Aspirin has been previously shown to reduce the in vivo virulence of Staphylococcus aureus in experimental endocarditis, through antiplatelet and antimicrobial mechanisms. In the present study, salicylic acid, the major in vivo metabolite of aspirin, mitigated two important virulence phenotypes in both clinical and laboratory S. aureus strains: α-hemolysin secretion and fibronectin binding in vitro. In addition, salicylic acid reduced the expression of the α-hemolysin gene promoter, hla, and the fibronectin gene promoter, fnbA. Transcriptional analysis, fluorometry, and flow cytometry revealed evidence of salicylic acid-mediated activation of the stress-response gene sigB. Expression of the sigBrepressible global regulon sarA and the global regulon agr were also mitigated by salicylic acid, corresponding to the reduced expression of the hla and fnbA genes in vitro. Studies in experimental endocarditis confirmed the key roles of both sarA and sigB in mediating the antistaphylococcal effects of salicylic acid in vivo. Therefore, aspirin has the potential to be an adjuvant therapeutic agent against endovascular infections that result from S. aureus, by downmodulating key staphylococcal global regulons and structural genes in vivo, thus abrogating relevant virulence phenotypes.

Recent discoveries suggest cysteine-stabilized toxins and antimicrobial peptides have structure-activity parallels derived by common ancestry. Here, human antimicrobial peptide hBD-2 and rattlesnake venom-toxin crotamine were compared in phylogeny, 3D structure, target cell specificity, and mechanisms of action. Results indicate a striking degree of structural and phylogenetic congruence. Importantly, these polypeptides also exhibited functional reciprocity: (i) they exerted highly similar antimicrobial pH optima and spectra; (ii) both altered membrane potential consistent with ion channel-perturbing activities; and (iii) both peptides induced phosphatidylserine accessibility in eukaryotic cells. However, the Nav channel-inhibitor tetrodotoxin antagonized hBD-2 mechanisms, but not those of crotamine. As crotamine targets eukaryotic ion channels, computational docking was used to compare hBD-2 versus crotamine interactions with prototypic bacterial, fungal, or mammalian Kv channels. Models support direct interactions of each peptide with Kv channels. However, while crotamine localized to occlude Kv channels in eukaryotic but not prokaryotic cells, hBD-2 interacted with prokaryotic and eukaryotic Kv channels but did not occlude either. Together, these results support the hypothesis that antimicrobial and cytotoxic polypeptides have ancestral structurefunction homology, but evolved to preferentially target respective microbial versus mammalian ion channels via residue-specific interactions. These insights may accelerate development of antiinfective or therapeutic peptides that selectively target microbial or abnormal host cells.channel ͉ defensin ͉ host defense ͉ toxin