The Department of Microbiology and Immunology is proud to announce that Caroline Vermilya, Graduate Research Assistant, has been awarded a National Institutes of Health F31 NRSA Predoctoral Fellowship Award titled: "Elucidating novel mechanisms of phosphate liberation in Staphylococcus aureus and how phosphate-limitation impacts the host-pathogen interface." Ms. Vermilya is a PhD Candidate in the Microbiology Graduate Program under the mentorship of Dr. Thomas Kehl-Fie.

Project Abstract:

Phosphorous in the form of phosphate, with numerous critical roles, is essential for life. Inorganic phosphate (P i ) is the main assimilable form of phosphorus. Despite possessing dedicated P i uptake systems, Staphylococcus aureus and other pathogens experience P i starvation. In addition to Pi uptake systems based on studies primarily using non-pathogenic model systems, bacteria must possess additional strategies for overcoming P i starvation including metabolic adaptations and scavenging of P i from organophosphates. S. aureus lacks the established metabolic adaptation, thus, to overcome Pi limitation S. aureus obtains Pi from organophosphates. This is accomplished by cleaving the P i moiety from organophosphates and importing it using the P i-transporters. Alkaline phosphatase (PhoB) can liberate P i from many organophosphates. However, prior work revealed that S. aureus consumes numerous organophosphates including those that are highly abundant within the host such as nucleotides, in a PhoB-independent and P i-transporter-dependent manner. It is also clear that S. aureus must utilize more than Pi transporters to overcome Pi limitation during infection, as the ΔphoPR mutant has a more severe virulence defect than the Pi -transporter double mutant ΔpstSΔnptA. To better understand how S. aureus overcomes Pi limitation during infection, the staphylococcal genes induced by P i limitation and PhoPR were determined. This analysis identified AdsA (adenosine synthase), which catalyzes the conversion of extracellular adenosine mono-, di- and triphosphates into adenosine and free Pi . AdsA can also catalyze a similar reaction with other nucleotides. Further work confirmed that AdsA is expressed in a PhoPR-dependent manner in response to Pi limitation and that it enables S. aureus to use adenosine mono- and diphosphates as Pi sources. PhoB can catalyze the same reaction as AdsA and further investigation revealed that the relative contribution of these two enzymes to using nucleotides as a Pi source is environment dependent. Extracellular adenosine suppresses the immune response and deoxyadenosine triggers macrophage death, with previous work revealing that loss of AdsA reduces S. aureus virulence and increases the inflammatory response. Combined, these observations lead to the hypothesis that Pi limitation increases the expression of AdsA and PhoB, which contribute to both suppression of the immune response and acquisition of Pi during infection. The Aims of this this proposal will test this hypothesis: Aim 1: Establish if Pi limitation enhances immunomodulation by AdsA and PhoB. Aim 2: Determine if environment modulates the contribution of AdsA and PhoB to phosphate uptake. Aim 3: Elucidate the in vivo impact of AdsA and PhoB on Pi uptake and immunomodulation during infection. The support of this fellowship will facilitate my doctoral training in the Department of Microbiology and Immunology at the University of Iowa (UI). All my time will be dedicated to research, obtaining an MS in Science Education, presentation of my work, and professional development.