First-in-class inhibitors of SbnA reduce siderophores production in Staphylococcus aureus

Siderophores production, along with heme scavenging by hemophores, is one of the main mechanisms exploited by bacteria to achieve iron supply. Staphylococcus aureus produces two main siderophores, staphyloferrin A (SA) and staphyloferrin B (SB), with the latter produced only by the most invasive, coagulasepositive S. aureus strains. Along the seven steps of SB biosynthetic pathway, SbnA catalyzes the crucial formation of the intermediate N-2-amino-2-carboxyethyl-L-glutamate from L-O-phosphoserine and glutamate. Our functional characterization of the enzyme highlighted that citrate inhibits SbnA with a Ki in the order of magnitude of the physiological concentration of the metabolite. We searched for inhibitors of SbnA within citrate analogues and identified 2-phenylmaleic acid (2-PhMA) as the best hit, with a Ki of 16 ± 2 μM and a mechanism of inhibition which is competitive with L-O-phosphoserine for active-site binding. The methyl ester of 2-PhMA at 2 mM concentration resulted effective in inhibiting siderophore biosynthesis in S. aureus. These results pave the way for the discovery of promising inhibitors of iron acquisition that might find application as innovative antimicrobials.