There has been major interest by the scientific community in anti-virulence approaches against bacterial infections. However, partly due to a lack of viable lead compounds, anti-virulence therapeutics have yet to reach the clinic. Here we investigate the development of an anti-virulence lead targeting quorum sensing signal-biosynthesis, a process that is conserved in Gram positive bacterial pathogens. Some preliminary studies suggest that the small molecule ambuic acid is a signal biosynthesis inhibitor. To confirm this, we construct a methicillin-resistant Staphylococcus aureus (MRSA) strain that decouples AIP production from regulation, and demonstrate that AIP production is inhibited in this mutant. Quantitative mass spectrometric measurements show that ambuic acid inhibits signal biosynthesis (IC50 of 2.5 ± 0.1 μM) against a clinically relevant USA 300 MRSA strain. Quantitative real-time PCR confirms that this compound selectively targets the quorum sensing regulon. We show that a 5 μg dose of ambuic acid reduces MRSA-induced abscess formation in a mouse model, and verify its quorum sensing inhibitory activity in vivo. Finally, we employ mass spectrometry to identify or confirm the structure of quorum sensing signaling peptides in three strains of S. aureus, three strains of Staphylococcus epidermidis, Enterococcus faecalis, Listeria monocytogenes, Staphylococcus saphrophyticus, and Staphylococcus lugdunensis. By measuring AIP production by these strains, we show that ambuic acid possesses broad-spectrum efficacy against multiple Gram positive bacterial pathogens, but does not inhibit quorum sensing in some commensal bacteria. Collectively, these findings demonstrate the promise of ambuic acid as a lead for the development of anti-virulence therapeutics.
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