Cefiderocol: EUCAST criteria for disc diffusion and broth microdilution for antimicrobial susceptibility testing

Performance evaluation of Bruker UMIC® microdilution panel and disc diffusion to determine cefiderocol susceptibility in Enterobacterales, Acinetobacter baumannii, Pseudomonas aeruginosa, Stenotrophomonas maltophilia, Achromobacter xylosoxidans and Burkolderia species

PURPOSE

Cefiderocol susceptibility testing (AST) represents an open challenge for clinical microbiology. Herein, we evaluated the performance of the UMIC® Cefiderocol broth microdilution (BMD) test and disc diffusion on Gram-negative species.

METHODS

UMIC® Cefiderocol BMD test, disc diffusion and reference BMD were in parallel performed on a collection of 256 clinical isolates. Categorical agreement (CA), essential agreement (EA), bias, major errors (MEs) and very major errors (VMEs) were calculated for both AST methods.

RESULTS

The UMIC® Cefiderocol BMD strip exhibited an EA < 90% (85.5%), a CA higher than 90% (93.7%) and a low number of VMEs (n = 4, 4.2%) and MEs (n = 12, 7.4%). UMIC® Cefiderocol identified 96.2% of the resistant isolates [Enterobacterales, (39/40); P. aeruginosa (19/19); A. xylosoxidans (5/6); S. maltophilia (5/6); Burkholderia spp. (8/8)]. Disc diffusion showed a high CA (from 94.9 to 100%) regardless of disc manufacturer in Enterobacterales, P. aeuroginosa, A. baumannii and S. maltophilia. However, high rates of results falling in the area of technical uncertainty (ATU) were observed in Enterobacterales (34/90, 37.8%) and P. aeruginosa (16/40, 40%). Disc diffusion showed a poor performance in A. xylosoxidans and Burkholderia spp. if PK/PD breakpoint was used (overall, 5/9 VMEs; in contrast, the use of P. aeruginosa-specific breakpoints resulted in 100% of CA with 24.6% of results in the ATU).

CONCLUSION

In conclusion, disc diffusion and UMIC® Cefiderocol are valid methods for the determination of cefiderocol susceptibility. Given the high number of results in the ATU by disc diffusion, a combined use of both AST methods may represent a solution to overcome the challenge of cefiderocol susceptibility testing in routine microbiology laboratories.

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.

Assessment of cefiderocol disk diffusion versus broth microdilution results when tested against Acinetobacter baumannii complex clinical isolates

IMPORTANCE

Carbapenem-resistant Acinetobacter baumannii is a major global health concern due to its high prevalence and limited treatment options. Cefiderocol is the only novel Food and Drug Administration (FDA)-approved β-lactam agent for the salvage treatment of carbapenem-resistant A. baumannii infection. Currently, a commercial automated susceptibility testing panel of cefiderocol is unavailable. Both the preparation of iron-depleted cation-adjusted Mueller-Hinton broth and the performance of broth microdilution are cumbersome in routine microbiology laboratories. A disk diffusion method is convenient for cefiderocol antimicrobial susceptibility testing, but limited data are available specifically for A. baumannii clinical isolates. Moreover, the Clinical and Laboratory Standards Institute published revisions to the A. baumannii cefiderocol disk diffusion breakpoints in 2022. Hence, we evaluated the performance of cefiderocol disk diffusion compared with the reference BMD against A. baumannii clinical isolates, especially those with cefiderocol zone diameters ≤ 14 mm.

In Vitro Susceptibility of Burkholderia pseudomallei Isolates to Cefiderocol and Ceftazidime/Avibactam from Odisha, India

Introduction and Objectives  The availability of a limited arsenal of antibacterial agents effective against Burkholderia pseudomallei, the causative agent of melioidosis, together with sporadic reports of emergence of resistance necessitates an evaluation of in vitro activity of new antimicrobials against clinical B. pseudomallei isolates. Cefiderocol (CFDC), a novel siderophore cephalosporin, and ceftazidime-avibactam (CZA), a new β lactam combination agent, have shown promising results for the treatment of difficult-to-treat Gram-negative bacilli infections with limited treatment options. This study was conducted to determine the in vitro activity of CFDC and CZA against a contemporary collection of 60 B. pseudomallei clinical isolates. Materials and Methods  Minimum inhibitory concentrations (MIC) of CFDC and CZA were determined by broth microdilution and E-test, respectively. The performance of disk diffusion was also evaluated for CFDC. Results  All B. pseudomallei isolates were susceptible to CFDC and CZA with MIC range of 0.125 to 2 mg/L and 0.19 to 1 mg/L, respectively. Zone diameters for CFDC ranged from 31 to 40 mm. Conclusion  CFDC and CZA exhibited excellent in vitro activity against 60 B. pseudomallei isolates. Further pharmacokinetic-pharmacodynamics studies and clinical trials are needed to prove the clinical efficacy of CFDC and CZA in the treatment of melioidosis.

In vitro activity of cefiderocol against comparators (ceftazidime-avibactam, ceftazidime-avibactam/ aztreonam combination, and colistin) against clinical isolates of meropenem-resistant Klebsiella pneumoniae from India

Cefiderocol (FDC), a novel siderophore drug, is active against Gram-negative bacteria producing carbapenemases, including metallo-beta-lactamases. The objective of this study is to compare the in vitro activity of FDC with ceftazidime-avibactam (CZA), CZA/aztreonam (AT) combination, and colistin (CST), in clinical isolates of meropenem-resistant (MER-R) Klebsiella pneumoniae. From the 2,052 clinical specimens submitted for culture testing, 245 K. pneumoniae isolates were recovered within a 6-month period in 2021. One hundred three non-duplicate, non-outbreak, MER-R (minimum inhibitory concentration, MIC >4 µg/mL) strains were included in the study. Identification and susceptibility were performed using VITEK-2 (bioMérieux). Meropenem-susceptible isolates (n = 10) served as controls. For FDC, broth microdilution (BMD) was performed after in-house standardization. Disk diffusion (Liofilchem, Italy) and broth microdilution (ComASP, STC, Liofilchem, Italy) were used for susceptibility testing of CZA and CST, respectively. Synergy testing for CZA and AT was performed using disk approximation method. CLSI breakpoints were used for the interpretation of the results. For FDC, MIC50 and MIC90 were 2 and 8 µg/mL, respectively. A total of 80% of isolates were susceptible to FDC, 26.2% of isolates were susceptible to CZA, synergy testing with CZA/AT was positive for 74 (72%) of the isolates, and 89.3% were intermediate to CST. Nine (8.7%) were susceptible only to FDC. FDC is active in vitro against MER-R K. pneumoniae >CZA/AT > CZA > CST, as observed in this study, applying CLSI criteria. Clinico-microbiological studies should be performed to assess the clinical efficacy of this novel drug in this region with a high prevalence of carbapenem resistance among Gram-negative organisms. IMPORTANCE Management of infections with multi-drug resistant Klebsiella pneumoniae is a major challenge in hospital settings, with few treatment options. In this study, the authors aim to assess the in vitro susceptibility of these clinical isolates to cefiderocol, a novel siderophore. Comparators are colistin, ceftazidime-avibactam, and ceftazidime-avibactam/aztreonam synergy, which are currently available options for treatment in this region. Baseline-resistance rates against cefiderocol are higher than those in the previously published studies, with MIC50 and MIC90 at 2 and 8 µg/mL, respectively.

First case of persistent Stenotrophomonas maltophilia bacteraemia due to septic thrombosis successfully treated with a cefiderocol-containing regimen

INTRODUCTION

There is scarce evidence in literature of what should be the best antimicrobial treatment for bloodstream infections (BSIs) sustained by Stenotrophomonas maltophilia, a peculiar pathogen that intrinsically withstands to most of the available antibiotics.

RESULTS AND CONCLUSION

Here, we describe a challenging case of a persistent S. maltophilia BSI due to septic thrombosis successfully treated with the addition of the novel siderophore cephalosporin cefiderocol to an only partially effective levofloxacin regimen. Additionally, an intra-lock therapy with trimethoprim/sulfamethoxazole was selected as a strategy to prevent recurrence of infection since complete source control was not possible. The serum bactericidal assay was also used to corroborate the in vivo efficacy of the adopted combination therapy.

Higher rates of cefiderocol resistance among NDM producing Klebsiella bloodstream isolates applying EUCAST over CLSI breakpoints

BACKGROUND

Cefiderocol is generally active against carbapenem-resistant Klebsiella spp. (CRK) with higher MICs against metallo-beta-lactamase producers. There is a variation in cefiderocol interpretive criteria determined by EUCAST and CLSI. Our objective was to test CRK isolates against cefiderocol and compare cefiderocol susceptibilities using EUCAST and CLSI interpretive criteria.

METHODS

A unique collection (n = 254) of mainly OXA-48-like- or NDM-producing CRK bloodstream isolates were tested against cefiderocol with disc diffusion (Mast Diagnostics, UK). Beta-lactam resistance genes and multilocus sequence types were identified using bioinformatics analyses on complete bacterial genomes.

RESULTS

Median cefiderocol inhibition zone diameter was 24 mm (interquartile range [IQR] 24-26 mm) for all isolates and 18 mm (IQR 15-21 mm) for NDM producers. We observed significant variability between cefiderocol susceptibilities using EUCAST and CLSI breakpoints, such that 26% and 2% of all isolates, and 81% and 12% of the NDM producers were resistant to cefiderocol using EUCAST and CLSI interpretive criteria, respectively.

CONCLUSIONS

Cefiderocol resistance rates among NDM producers are high using EUCAST criteria. Breakpoint variability may have significant implications on patient outcomes. Until more clinical outcome data are available, we suggest using EUCAST interpretive criteria for cefiderocol susceptibility testing.

Phytochemical profiling of Salsola tetragona Delile by LC-HR/MS and investigation of the antioxidant, anti-inflammatory, cytotoxic, antibacterial and anti-SARS-CoV-2 activities

This study aimed to investigate the phytochemical composition and biological activity of Salsola tetragona Delile. (Amaranthaceae), a medicinal plant. The study evaluated the antioxidant potential of the crude extract and five fractions of S. tetragona using DPPH•, ABTS•+, CUPRAC, and metal chelating assays. The anti-inflammatory activity was determined using a protein denaturation assay, and the antibacterial activity was determined by the Minimum inhibitory concentrations (MICs) for the growth of Escherichia coli and Staphylococcus aureus strains. The MTT test and an in vitro scratch assay evaluated the effects on cell viability and cell migration. The potential anti-SARS-CoV-2 activity was assessed by analyzing the effects on the interaction between ACE2 and Spike protein. The bioactive compounds present in the plant were identified using LC-HR/MS analysis. The crude hydromethanolic extract (STM) and five fractions of S. tetragona, n-hexane (STH), dichloromethane (STD), ethyl acetate (STE), n-butanol (STB), and aqueous (STW) showed significant antioxidant activity in four different tests. In the anti-inflammatory assay, the ethyl acetate fraction exhibited significantly higher activity than Aspirin® (IC50 = 13 ± 5 µg/mL). The crude extract and its fractions showed positive antibacterial activity with similar MICs. In the cytotoxicity assay against the breast cancer cell line MCF7, the dichloromethane fractions (STD) were very effective and demonstrated superiority over the other fractions (IC50 = 98 µg/mL). Moreover, the potential of the extract and fractions as anti-SARS-CoV-2, the ethyl acetate, and dichloromethane fractions demonstrated important activity in this test. LC-HR/MS analysis identified 16 different phenolic compounds, Eleven of which had not been previously reported in the genus Salsola. The results suggest that the extracts of S. tetragona have the potential to become new sources for developing plant-based therapies for managing a range of diseases.

Cefiderocol-containing regimens for the treatment of carbapenem-resistant A. baumannii ventilator-associated pneumonia: a propensity-weighted cohort study

Background

Cefiderocol is a novel β-lactam with activity against carbapenem-resistant Acinetobacter baumannii (CRAB), but its role in CRAB pulmonary infections is controversial due to limited evidence.

Objectives

To assess the association between cefiderocol-containing regimens treatment and 28-day mortality in carbapenem-resistant A. baumannii ventilator-associated pneumonia (VAP).

Methods

An observational cohort study including critically ill COVID-19 patients with CRAB-VAP admitted to two ICUs of a large academic hospital in Rome between September 2020 and December 2022. The primary outcome was 28-day all-cause mortality. A propensity score was created to balance the cefiderocol- and non-cefiderocol-containing groups. A propensity-weighted multiple logistic regression model was calculated to evaluate risk factors for 28-day mortality. Survival curves were calculated using the Kaplan-Meier method.

Results

121 patients were enrolled, 55 were treated with cefiderocol- and 66 with non-cefiderocol-containing regimens. The 28-day all-cause mortality was 56% (68/121). A statistically significant difference in 28-day mortality was found between cefiderocol- and non-cefiderocol- containing regimens groups (44% versus 67%, P = 0.011). In the propensity-adjusted multiple logistic regression, cefiderocol (OR 0.35 95% CI 0.14, 0.83) was a predictor of 28-day survival, Charlson comorbidity index (OR 1.36 95% CI 1.16, 1.78), SOFA score (OR 1.24 95% CI 1.09, 1.57) and septic shock (OR 3.71 95% CI 1.44, 12.73) were all associated with increased 28-day mortality.

Conclusion

Cefiderocol-containing regimens were associated with reduced 28-day mortality in CRAB-VAP. The sample size and the observational design limit the study's conclusions. Future RCTs are needed to establish cefiderocol's definite role in these infections.

Performance of the Disc Diffusion Method, MTS Gradient Tests and Two Commercially Available Microdilution Tests for the Determination of Cefiderocol Susceptibility in Acinetobacter spp

Cefiderocol is a siderophore-conjugated cephalosporin with potent activity against multidrug-resistant Gram-negative pathogens including Acinetobacter baumannii. The aim of this study was to evaluate cefiderocol testing methods on a relevant collection of 97 Acinetobacter spp. isolates. Commercialized broth microdilution methods (ComASP®, Liofilchem and UMIC®, Bruker), MIC test strips (Liofilchem) and disc diffusion using discs of three different brands (Mast Diagnostic, Liofilchem and Oxoid-Thermo Fisher Scientific) were compared with the broth microdilution reference method. None of the methods tested fulfilled acceptable criteria (essential agreement [EA] ≥ 90%; bias = ±30%) but both BMD methods achieved acceptable categorical agreement rates (CA = 95.9% [93/97, 95% CI 89.9-98.4] and CA = 93.8% [91/97, 95% CI 87.2-97.1] for ComASP® and UMIC®, respectively) and bias < 30% (-7.2% and -25.2% for ComASP® and UMIC®, respectively). The use of MIC gradient testing is strongly discouraged due to misclassification of 55% (n = 23/42) of resistant strains. Finally, the disc diffusion method could be used to rapidly screen for susceptible strains by setting a critical diameter of 22 mm.

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.

Cefiderocol-Based versus Colistin-Based Regimens for Severe Carbapenem-Resistant Acinetobacter baumannii Infections: A Propensity Score-Weighted, Retrospective Cohort Study during the First Two Years of the COVID-19 Pandemic

BACKGROUND

A large increase in multi-drug-resistant Acinetobacter baumannii, especially carbapenem-resistant strains, occurred during the first two years of the COVID-19 pandemic, posing important challenges in its treatment. Cefiderocol appeared to be a good option for the treatment of Carbapenem-resistant Acinetobacter baumannii (CR-Ab), but to date, the guidelines and evidence available are conflicting.

METHODS

We retrospectively included a group of patients with CR-Ab infections (treated with colistin- or cefiderocol-based regimens) at Padua University Hospital (August 2020-July 2022) and assessed predictors of 30-day mortality, and differences in microbiological and clinical treatment. To evaluate the difference in outcomes, accounting for the imbalance in antibiotic treatment allocation, a propensity score weighting (PSW) approach was adopted.

RESULTS

We included 111 patients, 68% males, with a median age of 69 years (IQR: 59-78). The median duration of antibiotic treatment was 13 days (IQR:11-16). In total, 60 (54.1%) and 51 (45.9%) patients received cefiderocol- and colistin-based therapy, respectively. Notably, 53 (47.7%) patients had bloodstream infections, while 58 (52.3%) had pneumonia. Colistin was combined in 96.1%, 80.4%, and 5.8% of cases with tigecycline, meropenem, and fosfomycin, respectively. Cefiderocol was combined in 13.3%, 30%, and 18.3% of cases with fosfomycin, tigecycline, and meropenem, respectively. At the baseline, the two treatment groups significantly differed in age (patients treated with colistin were significantly older), the prevalence of diabetes and obesity (more frequent in the group treated with colistin), length of stay (longer in the group receiving cefiderocol), and type of infection (BSI were more frequent in the group receiving cefiderocol). The proportion of patients who developed acute kidney injury was significantly higher in the colistin group. By using PSW, no statistically significant differences emerged for mortality or clinical and microbiological cure between the two groups. No independent predictors were detected for hospital mortality or clinical cure, while for the length of stay, the only selected predictor was age, with a non-linear effect (p-value 0.025 for non-linearity) on the prolongation of hospital stay of 0.25 days (95% CI 0.10-0.39) at increasing ages (calculated over the IQR).

CONCLUSIONS

Cefiderocol treatment did not differ in terms of main outcomes and safety profile from colistin-based regimens. More prospective studies with a larger number of patients are required to confirm our results.

Comparison of Broth Microdilution, Disk Diffusion and Strip Test Methods for Cefiderocol Antimicrobial Susceptibility Testing on KPC-Producing Klebsiella pneumoniae

The aim of this study was to compare the reference broth microdilution (BMD) method with the Disk Diffusion (DD) test and strip test against a collection of 75 well-characterized Klebsiella pneumoniae carbapenemase (KPC)-producing Klebsiella pneumoniae (KPC-Kp) clinical strains to assess cefiderocol (CFD) antimicrobial activity. Whole-genome sequencing was performed on KPC-Kp strains by Illumina iSeq100 platform. The Categorical Agreement (CA) between the BMD method and DD test was 92% (69/75) with a Major Error (ME) of 16.7% (6/36). Additionally, the CA between the BMD method and test strip was 90.7% (68/75) with a Very Major Error (VME) of 17.9% (7/39) and 82.7% (62/75) between the strip test and DD with a ME of 30.2%. KPC-Kp strains showing resistance to CFD were 27 out of 75 (36%) by three methods. Specifically, 51.9% (14/27) of KPC-Kp resistant to CFD harbored bla_KPC-3, while 48.1% (13/27) harbored mutated bla_KPC-3. Moreover, KPC-Kp strains carrying a mutated bla_KPC-3 gene exhibited high MIC values (p value < 0.001) compared to wild-type bla_KPC-3. In conclusion, the DD test resulted as a valid alternative to the BMD method to determine the in vitro susceptibility to CFD, while the strip test exhibited major limitations.

Disc Diffusion and ComASP® Cefiderocol Microdilution Panel to Overcome the Challenge of Cefiderocol Susceptibility Testing in Clinical Laboratory Routine

Cefiderocol susceptibility testing represents a major challenge for clinical microbiology. Although disc diffusion showed robustness to test cefiderocol susceptibility, large areas of technical uncertainty (ATU) are reported by current EUCAST breakpoints. Herein, we evaluated the in vitro activity of cefiderocol on a collection of 286 difficult-to-treat Gram-negative isolates using disc diffusion and ComASP^® cefiderocol microdilution panel. Broth microdilution (BMD) in iron-depleted Mueller-Hinton broth was used as reference method. Following the EUCAST guidelines, disc diffusion allowed to determine cefiderocol susceptibility (susceptible or resistant) in 78.6%, 88.1%, 85.4% and 100% of Enterobacterales, P. aeruginosa, A. baumannii and S. maltophilia isolates tested, respectively. ComASP^® cefiderocol panel showed 94% and 84% of overall categorical agreement and essential agreement. Only one very major error and two major errors were observed, for MIC values nearly close to the resistance breakpoint (2 mg/L). Overall, 20.5% of the carbapenemase-producing Enterobacterales that achieved ATU results by the disc diffusion method tested resistant by both ComASP^® panel and reference BMD. Conversely, all VIM-producing P. aeruginosa showed MIC values in the susceptible range (≤2 mg/L). Lastly, only six out of seven (85.7%) A. baumannii isolates showing inhibition zones <17 mm tested resistant by both ComASP^® panel and the reference BMD suggesting that inhibition zone <17 mm are not unequivocally suggestive of resistance. Our results, although obtained on a limited number of isolates, suggest that the combination of disc diffusion with a ComASP^® cefiderocol microdilution panel could be a viable solution to overcome the challenge of cefiderocol susceptibility testing in routine microbiology laboratories.

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.

Analysis of In Vitro Activity of Cefiderocol Against Carbapenem-Resistant Gram-Negative Bacilli by Broth Microdilution and Disk Diffusion Method: A Single-Center Study in Odisha, India

Introduction

Cefiderocol (CFDC), a novel semi-synthetic siderophore cephalosporin has been developed to combat the menace of infections caused by carbapenem-resistant Gram-negative bacilli (CR-GNB) including Carbapenem-resistant Enterobacterales (CRE) and Carbapenem-resistant Nonfermenting Gram-negative bacilli (CR-NFGNB).

Methods

We determined the in vitro activity of CFDC against a contemporary collection of 503 CR-GNB isolates by the reference broth microdilution method (BMD) using Iron depleted cation adjusted Mueller-Hinton broth (ID-CAMHB). Performance of CFDC disk diffusion (DD) was evaluated against the reference BMD, as an alternative convenient testing method. Molecular characterization of carbapenemase in CR-GNB was performed by PCR targeting bla _NDM-1, bla _{OXA-48like alleles}, bla _KPC, bla _IMP, and, bla _VIM. Minimum inhibitory concentration (MIC) distribution of CFDC in CR-GNB harbouring different carbapenemase enzymes was also analyzed.

Results

In our study, 81.7% (411/503) of CR-GNB isolates [81.3%, (278/342) CRE and 82.6% (133/161) CR-NFGNB] were susceptible to CFDC (p>0.05). Categorical agreement (CA) of DD ranged from 79.8% to 87.5%, Minor error (mE) ranged from 0 to 14%, Major error (ME) ranged from 0 to 3.5%, and Very Major error (VME) ranged from 0 to 12.5% with variations by species tested. Overall CFDC MIC50 and MIC90 values of CR-GNB isolates without any carbapenemase genes were higher as compared to those with the presence of carbapenemase genes (4 µg/mL and 128 µg/mL versus 2 µg/mL and 16 µg/mL respectively).

Discussion

CFDC is not yet available for clinical use in India. Hence, multicentric studies are the need of the hour in India for standardization of CFDC susceptibility using disks and CAMHB from different manufacturers as well as understanding mechanisms of high MIC values.

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).

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.