Abstract
The Staphylococcus aureus type VII secretion system (T7SS) exports several proteins that are pivotal for bacterial virulence. The mechanisms underlying T7SS-mediated staphylococcal survival during infection nevertheless remain unclear. Here we show that EsxC, a small secreted effector implicated in bacterial persistence, contributes to S. aureus membrane architecture and fluidity. Interestingly, isogenic mutants lacking EsxC, other T7SS effectors (EsxA and EsxB) or the membrane-bound ATPase EssC are more sensitive to killing by the host-derived antimicrobial fatty acid, linoleic acid (LA), compared to the wild-type. We demonstrate that LA induces more cell membrane damage in the T7SS mutants, although they do not bind differentially to LA. Membrane lipid profiles show that T7SS mutants are also less able to incorporate LA into their membrane phospholipids. Proteomic analyses of wild-type and mutant cell fractions reveal that, in addition to compromising membranes, T7SS defects readily induce bacterial stress and hamper their response to LA challenge. Together, our findings indicate that T7SS is crucial for S. aureus membrane integrity and homeostasis, which is critical when bacteria encounter antimicrobial fatty acids.
Significance Staphylococcus aureus is a major hospital and community-acquired pathogen that can cause a range of serious infections. Paradoxically, S. aureus is a common commensal inhabitant of skin and nares of healthy individuals, where the bacterium overcomes colonisation barriers deployed by the innate immune system, which include antimicrobial, unsaturated fatty acids (FAs). These long chain unsaturated free FAs are potent activators of the type VII secretion system (T7SS), a molecular machine that secretes virulence factors that promote S. aureus persistence in its host. Here we demonstrate a role for the T7SS in maintaining staphylococcal membrane architecture, which enables the bacteria to survive FA toxicity. An increased vulnerability to FAs might explain why T7SS-defective mutants are less virulent in FA-rich environments. Thus, therapeutics targeting the T7SS may mitigate S. aureus virulence and render the bacterium more susceptible to FAs.
