Antimicrobial Agents and Chemotherapy

Treatment shortening in tuberculosis therapy is needed, but testing all novel antibiotic combinations is unfeasible. Especially the tuberculosis relapsing mouse model is time- and resource demanding. Therefore, our objective is to develop a computational model predictive of long-term relapse prevention in mice based on short-term biomarkers, increasing the number of regimens that can be tested and prioritize regimens for further development. The innovative ribosomal RNA synthesis (RS) ratio is utilized to characterize drug effect on Mycobacterium tuberculosis health and activity, together with colony forming units (CFU) in murine lungs. Nine datasets of 58 unique regimens with 843 short-term biomarker and 2,239 long-term relapse observations were leveraged for model development in 3 iterations with external validations. The final model included therapeutic predictors, such as CFU and RS ratio change from baseline, and corrected for experimental conditions, to enable unbiased ranking of regimens between experiments. Model performance was optimal without model structure change despite fully separate model development at each iteration. Final external validation had an area under the receiver operator curve of 0.90. Challenging the model by assessing removal of either biomarker showed that performance of CFU only was similar to CFU and RS ratio once the sterilizing contribution of individual drugs to the regimens was accounted for. New drugs without this contribution quantified could benefit from RS ratio determination to predict relapse. Our predictive model can successfully differentiate between 2-, 3-, and 4-month regimens in the relapsing mouse model based on 4-week data only, supporting acceleration of treatment-shortening regimen development. One Sentence SummaryOur predictive model ranks new drug regimens by tuberculosis relapse prevention based on 28-day CFU and RS ratio, or on CFU only for known drugs.

Heteroresistance (HR), the coexistence of a rare resistant subpopulation within a predominantly susceptible bacterial population, is a clinically relevant problem. The resistant subpopulation often escapes detection by standard susceptibility testing, which can lead to treatment failure. To investigate HR in Enterococcus faecalis, we performed population analysis profiling (PAP) on 40 clinical isolates against five clinically important antibiotics and whole-genome sequenced (WGS) the resistant subpopulations. HR was identified for daptomycin (20.0%), gentamicin (13.2%), and tigecycline (35.9%), but not for linezolid, and vancomycin. Genomic analysis revealed that daptomycin resistance was primarily caused by mutations affecting cell envelope integrity and stress-response pathways. Gentamicin resistance was linked to alterations in efflux regulation, ribosomal proteins synthesis, and transcriptional control. Tigecycline resistance involved deletions in the tet(M) leader peptide, resulting in a 25-fold increased tet(M) expression, as well as transposition of the transposon Tn916 carrying tet(M) to multiple chromosomal sites, causing an increased gene dosage of tet(M). These findings highlight the role of chromosomal mutations and mobile genetic elements in causing HR in E. faecalis Author SummaryAntibiotic resistance is one of the most critical challenges in modern medicine, undermining the efficacy of antimicrobial therapies and compromising patient safety. Bacterial strains that are heteroresistant (HR), show a minor subpopulation of resistant bacteria within a main susceptible bacterial population. HR strains are mostly classified as susceptible strains since the subpopulation are usually too small to be detected with standard susceptibility testing. Treatment with antibiotics will lead to survival and increase of the resistant subpopulation that can lead to treatment failure. In this study, we observed a high frequency of HR among clinical Enterococcus faecalis strains against gentamicin, daptomycin and tigecycline, three commonly used antibiotics for treatment of E. faecalis infections. Chromosomal mutations and changes in gene expression were the genetic mechanisms generating the resistant subpopulations.

Malaria remains a pressing global health challenge, with the continued emergence of resistance threatening the long-term efficacy of artemisinin-based combination therapies (ACTs). Piperaquine (PPQ), an important partner drug in artemisinin-based combination therapies exhibits a unique bimodal dose-response phenotype associated with reduced susceptibility, yet the biological mechanism underlying this phenotype remains unknown. This phenotype is strongly associated with mutations in pfcrt and copy number amplification of plasmepsin II/III (pm II/III). Given that plasmepsins play a central role in hemoglobin degradation within the blood stage parasite digestive vacuole, and that PPQ accumulates within this compartment and perturbs heme detoxification, this phenotype likely reflects alterations in fundamental biological processes alongside drug-specific effects. We used isogenic PPQ-resistant parasite clones differing only in pm II/III copy number to integrate phenotypes with metabolic changes, and transcriptional responses to ascertain the impact of genotype combinations on parasite response to PPQ. Across increasing PPQ concentrations, parasites with elevated pm II/III copy number exhibited distinct metabolic responses compared to single-copy parasites, specifically, an altered abundance of peptides derived from hemoglobin degradation, directly implicating a core biological pathway long associated with plasmepsin function. The combination of metabolic and transcriptional data with phenotypic measurements supports a model in which increased plasmepsin expression enhances the parasite's capacity to sustain hemoglobin digestion and associated metabolic activity under high PPQ concentrations. This points to a mechanistic basis for continued parasite survival, indicating that changes in hemoglobin processing within the digestive vacuole contribute to the bimodal response to PPQ. Molecular dynamics simulations further support a direct interaction between PPQ and PM II/III, as a mechanism by which these proteins impact PPQ response dynamics through both modulation of hemoglobin digestion and protein-drug interactions within the digestive vacuole.

The Project to Accelerate New Treatments for Tuberculosis (PAN-TB) aims to accelerate development of shorter, simpler and safer pan-TB combinations. We previously identified 3 out of 25 first-generation novel PAN-TB 4-drug combinations, that cured 90% of mice in less than 3 months, at clinically relevant doses in the relapsing mouse model of TB. These regimens include BPa830Sut, BPa286Sut and BQSut286 (B: bedaquiline; Pa: pretomanid; 830: GSK3211830; 286: GSK2556286; Sut: sutezolid; Q: quabodepistat). Here, we assess the efficacy of these combinations where the original candidates are substituted next-generation or more advanced compounds (ganfeborole (656) for 830, sorfequiline, S for B, TBD09 for Sut and TBD11 for 286) and the individual contributions of specific agents. Six novel regimens demonstrated bactericidal activity more rapid than comparators PHMZ (Rifapentine P, Isoniazid H, Moxifloxacin M, Pyrazinamide Z) and BPaMZ. Modelled cure/relapse data showed that SPa286Sut, SPaSut and SPa656Sut cured 90% of mice in about 1 month, while SPa286, SPaQTBD11 and SPaTBD09 in less than 2 months, faster than PHMZ. Consistent with our previous findings, the fastest-curing regimens centered on a diarylquinoline (S), a nitroimidazole (Pa) and an oxazolidinone (TBD09 or Sut) together with an Rv1625c agonist (TBD11 or 286), DprE1 inhibitor (Q) or a LeuRS inhibitor (656). Notably, significant contributions to sterilizing efficacy were demonstrated for S in all combinations and for Pa, Sut, TBD09, Q and TBD11 or 286 in specific S-containing combinations. These findings suggest potential for these novel agents and combinations to improve treatment of both DS-and DR-TB.

The carbapenemases KPC and NDM are the most widespread determinants of carbapenem resistance in Klebsiella pneumoniae. Whereas KPC is a soluble periplasmic serine-{beta}-lactamase, NDM is a membrane-anchored metallo-{beta}-lactamase (MBL), a feature that promotes its incorporation into outer membrane vesicles (OMVs). OMVs are naturally released nanoparticles that deliver diverse bioactive cargo, including enzymes, virulence factors, and signaling molecules, and may contribute to antibiotic resistance. Here, we investigated the export and activity of carbapenemases in OMVs produced by carbapenem-resistant Klebsiella pneumoniae clinical isolates expressing NDM-7, an emerging variant, or KPC-2, as well as in isogenic laboratory-derived K. pneumoniae strains producing NDM-1, NDM-7 or KPC-2. NDM enzymes were detected in vesicles released by NDM-producing strains, whereas KPC-2 remained confined to the cellular fraction and was not observed in OMVs. OMVs contained catalytically active NDM enzyme and conferred protection to susceptible K. pneumoniae against imipenem. Importantly, NDM-positive OMVs also partially restored bacterial growth in the presence of cefiderocol, a siderophore cephalosporin used to treat infections caused by MBL producers. This protective effect was more pronounced for NDM-7 than for NDM-1. Together, these findings show that the clinically emerging NDM-7 variant is efficiently packaged into OMVs in K. pneumoniae and remains enzymatically active, allowing extracellular antibiotic degradation and conferring protection to susceptible bacteria exposed to carbapenems and cefiderocol.

IntroductionLomentospora prolificans is a pathogenic filamentous fungus that causes disease primarily in people with severely compromised immune systems. It is pan-resistant to antifungal drugs, but the mechanism of its resistance to amphotericin B (AMB) is unknown. ObjectivesWe aimed to investigate the mechanism of resistance to AMB of L. prolificans. MethodsThe AMB susceptibility of L. prolificans protoplasts was measured using broth microdilution. L. prolificans, either intact, homogenized or fractionated was incubated with AMB in broth. The same activity was carried out with Aspergillus fumigatus as a control. This broth was then used to prepare microdilution plates with Saccharomyces cerevisiae to determine the activity of the conditioned AMB. ResultsAMB was 16-fold more effective in inhibiting the growth of L. prolificans protoplasts than conidia, but only two-fold more effective against A. fumigatus protoplasts than conidia. Incubation of L. prolificans hyphae with AMB in media diminished drug activity to a much greater extent than A. fumigatus, with 8-fold greater fungal mass of the latter required to achieve the effect of the former. Homogenization and fractionization of L. prolificans revealed that the factor inhibiting AMB activity was soluble with a mass >100 kda. DNase, trypsin, proteinase K, amyloglucosidase, SDS and 0.22 m had no effect on the AMB resistance factor, while treatment with urea, acetonitrile inactivated it. ConclusionWe report a different mechanism for AMB resistance based on the existence of a substance residing in the L. prolificans cell wall that can eliminate the antifungal activity of AMB.

Mycobacterium abscessus exhibits intrinsic resistance to many antimicrobial agents, including rifampicin, a frontline anti-tuberculosis drug, severely limiting treatment options. Here, we used transposon insertion sequencing (Tn-Seq) to perform a genomewide screen to identify genes required for intrinsic rifampicin resistance in M. abscessus. We uncovered candidate genes that confer intrinsic resistance to the rifamycin-class antibiotics, rifampicin and rifabutin. The genes we identified included previously reported genes such as arr, helR, and MAB_2807. By comparing our results with the rifampicin intrinsic resistance gene in Mycobacterium tuberculosis, we found that the mechanisms underlying rifampicin intrinsic resistance were distinct between the two species. The contribution of seven representative candidate genes to rifampicin resistance was confirmed by characterizing targeted gene deletion or transposon insertion mutants. Among these determinants, MAB_2807 was identified as a major efflux-based contributor to rifamycin resistance, and its disruption increased intracellular rifampicin accumulation. In addition to known resistance determinants, Tn-Seq revealed contributions from many genes involved in cell envelope processes to rifamycin resistance. Guided by this genetic signature, we evaluated the combined effects of cell wall-targeting antimicrobial agents with rifamycins. We found that rifamycins displayed selective synergistic interactions with specific {beta}-lactam antibiotics. Interestingly, we found that rifamycin exposure altered cell envelope ultrastructure and increased the accumulation of the peptidoglycan precursor UDP-N-acetylglucosamine, suggesting that rifampicin perturbs envelope-associated metabolic homeostasis. These findings highlight rifamycin-induced vulnerable cellular processes that may inform rational combination strategies. ImportanceRifamycins are cornerstone antibiotics for the treatment of mycobacterial infections, yet they are ineffective against Mycobacterium abscessus. Although several key resistance mechanisms have been described, the full genetic basis of intrinsic rifamycin resistance remains to be elucidated. Here, we used transposon insertion sequencing to systematically identify genes that contribute to intrinsic rifamycin resistance in M. abscessus. Because recent studies have shown that chemical modification of rifamycins can overcome specific intrinsic resistance mechanisms, a comprehensive understanding of these determinants could facilitate the development of next-generation rifamycins with improved activity against M. abscessus. In addition, we unexpectedly found that rifamycin exposure alters cell envelope structure, the intracellular levels of a key metabolic intermediate in peptidoglycan biosynthesis, and susceptibility to certain {beta}-lactams in M. abscessus. These findings may help guide the rational development of combination therapeutic strategies with next-generation rifamycins.

Azoles remain the most common antifungal therapy worldwide. However, Nakaseomyces glabratus (previously named Candida glabrata) has a high intrinsic tolerance against azole drugs. The organism can also accrue additional chromosomal mutations to elevate its resistant level during treatment. These genetic alterations often result in overexpression of the ABC transmembrane transporter Cdr1, which has been shown to directly transport drugs out of the fungal cells. Another resistant mechanism is the upregulation of the ergosterol biosynthesis pathway, which is the direct target of azoles. Although the mechanisms of azole resistance in N. glabratus are well defined, knowledge of their regulation remains limited. Here, we show that the protein kinase Fpk1 is required for optimal azole response in vitro and in an in vivo mouse infection model. Loss of Fpk1 gene or its kinase function significantly enhances azole sensitivity in both azole-susceptible and -resistant clinical isolates. Fpk1 function is required for optimal expression of Cdr1 upon azole challenge. It also influences the intracellular trafficking of ergosterol, without affecting its biosynthesis. Together, our data demonstrates the important role of Fpk1 function in the N. glabratus azole response and characterizes it as a new regulator of the efflux pump and ergosterol biosynthesis pathways. IMPORTANCEAntifungal treatment against life-threatening bloodstream Candida infection remains limited to azoles, echinocandins, and polyenes. Among them, azoles are the most prescribed therapy worldwide. However, the pathogenic yeast Nakaseomyces glabrataus has a high level of resistance against azoles (> 10%) (1). This often complicates treatment and increases mortality and morbidity rates. Therefore, understanding the mechanism of azole resistance would reinforce the treatment strategy and bolster future therapy development. Here, we identify the protein kinase Fpk1 as an important regulator of the drug efflux plump and ergosterol biosynthesis pathways. Disruption of the Fpk1 function significantly enhances the azole efficacy in vitro and in a mouse model of Candida systemic infection. Protein kinases are druggable targets, and our data presents Fpk1 as a viable candidate for future antifungal development.

Nakaseomyces glabratus (Candida glabrata) is a WHO high-priority fungal pathogen associated with fungal antimicrobial resistance (fAMR). Given nosocomial transmission occurs sporadically, resistant strains could be transmitted, a concern for critically ill patients. We conducted a genomic investigation and retrospective observational study of N. glabratus to identify any nosocomial transmission of fAMR and understand resistance mechanisms and clinical and demiological factors among patients at a quaternary hospital in Melbourne, Australia. We selected stored N. glabratus with and without fAMR associated with similar patient clinical characteristics and performed whole genome sequencing. Clinical and epidemiological data were extracted from medical records. Phylogenetic, mutational, copy-number variation (CNV) and mitochondrial genomic analyses were performed, with a focus on the fAMR gene PDR1. Of 54 isolates collected over seven years, 20 (37%) were fluconazole-resistant and four (7%) had elevated flucytosine minimum inhibitory concentrations (MICs) (range 2-32 g/ml). There were no significant clinical differences between patients with and without fluconazole resistance. Most (55%) fluconazole-resistant isolates carried PDR1 mutations. Resistance was distributed throughout the phylogeny suggesting predominantly independent acquisition. However, a cluster of four resistant isolates with the same PDR1 mutation suggested nosocomial transmission. One probable ERG11 gene duplication, and two petite variants with apparent mitochondrial genomic deletions, were seen in association with fluconazole resistance. In this study, we identified a small probable nosocomial fAMR transmission cluster, and novel variants in PDR1, ERG11 and FCY2 associated with fAMR phenotypes. Future study should confirm functional impacts and systematically investigate for nosocomial transmission of resistance, including colonisation states.

Background: Polymorphisms in the N-acetyltransferase 2 (NAT2) gene explain much of the interindividual variation in isoniazid (INH) metabolism and determine risk of toxicities. However, there is limited evidence to guide INH dose adjustment according to the NAT2 acetylator profile in weekly rifapentine-INH tuberculosis preventive therapy (TPT). Methods: In a prospective, multicenter, within-subject PK trial (NCT05413551), adults initiating 3HP in Brazil were assigned genotype-guided INH doses (slow: 5 mg/kg <=300 mg; intermediate: 15 mg/kg <=900 mg; rapid: 25 mg/kg <=1,500 mg) alongside a standard 900 mg flat dose on an alternate occasion. AUC0-24 and C24 were estimated from serial blood samples; a two-compartment Michaelis-Menten population PK model characterized NAT2 effects on clearance. Results: Among 228 participants, 47.4% (108/228) were intermediate, 43.4% (99/228) slow, and 9.2% (21/228) rapid acetylators. Genotype-guided dosing reduced AUC0-24 variability approximately two-fold versus standard dosing (CV 58.8% vs 76.8%) and increased exposure uniformity (median AUC0-24 27.2 [IQR 18.8-41.3] vs 43.2 [27.3-71.0] mg h/L). Among slow acetylators, C24 >0.15 ug/mL decreased from 27/42 (64%) with standard dosing to 1/42 (2%) with genotype-guided dosing (P<0.0001). In 104 participants with intensive PK sampling, rapid acetylators receiving guided doses had AUC0-24 similar to standard-dose intermediate acetylators (42.8 vs 39.5 mg h/L; P=.63). Monte Carlo simulations supported doses of 600, 900, and 1,200 mg for slow, intermediate, and rapid acetylators, respectively. Conclusions: NAT2-guided isoniazid dosing reduced variation in drug levels, averting very low and high AUC and C24. These findings inform genotype-stratified dosing of INH for TPT, which might reduce toxicities and improve outcomes.

Tuberculosis (TB) caused by Mycobacterium tuberculosis (Mtb) remains a potent threat to global public health. Moreover, the alarming surge in the number of multidrug-resistant (MDR) and extensively drug-resistant (XDR) Mtb strains will continue to imperil TB control efforts. Thus, the discovery of new TB agents with novel modes of action or resistance-reversing therapeutics is a pressing priority. In this study, we report the generation of spontaneous Mtb mutants exhibiting bedaquiline (BDQ) resistance and the subsequent evaluation of natural product-derived efflux inhibitors (EIs) that restored the antimicrobial efficacy of BDQ against the mutants. BDQ-resistant mutants were successfully isolated, and colonies were observed on agar plates with concentrations up to 100x the minimum inhibitory concentration (MIC). Upon screening against BDQ, the resistant strains exhibited MIC values ranging from 0.098 M to 3.136 M, corresponding to 1-32-fold increases relative to the wild type, with higher resistance observed on 50x- and 100x-selection plates. Genetic analysis identified point mutations and frameshifts in key resistance-related genes, including Rv0678, pepQ, and atpE. Notably, combining BDQ with EIs such as berberine (BER), reserpine (RES), piperine (PIP), and lyoniresinol (LYO) remarkably lowered the MIC in the selected mutant strain. Synergistic effects were observed for BDQ+BER (FICI = 0.188; 16-fold MIC reduction) and BDQ+RES (FICI = 0.37; 8-fold MIC reduction). For BDQ+LYO, the FICI could not be calculated because LYO did not have an MIC at the highest concentration tested; however, this combination produced the strongest effect, restoring susceptibility with a 64-fold MIC reduction and exhibiting bactericidal activity. These results highlight the role of efflux pumps in BDQ resistance and support the use of natural product-derived EIs as potential supplementary therapies against drug-resistant Mtb.

Quinolone antibiotics such as ciprofloxacin inhibit DNA gyrase, leading to DNA double-strand breaks that result in rapid bacterial killing and induction of the mutagenic SOS DNA repair response. By contrast, the ciprofloxacin analogue IMP-1700 inhibits ciprofloxacin-induced SOS, suggesting a novel mechanism of action. Here, we provide evidence that IMP-1700 targets the quinolone binding domain of DNA gyrase but triggers a significantly higher frequency of division septa in S. aureus compared with other DNA gyrase targeting antibiotics, including ciprofloxacin. In keeping with this finding, the lipopeptide antibiotic daptomycin, which targets the division septum, bound more strongly to IMP-1700-treated cells relative to S. aureus exposed to other DNA gyrase inhibitors, leading to increased bacterial killing. This finding extended to a panel of paired daptomycin susceptible and resistant clinical isolates. We conclude that the ciprofloxacin analogue IMP-1700 has distinct effects on the cell envelope of S. aureus, despite appearing to share the same target as the parent drug, which result in the resensitisation of daptomycin resistant bacteria to the lipopeptide antibiotic.

Nitrofurantoin is commonly prescribed as a first-line treatment for urinary tract infections (UTIs) and is effective against most uropathogens; however, resistance among Klebsiella spp. remains high. In this study, we investigated synergistic drug combinations--defined as pairs of drugs whose combined effect exceeds that of each agent alone--to enhance treatment efficacy against drug-resistant Klebsiella pneumoniae. A high-throughput drug screen identified six and two small molecules that exhibited >50% synergistic activity in tested strains in combination with nitrofurantoin and ciprofloxacin, respectively. We further validated the top three candidates using nitrofurantoin-susceptible and resistant clinical isolates. Notably, the combination of nitrofurantoin and dequalinium demonstrated strong synergistic activity, observed in 68% of susceptible Escherichia coli strains and 94% of resistant Klebsiella pneumoniae strains tested. Combined inhibition of the citric acid cycle by nitrofurantoin and F1-ATPase by dequalinium resulted in a significant reduction in ATP levels compared to either treatment alone. Importantly, decreased ATP levels did not increase persister cell formation, and dequalinium alone reduced persister cell populations more effectively than nitrofurantoin. However, nitrofurantoin is processed into a poorly understood reactive intermediate whose specific metabolic targets have not been fully elucidated. Using metabolomic analyses, we identified aconitase and isocitrate dehydrogenase--key enzymes in the citric acid cycle--as altered in response to nitrofurantoin. Together, these findings demonstrate that dual targeting of bacterial metabolism by nitrofurantoin and dequalinium represents a promising therapeutic strategy for treating drug-resistant Klebsiella spp. in UTIs.

ObjectiveTo investigate the combined effects of multiple drugs and provide more therapeutic options for invasive fungal infections, this study evaluated the in vitro susceptibility of Aspergillus spp., Candida auris, Cryptococcus neoformans, and Exophiala dermatitidis to pibrentasvir (PIB) in combination with itraconazole (ITR), voriconazole (VOR), posaconazole (POS), or fluconazole (FLU). MethodsAccording to the M27-A3 and M38-A2 guidelines established by the Clinical and Laboratory Standards Institute (CLSI), the in vitro antifungal activities of PIB combined with ITR, VOR, POS, or FLU against 78 clinical isolates, including Aspergillus spp., E. dermatitidis, C. auris, and C. neoformans, were determined. The minimum inhibitory concentrations (MICs) and fractional inhibitory concentration indices (FICIs) were calculated to evaluate the synergistic effects. ResultsPIB alone exhibited no antifungal activity. Significant synergistic effects were observed when PIB was combined with azole antifungal agents. The PIB-POS combination showed synergistic effects against Aspergillus spp. (27/41, 65.90%), C. auris (9/10, 90.00%), C. neoformans (2/9, 22.20%), and E. dermatitidis (11/18, 61.11%). The PIB-ITR combination also showed synergistic effects against Aspergillus spp. (18/41, 43.9%), C. auris (9/10, 90.0%), C. neoformans (2/9, 22.2%), and E. dermatitidis (10/18, 55.5%). Synergistic effects were less frequently observed with the PIB-VOR or PIB-FLU combinations, and no antagonistic effects were observed. ConclusionThis study demonstrates that PIB acts as an azole sensitizer, with the strongest synergistic effects observed when combined with POS or ITR, providing a new research direction for combination therapy against invasive fungal infections.

Rifamycins inhibit bacterial transcription by targeting RNA polymerase (RNAP), but their clinical effectiveness is limited by the rapid emergence of resistance caused by mutations within the rifamycin-binding pocket. Rifamycin B (Rif B), one of the earliest discovered members of this antibiotic family and a precursor of clinically used derivatives, has remained poorly characterized because of its chemical instability and relatively weak antibacterial activity. Here, we revisit Rif B using biochemical and structural approaches. We show that Rif B remains sufficiently stable under assay conditions and retains inhibitory activity against RNAP variants carrying clinically relevant rifampicin-resistance mutations. We report the first crystal structure of Rif B and determine the structure of Rif B bound to bacterial RNAP. The structures reveal that the distinctive C-4 O-carboxymethyl substituent of Rif B forms an intramolecular interaction in the free molecule but establishes a salt bridge with fork loop 2 of the RNAP {beta}-subunit upon binding. This additional interaction explains the reduced sensitivity of Rif B to resistance-associated substitutions and identifies the C-4 position as an underexplored site for rational rifamycin modification. These findings redefine Rif B as a mechanistically distinct rifamycin scaffold and provide new insights for developing inhibitors targeting rifampicin-resistant RNAP.

Klebsiella aerogenes, a member of the Enterobacteriaceae, is a causative agent of healthcare-associated infections, and outbreaks caused by drug-resistant K. aerogenes have been reported worldwide. The range of antimicrobial agents available for treating infections caused by carbapenem-resistant K. aerogenes is limited. While in vivo animal experiments using clinical K. aerogenes isolates to evaluate antimicrobial therapy could facilitate selection of the most effective treatment, conducting infection experiments involving large numbers of mammals such as mice is challenging due to ethical concerns related to animal welfare. Silkworms are invertebrates increasingly used as experimental models for infectious disease research to evaluate antimicrobial efficacy. In this study, we aimed to establish a silkworm infection model using a clinical K. aerogenes isolate to evaluate its utility for determining effective antimicrobial doses. K. aerogenes strains were isolated from a patient at a Japanese hospital, and a silkworm infection model was established using the clinical isolate. The non-metallo-beta-lactamase-producing strain K. aerogenes TUM25562, isolated from a patient with a complicated urinary tract infection, was susceptible to meropenem (MEPM) and gentamicin (GM) in vitro. During treatment, additional isolates with increased resistance to MEPM and subsequently to both MEPM and GM emerged. K. aerogenes TUM25562 caused dose-dependent mortality in silkworms. Treatment with clinically equivalent weight-based doses of MEPM or GM did not cure the infected silkworms. The median effective (ED50) doses of MEPM and GM were therefore investigated using the silkworm infection model. Administration of higher doses corresponding to four times the ED50 significantly prolonged the survival of infected silkworms. These results suggest that a silkworm infection model using clinical K. aerogenes isolates may provide a practical approach for evaluating antimicrobial efficacy and determining effective antimicrobial doses.

Pharmacodynamic curves describe how changes in drug concentration affect pathogen growth. They are essential for designing treatments that promote pathogen eradication and minimize the evolution of antibiotic resistance. The classical function for modelling pharmacodynamics is a phenomenological, S-shaped curve with stable growth and death rates separated by a single drop. In this study, we characterized the pharmacodynamic curve of the {beta}-lactam antibiotic cefotaxime (CTX) acting against Escherichia coli. We found that the relationship between CTX concentration and net growth rate diverged from classical model predictions, instead yielding a two-step curve defined by distinct phases of growth, population maintenance, and killing. We hypothesized that the intermediate phase arose from antibiotic tolerance conferred by bacterial filaments. Microscopic assessment of treated cells indeed showed a difference in degree of filamentation with concentration. We further sought to explain this with a semi-mechanistic pharmacodynamic function, modelling the binding of CTX to its cellular targets, penicillin binding proteins (PBP) 1 and 3. By incorporating the preferential concentration-dependent binding of CTX to PBP3 and then PBP1, yielding filaments or lysed cells respectively, we replicated the two-step curve in silico. We also assessed the pharmacodynamics of CTX against mutants conferring resistance; these displayed further altered curves, in line with their fitness costs. Altogether, our results show that CTX has a two-step pharmacodynamic curve against E. coli arising from multiple targets separated in their affinity for the antibiotic. We present a model offering a mechanistically grounded framework for capturing such dynamics. These pharmacodynamic curves deserve careful consideration when defining optimal dosing.

BackgroundAztreonam-avibactam (AZA) is a recently approved agent active against metallo-{beta}-lactamase-producing Enterobacterales. However, independent evaluations of commercially available antimicrobial susceptibility testing (AST) products remain limited. MethodsWe evaluated 56 carbapenem-resistant Enterobacterales isolates using four newly available AZA AST products, including two gradient diffusion (bioMerieux Etest; Liofilchem MIC Test Strip) and two disk diffusion (Hardy, Liofilchem) tests across three Mueller-Hinton agar manufacturers at two independent clinical laboratories. A diverse isolate collection enriched for nonsusceptible and breakpoint-adjacent MICs was used to rigorously assess categorical agreement. Broth microdilution (BMD) was used as the reference method. Essential agreement (EA), categorical agreement (CA), error rates, and reproducibility were analyzed. ResultsAll four commercial AST products demonstrated acceptable analytical performance compared with reference BMD. Gradient diffusion demonstrated high EA (bioMerieux 94.3%, Liofilchem 90.5%), whereas CA ranged from 83.3% to 89.0% across all four products. Minor errors accounted for all categorical discrepancies, and occurred almost exclusively among breakpoint-adjacent isolates. Media-related variability was modest, and quality control performance was generally high, with strong within-site precision and consistent performance across laboratories. ConclusionsThis study provides the first comprehensive head-to-head comparison of commercial AZA susceptibility testing products and supports their implementation in clinical microbiology laboratories.

The global emergence of multidrug-resistant (MDR) ESKAPE pathogens has significantly reduced the effectiveness of existing antibiotics, highlighting the urgent need for new strategies to restore antimicrobial susceptibility. Here, we report the discovery and mechanism of BMM_1567, a peptide potentiator that enhances aminoglycoside efficacy against MDR pathogens. A genetically defined reporter-based screen identified BMM_1567 as a potent inhibitor of aminoglycoside resistance, potentiating spectinomycin activity against MDR Gram-negative ESKAPE isolates at low micromolar concentrations. Structural modeling and molecular dynamics simulations indicated that BMM_1567 interacts with residues lining the antibiotic-binding groove of aminoglycoside-modifying enzymes (ANT, APH, AAC), with highest affinity for ANT ({Delta}G_bind = -62.25 kcal/mol), suggesting competitive inhibition of substrate binding. Site-directed mutagenesis of key ANT residues identified critical amino acids involved in BMM_1567 binding, confirming their role in mediating spectinomycin potentiation. In murine abscess model using XDR E. coli, BMM_1567 in combination with spectinomycin significantly reduced bacterial burden and pro-inflammatory cytokine levels, comparable to colistin. Collectively, these findings establish BMM_1567 as a promising aminoglycoside potentiator that restores antibiotic activity against MDR pathogens through direct inhibition of resistance enzymes, while exhibiting in vivo efficacy and a remarkably low propensity for resistance development.

The New Delhi metallo-{beta}-lactamase (NDM) is a major determinant of carbapenem resistance. This has prompted the development of novel therapeutic strategies to treat NDM producers. Since these therapies impose new selective pressures, their clinical deployment may favor NDM variants carrying escape mutations. To anticipate these events and inform future therapeutic decisions, we explored the evolutionary landscape of NDM-1 under different selective constraints. To this end, we generated a highly diverse library of blaNDM variants and challenged it with zinc starvation, antibiotics or {beta}-lactam/{beta}-lactamase inhibitor combinations that represent novel and emerging therapies. Selection under zinc limitation and cefotaxime identified mutational trajectories that recapitulate clinical NDM evolution, validating the diversity of the library and its predictive nature. Drug-specific selections revealed sharply different evolutionary pathways. Mecillinam selected a narrow evolutionary pathway centered on residues N220 and M67 which enhance productive active-site interactions with this penicillin. Cefepime/taniborbactam selected multiple escape routes, dominated by substitutions at E152 and, secondarily, K211, that impair productive interaction with taniborbactam. In contrast, cefiderocol/xeruborbactam and aztreonam/avibactam failed to select NDM variants conferring an improved resistance phenotype. These results show that NDM evolution is constrained by the chemistry of each therapeutic challenge. Substrate adaptation is possible for mecillinam, inhibitor escape is readily accessible for taniborbactam, whereas aztreonam- and xeruborbactam-based strategies impose high evolutionary barriers on NDM. Mapping drug-specific evolutionary landscapes can help anticipate resistance before clinical deployment and prioritize therapeutic strategies less likely to drive NDM-mediated escape. ImportanceCarbapenem-resistant infections by Enterobacterales are increasingly difficult to treat because bacteria can destroy some of the most powerful antibiotics used in the clinic. This study focuses on Escherichia coli carrying variants of the New Delhi metallo-{beta}-lactamase, an enzyme that enables bacteria to resist carbapenem antibiotics. New therapies are being developed to overcome this problem, but bacteria may evolve again when exposed to these treatments. Here, we tested how New Delhi metallo-{beta}-lactamase can adapt under the evolutionary pressure of last-resort therapies. The results show that not all treatments carry the same evolutionary risk. Some drugs allow the enzyme to adapt through specific mutations, whereas other drug combinations make such escape much harder. This work helps predict which treatments are more likely to remain effective and which may more readily select resistance. This information can guide the design and use of future therapies against resistant bacterial infections.