Evaluating the role of GraS and FadXDEBA in promoting Staphylococcus aureus adaptation to host-derived fatty acids encountered at sites of colonization and infection
Abstract
Staphylococcus aureus is an opportunistic pathogen that asymptomatically colonizes 30% of humans, where it is well adapted to survive on the skin in the presence of innate defense mechanisms such as antimicrobial free fatty acids (FFA). While antimicrobial FFA function to inhibit the growth of S. aureus, they also provide a valuable source of lipids for membrane synthesis and energy production. We hypothesized that S. aureus possesses a novel antimicrobial FFA resistance pathway that is activated under conditions found on human skin, and that under these conditions, S. aureus can metabolize exogenous fatty acids to fuel growth and virulence expression. Working with the endemic community-associated methicillin resistant S. aureus strain, USA300, our data show that when grown with cationic antimicrobial peptides or at an acidic pH, conditions encountered on human skin, S. aureus becomes extremely resistant to antimicrobial FFA. This resistance is dependent on activation of the sensor kinase GraS, as well as the downstream effector protein MprF. While MprF is known for synthesizing lysyl-phosphatidylglycerol, this antimicrobial FFA resistance is independent of this synthase activity, highlighting a novel function for MprF. Once resistant to high levels of host derived fatty acids, we see expression of putative ß-oxidation genes, fadXDEBA, occur. Expression is upregulated by exogenous FFA in a concentration dependent manner, and is repressed by glucose. Additionally, expression appears to be regulated by the gene directly upstream of the fad locus, prsW, which is a membrane protease proposed to modulate the function of a stress response Sigma Factor. Interestingly, growth with exogenous FFA enhances the growth and protease expression of wildtype S. aureus, but severely impairs growth and viability in a fadXDEBA deletion mutant. Finally, we show that knocking out either graS or fadXEDBA results in reduced virulence in a murine abscess infection model, indicating both resistance and metabolism of host derived fatty acids are important during infection. While antimicrobial FFA encountered during colonization and infection of a host normally function to inhibit bacterial growth, S. aureus has evolved to thrive in this environmental niche through the use of GraS, MprF, and FadXEDBA.