Ironing out New Antibiotic Mechanisms with Xanthocillin X

Abstract

Antibiotic resistance is a growing health crisis, as the rate of antibiotic resistance is developing more rapidly than the approval of new antibiotic therapies. Even more troubling is the emergence of multi-drug-resistant pathogens and “super bugs” that are resistant to many common antibiotics used in the clinic. To address this health crisis, new antibiotics that have unique targets are needed, so that a strain of bacteria that is resistant to one antibiotic will likely be sensitive to a novel antibiotic treatment. In this issue of ACS Central Science, Hübner and colleagues uncover a new antibiotic mechanism of action that could meet this demand. Unique natural products are a promising source of new antibiotic discovery, but without an understanding of their mechanism, the modification and optimization of those compounds are challenging. Several hundred isonitrile antibacterial natural products have been discovered since the first isolation of Xanthocillin X (Xan) in 1948; yet, a cellular target for these compounds has been missing until now. Hübner and team discovered heme biosynthesis as a novel antibacterial target of Xan. The authors began with an evaluation of Xan’s effectiveness against several strains of bacteria and found that it had broad spectrum activity including against the most challenging Gram-negative bacteria strains. Excitingly, Xan was most effective against Acinetobacter baumannii, a clinically relevant pathogen identified as a high-priority health threat by the WHO. Previous studies identified copper binding and disruption of copper-dependent enzymes as a mechanistic target of isonitrile compounds. To see if Xan acts similarly, the authors began by testing its ability to bind metals in solution. They found that Xan exclusively bound to copper(II) and that an inactive Xan analogue (XanDME) did not bind to copper(II). These observations suggested that copper binding was involved in Xan’s antibacterial mechanism; however, a key control experiment showed that XanDME was being quickly exported from the bacteria and regained antibiotic activity in a mutant strain of A. baumannii with two efflux pumps knocked out. This suggested that copper chelation was not essential to Xan or XanDME’s activity and that these compounds were likely proceeding through another, unreported mechanism. The authors next evaluated whether Xan has a protein target using a chemical proteomics approach to identify proteins that it binds to or covalently labels in A. baumannii. The authors identified several covalent protein targets using an alkyne-modified Xan probe (XP); however, none of the identified proteins were essential for cellular function.