Staphylococcus aureus is a gram-positive bacteria found commonly in the nares of humans. However, it is also a prolific pathogen that can cause an array of infections, ranging from mild skin and soft tissue infections to severe diseases such as toxic shock syndrome, scalded skin syndrome, necrotizing pneumonia, necrotizing fasciitis, abscesses, and endocarditis. While once considered an opportunistic pathogen, the recent emergence of particular CA-MRSA strains that can infect otherwise healthy individuals has challenged this notion. In a very simplistic view, S. aureus infections can be grouped in two categories: 1) acute invasive disease dependent on the production of secreted factors such as toxins, and 2) chronic infections associated with biofilm formation which may or may not form on implanted medical devices. The research of my lab focuses on dissecting the molecular mechanisms behind the ability of this bacterium to cause disease.
FakA in virulence factor regulation:Bacteria have the ability to produce fatty acids endogenously for the production of many metabolites and membranes. In addition, bacteria can also acquire fatty acids from the environment and the use of these exogenous fatty acids requires an exogenous fatty acid utilization pathway. In Staphylococcus aureus, and likely other Gram-positive bacteria, this pathway consists of two fatty acid carrier proteins and the fatty acid kinase FakA. Our studies initially identified FakA (previously called VfrB) as an uncharacterized regulator of virulence factors such as Hla and proteases. The regulation of virulence factors is due, in part, to the necessity of FakA for activation of the SaeRS two component system. Indeed, the presence of a constitutively-active SaeS bypasses the need for FakA in hla and coa expression. Furthermore, stimulation of SaeS with HNP-1 decreases FakA requirement for coa expression. In addition to this, a fakA mutant have altered cellular metabolism including carbon and amino acid metabolism, as well as changes in cytoplasmic redox and energy status. This change in cellular metabolism results in altered growth kinetics. Interestingly, altered growth kinetics is independent of Sae, demonstrating that the inability to use exogenous fatty acids impacts multiple pathways. While initially expected to survive poorer during infection, the fakA mutant causes hyper-necrotic lesions in a murine model of skin infection, despite similar bacterial titers. By 7-days post-infection, the wild-type infection begins to resolve while the fakA mutant-infected mice still have necrotic skin lesions and high titers of bacteria. Interesting, these changes in bacterial numbers and pathology correlate with altered immune response at the site of infection. Together, these data demonstrate that the fatty acid kinase FakA modulates both the physiology and virulence of S. aureus.
YjbH/Spx in virulence factor production. To persist within the host and cause disease, Staphylococcus aureus relies on its ability to precisely fine-tune virulence factor expression in response to rapidly changing environments. During an unbiased transposon mutant screen, we observed that disruption of the two-gene operon, yjbIH, resulted in decreased pigmentation and aureolysin activity relative to the wild-type strain. Further analyses revealed that YjbH, a predicted thioredoxin-like oxidoreductase, is mostly responsible for the observed yjbIH mutant phenotypes, though a minor role exists for the putative truncated hemoglobin, YjbI. These differences were due to significantly decreased expression of crtOPQMN and aur. Previous studies found that YjbH targets the disulfide- and oxidative-stress responsive regulator Spx for degradation by ClpXP. The absence of yjbH or yjbI resulted in altered sensitivities to nitrosative and oxidative stress and iron deprivation. Additionally, aconitase activity was altered in the yjbH and yjbI mutant strains. Decreased pigmentation and Aur activity in the yjbH mutant was found to be Spx-dependent and to involve the alternative sigma factor, σB. Lastly, we used a murine sepsis model to determine the effect of the yjbIH deletion on pathogenesis and found that the mutant was better able to colonize the kidneys and spleens during an acute infection than the wild-type strain. These studies identify changes in pigmentation and protease activity in response to YjbIH and are the first to show a role for these proteins during infection.
New tools for the research community: While a lot of recent successes have enhanced our ability to genetically manipulate Staphylococcus to make mutants or track gene/protein expression, more tools are necessary. To accomplish our studies, we are heavily invested in the development of new genetic tools for studying S. aureus and closely related bacteria.
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