Cell-Based Assay to Determine Type 3 Secretion System Translocon Assembly in Pseudomonas aeruginosa Using Split Luciferase.

Multi-drug-resistant Pseudomonas aeruginosa poses a serious threat to hospitalized patients. This organism expresses an arsenal of virulence factors that enables it to readily establish infections and disseminate in the host. The Type 3 secretion system (T3SS) and its associated effectors play a crucial role in the pathogenesis of P. aeruginosa, making them attractive targets for the development of novel therapeutic agents. The T3SS translocon, composed of PopD and PopB, is an essential component of the T3SS secretion apparatus. In the properly assembled translocon, the N-terminus of PopD protrudes into the cytoplasm of the target mammalian cell, which can be exploited as a molecular indicator of functional translocon assembly. In this article, we describe a novel whole-cell-based assay that employs the split NanoLuc luciferase detection system to provide a readout for translocon assembly. The assay demonstrates a favorable signal/noise ratio (13.6) and robustness (Z' = 0.67), making it highly suitable for high-throughput screening of small-molecule inhibitors targeting T3SS translocon assembly.

Porin-independent accumulation in Pseudomonas enables antibiotic discovery.

Gram-negative antibiotic development has been hindered by a poor understanding of the types of compounds that can accumulate within these bacteria1,2. The presence of efflux pumps and substrate-specific outer-membrane porins in Pseudomonas aeruginosa renders this pathogen particularly challenging3. As a result, there are few antibiotic options for P. aeruginosa infections4 and its many porins have made the prospect of discovering general accumulation guidelines seem unlikely5. Here we assess the whole-cell accumulation of 345 diverse compounds in P. aeruginosa and Escherichia coli. Although certain positively charged compounds permeate both bacterial species, P. aeruginosa is more restrictive compared to E. coli. Computational analysis identified distinct physicochemical properties of small molecules that specifically correlate with P. aeruginosa accumulation, such as formal charge, positive polar surface area and hydrogen bond donor surface area. Mode of uptake studies revealed that most small molecules permeate P. aeruginosa using a porin-independent pathway, thus enabling discovery of general P. aeruginosa accumulation trends with important implications for future antibiotic development. Retrospective antibiotic examples confirmed these trends and these discoveries were then applied to expand the spectrum of activity of a gram-positive-only antibiotic, fusidic acid, into a version that demonstrates a dramatic improvement in antibacterial activity against P. aeruginosa. We anticipate that these discoveries will facilitate the design and development of high-permeating antipseudomonals.

Correction to "An Iterative Approach Guides Discovery of the FabI Inhibitor Fabimycin, a Late-Stage Antibiotic Candidate with In Vivo Efficacy against Drug-Resistant Gram-Negative Infections".

[This corrects the article DOI: 10.1021/acscentsci.2c00598.].

An Iterative Approach Guides Discovery of the FabI Inhibitor Fabimycin, a Late-Stage Antibiotic Candidate with In Vivo Efficacy against Drug-Resistant Gram-Negative Infections.

Genomic studies and experiments with permeability-deficient strains have revealed a variety of biological targets that can be engaged to kill Gram-negative bacteria. However, the formidable outer membrane and promiscuous efflux pumps of these pathogens prevent many candidate antibiotics from reaching these targets. One such promising target is the enzyme FabI, which catalyzes the rate-determining step in bacterial fatty acid biosynthesis. Notably, FabI inhibitors have advanced to clinical trials for Staphylococcus aureus infections but not for infections caused by Gram-negative bacteria. Here, we synthesize a suite of FabI inhibitors whose structures fit permeation rules for Gram-negative bacteria and leverage activity against a challenging panel of Gram-negative clinical isolates as a filter for advancement. The compound to emerge, called fabimycin, has impressive activity against >200 clinical isolates of Escherichia coli, Klebsiella pneumoniae, and Acinetobacter baumannii, and does not kill commensal bacteria. X-ray structures of fabimycin in complex with FabI provide molecular insights into the inhibition. Fabimycin demonstrates activity in multiple mouse models of infection caused by Gram-negative bacteria, including a challenging urinary tract infection model. Fabimycin has translational promise, and its discovery provides additional evidence that antibiotics can be systematically modified to accumulate in Gram-negative bacteria and kill these problematic pathogens.

Rationalizing the generation of broad spectrum antibiotics with the addition of a positive charge.

Antibiotic resistance of Gram-negative bacteria is largely attributed to the low permeability of their outer membrane (OM). Recently, we disclosed the eNTRy rules, a key lesson of which is that the introduction of a primary amine enhances OM permeation in certain contexts. To understand the molecular basis for this finding, we perform an extensive set of molecular dynamics (MD) simulations and free energy calculations comparing the permeation of aminated and amine-free antibiotic derivatives through the most abundant OM porin of E. coli, OmpF. To improve sampling of conformationally flexible drugs in MD simulations, we developed a novel, Monte Carlo and graph theory based algorithm to probe more efficiently the rotational and translational degrees of freedom visited during the permeation of the antibiotic molecule through OmpF. The resulting pathways were then used for free-energy calculations, revealing a lower barrier against the permeation of the aminated compound, substantiating its greater OM permeability. Further analysis revealed that the amine facilitates permeation by enabling the antibiotic to align its dipole to the luminal electric field of the porin and form favorable electrostatic interactions with specific, highly-conserved charged residues. The importance of these interactions in permeation was further validated with experimental mutagenesis and whole cell accumulation assays. Overall, this study provides insights on the importance of the primary amine for antibiotic permeation into Gram-negative pathogens that could help the design of future antibiotics. We also offer a new computational approach for calculating free-energy of processes where relevant molecular conformations cannot be efficiently captured.

An LC-MS/MS assay and complementary web-based tool to quantify and predict compound accumulation in E. coli.

Novel classes of broad-spectrum antibiotics have been extremely difficult to discover, largely due to the impermeability of the Gram-negative membranes coupled with a poor understanding of the physicochemical properties a compound should possess to promote its accumulation inside the cell. To address this challenge, numerous methodologies for assessing intracellular compound accumulation in Gram-negative bacteria have been established, including classic radiometric and fluorescence-based methods. The recent development of accumulation assays that utilize liquid chromatography-tandem mass spectrometry (LC-MS/MS) have circumvented the requirement for labeled compounds, enabling assessment of a substantially broader range of small molecules. Our unbiased study of accumulation trends in Escherichia coli using an LC-MS/MS-based assay led to the development of the eNTRy rules, which stipulate that a compound is most likely to accumulate in E. coli if it has an ionizable Nitrogen, has low Three-dimensionality and is relatively Rigid. To aid in the implementation of the eNTRy rules, we developed a complementary web tool, eNTRyway, which calculates relevant properties and predicts compound accumulation. Here we provide a comprehensive protocol for analysis and prediction of intracellular accumulation of small molecules in E. coli using an LC-MS/MS-based assay (which takes ~2 d) and eNTRyway, a workflow that is readily adoptable by any microbiology, biochemistry or chemical biology laboratory.

Compound Uptake into E. coli Can Be Facilitated by N-Alkyl Guanidiniums and Pyridiniums.

Multidrug-resistant Gram-negative bacterial infections are on the rise, and with no FDA approvals for new classes of broad-spectrum antibiotics in over 50 years, these infections constitute a major threat to human health. A significant challenge is the inability of most compounds to accumulate in Gram-negative bacteria. Recently developed predictive guidelines show that appending a primary amine to an appropriately shaped compound can enhance Gram-negative accumulation. Here, we report that other positively charged nitrogen functional groups, namely, N-alkyl guanidiniums and pyridiniums, can also facilitate compound uptake into Gram-negative bacteria. The accumulation of a set of 60 nonantibiotic compounds, consisting of 20 primary amines and their corresponding guanidiniums and pyridiniums, was assessed in Escherichia coli. We also installed these alternate functional groups onto antibiotic scaffolds and assessed their accumulation and antibacterial activity in Gram-negative bacteria. The results suggest that other positively-charged, nitrogen-containing functional groups should be considered when designing antibiotics with Gram-negative activity.

Gram-Negative Antibiotic Active Through Inhibition of an Essential Riboswitch.

Multidrug-resistant Gram-negative (GN) infections for which there are few available treatment options are increasingly common. The development of new antibiotics for these pathogens is challenging because of the inability of most small molecules to accumulate inside GN bacteria. Using recently developed predictive guidelines for compound accumulation in Escherichia coli, we have converted the antibiotic Ribocil C, which targets the flavin mononucleotide (FMN) riboswitch, from a compound lacking whole-cell activity against wild-type GN pathogens into a compound that accumulates to a high level in E. coli, is effective against Gram-negative clinical isolates, and has efficacy in mouse models of GN infections. This compound allows for the first assessment of the translational potential of FMN riboswitch binders against wild-type Gram-negative bacteria.

Implementation of permeation rules leads to a FabI inhibitor with activity against Gram-negative pathogens.

Gram-negative bacterial infections are a significant public health concern, and the lack of new drug classes for these pathogens is linked to the inability of most drug leads to accumulate inside Gram-negative bacteria1-7. Here, we report the development of a web application-eNTRyway-that predicts compound accumulation (in Escherichia coli) from its structure. In conjunction with structure-activity relationships and X-ray data, eNTRyway was utilized to re-design Debio-1452-a Gram-positive-only antibiotic8-into versions that accumulate in E. coli and possess antibacterial activity against high-priority Gram-negative pathogens. The lead compound Debio-1452-NH3 operates as an antibiotic via the same mechanism as Debio-1452, namely potent inhibition of the enoyl-acyl carrier protein reductase FabI, as validated by in vitro enzyme assays and the generation of bacterial isolates with spontaneous target mutations. Debio-1452-NH3 is well tolerated in vivo, reduces bacterial burden in mice and rescues mice from lethal infections with clinical isolates of Acinetobacter baumannii, Klebsiella pneumoniae and E. coli. This work provides tools for the facile discovery and development of high-accumulating compounds in E. coli, and a general blueprint for the conversion of Gram-positive-only compounds into broad-spectrum antibiotics.