Branched-chain fatty acids (BCFAs) are an essential component of staphylococcal membranes. Of the BCFAs, 15:0 anteiso BCFAs (a15:0) derived from isoleucine are especially important for viability. S. aureus does not synthesize unsaturated fatty acids (uFAs) nor does it have the enzyme to desaturate saturated fatty acids. The bacterium assimilates exogenous host uFAs during infection, but these fatty acids do not replace the requirement for a15:0 that predominantly occupies the 2 position of S. aureus phospholipids. We also found that BCFAs are required for virulence signaling. Thus, S. aureus uses BCFAs to trigger virulence gene expression via the Sae TCS and to fluidize the membrane to avoid membrane phase separation, protein segregation, and cell death.

We know that the branched-chain α-keto acid dehydrogenase complex (BKDH) is used by some Gram-negative and Gram-positive bacteria to catabolize branched-chain amino acids like isoleucine to generate energy. Other bacteria like S. aureus use the BKDH to make BCFAs. The BKDH is essential for S. aureus growth in vitro and in SSTIs; eliminating the dihydrolipomide component (lpdA) inactivates BKDH and results in BCFA auxotrophy. We discovered that growing the ΔlpdA mutant in laboratory medium selects for mutations that restore growth and reveal a previously unknown pathway in human-associated staphylococci that catalyzes the formation of BCFA precursors that feed FA synthesis (the MbcS pathway). Unlike the BKDH pathway, the MbcS pathway can facilitate salvaging of intermediates secreted by polymicrobial communities during infection. We are working to identify and characterize the genes and steps that compose this new route to BCFA synthesis.