Synthesis of Bisindole Alkaloids and Their Mode of Action against Methicillin-Resistant Staphylococcus Aureus.

About 100,000 deaths are attributed annually to infections with methicillin-resistant Staphylococcus aureus (MRSA) despite concerted efforts toward vaccine development and clinical trials involving several preclinically efficacious drug candidates. This necessitates the development of alternative therapeutic options against this drug-resistant bacterial pathogen. Using the Masuda borylation-Suzuki coupling (MBSC) sequence, we previously synthesized and modified naturally occurring bisindole alkaloids, alocasin A, hyrtinadine A and scalaradine A, resulting in derivatives showing potent in vitro and in vivo antibacterial efficacy. Here, we report on a modified one-pot MBSC protocol for the synthesis of previously reported and several undescribed N-tosyl-protected bisindoles with anti-MRSA activities and moderate cytotoxicity against human monocytic and kidney cell lines. In continuation of the mode of action investigation of the previously synthesized membrane-permeabilizing hit compounds, mechanistic studies reveal that bisindoles impact the cytoplasmic membrane of Gram-positive bacteria by promiscuously interacting with lipid II and membrane phospholipids while rapidly dissipating membrane potential. The bactericidal and lipid II-interacting lead compounds 5c and 5f might be interesting starting points for drug development in the fight against MRSA.

An antibiotic from an uncultured bacterium binds to an immutable target.

Antimicrobial resistance is a leading mortality factor worldwide. Here, we report the discovery of clovibactin, an antibiotic isolated from uncultured soil bacteria. Clovibactin efficiently kills drug-resistant Gram-positive bacterial pathogens without detectable resistance. Using biochemical assays, solid-state nuclear magnetic resonance, and atomic force microscopy, we dissect its mode of action. Clovibactin blocks cell wall synthesis by targeting pyrophosphate of multiple essential peptidoglycan precursors (C55PP, lipid II, and lipid IIIWTA). Clovibactin uses an unusual hydrophobic interface to tightly wrap around pyrophosphate but bypasses the variable structural elements of precursors, accounting for the lack of resistance. Selective and efficient target binding is achieved by the sequestration of precursors into supramolecular fibrils that only form on bacterial membranes that contain lipid-anchored pyrophosphate groups. This potent antibiotic holds the promise of enabling the design of improved therapeutics that kill bacterial pathogens without resistance development.

A new antibiotic from an uncultured bacterium binds to an immutable target.

Antimicrobial resistance is a leading mortality factor worldwide. Here we report the discovery of clovibactin, a new antibiotic, isolated from uncultured soil bacteria. Clovibactin efficiently kills drug-resistant bacterial pathogens without detectable resistance. Using biochemical assays, solid-state NMR, and atomic force microscopy, we dissect its mode of action. Clovibactin blocks cell wall synthesis by targeting pyrophosphate of multiple essential peptidoglycan precursors (C 55 PP, Lipid II, Lipid WTA ). Clovibactin uses an unusual hydrophobic interface to tightly wrap around pyrophosphate, but bypasses the variable structural elements of precursors, accounting for the lack of resistance. Selective and efficient target binding is achieved by the irreversible sequestration of precursors into supramolecular fibrils that only form on bacterial membranes that contain lipid-anchored pyrophosphate groups. Uncultured bacteria offer a rich reservoir of antibiotics with new mechanisms of action that could replenish the antimicrobial discovery pipeline.

Impacts of NaHCO3 on ß-Lactam Binding to PBP2a Protein Variants Associated with the NaHCO3-Responsive versus NaHCO3-Non-Responsive Phenotypes.

Methicillin-resistant Staphylococcus aureus (MRSA) regulates resistance to ß-lactams via preferential production of an alternative penicillin-binding protein (PBP), PBP2a. PBP2a binds many ß-lactam antibiotics with less affinity than PBPs which are predominant in methicillin-susceptible (MSSA) strains. A novel, rather frequent in vitro phenotype was recently identified among clinical MRSA bloodstream isolates, termed "NaHCO3-responsiveness". This phenotype features ß-lactam susceptibility of certain MRSA strains only in the presence of NaHCO3. Two distinct PBP2a variants, 246G and 246E, have been linked to the NaHCO3-responsive and NaHCO3-non-responsive MRSA phenotypes, respectively. To determine the mechanistic impact of PBP2a variants on ß-lactam susceptibility, binding profiles of a fluorescent penicillin probe (Bocillin-FL) to each purified PBP2a variant were assessed and compared to whole-cell binding profiles characterized by flow cytometry in the presence vs. absence of NaHCO3. These investigations revealed that NaHCO3 differentially influenced the binding of the fluorescent penicillin, Bocillin-FL, to the PBP2a variants, with binding intensity and rate of binding significantly enhanced in the 246G compared to the 246E variant. Of note, the NaHCO3-ß-lactam (oxacillin)-responsive JE2 strain, which natively harbors the 246G variant, had enhanced Bocillin-FL whole-cell binding following exposure to NaHCO3. This NaHCO3-mediated increase in whole-cell Bocillin-FL binding was not observed in the NaHCO3-non-responsive parental strain, COL, which contains the 246E PBP2a variant. Surprisingly, genetic swaps of the mecA coding sites between JE2 and COL did not alter the NaHCO3-enhanced binding seen in JE2 vs. COL. These data suggest that the non-coding regions of mecA may be involved in NaHCO3-responsiveness. This investigation also provides strong evidence that the NaHCO3-responsive phenotype in MRSA may involve NaHCO3-mediated increases in both initial cell surface ß-lactam binding, as well as ultimate PBP2a binding of ß-lactams.

THCz: Small molecules with antimicrobial activity that block cell wall lipid intermediates.

Emerging antibiotic resistance demands identification of novel antibacterial compound classes. A bacterial whole-cell screen based on pneumococcal autolysin-mediated lysis induction was developed to identify potential bacterial cell wall synthesis inhibitors. A hit class comprising a 1-amino substituted tetrahydrocarbazole (THCz) scaffold, containing two essential amine groups, displayed bactericidal activity against a broad range of gram-positive and selected gram-negative pathogens in the low micromolar range. Mode of action studies revealed that THCz inhibit cell envelope synthesis by targeting undecaprenyl pyrophosphate-containing lipid intermediates and thus simultaneously inhibit peptidoglycan, teichoic acid, and polysaccharide capsule biosynthesis. Resistance did not readily develop in vitro, and the ease of synthesizing and modifying these small molecules, as compared to natural lipid II-binding antibiotics, makes THCz promising scaffolds for development of cell wall-targeting antimicrobials.

Host Defense Peptides: Dual Antimicrobial and Immunomodulatory Action.

The rapid rise of multidrug-resistant (MDR) bacteria has once again caused bacterial infections to become a global health concern. Antimicrobial peptides (AMPs), also known as host defense peptides (HDPs), offer a viable solution to these pathogens due to their diverse mechanisms of actions, which include direct killing as well as immunomodulatory properties (e.g., anti-inflammatory activity). HDPs may hence provide a more robust treatment of bacterial infections. In this review, the advent of and the mechanisms that lead to antibiotic resistance will be described. HDP mechanisms of antibacterial and immunomodulatory action will be presented, with specific examples of how the HDP aurein 2.2 and a few of its derivatives, namely peptide 73 and cG4L73, function. Finally, resistance that may arise from a broader use of HDPs in a clinical setting and methods to improve biocompatibility will be briefly discussed.

Biosynthesis and Mechanism of Action of the Cell Wall Targeting Antibiotic Hypeptin.

Hypeptin is a cyclodepsipeptide antibiotic produced by Lysobacter sp. K5869, isolated from an environmental sample by the iChip technology, dedicated to the cultivation of previously uncultured microorganisms. Hypeptin shares structural features with teixobactin and exhibits potent activity against a broad spectrum of gram-positive pathogens. Using comprehensive in vivo and in vitro analyses, we show that hypeptin blocks bacterial cell wall biosynthesis by binding to multiple undecaprenyl pyrophosphate-containing biosynthesis intermediates, forming a stoichiometric 2:1 complex. Resistance to hypeptin did not readily develop in vitro. Analysis of the hypeptin biosynthetic gene cluster (BGC) supported a model for the synthesis of the octapeptide. Within the BGC, two hydroxylases were identified and characterized, responsible for the stereoselective ß-hydroxylation of four building blocks when bound to peptidyl carrier proteins. In vitro hydroxylation assays corroborate the biosynthetic hypothesis and lead to the proposal of a refined structure for hypeptin.

Ca2+-Daptomycin targets cell wall biosynthesis by forming a tripartite complex with undecaprenyl-coupled intermediates and membrane lipids.

The lipopeptide daptomycin is used as an antibiotic to treat severe infections with gram-positive pathogens, such as methicillin resistant Staphylococcus aureus (MRSA) and drug-resistant enterococci. Its precise mechanism of action is incompletely understood, and a specific molecular target has not been identified. Here we show that Ca2+-daptomycin specifically interacts with undecaprenyl-coupled cell envelope precursors in the presence of the anionic phospholipid phosphatidylglycerol, forming a tripartite complex. We use microbiological and biochemical assays, in combination with fluorescence and optical sectioning microscopy of intact staphylococcal cells and model membrane systems. Binding primarily occurs at the staphylococcal septum and interrupts cell wall biosynthesis. This is followed by delocalisation of components of the peptidoglycan biosynthesis machinery and massive membrane rearrangements, which may account for the pleiotropic cellular events previously reported. The identification of carrier-bound cell wall precursors as specific targets explains the specificity of daptomycin for bacterial cells. Our work reconciles apparently inconsistent previous results, and supports a concise model for the mode of action of daptomycin.

The Lasso Peptide Siamycin-I Targets Lipid II at the Gram-Positive Cell Surface.

Ribosomally synthesized post-translationally modified peptides (RiPPs) are a diverse class of biologically active molecules produced by many environmental bacteria. While thousands of these compounds have been identified, mostly through genome mining, a relatively small number has been investigated at the molecular level. One less understood class of RiPPs is the lasso peptides. These are 20-25 amino acid residue compounds bearing an N-terminal macrocyclic ring and a C-terminal tail that is threaded through the ring. We have carried out a detailed investigation on the mechanism of action of the siamycin-I lasso peptide. We demonstrate that siamycin-I interacts with lipid II, the central building block of the major cell wall component peptidoglycan, which is readily accessible on the outside of the cell. This interaction compromises cell wall biosynthesis in a manner that activates the liaI stress response. Additionally, resistance to siamycin-I can be brought about by mutations in the essential WalKR two-component system that causes thickening of the cell wall. Siamycin-I is the first lasso peptide that has been shown to inhibit cell wall biosynthesis.