Antimicrobial lipopeptide tridecaptin A1 selectively binds to Gram-negative lipid II

Tridecaptin A₁ (TriA₁) is a nonribosomal lipopeptide with selective antimicrobial activity against Gram-negative bacteria. Here we show that TriA₁ exerts its bactericidal effect by binding to the bacterial cell-wall precursor lipid II on the inner membrane, disrupting the proton motive force. Biochemical and biophysical assays show that binding to the Gram-negative variant of lipid II is required for membrane disruption and that only the proton gradient is dispersed. The NMR solution structure of TriA₁ in dodecylphosphocholine micelles with lipid II has been determined, and molecular modeling was used to provide a structural model of the TriA₁–lipid II complex. These results suggest that TriA₁ kills Gram-negative bacteria by a mechanism of action using a lipid-II–binding motif.Recently, a lot of media coverage has been focused on the problem of antimicrobial resistance. A report commissioned by the UK government predicts that by 2050 antimicrobial resistance will have caused 300 million premature deaths and cost the global economy over $100 trillion (1). Even more worrying is the lack of new classes of antibiotics active against Gram-negative bacteria. In the past 50 y, only a few structurally and mechanistically distinct classes of antibiotics have been clinically approved to treat systemic infections (including fidaxomicin, bedaquiline, linezolid, and daptomycin), yet none of these are active against Gram-negative bacteria (2, 3). Two new classes of Gram-negative targeting antibiotics in the clinical pipeline are POL7080 and brilacidin (4, 5). Both of these compounds are modeled on antimicrobial peptides, which are becoming increasingly important in the fight against antibiotic resistance (6). Bacteria produce a wealth of antimicrobial peptides, both ribosomally, including the lantibiotics (7, 8), and nonribosomally, including lipopeptides (9). In particular, lipopeptides are a rich source of antimicrobial compounds, and several examples with activity against Gram-positive (10, 11) and/or Gram-negative bacteria (12) have been recently characterized.Tridecaptin A₁ (TriA₁) is a member of the tridecaptin family, a group of nonribosomal lipopeptides produced by Bacillus and Paenibacillus species (Fig. 1) (13–15). This acylated tridecapeptide displays strong and selective antimicrobial activity against Gram-negative bacteria, including multidrug-resistant strains of Klebsiella pneumoniae, Acinetobacter baumannii, and Escherichia coli (16). TriA₁ analogs have low cytotoxicity and have been shown to treat K. pneumoniae infections in mice (16, 17). Therefore, we believe that tridecaptin A₁ could be an excellent antibiotic candidate. However, before our investigations little was known about how TriA₁ exerts its selective bactericidal effect against Gram-negative bacteria. A previous structure–activity relationship study by our group suggested that TriA₁, akin to many other lipopeptides, is a membrane-targeting agent. We found that removal of the N-terminal lipid tail abolishes antimicrobial activity; however, the chiral lipid tail could be replaced with an octanoyl chain to give Oct-TriA₁ (Fig. 1), which retains full activity (16). We therefore sought to identify the precise mode and mechanism of action by which TriA₁ kills Gram-negative bacteria.Open in a separate windowFig. 1.Structures of the tridecaptin analogs TriA₁ and Oct-TriA₁.