Skin and soft tissue infections (SSTIs) and their associated complications represent a significant source of morbidity and mortality in the US, particularly when caused by multidrug-resistant (MDR) pathogens. The most common source of MDR associated SSTI is methicillin-resistant Staphylococcus aureus (MRSA). The host response during MRSA SSTIs involves two general phases: early on, a classical inflammatory response ensues including infiltrating leukocytes generating NO* in an attempt to sterilize the wound. Later, the host shifts into a wound-resolution and regenerative phase associated with anti-inflammatory mediators and growth promoting signals including the production of a class of compounds known as polyamines (i.e. putrescine, spermidine, and spermine). Both NO* and polyamines are synthesized from host arginine and the fate of arginine profoundly affects the outcomes of S. aureus infections. S. aureus is uniquely resistant to the cytotoxic effects of NO*, however polyamines are highly toxic to this pathogen. In fact, given the highly proliferative nature of a resolving skin lesion, polyamines become abundant enough during the resolution phase to directly kill MRSA. The only exceptions are isolates belonging to the emerging community-associated MRSA USA300 clones. These strains acquired a spermine/spermidine acetyltransferase (SpeG) that confers complete resistance to host polyamines and contributes to the remarkable success of these clones. Here we propose to develop novel inhibitors of bacterial SpeG-homologues and simultaneously optimize host polyamine levels in infected skin abscesses. This antimicrobial/anti-resistance approach would allow for the treatment of complicated MRSA SSTIs regardless of whether they are caused by SpeG-expressing USA300 strains. Interestingly, we found that eliminating SpeG-activity from a variety of wound-related MDR pathogens (e.g. Vancomycin-resistant Enterococcus faecalis, Acinetobacter baumannii, and E. coli) generally leads to polyamine sensitivity. Thus, this approach can be extended to treat wound infections caused by a many MDR bacteria. We outline our plan to develop a fluorescence-based assay to quantify USA300 SpeG activity that can be screened with a small molecule library targeted to define structure-activity relationships to previously identified low-affinity SpeG-inhibitors. Our goal is to find effective SpeG antagonists that can be safely applied topically to skin lesions in which we have optimized polyamine levels. Polyamine optimization can be accomplished by direct topical administration of spermine and/or spermidine simultaneously with FDA-approved agents that limit polyamine catabolism. Alternatively, we propose a series of experiments that test the feasibility of immunomodulation as a mechanism of polyamine optimization. Upon completing our aims, we will determine the safest and most effective mechanism of optimizing tissue polyamines during an SSTI and combine this with potent anti-SpeG compounds that will render a number of wound-related pathogens susceptible to elevated polyamines. We will ascertain whether this is an effective treatment on its own, or better serves to augment current treatment strategies given the documented synergy between polyamine- killing and traditional antibiotics. In the end, we hope to harness the anti-inflammatory nature of polyamines and at the same time exploit their antibacterial effects to improve disease outcomes of SSTIs caused by a variety of MDR pathogens.
This project focuses on exploiting the direct effects of polyamines, natural compounds copiously produced by the host, to improve the clearance of skin and wound infections caused by Staphylococcus aureus. A thorough understanding of polyamine mechanism of action against this important human pathogen will help augment current treatment regimens. Furthermore, new compounds targeting a newly discovered resistance mechanism can potentially render many wound and skin pathogens sensitive to polyamines thus providing a broad spectrum treatment or adjunctive therapy.