Reproducibility of broth microdilution MICs for the novel siderophore cephalosporin, cefiderocol, determined using iron-depleted cation-adjusted Mueller-Hinton broth

Use of Cefiderocol in Adult Patients: Descriptive Analysis from a Prospective, Multicenter, Cohort Study

INTRODUCTION

Cefiderocol is a siderophore cephalosporin showing activity against various carbapenem-resistant Gram-negative bacteria (CR-GNB). No data currently exist about real-world use of cefiderocol in terms of types of therapy (e.g., empirical or targeted, monotherapy or combined regimens), indications, and patient characteristics.

METHODS

In this multicenter, prospective study, we aimed at describing the use of cefiderocol in terms of types of therapy, indications, and patient characteristics.

RESULTS

Cefiderocol was administered as empirical and targeted therapy in 27.5% (55/200) and 72.5% (145/200) of cases, respectively. Overall, it was administered as monotherapy in 101/200 cases (50.5%) and as part of a combined regimen for CR-GNB infections in the remaining 99/200 cases (49.5%). In multivariable analysis, previous isolation of carbapenem-resistant Acinetobacter baumannii odds ratio (OR) 2.56, with 95% confidence interval (95% CI) 1.01-6.46, p = 0.047] and previous hematopoietic stem cell transplantation (OR 8.73, 95% CI 1.05-72.54, p = 0.045) were associated with administration of cefiderocol as part of a combined regimen, whereas chronic kidney disease was associated with cefiderocol monotherapy (OR 0.38 for combined regimen, 95% CI 0.16-0.91, p = 0.029). Cumulative 30-day mortality was 19.8%, 45.0%, 20.7%, and 22.7% in patients receiving targeted cefiderocol for infections by Enterobacterales, A. baumannii, Pseudomonas aeruginosa, and any metallo-β-lactamase producers, respectively.

CONCLUSIONS

Cefiderocol is mainly used for targeted treatment, although empirical therapies account for more than 25% of prescriptions, thus requiring dedicated standardization and guidance. The almost equal distribution of cefiderocol monotherapy and cefiderocol-based combination therapies underlines the need for further study to ascertain possible differences in efficacy between the two approaches.

Should we, and how to, optimize cefiderocol administration during severe nosocomial pneumonia due to carbapenem-resistant Acinetobacter baumanii? A viewpoint

OBJECTIVES

Acinetobacter baumannii is classified by the centre for Disease Control and Prevention (CDC) as an "urgent threat" due to its ability to acquire and develop resistance to multiple classes of antibiotics. As a result, it is one of the most concerning pathogens in healthcare settings, with increasing incidence of infections due to carbapenem-resistant Acinetobacter baumannii (CRAB) associated with high morbidity and mortality rates. Therefore, there are ongoing efforts to find novel treatment options, one of which is cefiderocol. We aim to review available evidence on cefiderocol use for severe nosocomial pneumonia due to carbapenem-resistant Acinetobacter baumannii.

METHODS

A comprehensive review was conducted from 2017 to 2023, covering articles from databases such as Pubmed, Scopus, and Embase, along with conference proceedings from ECCMID 2023. The primary focus was on severe nosocomial pneumonia due A. baumannii and cefiderocol.

DISCUSSION

Cefiderocol, targeting periplasmic space Penicillin-Binding Proteins (PBPs) via siderophore transport pathways, exhibits promise against multi-drug resistant Gram-negative bacilli. Its effectiveness in treating CRAB pneumonia remains debated. The CREDIBLE trial reported higher mortality with cefiderocol compared to the best available treatment, while other cohort studies showed contrasting outcomes. Patient variations and pharmacokinetic factors may underlie these discrepancies. The recommended cefiderocol dosage regimen may fall short of desired pharmacokinetic targets, especially in critically ill patients and lung infections. Pulmonary factors hindering cefiderocol's entry into bacteria through iron transporters are overlooked in clinical breakpoints. Optimized dosing or combination regimens may enhance infection site exposure and outcomes.

CONCLUSIONS

Further research is needed to determine the optimal cefiderocol dosage and administration (mono vs. dual therapy, continuous vs. intermittent infusion), in severe Acinetobacter baumannii nosocomial pneumonia.

Commercially available tests for determining cefiderocol susceptibility display variable performance in the Achromobacter genus

BACKGROUND

Cefiderocol is a siderophore-conjugated cephalosporin increasingly used in the management of Achromobacter infections. Testing for cefiderocol susceptibility is challenging with distinct recommendations depending on the pathogens.

OBJECTIVES

We evaluated the performance of commercial tests for testing cefiderocol susceptibility in the Achromobacter genus and reviewed the literature.

METHODS

Diffusion (disks, MIC gradient test strips [MTS], Liofilchem) and broth microdilution (BMD) methods (ComASP™, Liofilchem; UMIC®, Bruker) were compared with the BMD reference method according to the EUCAST guidelines on 143 Achromobacter strains from 14 species with MIC50/90 of ≤ 0.015/0.5 mg/L. A literature search was conducted regardless of method or species.

RESULTS

None of the methods tested fulfilled an acceptable essential agreement (EA). MTS displayed the lowest EA (30.8%) after UMIC® (49%) and ComASP™ (76.9%). All methods achieved an acceptable bias, with MICs either underestimated using MTS (-1.3%) and ComASP™ (-14.2%) or overestimated with UMIC® (+ 9.1%). Inhibition zone diameters ranged from 6 to 38 mm (IZD50/90=33/30 mm). UMIC® and ComASP™ failed to categorize one or the two cefiderocol-resistant strains of this study as resistant unlike the diffusion-based methods. The literature review highlighted distinct performance of the available methods according to pathogens and testing conditions.

CONCLUSIONS

The use of MTS is discouraged for Achromobacter spp. Disk diffusion can be used to screen for susceptible strains by setting a threshold diameter of 30 mm. UMIC® and ComASP™ should not be used as the sole method but have to be systematically associated with disk diffusion to detect the yet rarely described cefiderocol-resistant Achromobacter sp. strains.

Host defence peptide plectasin targets bacterial cell wall precursor lipid II by a calcium-sensitive supramolecular mechanism

Antimicrobial resistance is a leading cause of mortality, calling for the development of new antibiotics. The fungal antibiotic plectasin is a eukaryotic host defence peptide that blocks bacterial cell wall synthesis. Here, using a combination of solid-state nuclear magnetic resonance, atomic force microscopy and activity assays, we show that plectasin uses a calcium-sensitive supramolecular killing mechanism. Efficient and selective binding of the target lipid II, a cell wall precursor with an irreplaceable pyrophosphate, is achieved by the oligomerization of plectasin into dense supra-structures that only form on bacterial membranes that comprise lipid II. Oligomerization and target binding of plectasin are interdependent and are enhanced by the coordination of calcium ions to plectasin's prominent anionic patch, causing allosteric changes that markedly improve the activity of the antibiotic. Structural knowledge of how host defence peptides impair cell wall synthesis will likely enable the development of superior drug candidates.

Cefiderocol susceptibility of Achromobacter spp.: study of an accurately identified collection of 230 strains

BACKGROUND

Achromobacter spp. are opportunistic pathogens, mostly infecting immunocompromised patients and patients with cystic fibrosis (CF) and considered as difficult-to-treat pathogens due to both intrinsic resistance and the possibility of acquired antimicrobial resistance. Species identification remains challenging leading to imprecise descriptions of resistance in each taxon. Cefiderocol is a broad-spectrum siderophore cephalosporin increasingly used in the management of Achromobacter infections for which susceptibility data remain scarce. We aimed to describe the susceptibility to cefiderocol of a collection of Achromobacter strains encompassing different species and isolation sources from CF or non-CF (NCF) patients.

METHODS

We studied 230 Achromobacter strains (67 from CF, 163 from NCF patients) identified by nrdA gene-based analysis, with available susceptibility data for piperacillin-tazobactam, meropenem and trimethoprim-sulfamethoxazole. Minimal inhibitory concentrations (MICs) of cefiderocol were determined using the broth microdilution reference method according to EUCAST guidelines.

RESULTS

Strains belonged to 15 species. A. xylosoxidans represented the main species (71.3%). MICs ranged from ≤ 0.015 to 16 mg/L with MIC50/90 of ≤ 0.015/0.5 mg/L overall and 0.125/2 mg/L against 27 (11.7%) meropenem-non-susceptible strains. Cefiderocol MICs were not related to CF/NCF origin or species although A. xylosoxidans MICs were statistically lower than those of other species considered as a whole. Considering the EUCAST non-species related breakpoint (2 mg/L), 228 strains (99.1%) were susceptible to cefiderocol. The two cefiderocol-resistant strains (A. xylosoxidans from CF patients) represented 3.7% of meropenem-non-susceptible strains and 12.5% of MDR strains.

CONCLUSIONS

Cefiderocol exhibited excellent in vitro activity against a large collection of accurately identified Achromobacter strains, irrespective of species and origin.

Cefiderocol for treating severe aerobic Gram-negative bacterial infections: technology evaluation to inform a novel subscription-style payment model

Background

To limit the use of antimicrobials without disincentivising the development of novel antimicrobials, there is interest in establishing innovative models that fund antimicrobials based on an evaluation of their value as opposed to the volumes used. The aim of this project was to evaluate the population-level health benefit of cefiderocol in the NHS in England, for the treatment of severe aerobic Gram-negative bacterial infections when used within its licensed indications. The results were used to inform the National Institute for Health and Care Excellence guidance in support of commercial discussions regarding contract value between the manufacturer and NHS England.

Methods

The health benefit of cefiderocol was first derived for a series of high-value clinical scenarios. These represented uses that were expected to have a significant impact on patients' mortality risks and health-related quality of life. The clinical effectiveness of cefiderocol relative to its comparators was estimated by synthesising evidence on susceptibility of the pathogens of interest to the antimicrobials in a network meta-analysis. Patient-level costs and health outcomes of cefiderocol under various usage scenarios compared with alternative management strategies were quantified using decision modelling. Results were reported as incremental net health effects expressed in quality-adjusted life-years, which were scaled to 20-year population values using infection number forecasts based on data from Public Health England. The outcomes estimated for the high-value clinical scenarios were extrapolated to other expected uses for cefiderocol.

Results

Among Enterobacterales isolates with the metallo-beta-lactamase resistance mechanism, the base-case network meta-analysis found that cefiderocol was associated with a lower susceptibility relative to colistin (odds ratio 0.32, 95% credible intervals 0.04 to 2.47), but the result was not statistically significant. The other treatments were also associated with lower susceptibility than colistin, but the results were not statistically significant. In the metallo-beta-lactamase Pseudomonas aeruginosa base-case network meta-analysis, cefiderocol was associated with a lower susceptibility relative to colistin (odds ratio 0.44, 95% credible intervals 0.03 to 3.94), but the result was not statistically significant. The other treatments were associated with no susceptibility. In the base case, patient-level benefit of cefiderocol was between 0.02 and 0.15 quality-adjusted life-years, depending on the site of infection, the pathogen and the usage scenario. There was a high degree of uncertainty surrounding the benefits of cefiderocol across all subgroups. There was substantial uncertainty in the number of infections that are suitable for treatment with cefiderocol, so population-level results are presented for a range of scenarios for the current infection numbers, the expected increases in infections over time and rates of emergence of resistance. The population-level benefits varied substantially across the base-case scenarios, from 896 to 3559 quality-adjusted life-years over 20 years.

Conclusion

This work has provided quantitative estimates of the value of cefiderocol within its areas of expected usage within the NHS.

Limitations

Given existing evidence, the estimates of the value of cefiderocol are highly uncertain.

Future work

Future evaluations of antimicrobials would benefit from improvements to NHS data linkages; research to support appropriate synthesis of susceptibility studies; and application of routine data and decision modelling to assess enablement value.

Study registration

No registration of this study was undertaken.

Funding

This award was funded by the National Institute for Health and Care Research (NIHR) Health Technology Assessment Policy Research Programme (NIHR award ref: NIHR135591), conducted through the Policy Research Unit in Economic Methods of Evaluation in Health and Social Care Interventions, PR-PRU-1217-20401, and is published in full in Health Technology Assessment; Vol. 28, No. 28. See the NIHR Funding and Awards website for further award information.

Discovery of YFJ-36: Design, Synthesis, and Antibacterial Activities of Catechol-Conjugated β-Lactams against Gram-Negative Bacteria

Cefiderocol is the first approved catechol-conjugated cephalosporin against multidrug-resistant Gram-negative bacteria, while its application was limited by poor chemical stability associated with the pyrrolidinium linker, moderate potency against Klebsiella pneumoniae and Acinetobacter baumannii, intricate procedures for salt preparation, and potential hypersensitivity. To address these issues, a series of novel catechol-conjugated derivatives were designed, synthesized, and evaluated. Extensive structure-activity relationships and structure-metabolism relationships (SMR) were conducted, leading to the discovery of a promising compound 86b (Code no. YFJ-36) with a new thioether linker. 86b exhibited superior and broad-spectrum in vitro antibacterial activity, especially against A. baumannii and K. pneumoniae, compared with cefiderocol. Potent in vivo efficacy was observed in a murine systemic infection model. Furthermore, the physicochemical stability of 86b in fluid medium at pH 6-8 was enhanced. 86b also reduced potential the risk of allergy owing to the quaternary ammonium linker. The improved properties of 86b supported its further research and development.

Understanding blaNDM-1 gene regulation in CRKP infections: toward novel antimicrobial strategies for hospital-acquired pneumonia

BACKGROUND

The escalating challenge of Carbapenem-resistant Klebsiella pneumoniae (CRKP) in hospital-acquired pneumonia (HAP) is closely linked to the blaNDM-1 gene. This study explores the regulatory mechanisms of blaNDM-1 expression and aims to enhance antibacterial tactics to counteract the spread and infection of resistant bacteria.

METHODS

KP and CRKP strains were isolated from HAP patients' blood samples. Transcriptomic sequencing (RNA-seq) identified significant upregulation of blaNDM-1 gene expression in CRKP strains. Bioinformatics analysis revealed blaNDM-1 gene involvement in beta-lactam resistance pathways. CRISPR-Cas9 was used to delete the blaNDM-1 gene, restoring sensitivity. In vitro and in vivo experiments demonstrated enhanced efficacy with Imipenem and Thanatin or Subatan combination therapy.

RESULTS

KP and CRKP strains were isolated with significant upregulation of blaNDM-1 in CRKP strains identified by RNA-seq. The Beta-lactam resistance pathway was implicated in bioinformatics analysis. Knockout of blaNDM-1 reinstated sensitivity in CRKP strains. Further, co-treatment with Imipenem, Thanatin, or Subactam markedly improved antimicrobial effectiveness.

CONCLUSION

Silencing blaNDM-1 in CRKP strains from HAP patients weakens their Carbapenem resistance and optimizes antibacterial strategies. These results provide new theoretical insights and practical methods for treating resistant bacterial infections.

Novel resistance ICEs carrying the blaFIM-1 metallo-β-lactamase gene from an ST235 Pseudomonas aeruginosa sublineage

FIM-1 is an acquired metallo-β-lactamase identified in a multidrug-resistant Pseudomonas aeruginosa (index strain FI-14/157) of clinical origin isolated in 2007 in Florence, Italy. Here we report on a second case of infection by FIM-1-positive P. aeruginosa (FI-17645), which occurred in 2020 in the same hospital. Both FIM-1-positive strains exhibited resistance to all anti-Pseudomonas antibiotics except colistin and cefiderocol. Comparative genomic characterization revealed that the two FIM-positive strains were closely related [core genome difference, 16 single nucleotide polymorphisms (SNPs)], suggesting a local circulation of similar strains. In the FI-14/157 index strain, the blaFIM-1 gene was associated with an ISCR19-like element that likely contributed to its capture downstream an integron platform inserted aboard a Tn21-like transposon, named Tn7703.1, which was associated with a large integrative and conjugative element (ICE) named ICE7705.1, integrated into an att site located within the 3'-end of tRNAGly CCC gene of the P. aeruginosa chromosome. In strain FI-17645, blaFIM-1 was associated with a closely related ICE, named ICE7705.2, integrated in the same chromosomal site. Similar ICE platforms, lacking the blaFIM-1-containing region, were detected in other ST235 P. aeruginosa strains from different geographic areas, suggesting a common ancestry and underscoring the role of these elements in the dissemination of resistance genes in P. aeruginosa. Sequence database mining revealed two draft P. aeruginosa genomes, one from Italy and one from the USA (both isolated in 2012), including a contig with blaFIM-1, suggesting that this resistance gene could have a broader distribution than originally anticipated.

Reduced susceptibility to aztreonam-avibactam conferred by acquired AmpC-type β-lactamases in PBP3-modified Escherichia coli

PURPOSE

Carbapenemase-producing Enterobacterales are a growing threat, and very few therapeutic options remain active against those multidrug resistant bacteria. Aztreonam is the molecule of choice against metallo-beta-lactamases (MBL) producers since it is not hydrolyzed by those enzymes, but the co-production of acquired plasmidic cephalosporinases or extended-spectrum β-lactamases leading to aztreonam resistance may reduce the efficacy of this molecule. Hence, the development of the aztreonam-avibactam (AZA) combination provides an interesting therapeutic alternative since avibactam inhibits the activity of both cephalosporinases and extended-spectrum β-lactamases. However, structural modifications of penicillin binding protein PBP3, the target of aztreonam, may lead to reduced susceptibility to aztreonam-avibactam.

METHODS

Here the impact of various plasmid-encoded AmpC-type β-lactamases (ACC-1, ACT-7, ACT-17, CMY-2, CMY-42, DHA-1, FOX-1, and FOX-5) on susceptibility to aztreonam-avibactam was evaluated using isogenic E. coli MG1655 strains harboring insertions in PBP3 (YRIN and YRIK). The inhibitory activity of various β-lactamase inhibitors (clavulanic acid, tazobactam, avibactam, relebactam, and vaborbactam) were also compared against these enzymes.

RESULTS

Hence, we showed that reduced susceptibility to AZA was due to the combined effect of both AmpC production and amino acid insertions in PBP3. The highest resistance level was achieved in strains possessing the insertions in PBP3 in association with the production of ACT-7, ACC-1, or CMY-42.

CONCLUSION

Although none of the recombinant strains tested displayed clinical resistance to aztreonam-avibactam, our data emphasize that the occurrence of such profile might be of clinical relevance for MBL-producing strains.

Global prevalence of cefiderocol non-susceptibility in Enterobacterales, Pseudomonas aeruginosa, Acinetobacter baumannii, and Stenotrophomonas maltophilia: a systematic review and meta-analysis

BACKGROUND

Cefiderocol is a last resort option for carbapenem-resistant (CR) Gram-negative bacteria, especially metallo-β-lactamase-producing Pseudomonas aeruginosa and CR Acinetobacter baumannii. Monitoring global levels of cefiderocol non-susceptibility (CFDC-NS) is important.

OBJECTIVES

To systematically collate and examine studies investigating in vitro CFDC-NS and estimate the global prevalence of CFDC-NS against major Gram-negative pathogens.

DATA SOURCES

PubMed and Scopus, up to May 2023.

STUDY ELIGIBILITY CRITERIA

Eligible were studies reporting CFDC-NS in Enterobacterales, P. aeruginosa, A. baumannii, or Stenotrophomonas maltophilia clinical isolates.

RISK-OF-BIAS ASSESSMENT

Two independent reviewers extracted study data and assessed the risk of bias on the population, setting, and measurement (susceptibility testing) domains.

DATA SYNTHESIS

Binomial-Normal mixed-effects models were applied to estimate CFDC-NS prevalence by species, coresistance phenotype, and breakpoint definition (EUCAST, CLSI, and FDA). Sources of heterogeneity were investigated by subgroup and meta-regression analyses.

RESULTS

In all, 78 studies reporting 82 035 clinical isolates were analysed (87% published between 2020 and 2023). CFDC-NS prevalence (EUCAST breakpoints) was low overall but varied by species (S. maltophilia 0.4% [95% CI 0.2-0.7%], Enterobacterales 3.0% [95% CI 1.5-6.0%], P. aeruginosa 1.4% [95% CI 0.5-4.0%]) and was highest for A. baumannii (8.8%, 95% CI 4.9-15.2%). CFDC-NS was much higher in CR Enterobacterales (12.4%, 95% CI 7.3-20.0%) and CR A. baumannii (13.2%, 95% CI 7.8-21.5%), but relatively low for CR P. aeruginosa (3.5%, 95% CI 1.6-7.8%). CFDC-NS was exceedingly high in New Delhi metallo-β-lactamase-producing Enterobacterales (38.8%, 95% CI 22.6-58.0%), New Delhi metallo-β-lactamase-producing A. baumannii (44.7%, 95% CI 34.5-55.4%), and ceftazidime/avibactam-resistant Enterobacterales (36.6%, 95% CI 22.7-53.1%). CFDC-NS varied considerably with breakpoint definition, predominantly among CR bacteria. Additional sources of heterogeneity were single-centre investigations and geographical regions.

CONCLUSIONS

CFDC-NS prevalence is low overall, but alarmingly high for specific CR phenotypes circulating in some institutions or regions. Continuous surveillance and updating of global CFDC-NS estimates are imperative while cefiderocol is increasingly introduced into clinical practice. The need to harmonize EUCAST and CLSI breakpoints was evident.

Multidrug-resistant Gram-negative clinical isolates with reduced susceptibility/resistance to cefiderocol: which are the best present and future therapeutic alternatives?

PURPOSE

To evaluate the different present and future therapeutic β-lactam/β-lactamase inhibitor (BL/BLI) alternatives, namely aztreonam-avibactam, imipenem-relebactam, meropenem-vaborbactam, cefepime-zidebactam, cefepime-taniborbactam, meropenem-nacubactam, and sulbactam-durlobactam against clinical isolates showing reduced susceptibility or resistance to cefiderocol in Enterobacterales, Acinetobacter baumannii, and Pseudomonas aeruginosa.

METHODS

MIC values of aztreonam, aztreonam-avibactam, cefepime, cefepime-taniborbactam, cefepime-zidebactam, imipenem, imipenem-relebactam, meropenem, meropenem-vaborbactam, meropenem-nacubactam, sulbactam-durlobactam, and cefiderocol combined with a BLI were determined for 67, 9, and 11 clinical Enterobacterales, P. aeruginosa or A. baumannii isolates, respectively, showing MIC values of cefiderocol being ≥1 mg/L. If unavailable, the respective β-lactam breakpoints according to EUCAST were used for BL/BLI combinations.

RESULTS

For Enterobacterales, the susceptibility rates for aztreonam, cefepime, imipenem, and meropenem were 7.5%, 0%, 10.4%, and 10.4%, respectively, while they were much higher for cefepime-zidebactam (91%), cefiderocol-zidebactam (91%), meropenem-nacubactam (71.6%), cefiderocol-nacubactam (74.6%), and cefiderocol-taniborbactam (76.1%), as expected. For P. aeruginosa isolates, the higher susceptibility rates were observed for imipenem-relebactam, cefiderocol-zidebactam, and meropenem-vaborbactam (56% for all combinations). For A. baumannii isolates, lower susceptibility rates were observed with commercially or under development BL/BLI combos; however, a high susceptibility rate (70%) was found for sulbactam-durlobactam and when cefiderocol was associated to some BLIs.

CONCLUSIONS

Zidebactam- and nacubactam-containing combinations showed a significant in vitro activity against multidrug-resistant Enterobacterales clinical isolates with reduced susceptibility to cefiderocol. On the other hand, imipenem-relebactam and meropenem-vaborbactam showed the highest susceptibility rates against P. aeruginosa isolates. Finally, sulbactam-durlobactam and cefiderocol combined with a BLI were the only effective options against A. baumannii tested isolates.

Role of transcriptomic and genomic analyses in improving the comprehension of cefiderocol activity in Acinetobacter baumannii

The mechanisms of action and resistance of cefiderocol (FDC) in Acinetobacter baumannii are still not fully elucidated, but iron transport systems have been evoked in its entry into the cell to reach the penicillin-binding proteins (PBPs). To capture the dynamics of gene expression related to FDC action in various conditions, we report on the genomic and transcriptomic features of seven A. baumannii strains with different FDC susceptibility, focusing on the variants in genes associated with β-lactam resistance and the expression of the siderophore biosynthesis and transport systems acinetobactin and baumannoferrin. We also investigated the expression of the TonB energy transduction system (ETS) and siderophore receptors piuA and pirA. The four clinical samples belonged to the same clonal complex (CC2), and the two strains with the highest FDC MICs showed peculiar variants in PBP2 and ampC. Similarly, the two clinical strains with the lowest MICs shared variants in an outer membrane protein as well as ampC. Gene expression analyses highlighted the up-regulation of the acinetobactin and baumannoferrin genes in response to iron depletion and a down-regulation in the presence of high iron concentrations. In response to FDC, gene expression seemed strain-dependent, probably due to the different metabolic features of each strain. Overall, FDC activates the ETS, confirming the active import of the drug; baumannoferrin, more than acinetobactin, appeared stimulated by FDC in an iron-depleted medium. In conclusion, iron transport systems play a clear role in the FDC uptake, and their expression likely contributes to MIC variation together with β-lactam resistance determinants.IMPORTANCEAcinetobacter baumannii poses a threat to healthcare due to its ability to give difficult-to-treat infections as a consequence of our shortage of antibiotic molecules active on this multidrug-resistant bacterium. Cefiderocol (FDC) represents one of the few drugs active on A. baumannii, and to preserve its activity, this study explored the transcriptomic and genomic features of seven strains with varying susceptibility to FDC. Transcriptomic analyses revealed the different effects of FDC on iron transport systems, promoting mainly baumannoferrin expression-thus more likely related to FDC entry-and the energy transduction systems. These findings suggest that not all iron transport systems are equally involved in FDC entry into A. baumannii cells. Finally, mutations in PBPs and β-lactamases may contribute to the resistance onset. Overall, the study sheds light on the importance of iron availability and metabolic differences in FDC resistance, offering insights into understanding the evolution of resistance in A. baumannii strains.

Impact of pH on the activity of novel cephalosporin cefiderocol in human urine

BACKGROUND

Antimicrobial activity of antibiotics can be impacted by pH, enhancing or reducing their bactericidal properties. Cefiderocol, a novel cephalosporin antibiotic that is among others indicated for the treatment of complicated urinary tract infections (cUTIs), lacks data on activity in urine.

METHODS

Pooled human urine (iron levels ∼0.05 mg/L/24 h), CAMHB and iron-depleted CAMHB (ID-CAMHB) at pH 5, 7 and 8 served as media. MIC testing was done according to EUCAST with the broth microdilution method for 17 clinical isolates of Escherichia coli and ATCC 25922 (including isolates with ESBL activity), 17 clinical isolates of Klebsiella pneumoniae and ATCC 700603 (also with ESBL), and 6 clinical isolates of Pseudomonas aeruginosa and ATCC 27853. Time-kill curves (TKCs) were performed for selected strains at pH 5, 7 and 8 in urine.

RESULTS

MIC values in urine, CAMHB and ID-CAMHB exhibited isolate-specific variations when assessed under identical pH conditions, ranging from a 1-fold dilution to changes of up to 4-fold dilutions in either direction. Median MICs of cefiderocol were up to 50-fold higher in pH 5 than in pH 7 for P. aeruginosa isolates and 32-fold higher in E. coli and K. pneumoniae isolates. TKCs with 650 and 1300 mg/L cefiderocol in urine showed that ATCC strains were efficiently eradicated despite the pH set.

CONCLUSIONS

Acidic pH had a significant negative impact on cefiderocol activity. Yet, after a recommended IV administration of 2 g cefiderocol every 8 h, a concentration of approximately 1300 mg/L can be achieved in urine, suggesting that efficient killing of all tested pathogens could have been possible even under acidic conditions in vivo.

Functional features of KPC-109, a novel 270-loop KPC-3 mutant mediating resistance to avibactam-based β-lactamase inhibitor combinations and cefiderocol

OBJECTIVES

To investigate a ceftazidime/avibactam (CZA)-resistant Klebsiella pneumoniae (NE368), isolated from a patient exposed to CZA, expressing a novel K. pneumoniae carbapenemase (KPC)-3 variant (KPC-109).

METHODS

Antimicrobial susceptibility testing was performed by reference broth microdilution. Whole-genome sequencing (WGS) analysis of NE368 was performed combining a short- and long-reads approach (Illumina and Oxford Nanopore Technologies). Functional characterization of KPC-109 was performed to investigate the impact of KPC-109 production on the β-lactam resistance phenotype of various Escherichia coli and Klebsiella pneumoniae strains, including derivatives of K. pneumoniae with OmpK35 and OmpK36 porin alterations. Horizontal transfer of the KPC-109-encoding plasmid was investigated by conjugation and transformation experiments.

RESULTS

K. pneumoniae NE368 was isolated from a patient after repeated CZA exposure, and showed resistance to CZA, fluoroquinolones, piperacillin/tazobactam, expanded-spectrum cephalosporins, amikacin, carbapenems and cefiderocol. WGS revealed the presence of a large chimeric plasmid of original structure (pKPN-NE368), encoding a novel 270-loop mutated KPC-3 variant (KPC-109; ins_270_KYNKDD). KPC-109 production mediated resistance/decreased susceptibility to avibactam-based combinations (with ceftazidime, cefepime and aztreonam) and cefiderocol, with a trade-off on carbapenem resistance. However, in the presence of porin alterations commonly encountered in high-risk clonal lineages of K. pneumoniae, KPC-109 was also able to confer clinical-level resistance to carbapenems. Resistance of NE368 to cefiderocol was likely contributed by KPC-109 production acting in concert with a mutated EnvZ sensor kinase. The KPC-109-encoding plasmid did not appear to be conjugative.

CONCLUSIONS

These findings expand current knowledge about the diversity of emerging KPC enzyme variants with 270-loop alterations that can be encountered in the clinical setting.

Cefiderocol and Sulbactam-Durlobactam against Carbapenem-Resistant Acinetobacter baumannii

Infections caused by carbapenem-resistant Acinetobacter baumannii (CRAB) remain a clinical challenge due to limited treatment options. Recently, cefiderocol, a novel siderophore cephalosporin, and sulbactam-durlobactam, a bactericidal β-lactam-β-lactamase inhibitor combination, have been approved by the Food and Drug Administration for the treatment of A. baumannii infections. In this review, we discuss the mechanisms of action of and resistance to cefiderocol and sulbactam-durlobactam, the antimicrobial susceptibility of A. baumannii isolates to these drugs, as well as the clinical effectiveness of cefiderocol and sulbactam/durlobactam-based regimens against CRAB. Overall, cefiderocol and sulbactam-durlobactam show an excellent antimicrobial activity against CRAB. The review of clinical studies evaluating the efficacy of cefiderocol therapy against CRAB indicates it is non-inferior to colistin/other treatments for CRAB infections, with a better safety profile. Combination treatment is not associated with improved outcomes compared to monotherapy. Higher mortality rates are often associated with prior patient comorbidities and the severity of the underlying infection. Regarding sulbactam-durlobactam, current data from the pivotal clinical trial and case reports suggest this antibiotic combination could be a valuable option in critically ill patients affected by CRAB infections, in particular where no other antibiotic appears to be effective.

Global Phylogeography and Genomic Epidemiology of Carbapenem-Resistant blaOXA-232-Carrying Klebsiella pneumoniae Sequence Type 15 Lineage

Prevalence of carbapenem-resistant Klebsiella pneumoniae (CRKP) has compromised antimicrobial efficacy against severe infections worldwide. To monitor global spread, we conducted a comprehensive genomic epidemiologic study comparing sequences from 21 blaOXA-232-carrying CRKP isolates from China with K. pneumoniae sequence type (ST) 15 strains from 68 countries available in GenBank. Phylogenetic and phylogeographic analyses revealed all blaOXA-232-carrying CRKP isolates belonged to ST15 lineage and exhibited multidrug resistance. Analysis grouped 330 global blaOXA-232-carrying ST15 CRKP strains into 5 clades, indicating clonal transmission with small genetic distances among multiple strains. The lineage originated in the United States, then spread to Europe, Asia, Oceania, and Africa. Most recent common ancestor was traced back to 2000; mutations averaged ≈1.7 per year per genome. Our research helps identify key forces driving global spread of blaOXA-232-carrying CRKP ST15 lineage and emphasizes the importance of ongoing surveillance of epidemic CRKP.

A nested cohort 5-year Canadian surveillance of Gram-negative antimicrobial resistance for optimized antimicrobial therapy

We analyzed 5 years (2016-2020) of nested Canadian data from the Study for Monitoring Antimicrobial Resistance Trends (SMART) to identify pathogen predominance and antimicrobial resistance (AMR) patterns of adult Gram-negative infections in Canadian health care and to complement other public surveillance programs and studies in Canada. A total of 6853 isolates were analyzed from medical (44%), surgical (18%), intensive care (22%) and emergency units (15%) and from respiratory tract (36%), intra-abdominal (25%), urinary tract (24%) and bloodstream (15%) infections. Overall, E. coli (36%), P. aeruginosa (18%) and K. pneumoniae (12%) were the most frequent isolates and P. aeruginosa was the most common respiratory pathogen. 18% of Enterobacterales species were ESBL positive. Collective susceptibility profiles showed that P. aeruginosa isolates were highly susceptible (> 95%) to ceftolozane/tazobactam and colistin, though markedly less susceptible (58-74%) to other antimicrobials tested. Multi-drug resistance (MDR) was present in 10% of P. aeruginosa isolates and was more frequent in those from respiratory infections and from ICU than non-ICU locations. Of P. aeruginosa isolates that were resistant to combinations of ceftazidime, piperacillin/tazobactam and meropenem, 73-96% were susceptible to ceftolozane/tazobactam over the period of the study. These national data can now be combined with clinical prediction rules and genomic data to enable expert antimicrobial stewardship applications and guide treatment policies to optimize adult patient care.

A Selective Culture Medium for Screening Cefiderocol Resistance in Enterobacterales, Pseudomonas aeruginosa, and Acinetobacter baumannii

Cefiderocol (FDC) is a siderophore cephalosporin with a broad spectrum of activity against many multidrug-resistant Gram-negative bacteria. Acquired resistance to FDC has been already reported among Gram-negative isolates, thus highlighting the need for rapid and accurate identification of such resistant pathogens, in order to control their spread. Therefore, the SuperFDC medium was developed to screen FDC-resistant Enterobacterales, Pseudomonas aeruginosa, and Acinetobacter baumannii. After testing several culture conditions, a selective medium was set up by supplementing an iron-depleted agar medium with 8 μg/mL of FDC and evaluated with a collection of 68 FDC-susceptible and 33 FDC-resistant Gram-negative isolates exhibiting a variety of β-lactam resistance mechanisms. The sensitivity and specificity of detection of this medium were evaluated at 97% and 100%, respectively. In comparison with the reference broth microdilution method, only 3% very major errors were found. In addition, excellent detection performances were obtained by testing spiked stools with a lower limit of detection ranging between 100 and 103 CFU/mL. The SuperFDC medium allows detection of FDC-resistant Gram-negative isolates regardless of their corresponding resistance mechanisms.

Treatment of critically ill patients with cefiderocol for infections caused by multidrug-resistant pathogens: review of the evidence

Appropriate antibiotic treatment for critically ill patients with serious Gram-negative infections in the intensive care unit is crucial to minimize morbidity and mortality. Several new antibiotics have shown in vitro activity against carbapenem-resistant Enterobacterales (CRE) and difficult-to-treat resistant Pseudomonas aeruginosa. Cefiderocol is the first approved siderophore beta-lactam antibiotic with potent activity against multidrug-resistant, carbapenem-resistant, difficult-to-treat or extensively drug-resistant Gram-negative pathogens, which have limited treatment options. The spectrum of activity of cefiderocol includes drug-resistant strains of Acinetobacter baumannii, P. aeruginosa, Stenotrophomonas maltophilia, Achromobacter spp. and Burkholderia spp. and CRE that produce serine- and/or metallo-carbapenemases. Phase 1 studies established that cefiderocol achieves adequate concentration in the epithelial lining fluid in the lung and requires dosing adjustment for renal function, including patients with augmented renal clearance and continuous renal-replacement therapy (CRRT); no clinically significant drug-drug interactions are expected. The non-inferiority of cefiderocol versus high-dose, extended-infusion meropenem in all-cause mortality (ACM) rates at day 14 was demonstrated in the randomized, double-blind APEKS-NP Phase 3 clinical study in patients with nosocomial pneumonia caused by suspected or confirmed Gram-negative bacteria. Furthermore, the efficacy of cefiderocol was investigated in the randomized, open-label, pathogen-focused, descriptive CREDIBLE-CR Phase 3 clinical study in its target patient population with serious carbapenem-resistant Gram-negative infections, including hospitalized patients with nosocomial pneumonia, bloodstream infection/sepsis, or complicated urinary tract infections. However, a numerically greater ACM rate with cefiderocol compared with BAT led to the inclusion of a warning in US and European prescribing information. Cefiderocol susceptibility results obtained with commercial tests should be carefully evaluated due to current issues regarding their accuracy and reliability. Since its approval, real-world evidence in patients with multidrug-resistant and carbapenem-resistant Gram-negative bacterial infections suggests that cefiderocol can be efficacious in certain critically ill patient groups, such as those requiring mechanical ventilation for COVID-19 pneumonia with subsequently acquired Gram-negative bacterial superinfection, and patients with CRRT and/or extracorporeal membrane oxygenation. In this article, we review the microbiological spectrum, pharmacokinetics/pharmacodynamics, efficacy and safety profiles and real-world evidence for cefiderocol, and look at future considerations for its role in the treatment of critically ill patients with challenging Gram-negative bacterial infections.

Siderophore conjugates to combat antibiotic-resistant bacteria

Antimicrobial resistance (AMR) is a global threat to society due to the increasing emergence of multi-drug resistant bacteria that are not susceptible to our last line of defence antibiotics. Exacerbating this issue is a severe gap in antibiotic development, with no new clinically relevant classes of antibiotics developed in the last two decades. The combination of the rapidly increasing emergence of resistance and scarcity of new antibiotics in the clinical pipeline means there is an urgent need for new efficacious treatment strategies. One promising solution, known as the 'Trojan horse' approach, hijacks the iron transport system of bacteria to deliver antibiotics directly into cells - effectively tricking bacteria into killing themselves. This transport system uses natively produced siderophores, which are small molecules with a high affinity for iron. By linking antibiotics to siderophores, to make siderophore antibiotic conjugates, the activity of existing antibiotics can potentially be reinvigorated. The success of this strategy was recently exemplified with the clinical release of cefiderocol, a cephalosporin-siderophore conjugate with potent antibacterial activity against carbapenem-resistant and multi-drug resistant Gram-negative bacilli. This review discusses the recent advancements in siderophore antibiotic conjugates and the challenges associated with the design of these compounds that need to be overcome to deliver more efficacious therapeutics. Potential strategies have also been suggested for new generations of siderophore-antibiotics with enhanced activity.

Performance evaluation of the UMIC® Cefiderocol to determine MIC in Gram-negative bacteria

BACKGROUND

Cefiderocol is a catechol-substituted cephalosporin with potent in vitro activity against carbapenem-resistant (CR) Gram-negative bacteria (GNB). Cefiderocol susceptibility testing is complex because iron concentrations need to be taken into consideration. Here, we assessed the clinical performance of Bruker's UMIC® Cefiderocol and corresponding iron-depleted CAMHB to determine MIC by broth microdilution (BMD) for clinically relevant GNB.

METHODS

MICs of cefiderocol for 283 GN clinical isolates were determined by BMD using iron-depleted CAMHB. Frozen panels were used as a reference. The concentration range of cefiderocol was 0.03-32 mg/L. The isolates, with different degrees of susceptibility to cefiderocol, included Enterobacterales (n = 180), Pseudomonas aeruginosa (n = 49), Acinetobacter baumannii (n = 44) and Stenotrophomonas maltophilia (n = 10).

RESULTS

The rates of categorical agreement (CA), essential agreement (EA) and bias were calculated to evaluate the performance of the UMIC® Cefiderocol, as compared with the reference method. Overall, the UMIC® Cefiderocol showed 90.8% EA (95% CI: 86.9%-93.7%) with a bias of -14.5% and a CA of 90.1% (95% CI: 86.1%-93.1%). For Enterobacterales, the UMIC® Cefiderocol showed 91.7% EA (95% CI: 86.7%-94.9%) with a bias of -25.0% and a CA of 87.8% (95% CI: 82.2%-91.8%). For non-fermenters, the UMIC® Cefiderocol showed 89.3% EA (95% CI: 81.9%-93.9%) (not significantly different from 90.0%, Student t-test) with a bias of -3.9% and a CA of 94.2% (95% CI: 87.7%-97.3%).

CONCLUSIONS

UMIC® Cefiderocol is a valid method for the determination of cefiderocol MICs even if higher than expected discrepancies were observed with NDM-producing Enterobacterales, which presented in most cases MIC values close to the breakpoint.

Genomic Modification of TonB and Emergence of Small-Colony Phenotype in VIM- and NDM-Producing Escherichia coli following Cefiderocol Exposure In Vitro

Knowledge on resistance mechanisms toward cefiderocol, a novel siderophore-conjugated cephalosporin antibiotic, is still limited. Although the presence of New-Delhi metallo-β-lactamase has been demonstrated to facilitate the resistance development toward cefiderocol via siderophore receptor mutations in Enterobacter cloacae and Klebsiella pneumoniae, the impact of metallo-β-lactamases on facilitating such mutations in Escherichia coli is not yet elucidated. Our study aimed to study the effect of the presence of various β-lactamases, such as NDM-5, VIM-1, KPC-2, and OXA-48, on the development of cefiderocol resistance in E. coli. To this end, we performed liquid mating to transfer these β-lactamases onto a defined K-12 E. coli background (J53) and exposed these transconjugants to increasing cefiderocol concentrations in a serial passage experiment. Cefiderocol-resistant isolates were genotyped by whole-genome sequencing to investigate the underlying resistance mechanism. Cefiderocol-resistant isolates emerged only in isolates producing VIM-1 and NDM-5 metallo-β-lactamase, but not in those producing the serine β-lactamases KPC-2 and OXA-48. We observed two distinct morphological changes of the J53 E. coli strain exhibiting reduced colony size after insertions of transposable elements in the tonB gene leading to alterations in the TonB binding site and morphological changes consistent with the small-colony variant (SCV) phenotype due to mutations in the hemB and hemH genes. Passaging experiments suggested that these phenotypes were highly plastic. The SCV phenotype is attributed to immune evasion and decreased susceptibility toward antibiotics. The emergence of SCV following cefiderocol exposure may have clinical implications for bacterial clearance and warrants further investigation.

Molecular Signatures of the Eagle Effect Induced by the Artificial Siderophore Conjugate LP-600 in E. coli

Achieving cellular uptake is a central challenge for novel antibiotics targeting Gram-negative bacterial pathogens. One strategy is to hijack the bacterial iron transport system by siderophore-antibiotic conjugates that are actively imported into the cell. This was realized with the MECAM-ampicillin conjugate LP-600 we recently reported that was highly active against E. coli. In the present study, we investigate a paradoxical regrowth of E. coli upon treatment of LP-600 at concentrations 16-32 times above the minimum inhibitory concentration (MIC). The phenomenon, coined "Eagle-effect" in other systems, was not due to resistance formation, and it occurred for the siderophore conjugate but not for free ampicillin. To investigate the molecular imprint of the Eagle effect, a combined transcriptome and untargeted metabolome analysis was conducted. LP-600 induced the expression of genes involved in iron acquisition, SOS response, and the e14 prophage upon regrowth conditions. The Eagle effect was diminished in the presence of sulbactam, which we ascribe to a putative synergistic antibiotic action but not to β-lactamase inhibition. The study highlights the relevance of the Eagle effect for siderophore conjugates. Through the first systematic -omics investigations, it also demonstrates that the Eagle effect manifests not only in a paradoxical growth but also in unique gene expression and metabolite profiles.

Evaluation of Variability in Interpretation of Disk Diffusion Testing for Cefiderocol Using Different Brands of Mueller-Hinton Agar

BACKGROUND

Cefiderocol is a new antibiotic used to treat infections with antibiotic resistant Gram-negative bacilli. The impact of differences between Mueller-Hinton agar (MHA) brands on susceptibility testing is underexplored. Compounding the implementation of cefiderocol susceptibility testing is a lack of harmonization between different regulatory body breakpoint criteria.

METHODS

We performed Kirby-Bauer disk diffusion using BD, Hardy, and Remel MHA, in addition to broth microdilution for Acinetobacter baumannii (n = 25), Enterobacterales (n = 25), Stenotrophomonas maltophilia (n = 24), and Pseudomonas aeruginosa (n = 23). We analyzed disk diffusion diameters and minimum inhibitory concentrations using interpretive criteria from the Clinical and Laboratory Standards Institute (CLSI), US Food and Drug Administration (FDA), and the European Committee on Antimicrobial Susceptibility Testing (EUCAST).

RESULTS

Breakpoint criteria impacted interpretation of susceptibly testing results, for example with the broth microdilution we found 8% (2/25) of A. baumannii isolates change interpretation between CLSI and EUCAST and 32% (8/25) change between CLSI and FDA, 12% (3/25) of Enterobacterales change between CLSI and EUCAST, 13% (3/23) of P. aeruginosa interpretations change between CLSI and FDA, and 4% (1/25) S. maltophilia change between CLSI and FDA. There was a significant difference between the zone disk diffusion diameters for P. aeruginosa and S. maltophilia between Hardy and BD; which changed interpretation (using CLSI criteria) for 8.7% (2/23) for P. aeruginosa but 0% (0/24) for S. maltophilia.

CONCLUSIONS

Breakpoint criteria impact cefiderocol susceptibility testing interpretation for broth microdilution and disk diffusion. Choice of MHA brand can also affect result interpretation.

Progressive in vivo development of resistance to cefiderocol in Pseudomonas aeruginosa

We report in vivo development of cefiderocol (FDC) resistance among four sequential Pseudomonas aeruginosa clinical isolates ST244 recovered from a single patient, without exposure to FDC, which raises concern about the effectiveness of this novel drug. The first recovered P. aeruginosa isolate (P-01) was susceptible to FDC (2 μg/mL), albeit this MIC value was higher than that of a wild-type P. aeruginosa (0.12-0.25 μg/ml). The subsequent isolated strains (P-02, P-03, P-04) displayed increasing levels of FDC MICs (8, 16, and 64 μg/ml, respectively). Those isolates also showed variable and gradual increasing levels of resistance to most β-lactams tested in this study. Surprisingly, no acquired β-lactamase was identified in any of those isolates. Whole-genome sequence analysis suggested that this resistance was driven by multifactorial mechanisms including mutational changes in iron transporter proteins associated with FDC uptake, ampC gene overproduction, and mexAB-oprM overexpression. These findings highlight that a susceptibility testing to FDC must be performed prior to any prescription.

Performance of discs, sensititre EUMDROXF microplates and MTS gradient strips for the determination of the susceptibility of multidrug-resistant Pseudomonas aeruginosa to cefiderocol

OBJECTIVES

To evaluate the performance of commercially available tests to determine the susceptibility of multidrug-resistant (MDR) clinical Pseudomonas aeruginosa strains to cefiderocol.

METHODS

A collection of 150 clinical strains of P. aeruginosa resistant to ceftazidime, (MIC, Minimal Inhibitory Concentration, MIC > 8 mg/L) imipenem (MIC> 4 mg/L) and ceftolozane/tazobactam (MIC> 4/4 mg/L), isolated from 2015 to 2022 was selected. Cefiderocol susceptibility was determined in parallel (a) by disc diffusion using Mast, Oxoid and Liofilchem discs deposited on Mueller-Hinton agar batches from Bio-Rad, BioMérieux, Mast, Becton Dickinson, I2A and Oxoid; (b) by MIC gradient test strips (MTS) (Liofilchem); and (c) by EUMDROXF Sensititre microplates. MICs and inhibition zones were compared with the broth microdilution reference method (BMD) MICs.

RESULTS

The MIC₅₀ and MIC₉₀ of cefiderocol were 1 mg/L and 8 mg/L by BMD, respectively, including 21.3% (32/150) resistant strains. None of the methods tested fulfilled acceptable criteria (essential agreement [EA] ≥ 90%; bias = ± 30%). Although the Sensititre EUMDROXF microplates overestimated MIC values (categorical agreement [CA] = 86.7% [130/150, 95% CI 80.3-91.2]; EA = 69.3% [104/150, 95% CI 61.6-76.2]; bias = 68.2%), MTS strips underestimated the MIC values for many strains (CA = 86.7%, 130/150, 95% CI 80.3-91.2; EA = 69.3%, 104/150, 95% CI 61.6-76.2; bias = -30.4%), classifying properly only 50% (16/32) of resistant strains. Finally, many cefiderocol-resistant strains were not identified by the disc method, although the CA ranged from 78.0% (117/150, 95% CI 70.7-83.0) to 89.3% (134/150, 95% IC 83.4-93.3) according to Mueller-Hinton agar batches.

CONCLUSION

Determination of cefiderocol susceptibility in MDR P. aeruginosa clinical strains by Sensititre EUMDROXF microplates is an alternative to the reference BMD method. However, MIC values ± 1 dilution apart from the breakpoint (2 mg/L) should be controlled by BMD whereas the use of MTS gradient strips is discouraged. Disc diffusion might be useful for screening, unfortunately many cefiderocol-resistant strains are not detected.

Rapid cefiderocol NP test for detection of cefiderocol susceptibility/resistance in Enterobacterales

BACKGROUND

Cefiderocol is among the latest generation of commercialized antibiotics against a large variety of MDR Gram-negative bacteria including carbapenem-resistant Enterobacterales and non-fermenters such as Pseudomonas aeruginosa and Acinetobacter baumannii. Cefiderocol susceptibility testing, a key element for implementing rapidly a cefiderocol-based treatment, might be still challenging.

OBJECTIVES

To develop a rapid culture-based test, Rapid Cefiderocol NP test, for the identification of cefiderocol resistance among MDR Enterobacterales.

METHODS

The Rapid Cefiderocol NP test is based on glucose metabolization when bacterial growth occurs and the detection of bacterial growth in the presence of cefiderocol at 64 mg/L using iron-depleted CAMHB. Bacterial growth is visually detectable by a red-to-yellow colour change of red phenol, a pH indicator. A total of 74 clinical enterobacterial isolates from various clinical sources and of worldwide origin, among which 42 isolates were cefiderocol resistant, were used to evaluate the test performance.

RESULTS

The sensitivity and specificity of the test were found to be 98% and 91%, respectively, by comparison with the reference broth microdilution (BMD) method. All positive results were obtained within 3 h after incubation at 35°C ± 2°C, that is a gain of time of ca. 18 h (1 day) compared with currently used techniques for susceptibility testing (BMD method).

CONCLUSIONS

This novel test is rapid, highly sensitive, specific, easily interpretable, and easy to implement in routine microbiology laboratories. Such a test may rapidly and accurately provide the information needed for the implementation of adequate cefiderocol-based treatment.

Genomic Characterization of an Extensively Drug-Resistant Extra-Intestinal Pathogenic (ExPEC) Escherichia coli Clinical Isolate Co-Producing Two Carbapenemases and a 16S rRNA Methylase

An extensively drug-resistant Escherichia coli clinical isolate (N1606) belonging to Sequence Type 361 was recovered from the urine of a patient hospitalized in Switzerland. The strain showed resistance to virtually all β-lactams including the latest generation antibiotics cefiderocol and aztreonam-avibactam. Whole genome sequencing revealed that it possessed two carbapenemase-encoding genes, namely bla_NDM-5 and bla_KPC-3, and a series of additional β-lactamase genes, including bla_CTX-M-15 and bla_SHV-11 encoding extended-spectrum β-lactamases (ESBLs), bla_CMY-145 encoding an AmpC-type cephalosporinase, and bla_OXA-1 encoding a narrow-spectrum class D ß-lactamase. Most of these resistance genes were located on plasmids (IncFII-FIA, IncX3, IncIγ, IncFII). That strain exhibited also a four amino-acid insertion in its penicillin-binding protein 3 (PBP3) sequence, namely corresponding to YRIN. Complete genome analysis revealed that this E. coli isolate carried virulence factors (sitA, gad, hra, terC, traT, and cia) and many other non-β-lactam resistance determinants including rmtB, tet(A), dfrA17 (two copies), aadA1, aadA5 (two copies), sul1 (two copies), qacE (two copies), qepA, mdf(A), catA1, erm(B), mph(A), and qnrS1, being susceptible only to tigecycline, colistin and fosfomycin. In conclusion, we described here the phenotypic and genome characteristics of an extensively drug-resistant (XDR) E. coli ST361 being recognized as an emerging clone worldwide.

Acinetobacter baumannii and Cefiderocol, between Cidality and Adaptability

Among the bacterial species included in the ESKAPE group, Acinetobacter baumannii is of great interest due to its intrinsic and acquired resistance to many antibiotics and its ability to infect different body regions. Cefiderocol (FDC) is a novel cephalosporin that is active against Gram-negative bacteria, with promising efficacy for A. baumannii infections, but some studies have reported therapeutic failures even in the presence of susceptible strains. This study aims to investigate the interactions between FDC and 10 A. baumannii strains with different susceptibilities to this drug. We confirmed diverse susceptibility profiles, with resistance values close to the EUCAST-proposed breakpoints. The minimal bactericidal concentration (MBC)/MIC ratios demonstrated bactericidal activity of the drug, with ratio values of ≤4 for all of the strains except ATCC 19606; however, bacterial regrowth was evident after exposure to FDC, as were changes in the shapes of colonies and bacterial cells. A switch to a nonsusceptible phenotype in the presence of high FDC concentrations was found in 1 strain as an adaptation mechanism implemented to overcome the cidal activity of this antibiotic, which was confirmed by the presence of heteroresistant, unstable subpopulations in 8/10 samples. Genomic analyses revealed the presence of mutations in penicillin-binding protein 3 (PBP3) and TonB3 that were shared by all of the strains regardless of their resistance phenotype. Because our isolates harbored β-lactamase genes, β-lactamase inhibitors showed the ability to restore the antimicrobial activity of FDC despite the different nonsusceptibility levels of the tested strains. These in vitro results support the concept of using combination therapy to eliminate drug-adapted subpopulations and regain full FDC activity in this difficult-to-treat species. IMPORTANCE This work demonstrates the underrated presence of Acinetobacter baumannii heteroresistant subpopulations after exposure of A. baumannii strains to FDC and its instability. Both A. baumannii and FDC are of great interest for the scientific community, as well as for clinicians; the former represents a major threat to public health due to its resistance to antibiotics, with related costs of prolonged hospitalization, and the latter is a novel, promising cephalosporin currently under the magnifying glass.

Nosocomial outbreak by NDM-1-producing Klebsiella pneumoniae highly resistant to cefiderocol, Florence, Italy, August 2021 to June 2022

A nosocomial outbreak by cefiderocol (FDC)-resistant NDM-1-producing Klebsiella pneumoniae (NDM-Kp) occurred in a large tertiary care hospital from August 2021-June 2022 in Florence, Italy, an area where NDM-Kp strains have become endemic. Retrospective analysis of NDM-Kp from cases observed in January 2021-June 2022 revealed that 21/52 were FDC-resistant. The outbreak was mostly sustained by clonal expansion of a mutant with inactivated cirA siderophore receptor gene, which exhibited high-level resistance to FDC (MIC ≥ 32 mg/L) and spread independently of FDC exposure.

Antibacterial spectrum of cefiderocol

Cefiderocol, a siderophore catechol cephalosporin, recently introduced in the market has been developed to enhance the in vitro activity of extended spectrum cephalosporins and to avoid resistance mechanisms affecting cephalosporins and carbapenems. The in vitro study of cefiderocol in the laboratory requires iron depleted media when MIC values are determined by broth microdilution. Disk diffusion presents good correlation with MIC values. In surveillance studies and in clinical trials it has been demonstrated excellent activity against Gram-negatives, including carbapenemase producers and non-fermenters such as Pseudomonas aeruginosa, Acinetobacter baumannii and Stenotrophomonas maltophilia. Few cefiderocol resistant isolates have been found in surveillance studies. Resistance mechanisms are not directly associated with porin deficiency and or efflux pumps. On the contrary, they are related with gene mutations affecting iron transporters, AmpC mutations in the omega loop and with certain beta-lactamases such us KPC-variants determining also ceftazidime-avibactam resistance, certain infrequent extended-spectrum betalactamases (PER, BEL) and metallo-beta-lactamases (certain NDM variants and SPM enzyme).

Novel Insights into blaGES Mobilome Reveal Extensive Genetic Variation in Hospital Effluents

Mobile genetic elements contribute to the emergence and spread of multidrug-resistant bacteria by enabling the horizontal transfer of acquired antibiotic resistance among different bacterial species and genera. This study characterizes the genetic backbone of blaGES in Aeromonas spp. and Klebsiella spp. isolated from untreated hospital effluents. Plasmids ranging in size from 9 to 244 kb, sequenced using Illumina and Nanopore platforms, revealed representatives of plasmid incompatibility groups IncP6, IncQ1, IncL/M1, IncFII, and IncFII-FIA. Different GES enzymes (GES-1, GES-7, and GES-16) were located in novel class 1 integrons in Aeromonas spp. and GES-5 in previously reported class 1 integrons in Klebsiella spp. Furthermore, in Klebsiella quasipneumoniae, blaGES-5 was found in tandem as a coding sequence that disrupted the 3' conserved segment (CS). In Klebsiella grimontii, blaGES-5 was observed in two different plasmids, and one of them carried multiple IncF replicons. Three Aeromonas caviae isolates presented blaGES-1, one Aeromonas veronii isolate presented blaGES-7, and another A. veronii isolate presented blaGES-16. Multilocus sequence typing (MLST) analysis revealed novel sequence types for Aeromonas and Klebsiella species. The current findings highlight the large genetic diversity of these species, emphasizing their great adaptability to the environment. The results also indicate a public health risk because these antimicrobial-resistant genes have the potential to reach wastewater treatment plants and larger water bodies. Considering that they are major interfaces between humans and the environment, they could spread throughout the community to clinical settings. IMPORTANCE In the "One Health" approach, which encompasses human, animal, and environmental health, emerging issues of antimicrobial resistance are associated with hospital effluents that contain clinically relevant antibiotic-resistant bacteria along with a wide range of antibiotic concentrations, and lack regulatory status for mandatory prior and effective treatment. blaGES genes have been reported in aquatic environments despite the low detection of these genes among clinical isolates within the studied hospitals. Carbapenemase enzymes, which are relatively unusual globally, such as GES type inserted into new integrons on plasmids, are worrisome. Notably, K. grimontii, a newly identified species, carried two plasmids with blaGES-5, and K. quasipneumoniae carried two copies of blaGES-5 at the same plasmid. These kinds of plasmids are primarily responsible for multidrug resistance among bacteria in both clinical and natural environments, and they harbor resistant genes against antibiotics of key importance in clinical therapy, possibly leading to a public health problem of large proportion.

Comparison of disk diffusion, MIC test strip and broth microdilution methods for cefiderocol susceptibility testing on carbapenem-resistant enterobacterales

OBJECTIVES

Cefiderocol is a catechol-substituted cephalosporin dedicated to the treatment of infections caused by multidrug resistant gram-negative rods. Cefiderocol susceptibility testing might be complex. We compared cefiderocol susceptibility testing methods on a relevant collection of carbapenem-resistant Enterobacterales.

METHODS

CE-IVD (European CE marking required for all in vitro diagnostic (IVD)) broth microdilution (BMD) plate (ThermoFisher, Waltham, MA, USA) using regular Mueller-Hinton broth, MIC test strip (Liofilchem, Teramo, Italy), and disk diffusion (Liofilchem) were compared to a frozen BMD plate prepared with iron depleted Mueller-Hinton broth. First, a collection of 100 entirely sequenced carbapenem-resistant Enterobacterales was used to compare these methods. Then, a prospective comparison of disk diffusion and CE-IVD BMD was performed on 827 consecutive carbapenem non-susceptible Enterobacterales including 634 carbapenemase-producers.

RESULTS

Compared to reference method, CE-IVD BMD plate gave 95.0% (95% CI, 88.8-97.9) categorisation agreement (CA), 2.8% (95% CI, 0.4-14.2) very major errors (VME), and 1.6% (95% CI, 0.3-8.7) major errors (ME) with high reproducibility. MIC strip gave only 63% (95% CI, 53.2-71.8) of CA and 94.9% (95% CI, 83.1-98.6) of VME due to critical underestimation of the MICs. Disk diffusion gave 77% (95% CI, 67.9-84.2) CA with additional 8% of the isolates within the area of technical uncertainty of 18-22 mm. Prospectively, disk diffusion gave 81.7% (95% CI, 79.0-84.2) CA, 23.3% (95% CI, 15.1-34.2%)VME, and 4.9% (95% CI, 3.6-6.7) ME. Additionally, 21.3% (95% CI, 18.6-24.2) of CRE were within the area of technical uncertainty.

DISCUSSION

Commercial CE-IVD BMD (ThermoFisher) is accurate for cefiderocol MIC determination in difficult-to-treat Enterobacterales whereas MIC test strip (Liofilchem), that was formulated for Pseudomonas aeruginosa only, should be avoided. Disk diffusion might be useful for screening, but many of these CRE have to be re-tested using BMD to assess definitive categorization.

Occurrence of High Levels of Cefiderocol Resistance in Carbapenem-Resistant Escherichia coli before Its Approval in China: a Report from China CRE-Network

Cefiderocol has been approved in the United States and Europe but not in China. We aim to evaluate carbapenem-resistant Enterobacterales (CRE) susceptibility to cefiderocol to provide baseline data and investigate the resistance mechanism. From 2018 to 2019, 1,158 CRE isolates were collected from 23 provinces and municipalities across China. The MICs of antimicrobials were determined via the agar dilution and broth microdilution methods. Whole-genome sequencing was performed for 26 cefiderocol-resistant Escherichia coli isolates to investigate the resistance mechanism. Clone transformations were used to explore the function of cirA, pbp3, and blaNDM-5 in resistance. Among the 21 antimicrobials tested, aztreonam-avibactam had the highest antibacterial activity (98.3%), followed by cefiderocol (97.3%) and colistin (95.3%). A total of 26 E. coli isolates harboring New Delhi metallo-beta-lactamase 5 (NDM-5) showed high levels of cefiderocol resistance, of which sequence type 167 (ST167) accounted for 76.9% (20/26). We found 4 amino-acid insertions (YRIN/YRIK) at position 333 of penicillin-binding protein 3 (PBP3) in the 26 E. coli isolates, and 22 isolates had a siderophore receptor cirA premature stop codon. After obtaining the wild-type cirA supplementation, the MIC of the transformants decreased by 8 to 16 times in two cefiderocol-resistant isolates. A cefiderocol-susceptible isolate harboring NDM-5 has an MIC increased from 1 μg/mL to 64 μg/mL after cirA deletion, and the MIC decreased from 64 μg/mL to 0.5 μg/mL after blaNDM-5 deletion. The MIC of the E. coli DH5α, from which the pbp3 mutant was obtained, increased from 0.064 μg/mL to 0.25 μg/mL. Cefiderocol showed activity against most CRE in China. The resistance of ST167 E. coli to cefiderocol is a combination of the premature stop codon of cirA, pbp3 mutation, and blaNDM-5 existence. IMPORTANCE Cefiderocol, a new siderophore cephalosporin, has been approved in the United States and Europe but not in China. At present, there are almost no antimicrobial susceptibility evaluation data on cefiderocol in China. We evaluated the in vitro susceptibility of 1,158 strains of carbapenem-resistant Enterobacterales to cefiderocol and other antibiotics. We found that a high proportion of Escherichia coli showed high-level resistance to cefiderocol. Whole-genome sequencing (WGS) and molecular cloning experiments confirmed that the synergistic effect of the cirA gene premature stop codon, blaNDM-5 existence, and the pbp3 mutation is associated with high levels of cefiderocol resistance.

Genome Mining and Metabolomics Unveil Pseudonochelin: A Siderophore Containing 5-Aminosalicylate from a Marine-Derived Pseudonocardia sp. Bacterium

Pseudonochelin (1), a siderophore from a marine-derived Pseudonocardia sp. bacterium, was discovered using genome mining and metabolomics technologies. A 5-aminosalicylic acid (5-ASA) unit, not previously found in siderophore natural products, was identified in 1. Annotation of a putative psn biosynthetic gene cluster combined with bioinformatics and isotopic enrichment studies enabled us to propose the biosynthesis of 1. Moreover, 1 was found to display in vitro and in vivo antibacterial activity in an iron-dependent fashion.

Pharmacokinetic evaluation of cefiderocol for the treatment of multidrug resistant Gram-negative infections

INTRODUCTION

Cefiderocol is a siderophore cephalosporin antibiotic and first of its kind approved by the Food and Drug Administration for the treatment of complicated urinary tract infections (cUTI) and hospital-acquired and ventilator-associated bacterial pneumonia (HABP/VABP) in patients 18 years or older caused by susceptible organisms. Cefiderocol's unique mechanism of iron chelation improves Gram-negative membrane penetration as the bacteria's iron uptake mechanism recognizes the chelated iron antibiotic and iron for entry. This also allows for the evasion of cefiderocol from cell entry-related resistance mechanisms.

AREAS COVERED

This review covers the mechanism of action, resistance mechanisms, pharmacokinetics in various patient populations, and pharmacodynamics. Relevant literature evaluating efficacy and safety are discussed.

EXPERT OPINION

Limited treatment options are available for the treatment of carbapenem-resistantorganisms. Clinical trials have demonstrated that cefiderocol is no worse than alternative treatment options for cUTIs and HABP/VABP, but more data are currently available to support the use of beta-lactam beta-lactamase inhibitor agents, where susceptible. Mortality differences demonstrated in patients with pneumonia and bloodstream infections must further be explored and logistical and practical considerations regarding susceptibility testing and use as monotherapy vs. combination therapy must be considered prior to confidently recommending cefiderocol for regular use in systemic infections.

Rapid Development of Cefiderocol Resistance in Carbapenem-resistant Enterobacter cloacae During Therapy Is Associated With Heterogeneous Mutations in the Catecholate Siderophore Receptor cirA

We report a case of resistance development toward cefiderocol in a patient with intra-abdominal and bloodstream infections caused by carbapenemase-producing Enterobacter cloacae within 21 days of cefiderocol therapy. Whole genome sequencing revealed heterogeneous mutations in the cirA gene, encoding a catecholate siderophore receptor, conferring phenotypic resistance to cefiderocol.

New Delhi Metallo-Beta-Lactamase Facilitates the Emergence of Cefiderocol Resistance in Enterobacter cloacae

Cefiderocol is a promising novel siderophore cephalosporin for the treatment of multidrug-resistant Gram-negative bacilli and with stability against degradation by metallo-β-lactamases. Nonetheless, the emergence of cefiderocol in metallo-β-lactamase-producing Enterobacterales during therapy has been reported on more than one occasion. To understand the underlying mechanisms and factors facilitating the resistance development, we conducted an in vitro evolution experiment using clinical E. cloacae isolates via serial passaging under cefiderocol pressure. In this study, we showed that the presence of the New Delhi metallo-β-lactamase (NDM) facilitates the emergence of resistance via nonsynonymous mutations of the CirA catecholate siderophore receptor. Inhibition of metallo-β-lactamase activity using dipicolinic acid prevented the emergence of cefiderocol-resistant mutants successfully. This finding implies that caution should be taken when using cefiderocol for the treatment of infections caused by metallo-β-lactamase-producing bacteria.

Three-Dimensional Structure and Optimization of the Metallo-β-Lactamase Inhibitor Aspergillomarasmine A

The aminopolycarboxylic acid aspergillomarasmine A (AMA) is a natural Zn²⁺ metallophore and inhibitor of metallo-β-lactamases (MBLs) which reverses β-lactam resistance. The first crystal structure of an AMA coordination complex is reported and reveals a pentadentate ligand with distorted octahedral geometry. We report the solid-phase synthesis of 23 novel analogs of AMA involving structural diversification of each subunit (l-Asp, l-APA1, and l-APA2). Inhibitory activity was evaluated in vitro using five strains of Escherichia coli producing globally prevalent MBLs. Further in vitro assessment was performed with purified recombinant enzymes and intracellular accumulation studies. Highly constrained structure-activity relationships were demonstrated, but three analogs revealed favorable characteristics where either Zn²⁺ affinity or the binding mode to MBLs were improved. This study identifies compounds that can further be developed to produce more potent and broader-spectrum MBL inhibitors with improved pharmacodynamic/pharmacokinetic properties.

Variable Susceptibility to Gallium Compounds of Major Cystic Fibrosis Pathogens

The decreasing efficacy of existing antibiotics against pulmonary pathogens that affect cystic fibrosis (CF) patients calls for the development of novel antimicrobials. Iron uptake and metabolism are vital processes for bacteria, hence potential therapeutic targets. Gallium [Ga(III)] is a ferric iron-mimetic that inhibits bacterial growth by disrupting iron uptake and metabolism. In this work we evaluate the efficacy of three Ga(III) compounds, namely, Ga(NO₃)₃, (GaN), Ga(III)-maltolate (GaM), and Ga(III)-protoporphyrin IX (GaPPIX), against a collection of CF pathogens using both reference media and media mimicking biological fluids. All CF pathogens, except Streptococcus pneumoniae, were susceptible to at least one Ga(III) compound. Notably, Mycobacterium abscessus and Stenotrophomonas maltophilia were susceptible to all Ga(III) compounds. Achromobacter xylosoxidans, Burkholderia cepacia complex, and Pseudomonas aeruginosa were more susceptible to GaN and GaM, whereas Staphylococcus aureus and Haemophilus influenzae were more sensitive to GaPPIX. The results of this study support the development of Ga(III)-based therapy as a broad-spectrum strategy to treat CF lung infections.

Optimized protocols for assessing libraries of poorly soluble sortase A inhibitors for antibacterial activity against medically-relevant bacteria, toxicity and enzyme inhibition

Increasing antimicrobial resistance is a major global health concern. Conventional antibiotics apply selection pressures, which promote the accumulation of resistant microbes. Anti-virulence strategies, in contrast, are less potent antimicrobials, but are less likely to select for resistance, can be combined with existing antibiotics to improve their activity, and in some cases can overcome antimicrobial resistance towards other antimicrobials. Sortase A inhibitors (SrtAIs) represent an exciting example of this class; however, many reported examples demonstrate poor water solubility, which complicates their biological assessment and activity. This includes reports that use antimicrobial concentrations of organic solvents or conditions that fail to solubilise these compounds for minimal inhibitory concentration (MIC) assessments. Herein, we report the first study to optimise screening processes for a library of prospective SrtAIs (trans-chalcone (TC), berberine (BR), curcumin (CUR), and quercetin (QC)), including comparative assessment of the effects of various co-solvent concentrations, along with comparative assessment of their antimicrobial activities against multiple disease relevant bacterial strains (methicillin-sensitive and resistant S. aureus, E. coli, and P. aeruginosa), inhibition of the sortase A enzyme, and toxicity towards mammalian cells (HEK-293), using these optimised conditions. Optimal solubility with minimal effect on bacterial viability was observed in the presence of 5% (v/v) dimethyl sulfoxide (DMSO)-Mueller-Hinton Broth. Three antimicrobial susceptibility tests (broth microdilution, agar dilution, and disk diffusion) were assessed for their ability to accurately determine minimal inhibitory concentration (MIC) data for each SrtAI. Broth microdilution and agar dilution were both effective; however, the broth microdilution assay required the addition of a colorimetric metabolic indicator (resazurin) to enable simple and reliable MIC determination due to the development of precipitants over time. In contrast, disk diffusion did not provide reliable zone of inhibition data. Identical MIC data was observed with methicillin-sensitive and -resistant S. aureus (MRSA; ATCC43300), with lower potency activity against E. coli and P. aeruginosa. Under these conditions, TC and CUR demonstrated significant toxicity towards human embryonic kidney (HEK-293) cells, with QC showing less toxicity and BR limited-to-no toxicity at its MIC. Overall, the findings of this work provide optimised processes, which will prove useful for the study of other poorly soluble antimicrobial agents and SrtAIs. The obtained data suggests that BR should be considered in preference to the other SrtAIs for the development of new antimicrobial formulations, based on its superior antimicrobial and SrtA inhibition potency, and greatly reduced toxicity.

In Vitro Antimicrobial Activity of the Siderophore Cephalosporin Cefiderocol against Acinetobacter baumannii Strains Recovered from Clinical Samples

BACKGROUND

Cefiderocol is a siderophore cephalosporin that exhibits antimicrobial activity against most multi-drug resistant Gram-negative bacteria, including Enterobacterales, Pseudomonas aeruginosa, Acinetobacter baumannii, and Stenotrophomonas maltophilia.

METHODS

A total of 20 multidrug-resistant A. baumannii strains were isolated from 2020 to 2021, molecularly characterized and tested to assess the in vitro antibacterial activity of cefiderocol. Thirteen strains were carbapenem-hydrolysing oxacillinase OXA-23-like producers, while seven were non-OXA-23-like producers. Minimum inhibitory concentrations (MICs) were determined by broth microdilution, considered as the gold standard method. Disk diffusion test was also carried out using iron-depleted CAMHB plates for cefiderocol.

RESULTS

Cefiderocol MICs ranged from 0.5 to 1 mg/L for OXA-23-like non-producing A. baumannii strains and from 0.25 to >32 mg/L for OXA-23-like producers, using the broth microdilution method. Cefiderocol MIC₉₀ was 8 mg/L. Diameter of inhibition zone of cefiderocol ranged from 18 to 25 mm for OXA-23-like non-producers and from 15 to 36 mm for OXA-23-like producers, using the diffusion disk method. A large variability and a low reproducibility were observed during the determination of diameter inhibition zone. Molecular characterization showed that all isolates presented the ISAba1 genetic element upstream the blaOXA-51. Among OXA-23-like non-producers, four were blaOXA-58 positive and two were negative for all the resistance determinants analyzed.

CONCLUSIONS

Cefiderocol showed in vitro antimicrobial activity against both carbapenem-susceptible and non-susceptible A. baumannii strains, although some OXA-23-like producers were resistant. Further clinical studies are needed to consolidate the role of cefiderocol as an antibiotic against MDR A. baumannii.

Antibacterial activity of gallium nitrate against polymyxin-resistant Klebsiella pneumoniae strains

Iron uptake and metabolism have become attractive targets for the development of new antibacterial drugs. In this scenario, the FDA-approved iron mimetic metal gallium [Ga (III)] has been successfully researched as an antimicrobial drug. Ga (III) inhibits microbial growth by disrupting ferric iron-dependent metabolic pathways. In this study, we revealed that gallium nitrate III (GaN) inhibits the growth of a collection of twenty polymyxin-resistant Klebsiella pneumoniae strains at concentrations ranging from 2 to 16µg/mL, using a medium, on which the low iron content and the presence of human serum better mimic the in vivo environment. GaN was also successful in protecting Caenorhabditis elegans from polymyxin-resistant K. pneumoniae strains lethal infection, with survival rates of >75%. GaN also exhibited synergism with polymyxin B, suggesting that a polymyxin B-GaN combination holds promise like as one alternative therapy for infections caused by resistant polymyxin B K. pneumoniae strains.

New Perspectives on Antimicrobial Agents: Cefiderocol

Bacterial resistance to carbapenem agents has reached alarming levels. Accordingly, collaborative efforts between national and international organizations and the pharmaceutical industry have led to an impressive expansion of commercially available β-lactam agents in recent years. No available agent comes close to the broad range of activity afforded by cefiderocol, a novel siderophore-cephalosporin conjugate. The novelty of and need for cefiderocol are clear, but available clinical data are conflicting, leaving infectious diseases specialists puzzled as to when to prescribe this agent in clinical practice. After a brief overview of cefiderocol pharmacokinetics and pharmacodynamics, safety data, cefiderocol susceptibility testing, and putative mechanisms of cefiderocol resistance, this review focuses on determining cefiderocol's role in the management of specific pathogens, including carbapenem-resistant Acinetobacter baumannii complex, carbapenem-resistant Pseudomonas aeruginosa, carbapenem-resistant Enterobacterales, and less commonly identified glucose-nonfermenting organisms such as Stenotrophomonas maltophilia, Burkholderia species, and Achromobacter species. Available preclinical, clinical trial, and postmarketing data are summarized for each organism, and each section concludes with our opinions on where to position cefiderocol as a clinical therapeutic.

Development of a Broth Microdilution Method for Exebacase Susceptibility Testing

Exebacase (CF-301) belongs to a new class of protein-based antibacterial agents, known as lysins (peptidoglycan hydrolases). Exebacase, a novel lysin with antistaphylococcal activity, is in phase 3 of clinical development. To advance into the clinic, it was necessary to develop an accurate and reproducible method for exebacase MIC determination. The Clinical and Laboratory Standards Institute (CLSI) reference broth microdilution (BMD) method using cation-adjusted Mueller-Hinton broth (CAMHB) produced trailing MIC endpoints, and exebacase activity was diminished when frozen BMD panels were used. A modified BMD method was developed using CAMHB supplemented with 25% horse serum and 0.5 mM dl-dithiothreitol (CAMHB-HSD). Preliminary quality control (QC) ranges for Staphylococcus aureus ATCC 29213 of 0.25 to 1 μg/ml and for Enterococcus faecalis ATCC 29212 of 16 to 64 μg/ml were determined based on the results of a CLSI M23-defined MIC QC tier 1 study. These preliminary QC ranges validated the MIC data generated from a systematic study testing a discrete S. aureus strain collection using CAMHB-HSD to investigate the impact of parameters known to influence susceptibility test results and to evaluate the exebacase MIC distribution against clinical S. aureus isolates. Presentation of these data led to the CLSI Subcommittee on Antimicrobial Susceptibility Testing (AST) approval of the use of CAMHB-HSD to determine exebacase susceptibility and commencement of a multilaboratory (tier 2) QC study. Use of a standard BMD method and concomitant QC testing provides confidence in the assessment of test performance to generate accurate and reproducible susceptibility data during antibacterial drug development.

Evaluating Cefiderocol in the Treatment of Multidrug-Resistant Gram-Negative Bacilli: A Review of the Emerging Data

Infections due to multidrug-resistant Gram-negative bacteria (MDR-GNB), especially when carbapenem resistant, have been very difficult to manage in the last fifteen years, owing to the paucity of dependable therapeutic options. Cefiderocol is a siderophore cephalosporin recently approved by the Food and Drug Administration (FDA) and European Medicines Agency (EMA) that may have the potential to fill some of the remaining gaps in the treatment of MDR-GNB infections. Among others, cefiderocol demonstrated in vitro activity against carbapenem-resistant Acinetobacter baumannii and metallo-β-lactamases producers. Clinical data from both registrative studies and post-marketing experiences are essential to confirm whether these promises from in vitro studies could readily translate into clinical practice, as well as to delineate the precise place in therapy for cefiderocol for the treatment of MDR-GNB in the near future. Because of its unique potential, it is essential to provide both randomized controlled trials (RCT) and real-life data to improve the ability of clinicians to exploit its benefit in both empirical and targeted treatment of MDR-GNB infections. In this narrative review, we discuss the emerging data from pivotal RCT and initial real-life experiences on the use of cefiderocol for the treatment of MDR-GNB infections.

Comparison of Agar Dilution to Broth Microdilution for Testing In Vitro Activity of Cefiderocol against Gram-Negative Bacilli

Cefiderocol (CFDC) is a siderophore cephalosporin with activity against Gram-negative bacterial species that are resistant to carbapenems and other drugs. The MICs of CFDC were determined for 610 Gram-negative bacilli, including 302 multinational Enterobacterales isolates with characterized mechanisms of beta-lactam resistance, 180 clinical isolates from the Mayo Clinic and Mayo Clinic Laboratories not characterized for specific resistance mechanisms, and 128 isolates with CFDC MICs of ≥8 μg/ml obtained from International Health Management Associates, Inc. (IHMA, Schaumburg, IL). Broth microdilution using standard cation-adjusted Mueller-Hinton broth (BMD) and iron-depleted cation-adjusted Mueller-Hinton broth (ID-BMD), and agar dilution (AD) using standard Mueller-Hinton agar were performed according to Clinical and Laboratory Standards Institute (CLSI) guidelines. MICs were interpreted according to the investigational CLSI, FDA, and EUCAST breakpoints, and results were compared. MICs inhibiting 50 and 90% of organisms (MIC50 and MIC90, respectively), essential agreement (EA), categorical agreement (CA), and error of different types were determined. Results showed considerable discordance between AD and ID-BMD. CFDC showed low EA and CA rates and high error rates for AD in comparison to ID-BMD. Overall, this study does not support use of standard AD for determining CFDC MICs.