Target- and resistance-based mechanistic studies with TP-434, a novel fluorocycline antibiotic

Newly developed antibiotics against multidrug-resistant and carbapenem-resistant Gram-negative bacteria: action and resistance mechanisms

Antimicrobial resistance stands as one of the most urgent global health concerns in the twenty-first century, with projections suggesting that deaths related to drug-resistant infections could escalate to 10 million by 2050 if proactive measures are not implemented. In intensive care settings, managing infections caused by multidrug-resistant (MDR) Gram-negative bacteria is particularly challenging, posing a significant threat to public health and contributing substantially to both morbidity and mortality. There are numerous studies on the antibiotics responsible for resistance in Gram-negative bacteria, but comprehensive research on resistance mechanisms against new antibiotics is rare. Considering the possibility that antibiotics may no longer be effective in combating diseases, it is crucial to comprehend the problem of emerging resistance to newly developed antibiotics and to implement preventive measures to curb the spread of resistance. Mutations in porins and efflux pumps play a crucial role in antibiotic resistance by altering drug permeability and active efflux. Porin modifications reduce the influx of antibiotics, whereas overexpression of efflux pumps, particularly those in the resistance-nodulation-cell division (RND) family, actively expels antibiotics from bacterial cells, significantly lowering intracellular drug concentrations and leading to treatment failure.This review examines the mechanisms of action, resistance profiles, and pharmacokinetic/pharmacodynamic characteristics of newly developed antibiotics designed to combat infections caused by MDR and carbapenem-resistant Gram-negative pathogens. The antibiotics discussed include ceftazidime-avibactam, imipenem-relebactam, ceftolozane-tazobactam, meropenem-vaborbactam, aztreonam-avibactam, delafloxacin, temocillin, plazomicin, cefiderocol, and eravacycline.

Probing the interaction mechanism of tigecycline with γ-globulin and hemoglobin in the absence and presence of amikacin

The interaction mechanism of tigecycline with γ-globulin and hemoglobin in the absence and presence of amikacin was investigated through multipectral, molecular docking and molecular dynamics simulation. The results show that tigecycline and γ-globulin/hemoglobin forms a ground state complex without or with amikacin. The presence of amikacin slightly increases the binding constant of tigecycline to γ-globulin/hemoglobin, but all are of moderate binding affinity, at 104 L mol-1. The equilibrium fraction of unbound tigecycline fu is >90 %, but the presence of amikacin reduces the free concentration of tigecycline in γ-globulin and hemoglobin. Whether amikacin is present or not, the interaction between tigecycline and γ-globulin/hemoglobin is a synergistic interaction driven by enthalpy and entropy. Non-covalent forces are primarily hydrophobic interactions, but also include electrostatic forces and hydrogen bonds. In the presence of amikacin, the effect of tigecycline on the skeleton structure of γ-globulin/hemoglobin is more significant. The effect of tigecycline and/or amikacin on the secondary structure of γ-globulin/hemoglobin is not significant, while the secondary structure changes in different systems are not the same. Molecular docking shows that γ-globulin/hemoglobin-tigecycline (first)-amikacin ternary system is the most stable. Molecular dynamics simulation explores the stability and dynamic behavior of γ-globulin/hemoglobin-tigecycline complex without or with amikacin.

Paenilamicins are context-specific translocation inhibitors of protein synthesis

The paenilamicins are a group of hybrid nonribosomal peptide-polyketide compounds produced by the honey bee pathogen Paenibacillus larvae that display activity against Gram-positive pathogens, such as Staphylococcus aureus. While paenilamicins have been shown to inhibit protein synthesis, their mechanism of action has remained unclear. Here we determine structures of paenilamicin PamB2-stalled ribosomes, revealing a unique binding site on the small 30S subunit located between the A- and P-site transfer RNAs (tRNAs). In addition to providing a precise description of interactions of PamB2 with the ribosome, the structures also rationalize the resistance mechanisms used by P. larvae. We further demonstrate that PamB2 interferes with the translocation of messenger RNA and tRNAs through the ribosome during translation elongation, and that this inhibitory activity is influenced by the presence of modifications at position 37 of the A-site tRNA. Collectively, our study defines the paenilamicins as a class of context-specific translocation inhibitors.

Efficacy and safety of eravacycline (ERV) in treating infections caused by Gram-negative pathogens: a systematic review and meta-analysis

BACKGROUND

Eravacycline (ERV) is a novel synthetic fluorocycline antibiotic with broad-spectrum antibacterial efficacy against pathogens. This study sought to investigate ERV's effectiveness and safety in treating Gram-negative pathogens (GNPs) infections.

METHODS

We conducted a comprehensive search of PubMed, Cochrane Library, Embase, Web of Science, and ClinicalTrials.gov up to September 2023. Included in the review were studies assessing the efficacy or safety of ERV in treating GNP infections.

RESULTS

Three randomized controlled trials, seven cohort studies, and two case reports were included. There was no statistically significant difference between ERV and comparators in clinical cure (OR = 0.84, 95% CI = 0.59-1.19), microbiologic eradication (OR = 0.69, 95% CI = 0.36-1.33), and mortality (OR = 1.66, 95% CI = 0.81-3.41). However, a significantly higher rate of adverse events with ERV was observed compared to the control group (OR = 1.55, 95% CI = 1.21-1.99). Additionally, cohort studies reported a clinical cure rate of 73.2% (88.8% in RCTs), an AE rate of 4.5% (38.3% in RCTs), and mortality of 16.2% (1.5% in RCTs). Patients in RCTs received ERV monotherapy, whereas almost half of the patients in cohort studies were treated with ERV in combination with other antibiotics.

CONCLUSIONS

Further studies are warranted to investigate the safety and efficacy of ERV monotherapy or combination therapy in critically ill patients.

Comparison of antimicrobial activities and resistance mechanisms of eravacycline and tigecycline against clinical Acinetobacter baumannii isolates in China

Tigecycline (TGC) is currently used to treat carbapenem-resistant Acinetobacter baumannii (CRAB) infections, while eravacycline (ERV), a new-generation tetracycline, holds promise as a novel therapeutic option for these infections. However, differences in resistance mechanism between ERV and TGC against A. baumannii remain unclear. This study sought to compare the characteristics and mechanisms of ERV and TGC resistance among clinical A. baumannii isolates. A total of 492 isolates, including 253 CRAB and 239 carbapenem-sensitive A. baumannii (CSAB) isolates, were collected from hospitalized patients in China. The MICs of ERV and TGC against A. baumannii were determined by broth microdilution. Genetic mutations and expressions of adeB, adeG, adeJ, adeS, adeL, and adeN in resistant strains were examined by PCR and qPCR, respectively. The in vitro recombination experiments were used to verify the resistance mechanism of ERV and TGC in A. baumannii. The MIC90 of ERV in CRAB and CSAB isolates were lower than those of TGC. A total of 24 strains resistant to ERV and/or TGC were categorized into three groups: only ERV-resistant (n = 2), both ERV- and TGC-resistant (n = 7), and only TGC-resistant (n = 15). ST208 (75%, n = 18) was a major clone that has disseminated in all three groups. The ISAba1 insertion in adeS was identified in 66.7% (6/9) of strains in the only ERV-resistant and both ERV- and TGC-resistant groups, while the ISAba1 insertion in adeN was found in 53.3% (8/15) of strains in the only TGC-resistant group. The adeABC and adeRS expressions were significantly increased in the only ERV-resistant and both ERV- and TGC-resistant groups, while the adeABC and adeIJK expressions were significantly increased and adeN was significantly decreased in the only TGC-resistant group. Expression of adeS with the ISAba1 insertion in ERV- and TGC-sensitive strains significantly increased the ERV and TGC MICs and upregulated adeABC and adeRS expressions. Complementation of the wildtype adeN in TGC-resistant strains with the ISAba1 insertion in adeN restored TGC sensitivity and significantly downregulated adeIJK expression. In conclusion, our data illustrates that ERV is more effective against A. baumannii clinical isolates than TGC. ERV resistance is correlated with the ISAba1 insertion in adeS, while TGC resistance is associated with the ISAba1 insertion in adeN or adeS in A. baumannii.

Synthetic macrolides overcoming MLSBK-resistant pathogens

Conventional macrolide-lincosamide-streptogramin B-ketolide (MLSBK) antibiotics are unable to counter the growing challenge of antibiotic resistance that is conferred by the constitutive methylation of rRNA base A2058 or its G2058 mutation, while the presence of unmodified A2058 is crucial for high selectivity of traditional MLSBK in targeting pathogens over human cells. The absence of effective modes of action reinforces the prevailing belief that constitutively antibiotic-resistant Staphylococcus aureus remains impervious to existing macrolides including telithromycin. Here, we report the design and synthesis of a novel series of macrolides, featuring the strategic fusion of ketolide and quinolone moieties. Our effort led to the discovery of two potent compounds, MCX-219 and MCX-190, demonstrating enhanced antibacterial efficacy against a broad spectrum of formidable pathogens, including A2058-methylated Staphylococcus aureus, Streptococcus pneumoniae, Streptococcus pyogenes, and notably, the clinical Mycoplasma pneumoniae isolates harboring A2058G mutations which are implicated in the recent pneumonia outbreak in China. Mechanistic studies reveal that the modified quinolone moiety of MCX-190 establishes a distinctive secondary binding site within the nascent peptide exit tunnel. Structure-activity relationship analysis underscores the importance of this secondary binding, maintained by a sandwich-like π-π stacking interaction and a water-magnesium bridge, for effective engagement with A2058-methylated ribosomes rather than topoisomerases targeted by quinolone antibiotics. Our findings not only highlight MCX-219 and MCX-190 as promising candidates for next-generation MLSBK antibiotics to combat antibiotic resistance, but also pave the way for the future rational design of the class of MLSBK antibiotics, offering a strategic framework to overcome the challenges posed by escalating antibiotic resistance.

Third-Generation Tetracyclines: Current Knowledge and Therapeutic Potential

Tetracyclines constitute a unique class of antibiotic agents, widely prescribed for both community and hospital infections due to their broad spectrum of activity. Acting by disrupting protein synthesis through tight binding to the 30S ribosomal subunit, their interference is typically reversible, rendering them bacteriostatic in action. Resistance to tetracyclines has primarily been associated with changes in pump efflux or ribosomal protection mechanisms. To address this challenge, tetracycline molecules have been chemically modified, resulting in the development of third-generation tetracyclines. These novel tetracyclines offer significant advantages in treating infections, whether used alone or in combination therapies, especially in hospital settings. Beyond their conventional antimicrobial properties, research has highlighted their potential non-antibiotic properties, including their impact on immunomodulation and malignancy. This review will focus on third-generation tetracyclines, namely tigecycline, eravacycline, and omadacycline. We will delve into their mechanisms of action and resistance, while also evaluating their pros and cons over time. Additionally, we will explore their therapeutic potential, analyzing their primary indications of prescription, potential future uses, and non-antibiotic features. This review aims to provide valuable insights into the clinical applications of third-generation tetracyclines, thereby enhancing understanding and guiding optimal clinical use.

Unveiling the Secrets of Acinetobacter baumannii: Resistance, Current Treatments, and Future Innovations

Acinetobacter baumannii represents a significant concern in nosocomial settings, particularly in critically ill patients who are forced to remain in hospital for extended periods. The challenge of managing and preventing this organism is further compounded by its increasing ability to develop resistance due to its extraordinary genomic plasticity, particularly in response to adverse environmental conditions. Its recognition as a significant public health risk has provided a significant impetus for the identification of new therapeutic approaches and infection control strategies. Indeed, currently used antimicrobial agents are gradually losing their efficacy, neutralized by newer and newer mechanisms of bacterial resistance, especially to carbapenem antibiotics. A deep understanding of the underlying molecular mechanisms is urgently needed to shed light on the properties that allow A. baumannii enormous resilience against standard therapies. Among the most promising alternatives under investigation are the combination sulbactam/durlobactam, cefepime/zidebactam, imipenem/funobactam, xeruborbactam, and the newest molecules such as novel polymyxins or zosurabalpin. Furthermore, the potential of phage therapy, as well as deep learning and artificial intelligence, offer a complementary approach that could be particularly useful in cases where traditional strategies fail. The fight against A. baumannii is not confined to the microcosm of microbiological research or hospital wards; instead, it is a broader public health dilemma that demands a coordinated, global response.

Paenilamicins from the honey bee pathogen Paenibacillus larvae are context-specific translocation inhibitors of protein synthesis

The paenilamicins are a group of hybrid non-ribosomal peptide-polyketide compounds produced by the honey bee pathogen Paenibacillus larvae that display activity against Gram-positive pathogens, such as Staphylococcus aureus. While paenilamicins have been shown to inhibit protein synthesis, their mechanism of action has remained unclear. Here, we have determined structures of the paenilamicin PamB2 stalled ribosomes, revealing a unique binding site on the small 30S subunit located between the A- and P-site tRNAs. In addition to providing a precise description of interactions of PamB2 with the ribosome, the structures also rationalize the resistance mechanisms utilized by P. larvae. We could further demonstrate that PamB2 interferes with the translocation of mRNA and tRNAs through the ribosome during translation elongation, and that this inhibitory activity is influenced by the presence of modifications at position 37 of the A-site tRNA. Collectively, our study defines the paenilamicins as a new class of context-specific translocation inhibitors.

Production of antibody and development of enzyme-linked immunosorbent assay for therapeutic drug monitoring of eravacycline

Eravacycline (ERC) was approved for clinical use in 2018. It is more potent than other tetracyclines and can overcome resistance, making it an attractive option for combating multidrug-resistant bacterial infections. Intensive pharmacokinetic (PK) studies are currently being conducted to ensure the effectiveness and safety of ERC in various groups of patients, including those undergoing extracorporeal therapies. This study is the first attempt to develop a simple, efficient, and high-throughput immunoassay for quantifying ERC in human or animal serum. BSA-ERC conjugate as immunogen elicited antibody production in rabbits. Monitoring of the immune response and comparison of homologous and heterologous coating antigens allowed selection of immunoreagents and development of an assay that was selective for ERC possessing sensitivity (IC50), dynamic range (IC20-IC80) and detection limit equal to 3.3 ng/mL, 0.27-54 ng/mL and 0.09 ng/mL, respectively. The developed ELISA showed acceptable recovery of ERC (85-105 %) from rabbit and human serum in the clinically relevant concentration range of 0.1-3.0 mg/L. The method was used to quantify serum ERC concentration in the pilot PK study in Soviet chinchilla rabbits. The results were confirmed by HPLC-MS/MS.

Antimicrobial efficacy of eravacycline against emerging extensively drug-resistant (XDR) Acinetobacter baumannii isolates

PURPOSE

Drug-resistant Acinetobacter baumannii is an emerging threat. This study has been conducted to observe the efficacy of eravacycline along with the RND-efflux pump system.

METHODS

A cross-sectional study was done collecting 48 clinical isolates of Acinetobacter baumannii. MICs of 15 antibiotics were detected along with BMD of tigecycline and eravacycline. PCR products of drug-resistant regulatory genes were sequenced and analyzed.

RESULTS

Of the total 48 Isolates, 35 (72.91%) were XDR and 13 (27.08%) were MDR. Out of all, 60.41% of isolates were found to be susceptible to eravacycline by BMD according to both FDA and EUCAST guidelines. A 2-fold decline of MIC50/90 was observed with the use of eravacycline compared to tigecycline. RND-efflux genes like AdeC in 30 (62.5%) isolates and Regulatory gene AdeS in 29 (60.41%) isolates were detected, explaining the existing resistance mechanism.

CONCLUSIONS

XDR Acinetobacter poses an escalating threat due to its resistance to multiple antibiotics, raising serious concerns in healthcare settings. Eravacycline is an encouraging new drug for empirical use in severe infection caused due to the same. Molecular investigation and strict antimicrobial stewardship should be followed to control the emergence, and a better understanding of mechanisms of resistance to prevent the spread of drug-resistant isolates.

D-Ring Modifications of Tetracyclines Determine Their Ability to Induce Resistance Genes in the Environment

The widespread utilization of tetracyclines (TCs) in agriculture and medicine has led to the borderless spread of tetracycline resistance in humans, animals, and the environment, posing huge risks to both the ecosystem and human society. Changes in the functional group modifications resulted in a higher bacteriostatic efficacy of the new generation of TCs, but their effect on the emergence and evolution of antibiotic resistance genes (ARGs) is not yet known. To this end, four TCs from three generations were chosen to compare their structural effects on influencing the evolution of ARGs in soil microbial communities. The findings revealed that low-generation TCs, such as tetracycline and oxytetracycline, exhibited a greater propensity to stimulate the production and proliferation of ARGs than did high-generation tigecycline. Molecular docking analysis demonstrated that modifications of the D-ring functional group determined the binding capacity of TCs to the substrate-binding pocket of transcriptional regulators and efflux pumps mainly involved in drug resistance. This can be further evidenced by reverse transcription-quantitative polymerase chain reaction quantification and intracellular antibiotic accumulation assessment. This study sheds light on the mechanism of the structural effect of antibiotic-induced ARG production from the perspective of compound-protein binding, therefore providing theoretical support for controlling the dissemination of antibiotic resistance.

Eravacycline, the first four years: health outcomes and tolerability data for 19 hospitals in 5 U.S. regions from 2018 to 2022

IMPORTANCE

The rise of multidrug-resistant (MDR) pathogens, especially MDR Gram-negatives, poses a significant challenge to clinicians and public health. These resilient bacteria have rendered many traditional antibiotics ineffective, underscoring the urgency for innovative therapeutic solutions. Eravacycline, a broad-spectrum fluorocycline tetracycline antibiotic approved by the FDA in 2018, emerges as a promising candidate, exhibiting potential against a diverse array of MDR bacteria, including Gram-negative, Gram-positive, anaerobic strains, and Mycobacterium. However, comprehensive data on its real-world application remain scarce. This retrospective cohort study, one of the largest of its kind, delves into the utilization of eravacycline across various infectious conditions in the USA during its initial 4 years post-FDA approval. Through assessing clinical, microbiological, and tolerability outcomes, the research offers pivotal insights into eravacycline's efficacy in addressing the pressing global challenge of MDR bacterial infections.

In vitro activity of eravacycline and comparator agents against bacterial pathogens isolated from patients with cancer

Background

Bacterial infections are common in patients with cancer, and many bacteria have developed resistance to currently used antibiotics.

Objectives

We evaluated the in vitro activity of eravacycline (a recently developed fluorocycline) and comparators against bacterial pathogens isolated from patients with cancer.

Methods

Antimicrobial susceptibility testing was performed using CLSI-approved methodology and interpretive criteria for 255 Gram-positive and 310 Gram-negative bacteria. MIC and susceptibility percentage were calculated according to CLSI and FDA breakpoints when available.

Results

Eravacycline had potent activity against most Gram-positive bacteria, including MRSA. Of 80 Gram-positive isolates with available breakpoints, 74 (92.5%) were susceptible to eravacycline. Eravacycline had potent activity against most Enterobacterales, including ESBL-producing organisms. Of 230 Gram-negative isolates with available breakpoints, 201 (87.4%) were susceptible to eravacycline. Eravacycline had the best activity among comparators against carbapenem-resistant Enterobacterales, with 83% susceptibility. Eravacycline was also active against many non-fermenting Gram-negative bacteria, with the lowest MIC₉₀ value among comparators.

Conclusions

Eravacycline was active against many clinically significant bacteria isolated from patients with cancer, including MRSA, carbapenem-resistant Enterobacterales, and non-fermenting Gram-negative bacilli. Eravacycline might play an important role in the treatment of bacterial infections in patients with cancer, and additional clinical evaluation is warranted.

Antibiotics and Bacterial Resistance-A Short Story of an Endless Arms Race

Despite the undisputed development of medicine, antibiotics still serve as first-choice drugs for patients with infectious disorders. The widespread use of antibiotics results from a wide spectrum of their actions encompassing mechanisms responsible for: the inhibition of bacterial cell wall biosynthesis, the disruption of cell membrane integrity, the suppression of nucleic acids and/or proteins synthesis, as well as disturbances of metabolic processes. However, the widespread availability of antibiotics, accompanied by their overprescription, acts as a double-edged sword, since the overuse and/or misuse of antibiotics leads to a growing number of multidrug-resistant microbes. This, in turn, has recently emerged as a global public health challenge facing both clinicians and their patients. In addition to intrinsic resistance, bacteria can acquire resistance to particular antimicrobial agents through the transfer of genetic material conferring resistance. Amongst the most common bacterial resistance strategies are: drug target site changes, increased cell wall permeability to antibiotics, antibiotic inactivation, and efflux pumps. A better understanding of the interplay between the mechanisms of antibiotic actions and bacterial defense strategies against particular antimicrobial agents is crucial for developing new drugs or drug combinations. Herein, we provide a brief overview of the current nanomedicine-based strategies that aim to improve the efficacy of antibiotics.

Ribosome biogenesis in disease: new players and therapeutic targets

The ribosome is a multi-unit complex that translates mRNA into protein. Ribosome biogenesis is the process that generates ribosomes and plays an essential role in cell proliferation, differentiation, apoptosis, development, and transformation. The mTORC1, Myc, and noncoding RNA signaling pathways are the primary mediators that work jointly with RNA polymerases and ribosome proteins to control ribosome biogenesis and protein synthesis. Activation of mTORC1 is required for normal fetal growth and development and tissue regeneration after birth. Myc is implicated in cancer development by enhancing RNA Pol II activity, leading to uncontrolled cancer cell growth. The deregulation of noncoding RNAs such as microRNAs, long noncoding RNAs, and circular RNAs is involved in developing blood, neurodegenerative diseases, and atherosclerosis. We review the similarities and differences between eukaryotic and bacterial ribosomes and the molecular mechanism of ribosome-targeting antibiotics and bacterial resistance. We also review the most recent findings of ribosome dysfunction in COVID-19 and other conditions and discuss the consequences of ribosome frameshifting, ribosome-stalling, and ribosome-collision. We summarize the role of ribosome biogenesis in the development of various diseases. Furthermore, we review the current clinical trials, prospective vaccines for COVID-19, and therapies targeting ribosome biogenesis in cancer, cardiovascular disease, aging, and neurodegenerative disease.

Genome-wide association studies reveal distinct genetic correlates and increased heritability of antimicrobial resistance in Vibrio cholerae under anaerobic conditions

The antibiotic formulary is threatened by high rates of antimicrobial resistance (AMR) among enteropathogens. Enteric bacteria are exposed to anaerobic conditions within the gastrointestinal tract, yet little is known about how oxygen exposure influences AMR. The facultative anaerobe Vibrio cholerae was chosen as a model to address this knowledge gap. We obtained V. cholerae isolates from 66 cholera patients, sequenced their genomes, and grew them under anaerobic and aerobic conditions with and without three clinically relevant antibiotics (ciprofloxacin, azithromycin, doxycycline). For ciprofloxacin and azithromycin, the minimum inhibitory concentration (MIC) increased under anaerobic conditions compared to aerobic conditions. Using standard resistance breakpoints, the odds of classifying isolates as resistant increased over 10 times for ciprofloxacin and 100 times for azithromycin under anaerobic conditions compared to aerobic conditions. For doxycycline, nearly all isolates were sensitive under both conditions. Using genome-wide association studies, we found associations between genetic elements and AMR phenotypes that varied by oxygen exposure and antibiotic concentrations. These AMR phenotypes were more heritable, and the AMR-associated genetic elements were more often discovered, under anaerobic conditions. These AMR-associated genetic elements are promising targets for future mechanistic research. Our findings provide a rationale to determine whether increased MICs under anaerobic conditions are associated with therapeutic failures and/or microbial escape in cholera patients. If so, there may be a need to determine new AMR breakpoints for anaerobic conditions.

New antimicrobial treatment options for severe Gram-negative infections

PURPOSE OF REVIEW

This review will provide rationale for the development of new antibiotics to treat severe or multidrug-resistant (MDR) Gram-negative infections. It will also provide an overview of recently approved and pipeline antibiotics for severe/MDR Gram-negative infections.

RECENT FINDINGS

MDR Gram-negative infections are recognized as critical threats by global and national organizations and carry a significant morbidity and mortality risk. Increasing antibiotic resistance amongst Gram-negative bacteria, including carbapenem-resistant Acinetobacter baumannii , extended-spectrum β-lactamase-producing Enterobacterales, carbapenem-resistant Enterobacterales and Pseudomonas aeruginosa , with difficult-to-treat-resistance has made both empiric and definitive treatment of these infections increasingly problematic. In recent years, several antibiotics have been approved for treatment of MDR Gram-negative infections and ongoing clinical trials are poised to provide additional options to clinicians' armamentarium. These agents include various β-lactam/β-lactamase inhibitor combinations, eravacycline, plazomicin and cefiderocol.

SUMMARY

Severe/MDR Gram-negative infections continue to be important infections due to their impact on patient outcomes, especially in critically ill and immunocompromised hosts. The availability of new antibiotics offers an opportunity to improve empiric and definitive treatment of these infections.

In Vitro Susceptibility Testing of Eravacycline against Nontuberculous Mycobacteria

Nontuberculous mycobacteria (NTM) infections are increasing worldwide. Mycobacterium avium complex (MAC) and the M. abscessus species are the most commonly cultured NTM and treatment options are limited, especially for the M. abscessus species. In this study, the in vitro activity of eravacycline, a new tetracycline derivative, was tested against 110 clinical isolates of NTM. MIC testing was performed as recommended by the Clinical and Laboratory Standards Institute against 60 isolates of rapidly growing mycobacteria (RGM), of which ~70% were tetracycline resistant. These included M. abscessus subsp. abscessus (8 isolates), M. abscessus subsp. massiliense (5), M. chelonae (10), M. immunogenum (3), M. fortuitum group (20) including 12 doxycycline-resistant isolates, and M. mucogenicum group (10) including three doxycycline-resistant isolates. Due to trailing, eravacycline MICs were read at 80% and 100% inhibition. Eravacycline was active against all RGM species, with MIC₅₀ ranges of ≤0.015 to 0.5 and ≤0.015 to 0.12 μg/mL for 100% and 80% inhibition, respectively. For M. abscessus subsp. abscessus, MIC₅₀ values were 0.12 and 0.03 μg/mL with 100% and 80% inhibition, respectively. MICs for tigecycline were generally within 1 to 2 dilutions of the 100%-inhibition eravacycline MIC values. Fifty isolates of slowly growing mycobacteria (SGM) species, including 16 isolates of MAC, were also tested. While there was no trailing observed in most SGM, the eravacycline MICs were higher (MIC range of >8 μg/mL), except for M. kansasii and M. marinum which had MIC₅₀ values of 1 μg/mL. This study supports further evaluation of eravacycline, including clinical trials for the development of RGM treatment regimens, especially for M. abscessus.

OXA-48-Like β-Lactamases: Global Epidemiology, Treatment Options, and Development Pipeline

Modern medicine is threatened by the rising tide of antimicrobial resistance, especially among Gram-negative bacteria, where resistance to β-lactams is most often mediated by β-lactamases. The penicillin and cephalosporin ascendancies were, in their turn, ended by the proliferation of TEM penicillinases and CTX-M extended-spectrum β-lactamases. These class A β-lactamases have long been considered the most important. For carbapenems, however, the threat is increasingly from the insidious rise of a class D carbapenemase, OXA-48, and its close relatives. Over the past 20 years, OXA-48 and "OXA-48-like" enzymes have proliferated to become the most prevalent enterobacterial carbapenemases across much of Europe, Northern Africa, and the Middle East. OXA-48-like enzymes are notoriously difficult to detect because they often cause only low-level in vitro resistance to carbapenems, meaning that the true burden is likely underestimated. Despite this, they are associated with carbapenem treatment failures. A highly conserved incompatibility complex IncL plasmid scaffold often carries bla_OXA-48 and may carry other antimicrobial resistance genes, leaving limited treatment options. High conjugation efficiency means that this plasmid is sometimes carried by multiple Enterobacterales in a single patient. Producers evade most β-lactam-β-lactamase inhibitor combinations, though promising agents have recently been licensed, notably ceftazidime-avibactam and cefiderocol. The molecular machinery enabling global spread, current treatment options, and the development pipeline of potential new therapies for Enterobacterales that produce OXA-48-like β-lactamases form the focus of this review.

Total Synthesis and Biological Evaluation of Paenilamicins from the Honey Bee Pathogen Paenibacillus larvae

Paenilamicins are a group of complex polycationic peptide secondary metabolites with antibacterial and antifungal activities produced by the devastating honey bee brood pathogen Paenibacillus larvae causing the lethal brood disease American Foulbrood (AFB). Here, we report the convergent total synthesis and structural revision of paenilamicin B2. Specific stereoisomers of paenilamicin B2 were synthesized for unambiguous confirmation of the natural product structure and for evaluation of biological activities. These studies revealed the N-terminal fragment of paenilamicin as an important pharmacophore. Infection assays using bee larvae and the insect pathogen Bacillus thuringiensis demonstrated that paenilamicins outcompete bacterial competitors in the ecological niche of P. larvae. Finally, we show first data that classifies paenilamicins as potential ribosome inhibitors. Hence, our synthesis route is a further step for understanding the pathogenicity of P. larvae and for thorough structure-activity-relationship as well as mode-of-action studies in the near future.

The Development of Third-Generation Tetracycline Antibiotics and New Perspectives

The tetracycline antibiotic class has acquired new valuable members due to the optimisation of the chemical structure. The first modern tetracycline introduced into therapy was tigecycline, followed by omadacycline, eravacycline, and sarecycline (the third generation). Structural and physicochemical key elements which led to the discovery of modern tetracyclines are approached. Thus, several chemical subgroups are distinguished, such as glycylcyclines, aminomethylcyclines, and fluorocyclines, which have excellent development potential. The antibacterial spectrum comprises several resistant bacteria, including those resistant to old tetracyclines. Sarecycline, a narrow-spectrum tetracycline, is notable for being very effective against Cutinebacterium acnes. The mechanism of antibacterial action from the perspective of the new compound is approached. Several severe bacterial infections are treated with tigecycline, omadacycline, and eravacycline (with parenteral or oral formulations). In addition, sarecycline is very useful in treating acne vulgaris. Tetracyclines also have other non-antibiotic properties that require in-depth studies, such as the anti-inflammatory effect effect of sarecycline. The main side effects of modern tetracyclines are described in accordance with published clinical studies. Undoubtedly, this class of antibiotics continues to arouse the interest of researchers. As a result, new derivatives are developed and studied primarily for the antibiotic effect and other biological effects.

Treatment of patients with serious infections due to carbapenem-resistant Acinetobacter baumannii: How viable are the current options?

This review critically appraises the published microbiologic and clinical data on the treatment of patients with carbapenem-resistant Acinetobacter baumannii infections. Despite being recognized as an urgent threat pathogen by the CDC and WHO, optimal treatment of patients with serious CRAB infections remains ill-defined. Few commercially available agents exhibit reliable in vitro activity against CRAB. Historically, polymyxins have been the most active agents in vitro, though interpretations of susceptibility data are difficult given issues surrounding MIC testing methodologies and lack of correlation between MICs and clinical outcomes. Most available preclinical and clinical data involve use of polymyxins, tetracyclines, and sulbactam, alone and in combination. As the number of viable treatment options is limited, combination therapy with a polymyxin is often used for patients with CRAB infections, despite the significant risk of nephrotoxicity. However, no treatment regimen has been found to reduce mortality, which exceeds 40% across most studies, or substantially improve clinical response. While some newer agents, such as eravacycline and cefiderocol, have demonstrated in vitro activity, clinical efficacy has not been fully established. New agents with clinically relevant activity against CRAB isolates and favorable toxicity profiles are sorely needed.

Treatment for carbapenem-resistant Enterobacterales infections: recent advances and future directions

Carbapenem-resistant Enterobacterales (CRE) are a growing threat to human health worldwide. CRE often carry multiple resistance genes that limit treatment options and require longer durations of therapy, are more costly to treat, and necessitate therapies with increased toxicities when compared with carbapenem-susceptible strains. Here, we provide an overview of the mechanisms of resistance in CRE, the epidemiology of CRE infections worldwide, and available treatment options for CRE. We review recentlyapproved agents for the treatment of CRE, including ceftazidime-avibactam, meropenem-vaborbactam, imipenem-relebactam, cefiderocol, and novel aminoglycosides and tetracyclines. We also discuss recent advances in phage therapy and antibiotics that are currently in development targeted to CRE. The potential for the development of resistance to these therapies remains high, and enhanced antimicrobial stewardship is imperative both to reduce the spread of CRE worldwide and to ensure continued access to efficacious treatment options.

Efficacy and Tolerability of Eravacycline in Bacteremic Patients with Complicated Intra-Abdominal Infection: A Pooled Analysis from the IGNITE1 and IGNITE4 Studies

Background: Eravacycline is a novel, fully synthetic fluorocycline antibiotic that was evaluated for the treatment of complicated intra-abdominal infections (cIAI) in two phase 3 clinical trials. The objective of this analysis was to evaluate the clinical cure and microbiologic response at the test-of-cure (TOC) visit and the safety of eravacycline in patients with cIAI and baseline bacteremia who received eravacycline versus comparators. Patients and Methods: Pooled data of patients with bacteremia from the Investigating Gram-Negative Infections Treated with Eravacycline (IGNITE) 1 and IGNITE4 studies were analyzed. All patients were randomly assigned in a one-to-one ratio to receive eravacycline 1 mg/kg intravenously every 12 hours, ertapenem 1 g intravensouly every 24 hours (IGNITE1), or meropenem 1 g intravenously every eight hours (IGNITE4) for four to 14 days. Blood and intra-abdominal samples were collected from all patients at baseline. Clinical outcome and microbiologic eradiation at the TOC visit (28 days after randomization) and safety in the microbiologic-intent-to-treat population (micro-ITT) were assessed. Results: Of 415 patients treated with eravacycline and 431 treated with carbapenem comparators, concurrent bacteremia was identified in 32 (7.7%) and 31 (7.2%) patients, respectively. Demographic and baseline characteristics were similar among treatment groups. In the micro-ITT population, the pooled clinical response at the TOC visit for eravacycline was 28 of 32 (87.5%) and was 24 of 31 (77.0%) for comparators among the subgroup with baseline bacteremia (treatment difference 5.9; 95% confidence interval [CI], -6.5 to 17.4). At TOC, microbiologic eradication of pathogens isolated from blood specimens occurred for 34 of 35 (97.1%) pathogens with eravacycline and 35 of 36 (97.2%) pathogens with comparators. The incidence of adverse events was comparable between treated groups and similar to that observed in the non-bacteremic population. Conclusion: Eravacycline demonstrated a similar clinical outcome and microbiologic eradication rate as comparator carbapenems in patients with cIAI and associated secondary bacteremia. Future clinical trials of cIAI should report outcomes of this important clinical cohort (cIAI with concurrent bacteremia) given their high risk for adverse outcomes.

Cryo-EM Determination of Eravacycline-Bound Structures of the Ribosome and the Multidrug Efflux Pump AdeJ of Acinetobacter baumannii

Antibiotic-resistant strains of the Gram-negative pathogen Acinetobacter baumannii have emerged as a significant global health threat. One successful therapeutic option to treat bacterial infections has been to target the bacterial ribosome. However, in many cases, multidrug efflux pumps within the bacterium recognize and extrude these clinically important antibiotics designed to inhibit the protein synthesis function of the bacterial ribosome. Thus, multidrug efflux within A. baumannii and other highly drug-resistant strains is a major cause of failure of drug-based treatments of infectious diseases. We here report the first structures of the Acinetobacterdrug efflux (Ade)J pump in the presence of the antibiotic eravacycline, using single-particle cryo-electron microscopy (cryo-EM). We also describe cryo-EM structures of the eravacycline-bound forms of the A. baumannii ribosome, including the 70S, 50S, and 30S forms. Our data indicate that the AdeJ pump primarily uses hydrophobic interactions to bind eravacycline, while the 70S ribosome utilizes electrostatic interactions to bind this drug. Our work here highlights how an antibiotic can bind multiple bacterial targets through different mechanisms and potentially enables drug optimization by taking advantage of these different modes of ligand binding.

Efficacy and safety of eravacycline: A meta-analysis

OBJECTIVES

This study was conducted to evaluate the efficacy and safety of eravacycline, a recently approved fluorocycline for treatment of complicated intra-abdominal infections (cIAIs).

METHODS

PubMed, EMBASE and three trial registries were searched for randomised controlled trials (RCTs) comparing the efficacy and safety of eravacycline versus comparators. Odds ratios (ORs) with 95% confidence intervals (CIs) were estimated using random-effects models. The study outcomes included clinical response, all-cause mortality and adverse events (AEs).

RESULTS

Three RCTs (1128 patients) with cIAIs were included. There were no significant differences in clinical response in the modified intention-to-treat (ITT) (OR, 0.91, 95% CI 0.62-1.35; I² = 0%), microbiological ITT (OR, 0.93, 95% CI 0.61-1.41; I² = 0%) and clinically evaluable (OR, 0.98, 95% CI 0.55-1.75; I² = 0%) populations or in all-cause mortality (OR, 1.18, 95% CI 0.16-8.94; I² = 0%). Eravacycline was associated with significantly greater odds of total AEs (OR, 1.55, 95% CI 1.20-1.99; I² = 0%) and nausea (OR, 5.29, 95% CI 1.77-15.78; I² = 1.70%) but the increase in vomiting was non-significant (OR, 1.44, 95% CI 0.73-2.86; I² = 1.70%). There were no significant differences in serious AEs or discontinuation due to AEs.

CONCLUSION

This meta-analysis of RCTs found similar clinical efficacy and mortality for eravacycline compared with carbapenems for treatment of cIAIs. However, the odds of total AEs and specifically nausea was higher with eravacycline, while no significant differences were observed in vomiting (although numerically higher), serious AEs or discontinuation due to AEs.

The Antibiotic Resistome: A Guide for the Discovery of Natural Products as Antimicrobial Agents

The use of life-saving antibiotics has long been plagued by the ability of pathogenic bacteria to acquire and develop an array of antibiotic resistance mechanisms. The sum of these resistance mechanisms, the antibiotic resistome, is a formidable threat to antibiotic discovery, development, and use. The study and understanding of the molecular mechanisms in the resistome provide the basis for traditional approaches to combat resistance, including semisynthetic modification of naturally occurring antibiotic scaffolds, the development of adjuvant therapies that overcome resistance mechanisms, and the total synthesis of new antibiotics and their analogues. Using two major classes of antibiotics, the aminoglycosides and tetracyclines as case studies, we review the success and limitations of these strategies when used to combat the many forms of resistance that have emerged toward natural product-based antibiotics specifically. Furthermore, we discuss the use of the resistome as a guide for the genomics-driven discovery of novel antimicrobials, which are essential to combat the growing number of emerging pathogens that are resistant to even the newest approved therapies.

Ribosome-targeting antibacterial agents: Advances, challenges, and opportunities

Ribosomes, which synthesize proteins, are critical organelles for the survival and growth of bacteria. About 60% of approved antibiotics discovered so far combat pathogenic bacteria by targeting ribosomes. However, several issues, such as drug resistance and toxicity, have impeded the clinical use of ribosome-targeting antibiotics. Moreover, the complexity of the bacteria ribosome structure has retarded the discovery of new ribosome-targeting agents that are considered as the key to the drug-resistance and toxicity. To deal with these challenges, efforts such as medicinal chemistry optimization, combination treatment, and new drug delivery system have been developed. But not enough, the development of structural biology and new screening methods bring powerful tools, such as cryo-electron microscopy technology, advanced computer-aided drug design, and cell-free in vitro transcription/translation systems, for the discovery of novel ribosome-targeting antibiotics. Thus, in this paper, we overview the research on different aspects of bacterial ribosomes, especially focus on discussing the challenges in the discovery of ribosome-targeting antibacterial drugs and advances made to address issues such as drug-resistance and selectivity, which, we believe, provide perspectives for the discovery of novel antibiotics.

Recent advances in antibacterial agents

Antimicrobial resistance is a global challenge and the effectiveness of old antibiotics is decreasing. Discovery and development of antibacterial agents have been accelerated to replenish the arsenal of antibiotics which is limited and shrinking. In recent years, significant advances have achieved in the antibacterial area, including new compounds of known classes and new compounds with new mechanisms. This review summarizes these advances and provides perspective on future directions of antibacterial agents.

The search for novel treatment strategies for Streptococcus pneumoniae infections

This review provides an overview of the most important novel treatment strategies against Streptococcus pneumoniae infections published over the past 10 years. The pneumococcus causes the majority of community-acquired bacterial pneumonia cases, and it is one of the prime pathogens in bacterial meningitis. Over the last 10 years, extensive research has been conducted to prevent severe pneumococcal infections, with a major focus on (i) boosting the host immune system and (ii) discovering novel antibacterials. Boosting the immune system can be done in two ways, either by actively modulating host immunity, mostly through administration of selective antibodies, or by interfering with pneumococcal virulence factors, thereby supporting the host immune system to effectively overcome an infection. While several of such experimental therapies are promising, few have evolved to clinical trials. The discovery of novel antibacterials is hampered by the high research and development costs versus the relatively low revenues for the pharmaceutical industry. Nevertheless, novel enzymatic assays and target-based drug design, allow the identification of targets and the development of novel molecules to effectively treat this life-threatening pathogen.

In vitro activity of eravacycline and mechanisms of resistance in enterococci

Eravacycline (ERC), the first fluorocycline, is a new tetracycline with superior activity to tigecycline (TGC) against many bacterial species. This work aimed to determine the in vitro activity of ERC compared with other tetracyclines against enterococcal clinical isolates and to analyse corresponding resistance mechanisms. A collection of 60 enterococcal strains was studied: 54 epidemiologically unrelated clinical isolates (46 Enterococcus faecium and 8 Enterococcus faecalis) including 42 vancomycin-resistant enterococci (VRE) (33 vanA and 9 vanB), 3 in vitro TGC-resistant mutants (E. faecium AusTig, HMtig1 and HMtig2) and 3 reference wild-type strains (E. faecium Aus0004 and HM1070, E. faecalis ATCC 29212). In vitro susceptibility was determined using Etest strips (for ERC) or by broth microdilution (for TGC, doxycycline, minocycline and tetracycline). Resistance genes [tet(M), tet(L), tet(O) and tet(S)] were screened by PCR for TGC- and/or ERC-resistant strains as well as sequencing of the rpsJ gene (encoding ribosomal protein S10). MIC_50/90 values were 0.016/0.08, ≤0.03/0.5, 4/32, 8/16 and 32/>32 mg/L for ERC, TGC, doxycycline, minocycline and tetracycline, respectively. According to EUCAST guidelines, nine strains were categorised as resistant to TGC (MIC, 0.5-8 mg/L), including four E. faecium vanA(+) strains also resistant to ERC (MIC, 0.19-1.5 mg/L). These four strains all possessed at least one mutation in rpsJ and two tet determinants: tet(M) + tet(L) (n = 2); and tet(M) + tet(S) (n = 2). Although ERC has excellent in vitro activity against enterococci (including VRE), emergence of resistance is possible due to combined mechanisms (rpsJ mutations + tet genes).

A Real-World Assessment of Clinical Outcomes and Safety of Eravacycline: A Novel Fluorocycline

Background

Eravacycline is a novel fluorocycline approved for treatment of intraabdominal infections, with a broad spectrum of activity against a range of pathogens including multidrug-resistant species, including ESBL- or KPC-producing isolates. It is approved for twice-daily dosing with no need for adjustment in renal dysfunction. In the concomitant administration with CYP 3A4-inducing drugs, eravacycline dosing should be modified.

Objective

To evaluate the efficacy and safety of eravacycline in a range of infections such as intraabdominal infections, pneumonia and diabetic foot infections in seriously ill patients.

Methods

A retrospective observational cohort study using electronic patient records of 50 consecutive patients administered eravacycline during inpatient acute care admission or as part of outpatient antibiotic therapy (OPAT).

Results

Therapy of 1.5 mg/kg q24h was initiated in the hospital in most patients, although some of the less sick were managed in the office or OPAT setting. All patients concluded their management outside of the hospital. Of the 50 patients, 47 (94%) achieved clinical resolution of their infection and 3 (6%) clinical failures occurred. Only three (6%) patients did not have comorbidities, three had a single comorbidity (6%), and the majority (88%) of patients had two or more comorbidities. Most common infections were intraabdominal (36%), pneumonia (18%), diabetic foot (12%), spontaneous bacterial peritonitis (8%) and empyema (8%). Almost half of infections had more than one pathogen isolated, and resistant isolates were frequent. The drug was well tolerated with only two reports of nausea, which did not result in treatment discontinuation, and in 30 days of post-eravacycline therapy only one case of Clostridiodes difficile.

Conclusions

In this real-world setting, eravacycline demonstrated a similar high level of clinical efficacy as seen in clinical trials, 94%, in a variety of infections, including against multidrug-resistant bacteria, and was well tolerated.

In vitro activity of eravacycline against common ribotypes of Clostridioides difficile

Background

Eravacycline is a novel synthetic fluorocycline antibacterial approved for complicated intra-abdominal infections.

Objectives

The purpose of this study was to assess the in vitro activities of eravacycline and comparator antibiotics against contemporary clinical isolates of Clostridioides difficile representing common ribotypes, including isolates with decreased susceptibility to metronidazole and vancomycin.

Methods

Clinical C. difficile strains from six common or emerging ribotypes were used to test the in vitro activities of eravacycline and comparator antibiotics (fidaxomicin, vancomycin and metronidazole) by broth microdilution. In addition, MBC experiments, time–kill kinetic studies and WGS experiments were performed.

Results

A total of 234 isolates were tested, including ribotypes RT001 (n = 37), RT002 (n = 41), RT014-020 (n = 39), RT027 (n = 42), RT106 (n = 38) and RT255 (n = 37). MIC_50/90 values were lowest for eravacycline (≤0.0078/0.016 mg/L), followed by fidaxomicin (0.016/0.063 mg/L), metronidazole (0.25/1.0 mg/L) and vancomycin (2.0/4.0 mg/L). MBCs were lower for eravacycline compared with vancomycin for all ribotypes tested. Both vancomycin and eravacycline demonstrated bactericidal killing, including for epidemic RT027. The presence of the tetM or tetW resistance genes did not affect the MIC of eravacycline.

Conclusions

This study demonstrated potent in vitro activity of eravacycline against a large collection of clinical C. difficile strains that was not affected by ribotype, susceptibility to vancomycin or the presence of certain tet resistance genes. Further development of eravacycline as an antibiotic to be used in patients with Clostridioides difficile infection is warranted.

IGNITE4: Results of a Phase 3, Randomized, Multicenter, Prospective Trial of Eravacycline vs Meropenem in the Treatment of Complicated Intraabdominal Infections

Background

Increasing antimicrobial resistance among pathogens that cause complicated intraabdominal infections (cIAIs) supports the development of new antimicrobials. Eravacycline, a novel member of the fluorocycline family, is active against multidrug-resistant bacteria including extended-spectrum β-lactamase (ESBL) and carbapenem-resistant Enterobacteriaceae.

Methods

IGNITE4 was a prospective, randomized, double-blind trial. Hospitalized patients with cIAI received either eravacycline 1 mg/kg every 12 hours or meropenem 1 g every 8 hours intravenously for 4–14 days. The primary objective was to demonstrate statistical noninferiority (NI) in clinical cure rates at the test-of-cure visit (25–31 days from start of therapy) in the microbiological intent-to-treat population using a NI margin of 12.5%. Microbiological outcomes and safety were also evaluated.

Results

Eravacycline was noninferior to meropenem in the primary endpoint (177/195 [90.8%] vs 187/205 [91.2%]; difference, –0.5%; 95% confidence interval [CI], –6.3 to 5.3), exceeding the prespecified margin. Secondary endpoints included clinical cure rates in the modified ITT population (231/250 [92.4%] vs 228/249 [91.6%]; difference, 0.8; 95% CI, –4.1, 5.8) and the clinically evaluable population (218/225 [96.9%] vs 222/231 [96.1%]; (difference, 0.8; 95% CI –2.9, 4.5). In patients with ESBL-producing Enterobacteriaceae, clinical cure rates were 87.5% (14/16) and 84.6% (11/13) in the eravacycline and meropenem groups, respectively. Eravacycline had relatively low rates of adverse events for a drug of this class, with less than 5%, 4%, and 3% of patients experiencing nausea, vomiting, and diarrhea, respectively.

Conclusions

Treatment with eravacycline was noninferior to meropenem in adult patients with cIAI, including infections caused by resistant pathogens.

Clinical Trials Registration

NCT01844856.

In Vitro Activities of Eravacycline and Other Antimicrobial Agents against Human Mycoplasmas and Ureaplasmas

We performed in vitro susceptibility testing for eravacycline in comparison to 4 other antimicrobials against 10 Mycoplasma genitalium, 40 Mycoplasma hominis, 44 Mycoplasma pneumoniae, 20 Ureaplasma parvum, and 20 Ureaplasma urealyticum isolates. All eravacycline MICs were ≤0.25 μg/ml, except that for one isolate of M. genitalium, for which the MIC was 2 μg/ml. Eravacycline was markedly more potent than tetracycline, azithromycin, moxifloxacin, and clindamycin against all isolates tested, which included 37 macrolide, tetracycline, and/or fluoroquinolone-resistant organisms.

Investigations into the Antibacterial Mechanism of Action of Viridicatumtoxins

Viridicatumtoxins are a rare class of tetracycline-like antibiotics that strongly inhibit drug-resistant Gram-positive bacteria. Although reported to exhibit in vitro inhibition activity to undecaprenyl pyrophosphate synthase (UPPS), an essential enzyme in bacterial cell wall synthesis, the biological targets and mechanism of action of viridicatumtoxins, especially the drug-target interactions, remain largely unknown. In this study, the structure of Enterococcus faecalis UPPS (EfaUPPS) was first determined, uncovering that EfaUPPS can form not only a typical functional dimer but also an unexpected atypical dimer. We then observed that viridicatumtoxins A (VirA) and B (VirB) are able to bind to UPPSs of E. faecalis, S. aureus, and E. coli in a direct and high-affinity manner as evidenced by in vitro enzyme inhibition assay, surface plasmon resonance (SPR) binding analysis, and in vivo growth inhibition assay, demonstrating that viridicatumtoxins exert antibacterial effects through UPPS binding. The key amino acid residues involved in the interactions with VirA and VirB in EfaUPPS binding pocket were revealed by molecular docking studies, and further validated by site-directed mutagenesis. A single mutation of EfaUPPS at D29A, N31A, and R42A can obviously increase their affinities to VirA, while a single mutation at W228A conferred significant resistance to VirA. Moreover, translation inhibition assay showed that VirA and VirB can weakly inhibit E. coli 70S ribosome. The weak inhibition of ribosome was proposed to be attributed to steric hindrance between viridicatumtoxin ring F and 70S ribosome helix 34 by molecular docking study. Our structural, biochemical, and computational investigations on the interactions of viridicatumtoxins with UPPS and 70S ribosome not only disclosed the potential biological targets of viridicatumtoxins, but also provided a theoretical basis for structural optimization to make new viridicatumtoxin derivatives with improved antimicrobial activities.

In vitro activity of new tetracycline analogues omadacycline and eravacycline against clinical isolates of Helicobacter pylori collected in China

Omadacycline and eravacycline are newly approved tetracycline analogues with excellent activity against a broad spectrum of Gram-positive and Gram-negative microorganisms; however, no data are available regarding Helicobacter pylori. The susceptibility of 201 clinical isolates of H. pylori collected in China to omadacycline, eravacycline, and the comparator tetracycline was determined by an agar dilution method. They showed greater activity than tetracycline. The MIC_50/90 values of omadacycline, eravacycline, and tetracycline were 0.125/0.25 μg/mL, 0.063/0.125 μg/mL, and 0.25/1 μg/mL, respectively. Omadacycline and eravacycline were potent in vitro against all the isolates tested, including tetracycline-resistant strains, and warrant further investigation as potential antibiotics for H. pylori treatment.

Target protection as a key antibiotic resistance mechanism

Antibiotic resistance is mediated through several distinct mechanisms, most of which are relatively well understood and the clinical importance of which has long been recognized. Until very recently, neither of these statements was readily applicable to the class of resistance mechanism known as target protection, a phenomenon whereby a resistance protein physically associates with an antibiotic target to rescue it from antibiotic-mediated inhibition. In this Review, we summarize recent progress in understanding the nature and importance of target protection. In particular, we describe the molecular basis of the known target protection systems, emphasizing that target protection does not involve a single, uniform mechanism but is instead brought about in several mechanistically distinct ways.

Mechanism of Eravacycline Resistance in Clinical Enterococcus faecalis Isolates From China

Opportunistic infections caused by multidrug-resistant Enterococcus faecalis strains are a significant clinical challenge. Eravacycline (Erava) is a synthetic fluorocycline structurally similar to tigecycline (Tige) that exhibits robust antimicrobial activity against Gram-positive bacteria. This study investigated the in vitro antimicrobial activity and heteroresistance risk of Eravacycline (Erava) in clinical E. faecalis isolates from China along with the mechanism of Erava resistance. A total of 276 non-duplicate E. faecalis isolates were retrospectively collected from a tertiary care hospital in China. Heteroresistance to Erava and the influence of tetracycline (Tet) resistance genes on Erava susceptibility were examined. To clarify the molecular basis for Erava resistance, E. faecalis variants exhibiting Erava-induced resistance were selected under Erava pressure. The relative transcript levels of six candidate genes linked to Erava susceptibility were determined by quantitative reverse-transcription PCR, and their role in Erava resistance and heteroresistance was evaluated by in vitro overexpression experiments. We found that Erava minimum inhibitory concentrations (MICs) against clinical E. faecalis isolates ranged from ≤0.015 to 0.25 mg/l even in strains harboring Tet resistance genes. The detection frequency of Erava heteroresistance in isolates with MICs ≤ 0.06, 0.125, and 0.25 mg/l were 0.43% (1/231), 7.5% (3/40), and 0 (0/5), respectively. No mutations were detected in the 30S ribosomal subunit gene in Erava heteroresistance-derived clones, although mutations in this subunit conferred cross resistance to Tige in Erava-induced resistant E. faecalis. Overexpressing RS00630 (encoding a bone morphogenetic protein family ATP-binding cassette transporter substrate-binding protein) in E. faecalis increased the frequency of Erava and Tige heteroresistance, whereas RS12140, RS06145, and RS06880 overexpression conferred heteroresistance to Tige only. These results indicate that Erava has potent in vitro antimicrobial activity against clinical E. faecalis isolates from China and that Erava heteroresistance can be induced by RS00630 overexpression.

In Vitro Activity of Eravacycline against Gram-Positive Bacteria Isolated in Clinical Laboratories Worldwide from 2013 to 2017

Eravacycline is a novel, fully synthetic fluorocycline antibiotic being developed for the treatment of serious infections, including those caused by resistant Gram-positive pathogens. Here, we evaluated the in vitro activities of eravacycline and comparator antimicrobial agents against a recent global collection of frequently encountered clinical isolates of Gram-positive bacteria. The CLSI broth microdilution method was used to determine in vitro MIC data for isolates of Enterococcus spp. (n = 2,807), Staphylococcus spp. (n = 4,331), and Streptococcus spp. (n = 3,373) isolated primarily from respiratory, intra-abdominal, urinary, and skin specimens by clinical laboratories in 37 countries on three continents from 2013 to 2017. Susceptibilities were interpreted using both CLSI and EUCAST breakpoints. There were no substantive differences (a >1-doubling-dilution increase or decrease) in eravacycline MIC₉₀ values for different species/organism groups over time or by region. Eravacycline showed MIC₅₀ and MIC₉₀ results of 0.06 and 0.12 μg/ml, respectively, when tested against Staphylococcus aureus, regardless of methicillin susceptibility. The MIC₉₀ values of eravacycline for Staphylococcus epidermidis and Staphylococcus haemolyticus were equal (0.5 μg/ml). The eravacycline MIC₉₀s for Enterococcus faecalis and Enterococcus faecium were 0.06 μg/ml and were within 1 doubling dilution regardless of the vancomycin susceptibility profile. Eravacycline exhibited MIC₉₀ results of ≤0.06 μg/ml when tested against Streptococcus pneumoniae and beta-hemolytic and viridans group streptococcal isolates. In this surveillance study, eravacycline demonstrated potent in vitro activity against frequently isolated clinical isolates of Gram-positive bacteria (Enterococcus, Staphylococcus, and Streptococcus spp.), including isolates collected over a 5-year period (2013 to 2017), underscoring its potential benefit in the treatment of infections caused by common Gram-positive pathogens.

In Vitro Activity of Eravacycline against Gram-Negative Bacilli Isolated in Clinical Laboratories Worldwide from 2013 to 2017

Eravacycline is a novel, fully synthetic fluorocycline antibiotic developed for the treatment of serious infections, including those caused by multidrug-resistant (MDR) pathogens. Here, we evaluated the in vitro activities of eravacycline and comparator antimicrobial agents against a global collection of frequently encountered clinical isolates of Gram-negative bacilli. The CLSI broth microdilution method was used to determine MIC data for isolates of Enterobacterales (n = 13,983), Acinetobacter baumannii (n = 2,097), Pseudomonas aeruginosa (n = 1,647), and Stenotrophomonas maltophilia (n = 1,210) isolated primarily from respiratory, intra-abdominal, and urinary specimens by clinical laboratories in 36 countries from 2013 to 2017. Susceptibilities were interpreted using both CLSI and EUCAST breakpoints. Multidrug-resistant (MDR) isolates were defined by resistance to agents from ≥3 different antimicrobial classes. The MIC₉₀s ranged from 0.25 to 1 μg/ml for Enterobacteriaceae and were 1 μg/ml for A. baumannii and 2 μg/ml for S. maltophilia, Proteus mirabilis, and Serratia marcescens Eravacycline's potency was up to 4-fold greater than that of tigecycline against genera/species of Enterobacterales, A. baumannii, and S. maltophilia The MIC₉₀s for five of six individual genera/species of Enterobacterales and A. baumannii were within 2-fold of the MIC₉₀s for their respective subsets of MDR isolates, while the MDR subpopulation of Klebsiella spp. demonstrated 4-fold higher MIC₉₀s. Eravacycline demonstrated potent in vitro activity against the majority of clinical isolates of Gram-negative bacilli, including MDR isolates, collected over a 5-year period. This study further underscores the potential benefit of eravacycline in the treatment of infections caused by MDR Gram-negative pathogens.

Evaluation of Eravacycline: A Novel Fluorocycline

Eravacycline (ERV), formerly known as TP-434, is a novel tetracycline (TET) antibiotic that exhibits in vitro activity against various gram-positive, gram-negative aerobic and anaerobic pathogens, including those exhibiting TET-specific acquired resistance mechanisms. Similar to other TETs, it inhibits protein synthesis through binding to the 30S ribosomal subunit. Eravacycline was approved by the United States Food and Drug Administration (FDA) in August 2018 for the treatment of complicated intraabdominal infections (cIAIs) in adults following the Investigating Gram-Negative Infections Treated with Eravacycline (IGNITE)1 and IGNITE4 phase III trials. In these two, double-blind, multicenter clinical trials, ERV was proven noninferior in terms of clinical response in comparison to ertapenem and meropenem, respectively. Eravacycline was well tolerated with nausea, vomiting, and infusion site reactions being the most commonly reported adverse reactions. Clinicians now have ERV as a novel therapeutic option for the treatment of adults with intraabdominal infections, allergies to β-lactam agents, Clostridioides difficile-associated diarrhea, or if tolerability to other agents is a concern.

Prevalence of Potentially Pathogenic Antibiotic-Resistant Aeromonas spp. in Treated Urban Wastewater Effluents versus Recipient Riverine Populations: a 3-Year Comparative Study

Antibiotic resistance continues to be an emerging threat both in clinical and environmental settings. Among the many causes, the impact of postchlorinated human wastewater on antibiotic resistance has not been well studied. Our study compared antibiotic susceptibility among Aeromonas spp. in postchlorinated effluents to that of the recipient riverine populations for three consecutive years against 12 antibiotics. Aeromonas veronii and Aeromonas hydrophila predominated among both aquatic environments, although greater species diversity was evident in treated wastewater. Overall, treated wastewater contained a higher prevalence of nalidixic acid-, trimethoprim-sulfamethoxazole (SXT)-, and tetracycline-resistant isolates, as well as multidrug-resistant (MDR) isolates compared to upstream surface water. After selecting for tetracycline-resistant strains, 34.8% of wastewater isolates compared to 8.3% of surface water isolates were multidrug resistant, with nalidixic acid, streptomycin, and SXT being the most common. Among tetracycline-resistant isolates, efflux pump genes tetE and tetA were the most prevalent, though stronger resistance correlated with tetA. Over 50% of river and treated wastewater isolates exhibited cytotoxicity that was significantly correlated with serine protease activity, suggesting many MDR strains from effluent have the potential to be pathogenic. These findings highlight that conventionally treated wastewater remains a reservoir of resistant, potentially pathogenic bacterial populations being introduced into aquatic systems that could pose a threat to both the environment and public health.IMPORTANCE Aeromonads are Gram-negative, asporogenous rod-shaped bacteria that are autochthonous in fresh and brackish waters. Their pathogenic nature in poikilotherms and mammals, including humans, pose serious environmental and public health concerns especially with rising levels of antibiotic resistance. Wastewater treatment facilities serve as major reservoirs for the dissemination of antibiotic resistance genes (ARGs) and resistant bacterial populations and are, thus, a potential major contributor to resistant populations in aquatic ecosystems. However, few longitudinal studies exist analyzing resistance among human wastewater effluents and their recipient aquatic environments. In this study, considering their ubiquitous nature in aquatic environments, we used Aeromonas spp. as bacterial indicators of environmental antimicrobial resistance, comparing it to that in postchlorinated wastewater effluents over 3 years. Furthermore, we assessed the potential of these resistant populations to be pathogenic, thus elaborating on their potential public health threat.

Mechanism of eravacycline resistance in Acinetobacter baumannii mediated by a deletion mutation in the sensor kinase adeS, leading to elevated expression of the efflux pump AdeABC

Acinetobacter baumannii is an important pathogen and presents a major burden in healthcare as strains frequently cause hospital associated opportunistic infections with high mortality rates. Due to increasing numbers of drug resistant A. baumannii strains, newly developed antibiotics are being used to treat infections caused by such strains. One novel synthetic antibiotic of the tetracycline class with activity against A. baumannii is eravacycline. To investigate possible mechanisms of eravacycline resistance, we performed an in vitro evolution experiment to select for an eravacycline resistant strain, with the clinical isolate MDR-ZJ06 as parental strain. We obtained a strain designated MDR-ZJ06-E6 that was able to grow in 64-fold MIC. Genomic mutations were identified by whole genome sequencing, where we found a deletion mutation in the gene adeS. Using complementation experiments, including growth rate determination and antibiotics susceptibility testing, we could confirm that this mutation was responsible for eravacycline resistance of strain MDR-ZJ06-E6. As a mechanism of resistance, we identified a significant overexpression of the efflux pump AdeABC which seems to be regulated by the mutation in adeS in A. baumannii.

Multidrug Resistant Acinetobacter baumannii: Resistance by Any Other Name Would Still be Hard to Treat

PURPOSE OF REVIEW

Acinetobacter baumannii (AB) is an infamous nosocomial pathogen with a seemingly limitless capacity for antimicrobial resistance, leading to few treatment options and poor clinical outcomes. The debatably low pathogenicity and virulence of AB are juxtaposed by its exceptionally high rate of infection-related mortality, likely due to delays in time to effective antimicrobial therapy secondary to its predilection for resistance to first-line agents. Recent studies of AB and its infections have led to a burgeoning understanding of this critical microbial threat and provided clinicians with new ammunition for which to target this elusive pathogen. This review will provide an update on the virulence, resistance, diagnosis, and treatment of multidrug resistant (MDR) AB.

RECENT FINDINGS

Advances in bacterial genomics have led to a deeper understanding of the unique mechanisms of resistance often present in MDR AB and how they may be exploited by new antimicrobials or optimized combinations of existing agents. Further, improvements in rapid diagnostic tests (RDTs) and their more pervasive use in combination with antimicrobial stewardship interventions have allowed for more rapid diagnosis of AB and decreases in time to effective therapy. Unfortunately, there remains a paucity of high-quality clinical data for which to inform the optimal treatment of MDR AB infections. In fact, recently completed studies have failed to identify a combination regimen that is consistently superior to monotherapy, despite the benefits demonstrated in vitro. Encouragingly, new and updated guidelines offer strategies for the treatment of MDR AB and may help to harmonize the use of high toxicity agents such as the polymyxins. Finally, new antimicrobial agents such as eravacycline and cefiderocol have promising in vitro activity against MDR AB but their place in therapy for these infections remains to be determined. Notwithstanding available clinical trial data, polymyxin-based combination therapies with either a carbapenem, minocycline, or eravacycline remain the treatment of choice for MDR, particularly carbapenem-resistant, AB. Incorporating antimicrobial stewardship intervention with RDTs relevant to MDR AB can help avoid potentially toxic combination therapies and catalyze the most important modifiable risk factor for mortality-time to effective therapy. Further research efforts into pharmacokinetic/pharmacodynamic-based dose optimization and clinical outcomes data for MDR AB continue to be desperately needed.

Treatment Options for Carbapenem-resistant Gram-negative Bacterial Infections

Antimicrobial resistance has become one of the greatest threats to public health, with rising resistance to carbapenems being a particular concern due to the lack of effective and safe alternative treatment options. Carbapenem-resistant gram-negative bacteria of clinical relevance include the Enterobacteriaceae, Pseudomonas aeruginosa, Acinetobacter baumannii, and more recently, Stenotrophomonas maltophilia. Colistin and tigecycline have been used as first-line agents for the treatment of infections caused by these pathogens; however, there are uncertainties regarding their efficacy even when used in combination with other agents. More recently, several new agents with activity against certain carbapenem-resistant pathogens have been approved for clinical use or are reaching late-stage clinical development. They include ceftazidime-avibactam, ceftolozane-tazobactam, meropenem-vaborbactam, imipenem-cilastatin-relebactam, plazomicin, eravacycline, and cefiderocol. In addition, fosfomycin has been redeveloped in a new intravenous formulation. Data regarding the clinical efficacy of these new agents specific to infections caused by carbapenem-resistant pathogens are slowly emerging and appear to generally favor newer agents over previous best available therapy. As more treatment options become widely available for carbapenem-resistant gram-negative infections, the role of antimicrobial stewardship will become crucial in ensuring appropriate and rationale use of these new agents.

Eravacycline: A Review in Complicated Intra-Abdominal Infections

Eravacycline (Xerava™), a novel fully synthetic fluorocycline, consists of the tetracyclic core scaffold with unique modifications in the tetracyclic D ring; consequently, it exhibits potent in vitro activity against Gram-positive and -negative bacterial strains expressing certain common tetracycline-specific acquired resistance mechanisms. In vitro, eravacycline exhibits potent activity against a broad spectrum of clinically relevant Gram-positive and -negative aerobic and anaerobic bacteria. Intravenous eravacycline is approved in several countries for the treatment of complicated intra-abdominal infections (cIAIs) in adult patients. In two pivotal double-blind, multinational trials in this patient population, eravacycline (infusion ≈ 1 h) was noninferior to intravenous ertapenem or meropenem at the test-of-cure visit in terms of clinical response rates in all prespecified populations. Eravacycline had an acceptable tolerability profile, with infusion site reactions, nausea, vomiting and diarrhoea the most commonly reported adverse reactions, most of which were of mild to moderate severity. Given its broad spectrum of activity against common clinically relevant pathogens (including those expressing certain tetracycline- and other antibacterial-specific acquired resistance mechanisms) and its more potent in vitro activity and better tolerability profile than tigecycline, eravacycline provides a novel emerging option for the treatment of adult patients with cIAIs, especially as empirical therapy when coverage of resistant pathogens is required.

Eravacycline: a new treatment option for complicated intra-abdominal infections in the age of multidrug resistance

Aim: Recently approved for use in complicated intra-abdominal infection, eravacycline is a novel fluorocycline with broad spectrum of activity against resistant Gram-negative pathogens. This manuscript is a pooled analysis of two Phase III trials. Clinical efficacy: Clinical cure rates were 86.8% for eravacycline versus 87.6% for ertapenem, and 90.8% for eravacycline versus 91.2% for meropenem in the Intent to Treat (micro-ITT) populations, and 87.0% for eravacycline versus 88.8% ertapenem, and 92.4 versus 91.6% for meropenem in the Modified Intent to Treat (MITT) populations. Safety: Eravacycline is well tolerated, with lower rates of nausea, vomiting and diarrhea than other tetracyclines. Conclusion: Eravacycline is an effective new option for use in complicated intra-abdominal infections, and in particular, for the treatment of extended-spectrum β-lactamase- and carbapenem-resistant Enterobacteriaceae-expressing organisms.