Our results indicate that a shift in membrane fatty acid repertoire away from branched chain 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. [ABSTRACT FROM AUTHOR]