Doctor of Philosophy
Microbiology and Immunology
McGavin, Martin J.
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.
Summary for Lay Audience
Staphylococcus aureus is an opportunistic bacterial pathogen that asymptotically colonizes approximately 30% of the population, primarily in our nose or on our skin. Those who are colonized have a much greater risk of then become infected by S. aureus, and it is frequently the strain that colonizes us that then subsequently infects us. To combat S. aureus colonization, our skin produces a variety of antimicrobial compounds that function to inhibit the growth of this bacteria. Of these, are antimicrobial fatty acids, which can compromise the membrane integrity of S. aureus. However, these fatty acids are also a valuable energy source for the bacteria on our skin. We found there is a protein in the membrane of S. aureus, GraS, that can sense the antimicrobial conditions of human skin, and activate a robust response to resist these compounds. Specifically, activation of the protein GraS leads to high levels of resistance to antimicrobial fatty acids. Furthermore, once S. aureus is resistant to these fatty acids, we have identified a pathway in S. aureus that can metabolize these fatty acids to provide energy for the bacteria. This metabolism occurs through the proteins FadDEBA, which are predicted to conduct a metabolic process known as β-oxidation. Together, our findings show that although our skin produces antimicrobial fatty acids to inhibit the growth S. aureus, the bacteria has evolved to sense the environment of human skin and upregulate a robust resistance to these antimicrobial fatty acids, as well as metabolize these fatty acids as an energy source.
Kuiack, Robert C., "Evaluating the role of GraS and FadXDEBA in promoting Staphylococcus aureus adaptation to host-derived fatty acids encountered at sites of colonization and infection" (2023). Electronic Thesis and Dissertation Repository. 9248.
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