OXA-66 structure and oligomerisation of OXAAb enzymes.

The OXA ß-lactamases are responsible for hydrolysing ß-lactam antibiotics and contribute to the multidrug-resistant phenotype of several major human pathogens. The OXAAb enzymes are intrinsic to Acinetobacter baumannii and can confer resistance to carbapenem antibiotics. Here we determined the structure of the most prevalent OXAAb enzyme, OXA-66. The structure of OXA-66 was solved at a resolution of 2.1 Å and found to be very similar to the structure of OXA-51, the only other OXAAb enzyme that has had its structure solved. Our data contained one molecule per asymmetric unit, and analysis of positions responsible for dimer formation in other OXA enzymes suggest that OXA-66 likely exists as a monomer.

Virtual Screening Identifies Novel and Potent Inhibitors of Mycobacterium tuberculosis PknB with Antibacterial Activity.

Mycobacterium tuberculosis protein kinase B (PknB) is essential to mycobacterial growth and has received considerable attention as an attractive target for novel anti-tuberculosis drug development. Here, virtual screening, validated by biological assays, was applied to select candidate inhibitors of M. tuberculosis PknB from the Specs compound library (www.specs.net). Fifteen compounds were identified as hits and selected for in vitro biological assays, of which three indoles (2, AE-848/42799159; 4, AH-262/34335013; 10, AP-124/40904362) inhibited growth of M. tuberculosis H37Rv with minimal inhibitory concentrations of 6.2, 12.5, and 6.2 µg/mL, respectively. Two compounds, 2 and 10, inhibited M. tuberculosis PknB activity in vitro, with IC50 values of 14.4 and 12.1 µM, respectively, suggesting this to be the likely basis of their anti-tubercular activity. In contrast, compound 4 displayed anti-tuberculosis activity against M. tuberculosis H37Rv but showed no inhibition of PknB activity (IC50 > 128 µM). We hypothesize that hydrolysis of its ethyl ester to a carboxylate moiety generates an active species that inhibits other M. tuberculosis enzymes. Molecular dynamics simulations of modeled complexes of compounds 2, 4, and 10 bound to M. tuberculosis PknB indicated that compound 4 has a lower affinity for M. tuberculosis PknB than compounds 2 and 10, as evidenced by higher calculated binding free energies, consistent with experiment. Compounds 2 and 10 therefore represent candidate inhibitors of M. tuberculosis PknB that provide attractive starting templates for optimization as anti-tubercular agents.

Cladobotric Acids: Metabolites from Cultures of Cladobotryum sp., Semisynthetic Analogues and Antibacterial Activity.

Three new polyketide-derived natural products, cladobotric acids G-I (1-3), and six known metabolites (4, 5, 8-11) were isolated from fermentation of the fungus Cladobotryum sp. grown on rice. Their structures were elucidated by extensive spectroscopic methods. Two metabolites, cladobotric acid A (4) and pyrenulic acid A (10), were converted to a series of new products (12-20) by semisynthesis. The antibacterial activities of all these compounds were investigated against the Gram-positive pathogen Staphylococcus aureus including methicillin-susceptible (MSSA), methicillin-resistant and vancomycin-intermediate (MRSA/VISA), and heterogeneous vancomycin-intermediate (hVISA) strains. Results of these antibacterial assays revealed structural features of the unsaturated decalins important for biological activity.

Fast Identification and Quantification of Uropathogenic E. coli through Cluster Analysis.

Rapid diagnostic tools to detect, identify, and enumerate bacteria are key to maintaining effective antibiotic stewardship and avoiding the unnecessary prescription of broad-spectrum agents. In this study, a 15 min agglutination assay is developed that relies on the use of mannose-functionalized polymeric microspheres in combination with cluster analysis. This allows for the identification and enumeration of laboratory (BW25113), clinical isolate (NCTC 12241), and uropathogenic Escherichia coli strains (NCTC 9001, NCTC 13958, J96, and CFT073) at clinically relevant concentrations in tryptic soy broth (103-108 CFU/mL) and in urine (105-108 CFU/mL). This fast, simple, and efficient assay offers a step forward toward efficient point-of-care diagnostics for common urinary tract infections.

Trade-Offs between Antibacterial Resistance and Fitness Cost in the Production of Metallo-ß-Lactamases by Enteric Bacteria Manifest as Sporadic Emergence of Carbapenem Resistance in a Clinical Setting.

Meropenem is a clinically important antibacterial reserved for treatment of multiresistant infections. In meropenem-resistant bacteria of the family Enterobacterales, NDM-1 is considerably more common than IMP-1, despite both metallo-ß-lactamases (MBLs) hydrolyzing meropenem with almost identical kinetics. We show that blaNDM-1 consistently confers meropenem resistance in wild-type Enterobacterales, but blaIMP-1 does not. The reason is higher blaNDM-1 expression because of its stronger promoter. However, the cost of meropenem resistance is reduced fitness of blaNDM-1-positive Enterobacterales. In parallel, from a clinical case, we identified multiple Enterobacter spp. isolates carrying a plasmid-encoded blaNDM-1 having a modified promoter region. This modification lowered MBL production to a level associated with zero fitness cost, but, consequently, the isolates were not meropenem resistant. However, we identified a Klebsiella pneumoniae isolate from this same clinical case carrying the same blaNDM-1 plasmid. This isolate was meropenem resistant despite low-level NDM-1 production because of a ramR mutation reducing envelope permeability. Overall, therefore, we show how the resistance/fitness trade-off for MBL carriage can be resolved. The result is sporadic emergence of meropenem resistance in a clinical setting.

Variability in carbapenemase activity of intrinsic OxaAb (OXA-51-like) ß-lactamase enzymes in Acinetobacter baumannii.

OBJECTIVES: To measure the variability in carbapenem susceptibility conferred by different OxaAb variants, characterize the molecular evolution of oxaAb and elucidate the contribution of OxaAb and other possible carbapenem resistance factors in the clinical isolates using WGS and LC-MS/MS. METHODS: Antimicrobial susceptibility tests were performed on 10 clinical Acinetobacter baumannii isolates. Carbapenem MICs were evaluated for all oxaAb variants cloned into A. baumannii CIP70.10 and BM4547, with and without their natural promoters. Molecular evolution analysis of the oxaAb variants was performed using FastTree and SplitsTree4. Resistance determinants were studied in the clinical isolates using WGS and LC-MS/MS. RESULTS: Only the OxaAb variants with I129L and L167V substitutions, OxaAb(82), OxaAb(83), OxaAb(107) and OxaAb(110) increased carbapenem MICs when expressed in susceptible A. baumannii backgrounds without an upstream IS element. Carbapenem resistance was conferred with the addition of their natural upstream ISAba1 promoter. LC-MS/MS analysis on the original clinical isolates confirmed overexpression of the four I129L and L167V variants. No other differences in expression levels of proteins commonly associated with carbapenem resistance were detected. CONCLUSIONS: Elevated carbapenem MICs were observed by expression of OxaAb variants carrying clinically prevalent substitutions I129L and L167V. To drive carbapenem resistance, these variants required overexpression by their upstream ISAba1 promoter. This study clearly demonstrates that a combination of IS-driven overexpression of oxaAb and the presence of particular amino acid substitutions in the active site to improve carbapenem capture is key in conferring carbapenem resistance in A. baumannii and other mechanisms are not required.

Discovery of New and Potent InhA Inhibitors as Antituberculosis Agents: Structure-Based Virtual Screening Validated by Biological Assays and X-ray Crystallography.

The enoyl-acyl carrier protein reductase InhA of Mycobacterium tuberculosis is an attractive, validated target for antituberculosis drug development. Moreover, direct inhibitors of InhA remain effective against InhA variants with mutations associated with isoniazid resistance, offering the potential for activity against MDR isolates. Here, structure-based virtual screening supported by biological assays was applied to identify novel InhA inhibitors as potential antituberculosis agents. High-speed Glide SP docking was initially performed against two conformations of InhA differing in the orientation of the active site Tyr158. The resulting hits were filtered for drug-likeness based on Lipinski's rule and avoidance of PAINS-like properties and finally subjected to Glide XP docking to improve accuracy. Sixteen compounds were identified and selected for in vitro biological assays, of which two (compounds 1 and 7) showed MIC of 12.5 and 25 µg/mL against M. tuberculosis H37Rv, respectively. Inhibition assays against purified recombinant InhA determined IC50 values for these compounds of 0.38 and 0.22 µM, respectively. A crystal structure of the most potent compound, compound 7, bound to InhA revealed the inhibitor to occupy a hydrophobic pocket implicated in binding the aliphatic portions of InhA substrates but distant from the NADH cofactor, i.e., in a site distinct from those occupied by the great majority of known InhA inhibitors. This compound provides an attractive starting template for ligand optimization aimed at discovery of new and effective compounds against M. tuberculosis that act by targeting InhA.

CreC Sensor Kinase Activation Enhances Growth of Escherichia coli in the Presence of Cephalosporins and Carbapenems.

Mutants with enhanced growth in the presence of an antibiotic are more difficult to identify than mutants where the antibiotic's MIC increases, because they are not amenable to lethal selection in vitro We report that activatory mutations in the CreC signal sensor enhance growth of Escherichia coli in the presence of cefoxitin, cefotaxime, and meropenem, without increasing their MICs. Enhanced growth is dependent on overproduction of the inner membrane cre regulon protein CreD.

ß-Lactamases and ß-Lactamase Inhibitors in the 21st Century.

The ß-lactams retain a central place in the antibacterial armamentarium. In Gram-negative bacteria, ß-lactamase enzymes that hydrolyze the amide bond of the four-membered ß-lactam ring are the primary resistance mechanism, with multiple enzymes disseminating on mobile genetic elements across opportunistic pathogens such as Enterobacteriaceae (e.g., Escherichia coli) and non-fermenting organisms (e.g., Pseudomonas aeruginosa). ß-Lactamases divide into four classes; the active-site serine ß-lactamases (classes A, C and D) and the zinc-dependent or metallo-ß-lactamases (MBLs; class B). Here we review recent advances in mechanistic understanding of each class, focusing upon how growing numbers of crystal structures, in particular for ß-lactam complexes, and methods such as neutron diffraction and molecular simulations, have improved understanding of the biochemistry of ß-lactam breakdown. A second focus is ß-lactamase interactions with carbapenems, as carbapenem-resistant bacteria are of grave clinical concern and carbapenem-hydrolyzing enzymes such as KPC (class A) NDM (class B) and OXA-48 (class D) are proliferating worldwide. An overview is provided of the changing landscape of ß-lactamase inhibitors, exemplified by the introduction to the clinic of combinations of ß-lactams with diazabicyclooctanone and cyclic boronate serine ß-lactamase inhibitors, and of progress and strategies toward clinically useful MBL inhibitors. Despite the long history of ß-lactamase research, we contend that issues including continuing unresolved questions around mechanism; opportunities afforded by new technologies such as serial femtosecond crystallography; the need for new inhibitors, particularly for MBLs; the likely impact of new ß-lactam:inhibitor combinations and the continuing clinical importance of ß-lactams mean that this remains a rewarding research area.

Impact of OqxR loss of function on the envelope proteome of Klebsiella pneumoniae and susceptibility to antimicrobials.

Background: In Klebsiella pneumoniae, loss-of-function mutations in the transcriptional repressors RamR and OqxR both have an impact on the production of efflux pumps and porins relevant to antimicrobial efflux/entry. Objectives: To define, in an otherwise isogenic background, the relative effects of OqxR and RamR loss-of-function mutations on envelope protein production, envelope permeability and antimicrobial susceptibility. We also investigated the clinical relevance of an OqxR loss-of-function mutation, particularly in the context of ß-lactam susceptibility. Methods: Envelope permeability was estimated using a fluorescent dye accumulation assay. Antimicrobial susceptibility was measured using disc testing. Total envelope protein production was quantified using LC-MS/MS proteomics and quantitative RT-PCR was used to measure transcript levels. Results: Loss of RamR or OqxR reduced envelope permeability in K. pneumoniae by 45%-55% relative to the WT. RamR loss activated AcrAB efflux pump production ∼5-fold and this reduced ß-lactam susceptibility, conferring ertapenem non-susceptibility even in the absence of a carbapenemase. In contrast, OqxR loss specifically activated OqxAB efflux pump production >10 000-fold. This reduced fluoroquinolone susceptibility but had little impact on ß-lactam susceptibility even in the presence of a ß-lactamase. Conclusions: Whilst OqxR loss and RamR loss are both seen in K. pneumoniae clinical isolates, only RamR loss significantly stimulates AcrAB efflux pump production. This means that only RamR mutants have significantly reduced ß-lactamase-mediated ß-lactam susceptibility and therefore represent a greater clinical threat.

Envelope proteome changes driven by RamA overproduction in Klebsiella pneumoniae that enhance acquired ß-lactam resistance.

OBJECTIVES: In Klebsiella pneumoniae, overproduction of RamA results in reduced envelope permeability and reduced antimicrobial susceptibility but clinically relevant resistance is rarely observed. Here we have tested whether RamA overproduction can enhance acquired ß-lactam resistance mechanisms in K. pneumoniae and have defined the envelope protein abundance changes upon RamA overproduction during growth in low and high osmolarity media. METHODS: Envelope permeability was estimated using a fluorescent dye accumulation assay. ß-Lactam susceptibility was measured using disc testing. Total envelope protein production was quantified using LC-MS/MS proteomics and transcript levels were quantified using real-time RT-PCR. RESULTS: RamA overproduction enhanced ß-lactamase-mediated ß-lactam resistance, in some cases dramatically, without altering ß-lactamase production. It increased production of efflux pumps and decreased OmpK35 porin production, though micF overexpression showed that OmpK35 reduction has little impact on envelope permeability. A survey of K. pneumoniae bloodstream isolates revealed ramA hyperexpression in 3 of 4 carbapenemase producers, 1 of 21 CTX-M producers and 2 of 19 strains not carrying CTX-M or carbapenemases. CONCLUSIONS: Whilst RamA is not a key mediator of antibiotic resistance in K. pneumoniae on its own, it is potentially important for enhancing the spectrum of acquired ß-lactamase-mediated ß-lactam resistance. LC-MS/MS proteomics analysis has revealed that this enhancement is achieved predominantly through activation of efflux pump production.

Prediction of Fluoroquinolone Susceptibility Directly from Whole-Genome Sequence Data by Using Liquid Chromatography-Tandem Mass Spectrometry To Identify Mutant Genotypes.

Fluoroquinolone resistance in Gram-negative bacteria is multifactorial, involving target site mutations, reductions in fluoroquinolone entry due to reduced porin production, increased fluoroquinolone efflux, enzymes that modify fluoroquinolones, and Qnr, a DNA mimic that protects the drug target from fluoroquinolone binding. Here we report a comprehensive analysis, using transformation and in vitro mutant selection, of the relative importance of each of these mechanisms for fluoroquinolone nonsusceptibility using Klebsiella pneumoniae as a model system. Our improved biological understanding was then used to generate 47 rules that can predict fluoroquinolone susceptibility in K. pneumoniae clinical isolates. Key to the success of this predictive process was the use of liquid chromatography-tandem mass spectrometry to measure the abundance of proteins in extracts of cultured bacteria, identifying which sequence variants seen in the whole-genome sequence data were functionally important in the context of fluoroquinolone susceptibility.

A conserved motif flags acyl carrier proteins for ß-branching in polyketide synthesis.

Type I polyketide synthases often use programmed ß-branching, via enzymes of a 'hydroxymethylglutaryl-CoA synthase (HCS) cassette', to incorporate various side chains at the second carbon from the terminal carboxylic acid of growing polyketide backbones. We identified a strong sequence motif in acyl carrier proteins (ACPs) where ß-branching is known to occur. Substituting ACPs confirmed a correlation of ACP type with ß-branching specificity. Although these ACPs often occur in tandem, NMR analysis of tandem ß-branching ACPs indicated no ACP-ACP synergistic effects and revealed that the conserved sequence motif forms an internal core rather than an exposed patch. Modeling and mutagenesis identified ACP helix III as a probable anchor point of the ACP-HCS complex whose position is determined by the core. Mutating the core affects ACP functionality, whereas ACP-HCS interface substitutions modulate system specificity. Our method for predicting ß-carbon branching expands the potential for engineering new polyketides and lays a basis for determining specificity rules.

YieJ (CbrC) mediates CreBC-dependent colicin E2 tolerance in Escherichia coli.

Colicin E2-tolerant (known as Cet2) Escherichia coli K-12 mutants overproduce an inner membrane protein, CreD, which is believed to cause the Cet2 phenotype. Here, we show that overproduction of CreD in a Cet2 strain results from hyperactivation of the CreBC two-component regulator, but CreD overproduction is not responsible for the Cet2 phenotype. Through microarray analysis and gene knockout and overexpression studies, we show that overexpression of another CreBC-regulated gene, yieJ (also known as cbrC), causes the Cet2 phenotype.