In Vitro Activity of Cefiderocol, a Siderophore Cephalosporin, Against Gram-Negative Bacilli Isolated by Clinical Laboratories in North America and Europe in 2015-2016: SIDERO-WT-2015

Carbapenem-resistant Acinetobacter baumannii raises global alarm for new antibiotic regimens

Carbapenem-resistant Acinetobacter baumannii (CRAB) is a top-priority pathogen causing a nosocomial infection that increases morbidity and mortality. Treatment options for CRAB are relatively limited by pharmacokinetic restrictions, such as substantial toxicity. Therefore, we must better understand this pathogen to develop new treatments and control strategies. The review aims to provide an overview of the current understanding of acquired, adaptive, and intrinsic Carbapenem-resistant pathways in A. baumannii, as well as its consequences on healthcare systems, particularly critical care units. The review also provides insights into how CRAB infections are currently managed worldwide and why novel therapeutic regimens are needed. The peculiarity of A. baumannii and its often reported virulence factors have been discussed further. In conclusion, the purpose of this review is to emphasize the current knowledge on CRAB, as it causes major worry in the field of nosocomial infections as well as overall public health.

In vivo development of resistance to novel β-lactam/β-lactamase inhibitor combinations in KPC-producing Klebsiella pneumoniae infections: a case series

INTRODUCTION

Understanding the dynamics that may characterize the emergence of KPC variants with resistance to novel β-lactam/β-lactamase inhibitor combinations (βL/βLICs) represents a challenge to be overcome in the appropriate use of recently introduced antibiotics.

METHODS

Retrospective case series describing development of multiple resistance to novel βL/βLICs in patients with KPC-producing Klebsiella pneumoniae (KPC-Kp) infections treated with these drugs. Clinical-microbiological investigation and characterization of longitudinal strains by Whole-Genome Sequencing were performed.

RESULTS

Four patients with KPC-Kp bloodstream infections were included. Most frequent clinical features were kidney disease, obesity, cardiac surgery as reason for admission, ICU stay, treatment with ceftazidime/avibactam, and pneumonia and/or acute kidney injury needing renal replacement therapy as KPC-Kp sepsis-associated complications. The development of resistance to ceftazidime/avibactam was observed in four longitudinal strains (three of which were co-resistant to aztreonam/avibactam and cefiderocol) following treatments with ceftazidime/avibactam (n = 3) or cefiderocol (n = 1). Resistance to meropenem/vaborbactam and imipenem/cilastatin/relebactam was observed in one case after exposure to ceftazidime/avibactam and imipenem/cilastatin/relebactam. Resistome analysis showed that resistance to novel βL/βLICs was related to specific mutations within blaKPC carbapenemase gene (D179Y mutation [KPC-33]; deletion Δ242-GT-243 [KPC-14]) in three longitudinal strains, while porin loss (truncated OmpK35 and OmpK36 porins) was observed in one case.

CONCLUSION

Therapy with novel βL/βLICs or cefiderocol may lead to the selection of resistant mutants in the presence of factors influencing the achievement of PK/PD targets. KPC variants are mainly associated with resistance to ceftazidime/avibactam, and some of them (e.g. KPC-14) may also be associated with reduced susceptibility to aztreonam/avibactam and/or cefiderocol. Loss of function of the OmpK35 and OmpK36 porins appears to play a role in the development of resistance to meropenem/vaborbactam and/or imipenem/relebactam, but other mechanisms may also be involved.

Overexpression of β-lactamase genes (blaKPC, blaSHV) and novel CirA deficiencies contribute to decreased cefiderocol susceptibility in carbapenem-resistant Klebsiella pneumoniae before its approval in China

Cefiderocol (FDC) is an effective antibiotic that is used to treat severe infections caused by carbapenem-resistant Klebsiella pneumoniae (CRKP). The mechanisms underlying FDC resistance and molecular epidemiology in China remain unclear. We collected 477 non-duplicate CRKP clinical isolates in central China and characterized their susceptibility to FDC, virulence genes, and sequence typing. The overall FDC susceptibility rate of CRKP was 99.2% in central China, which was higher than that in North America and Europe (96.1%), with MIC50/90 values of 1/2 mg/L. The decrease in FDC susceptibility in central China was concentrated in the ST11 CRKP-carrying virulence plasmids. Whole-genome sequencing (WGS) and quantitative reverse transcription PCR (qRT-PCR) experiments showed that serine β-lactamases, especially highly expressed KPC and SHV, substantially decreased FDC susceptibility in four FDC non-susceptible isolates (two resistant and two intermediate isolates). Notably, different CirA deficiencies, p.E450GfsTer16 and p.E133Ter, were found in both of the resistant isolates. In contrast, global WGS data indicate that the resistance mechanisms in North America and Europe were primarily associated with NDM and KPC variants, predominantly found in ST307 and ST147. Overall, FDC exhibits excellent activity against CRKP in central China, with resistance mechanisms primarily related to high KPC and SHV expression, along with deficiencies in CirA, frequently observed in ST11. This is remarkably different from the situation in North America and Europe and will directly impact the choice of clinical interventions. Additionally, the surveillance of FDC resistance in China is imperative.

Cefiderocol - An effective antimicrobial for MDR infections but a challenge for routine antimicrobial susceptibility testing

PURPOSE

Cefiderocol is a novel cephalosporin-siderophore conjugate antibiotic that holds promise to thwart infections caused by multi-drug-resistant gram-negative bacilli. Its antibacterial activity against normally susceptible species is not affected by most β-lactamases, including metallo-β-lactamases. Due to the siderophore-mediated entry into the cell, the activity of cefiderocol is less affected by porin loss or active efflux resistance than many other β-lactam antibiotics. The aim of this study was to assess in vitro susceptibility to the cefiderocol of carbapenemase-producing gram-negative bacilli from clinical samples of hospitalized patients.

MATERIALS AND METHODS

We analyzed 102 clinical strains of carbapenemase-producing Enterobacterales and non-fermentives from hospital centers in Łódź, Poland. Antimicrobial susceptibility to cefiderocol was tested by the minimum inhibitory concentration test strips and disc diffusion methods.

RESULTS

The obtained results turned out to be ambiguous, and the area of technical uncertainty made their interpretation very difficult.

CONCLUSIONS

The cost of therapy with this antibiotic, and difficulties in interpreting the drug susceptibility are the limitations to the use of cefiderocol. Intensive work should be carried out to finally standardize an easily accessible and reliable method for the determination of susceptibility to cefiderocol.

In vitro activity of cefiderocol against carbapenemase-producing and meropenem-non-susceptible Gram-negative bacteria collected in the Japan Antimicrobial Resistant Bacterial Surveillance

OBJECTIVES

The treatment options available for infections caused by multidrug-resistant Gram-negative pathogens are often limited. Cefiderocol (CFDC) is a novel siderophore cephalosporin that exhibits activity against these pathogens. Several studies have reported the in vitro activity of CFDC against isolates from Europe, the United States, and China, but the activity against carbapenem-resistant bacteria with IMP-type carbapenemase has not been extensively studied. We, therefore, studied the in vitro activities of CFDC against carbapenem-resistant bacteria with available genomic backgrounds based on whole-genome sequencing (WGS) in Japan, where the IMP-type is the predominant carbapenemase produced by Gram-negative rods.

METHODS

We selected 603 isolates (528 Enterobacterales, 18 Pseudomonas aeruginosa, and 57 Acinetobacter spp.) from a collection of Gram-negative clinical isolates collected during a Japan Antimicrobial Resistance Bacterial Surveillance program and evaluated the antimicrobial activities of CFDC, ceftolozane/tazobactam (CTLZ/TAZ), imipenem-relebactam (IPM/REL), and ceftazidime/avibactam (CAZ/AVI) against carbapenemase-producing Enterobacterales, carbapenemase-non-producing meropenem-non-susceptible Enterobacterales, and carbapenemase-producing nonfermentative bacteria.

RESULTS

Among these, 97.7% of carbapenemase-producing Enterobacterales (99.2% of IMP-type carbapenemase-producing Enterobacterales), 100% of carbapenemase-producing P. aeruginosa, and 91.2% of carbapenemase-producing Acinetobacter spp. were susceptible to CFDC, showing better antimicrobial activity than the other antimicrobial agents evaluated in this study. CFDC was highly effective against class A-, B-, and D β-lactamase-harbouring isolates when compared to the other antimicrobial agents. In addition, the relationship between CFDC resistance and three genetic factors involved in resistance was discussed.

CONCLUSIONS

This is the first large-scale study to systematically demonstrate the efficacy of CFDC against IMP-type carbapenemase-producing strains with known genomic backgrounds.

Guidelines for Antibacterial Treatment of Carbapenem-Resistant Enterobacterales Infections

This guideline aims to promote the prudent use of antibacterial agents for managing carbapenem-resistant Enterobacterales (CRE) infections in clinical practice in Korea. The general section encompasses recommendations for the management of common CRE infections and diagnostics, whereas each specific section is structured with key questions that are focused on antibacterial agents and disease-specific approaches. This guideline covers both currently available and upcoming antibacterial agents in Korea.

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.

Optimizing Treatment for Carbapenem-Resistant Acinetobacter baumannii Complex Infections: A Review of Current Evidence

Carbapenem-resistant Acinetobacter baumannii complex (CRAB) poses a significant global health challenge owing to its resistance to multiple antibiotics and limited treatment options. Polymyxin-based therapies have been widely used to treat CRAB infections; however, they are associated with high mortality rates and common adverse events such as nephrotoxicity. Recent developments include numerous observational studies and randomized clinical trials investigating antibiotic combinations, repurposing existing antibiotics, and the development of novel agents. Consequently, recommendations for treating CRAB are undergoing significant changes. The importance of colistin is decreasing, and the role of sulbactam, which exhibits direct antibacterial activity against A. baumannii complex, is being reassessed. High-dose ampicillin-sulbactam-based combination therapies, as well as combinations of sulbactam and durlobactam, which prevent the hydrolysis of sulbactam and binds to penicillin-binding protein 2, have shown promising results. This review introduces recent advancements in CRAB infection treatment based on clinical trial data, highlighting the need for optimized treatment protocols and comprehensive clinical trials to combat the evolving threat of CRAB effectively.

Novel Siderophore Cephalosporin and Combinations of Cephalosporins with β-Lactamase Inhibitors as an Advancement in Treatment of Ventilator-Associated Pneumonia

In an era of increasing antibiotic resistance among pathogens, the treatment options for infectious diseases are diminishing. One of the clinical groups especially vulnerable to this threat are patients who are hospitalized in intensive care units due to ventilator-associated pneumonia caused by multidrug-resistant/extensively drug-resistant Gram-negative bacteria. In order to prevent the exhaustion of therapeutic options for this life-threatening condition, there is an urgent need for new pharmaceuticals. Novel β-lactam antibiotics, including combinations of cephalosporins with β-lactamase inhibitors, are proposed as a solution to this escalating problem. The unique mechanism of action, distinctive to this new group of siderophore cephalosporins, can overcome multidrug resistance, which is raising high expectations. In this review, we present the summarized results of clinical trials, in vitro studies, and case studies on the therapeutic efficacy of cefoperazone-sulbactam, ceftolozane-tazobactam, ceftazidime-avibactam, and cefiderocol in the treatment of ventilator-associated pneumonia. We demonstrate that treatment strategies based on siderophore cephalosporins and combinations of β-lactams with β-lactamases inhibitors show comparable or higher clinical efficacy than those used with classic pharmaceuticals, like carbapenems, colistin, or tigecycline, and are often associated with a lower risk of adverse events.

Effectiveness of cephalosporins in hydrolysis and inhibition of Staphylococcus aureus and Escherichia coli biofilms

IMPORTANCE

Staphylococcus aureus and Escherichia coli contribute to global health challenges by forming biofilms, a key virulence element implicated in the pathogenesis of several infections.

OBJECTIVE

The study examined the efficacy of various generations of cephalosporins against biofilms developed by pathogenic S. aureus and E. coli.

METHODS

The development of biofilms by both bacteria was assessed using petri-plate and microplate methods. Biofilm hydrolysis and inhibition were tested using first to fourth generations of cephalosporins, and the effects were analyzed by crystal violet staining and phase contrast microscopy.

RESULTS

Both bacterial strains exhibited well-developed biofilms in petri-plate and microplate assays. Cefradine (first generation) showed 76.78% hydrolysis of S. aureus biofilm, while significant hydrolysis (59.86%) of E. coli biofilm was observed by cefipime (fourth generation). Similarly, cefuroxime, cefadroxil, cefepime, and cefradine caused 78.8%, 71.63%, 70.63%, and 70.51% inhibition of the S. aureus biofilms, respectively. In the case of E. coli, maximum biofilm inhibition (66.47%) was again shown by cefepime. All generations of cephalosporins were more effective against S. aureus than E. coli, which was confirmed by phase contrast microscopy.

CONCLUSIONS AND RELEVANCE

Cephalosporins exhibit dual capabilities of hydrolyzing and inhibiting S. aureus and E. coli biofilms. First-generation cephalosporins exhibited the highest inhibitory activity against S. aureus, while the third and fourth generations significantly inhibited E. coli biofilms. This study highlights the importance of tailored antibiotic strategies based on the biofilm characteristics of specific bacterial strains.

In vitro activity of cefiderocol against Gram-negative pathogens isolated from people with cystic fibrosis and bronchiectasis

OBJECTIVES

Gram-negative pathogens causing respiratory infection in people with cystic fibrosis and bronchiectasis are becoming progressively more resistant to conventional antibiotics. Although cefiderocol is licenced for the treatment of infections due to Gram-negative organisms, there are limited data on the activity of cefiderocol against pathogens associated with chronic respiratory diseases. The aim of this study was to determine the susceptibility of Gram-negative pathogens from cystic fibrosis and bronchiectasis to cefiderocol and comparator antibiotics.

METHODS

Minimal inhibitory concentrations (MICs) of cefiderocol and 15 comparator antibiotics were determined by broth microdilution against 300 respiratory isolates: Burkholderia spp., Stenotrophomonas spp., Achromobacter spp., Ralstonia spp. and Pandoraea spp., and used to calculate the MIC of each antibiotic required to inhibit 50% (MIC50) and 90% (MIC90) of isolates.

RESULTS

The MIC50 and MIC90 of cefiderocol for all 300 isolates tested was 0.25 and 32 mg/L, with 232 (77.3%) isolates having an MIC value ≤2 mg/L. In addition, cefiderocol demonstrated excellent activity against Stenotrophomonas spp. and Achromobacter spp. isolates, with 86.7% and 87.2%, respectively, exhibiting an MIC of 2 mg/L. Tigecycline also demonstrated good activity against all isolates with an MIC50 of <0.5 mg/L.

CONCLUSIONS

These in vitro data demonstrated that cefiderocol had greater activity than most comparator antibiotics and could be an alternative treatment option for respiratory infection caused by these pathogens that has not responded to first-line therapy.

Cefiderocol: early clinical experience for multi-drug resistant gram-negative infections

Multi-drug resistant gram-negative bacteria present a significant global health threat. Cefiderocol (CFDC), a siderophore cephalosporin, has shown potential in combating this threat, but with the currently available data, its role in therapy remains poorly defined. This multi-center, retrospective cohort study evaluated the real-world application of CFDC across six U.S. medical centers from January 2018 to May 2023. Patients aged ≥18 years and who had received ≥72 hours of CFDC were included. The primary outcome was a composite of clinical success: survival at 30 days, absence of symptomatic microbiologic recurrence at 30 days following CFDC treatment initiation, and resolution of signs and symptoms. Secondary outcomes included time to CFDC therapy and on-treatment non-susceptibility to CFDC. A total of 112 patients were included, with median (interquartile range [IQR]) APACHE II scores of 15 (19-18). Clinical success was observed in 68.8% of patients, with a mortality rate of 16.1% and comparable success rates across patients infected with carbapenem-resistant gram-negative infections. The most common isolated organisms were Pseudomonas aeruginosa (61/112, 54.5%, of which 55/61 were carbapenem-resistant) and carbapenem-resistant Acinetobacter baumannii (32/112, 28.6%). Median (IQR) time to CFDC therapy was 77 (14-141) hours. Two patients experienced a non-anaphylactic rash as an adverse drug reaction. On-treatment non-susceptibility to CFDC was found in six patients, notably due to P. aeruginosa and A. baumannii.IMPORTANCECFDC was safe and clinically effective as a monotherapy or in combination in treating a variety of carbapenem-resistant gram-negative infections. Further prospective studies are warranted to confirm these findings.

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.

Reviewing novel treatment options for carbapenem-resistant Enterobacterales

INTRODUCTION

Carbapenem resistant Enterobacterales (CRE) are a major threat to global health and hospital-onset CRE infections have risen during the COVID-19 pandemic. Novel antimicrobials are now available for the treatment of CRE infections. There remains an urgent need for new antimicrobials for CRE, especially for those producing metallo-β-lactamases.

AREAS COVERED

This article discusses previously published research supporting currently available novel antimicrobials for the treatment of CRE infections. Newer compounds currently being evaluated in clinical trials are covered. A literature search was conducted in PubMed over all available dates for relevant published papers and conference abstracts with the search terms, 'CRE,' 'carbapenem-resistant Enterobacterales,' 'β-lactam-β-lactamase inhibitor,' 'KPC,' 'NDM,' 'metallo-β-lactamase,' 'ceftazidime-avibactam,' 'meropenem-vaborbactam,' 'imipenem-cilastatin-relebactam,' 'cefiderocol,' 'eravacycline,' 'plazomicin,' 'taniborbactam,' 'zidebactam,' and 'nacubactam.'

EXPERT OPINION

Novel antimicrobials for CRE infections have been developed, most notably the β-lactam-β-lactamase inhibitor combinations, though treatment options for infections with metallo-β-lactamase producing Enterobacterales remain few and have limitations. Development of antibiotics with activity against metallo-β-lactamase producing Enterobacterales is eagerly awaited, and there are promising new compounds in clinical trials. Finally, more clinical research is needed to optimize and individualize treatment approaches, which will help guide antimicrobial stewardship initiatives aimed at reducing the spread of CRE and development of further resistance.

In vitro activity of cefiderocol, a siderophore cephalosporin, against carbapenem-resistant hypervirulent Klebsiella pneumoniae in China

Cefiderocol is a siderophore cephalosporin that binds ferric iron and utilizes iron transporters to cross the cell membrane. Hypervirulent Klebsiella pneumoniae (hvKp) is known to produce more siderophores; in this case, the uptake of cefiderocol may be decreased. Therefore, the objective of this study was to evaluate the in vitro activity of cefiderocol against hvKp isolates. A total of 320 carbapenem-resistant K. pneumoniae (CRKp) isolates were collected in China between 2014 and 2022, including 171 carbapenem-resistant hvKp (CR-hvKp) and 149 carbapenem-resistant classical K. pneumoniae (CR-cKp). Quantitative detection of siderophores showed that the average siderophore production of CR-hvKp (234.6 mg/L) was significantly higher than that of CR-cKp (68.9 mg/L, P < 0.001). The overall cefiderocol resistance rate of CR-hvKp and CR-cKp was 5.8% (10/171) and 2.7% (4/149), respectively. The non-susceptible rates of both cefiderocol and siderophore production of CR-hvKp isolates were higher than those of CR-cKp in either NDM-1- or KPC-2-producing groups. The MIC90 and MIC50 for CR-hvKp and CR-cKp were 8 mg/L and 2 mg/L and 4 mg/L and 1 mg/L, respectively. The cumulative cefiderocol MIC distribution for CR-hvKp was significantly lower than that of CR-cKp isolates (P = 0.003). KL64 and KL47 consisted of 53.9% (83/154) and 75.7% (53/70) of the ST11 CR-hvKp and CR-cKp, respectively, and the former had significantly higher siderophore production. In summary, cefiderocol might be less effective against CR-hvKp compared with CR-cKp isolates, highlighting the need for caution regarding the prevalence of cefiderocol-resistant K. pneumoniae strains, particularly in CR-hvKp isolates.

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.

Emerging infections in vulnerable hosts: Stenotrophomonas maltophilia and Elizabethkingia anophelis

PURPOSE OF REVIEW

This systematic review aimed to explore the recent trends in the epidemiology, risk factors, and antimicrobial susceptibility of two emerging opportunistic pathogens, Stenotrophomonas maltophilia and Elizabethkingia anophelis .

RECENT FINDINGS

Since 2020, numerous outbreaks of S. maltophilia and E. anophelis have been reported worldwide. Most of these outbreaks have been associated with healthcare facilities, although one outbreak caused by E. anophelis in France was considered a community-associated infection. In terms of antimicrobial susceptibility, trimethoprim/sulfamethoxazole (TMP-SMZ), levofloxacin, and minocycline have exhibited good efficacy against S. maltophilia . Additionally, cefiderocol and a combination of aztreonam and avibactam have shown promising results in in vitro susceptibility testing. For E. anophelis , there is currently no consensus on the optimal treatment. Although some studies have reported good efficacy with rifampin, TMP-SMZ, piperacillin/tazobactam, and cefoperazone/sulbactam, minocycline had the most favourable in vitro susceptibility rates. Cefiderocol may serve as an alternative due to its low minimum inhibitory concentration (MIC) against E. anophelis . The role of vancomycin in treatment is still uncertain, although several successful cases with vancomycin treatment, even with high MIC values, have been reported.

SUMMARY

Immunocompromised patients are particularly vulnerable to infections caused by S. maltophilia and E. anophelis , but the optimal treatment strategy remains inconclusive. Further research is necessary to determine the most effective use of conventional and novel antimicrobial agents in combatting these multidrug-resistant pathogens.

In silico identification of potential inhibitors of vital monkeypox virus proteins from FDA approved drugs

The World Health Organization (WHO) recently declared the monkeypox outbreak 'A public health emergency of international concern'. The monkeypox virus belongs to the same Orthopoxvirus genus as smallpox. Although smallpox drugs are recommended for use against monkeypox, monkeypox-specific drugs are not yet available. Drug repurposing is a viable and efficient approach in the face of such an outbreak. Therefore, we present a computational drug repurposing study to identify the existing approved drugs which can be potential inhibitors of vital monkeypox virus proteins, thymidylate kinase and D9 decapping enzyme. The target protein structures of the monkeypox virus were modelled using the corresponding protein structures in the vaccinia virus. We identified four potential inhibitors namely, Tipranavir, Cefiderocol, Doxorubicin, and Dolutegravir as candidates for repurposing against monkeypox virus from a library of US FDA approved antiviral and antibiotic drugs using molecular docking and molecular dynamics simulations. The main goal of this in silico study is to identify potential inhibitors against monkeypox virus proteins that can be further experimentally validated for the discovery of novel therapeutic agents against monkeypox disease.

Comparative De Novo and Pan-Genome Analysis of MDR Nosocomial Bacteria Isolated from Hospitals in Jeddah, Saudi Arabia

Multidrug-resistant (MDR) bacteria are one of the most serious threats to public health, and one of the most important types of MDR bacteria are those that are acquired in a hospital, known as nosocomial. This study aimed to isolate and identify MDR bacteria from selected hospitals in Jeddah and analyze their antibiotic-resistant genes. Bacteria were collected from different sources and wards of hospitals in Jeddah City. Phoenix BD was used to identify the strains and perform susceptibility testing. Identification of selected isolates showing MDR to more than three classes on antibiotics was based on 16S rRNA gene and whole genome sequencing. Genes conferring resistance were characterized using de novo and pan-genome analyses. In total, we isolated 108 bacterial strains, of which 75 (69.44%) were found to be MDR. Taxonomic identification revealed that 24 (32%) isolates were identified as Escherichia coli, 19 (25.3%) corresponded to Klebsiella pneumoniae, and 17 (22.67%) were methicillin-resistant Staphylococcus aureus (MRSA). Among the Gram-negative bacteria, K. pneumoniae isolates showed the highest resistance levels to most antibiotics. Of the Gram-positive bacteria, S. aureus (MRSA) strains were noticed to exhibit the uppermost degree of resistance to the tested antibiotics, which is higher than that observed for K. pneumoniae isolates. Taken together, our results illustrated that MDR Gram-negative bacteria are the most common cause of nosocomial infections, while MDR Gram-positive bacteria are characterized by a wider antibiotic resistance spectrum. Whole genome sequencing found the appearance of antibiotic resistance genes, including SHV, OXA, CTX-M, TEM-1, NDM-1, VIM-1, ere(A), ermA, ermB, ermC, msrA, qacA, qacB, and qacC.

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.

The prevalence of colistin resistance in clinical Stenotrophomonas maltophilia isolates worldwide: a systematic review and meta-analysis

While trimethoprim-sulfamethoxazole (TMP-SMX) is the first-line therapy of Stenotrophomonas maltophilia infections, colistin is one of the therapeutic options in cases of allergy or resistance to TMP-SMX. However, understanding the global status of resistance to colistin amongst S. maltophilia isolates could be helpful for appropriate antibiotic prescription. This study aimed to conduct a systematic review and meta-analysis to examine the prevalence of colistin resistance in clinical S. maltophilia isolates worldwide. According to eligibility criteria, a total of 61 studies were included in the analysis. The pooled prevalence for colistin resistance was 42% (95% CI: 35-49%), ranging from 0.1 to 97%. Subgroups analysis indicated that, the pooled prevalence of colistin resistance was 44% (95% CI: 29-60%) in 15 studies during 2000-2010, and it was estimated to be 41% (95% CI: 33-50%) in 46 articles from 2011 to 2021. It was 46% (95% CI: 35-58%) in the studies that used broth microdilution method, and 39% (95% CI: 30-49%) in the studies with other used methods. The resistance rate in Asian countries was 45% (95% CI: 31-60%), in European countries was 45% (95% CI: 34-56%) and in the countries of North and South America was 33% (95% CI: 20-46%). Our review showed notable resistance to colistin in clinical S. maltophilia isolates. Given the estimated resistance rates, alternative antibiotics could be preferred to treat serious infections due to S. maltophilia.

An Overview of Cefiderocol's Therapeutic Potential and Underlying Resistance Mechanisms

Antimicrobial resistance continues to increase globally and treatment of difficult-to-treat (DTT) infections, mostly associated with carbapenem-resistant (CR) Pseudomonas aeruginosa, CR Acinetobacter baumannii, and CR- and third-generation-cephalosporins-resistant Enterobacterales remains a challenge for the clinician. The recent approval of cefiderocol has broaden the armamentarium for the treatment of patients with DTT infections. Cefiderocol is a siderophore cephalosporin that has shown excellent antibacterial activity, in part due to its innovative way of cell permeation. It is relatively stable compared to most commonly found carbapenamases. However, some resistant mechanisms to cefiderocol have already been identified and reduced susceptibility has developed during patient treatment, highlighting that the clinical use of cefiderocol must be rational. In this review, we summarize the current available treatments against the former resistant bacteria, and we revise and discuss the mechanism of action of cefiderocol, underlying the biological function of siderophores, the therapeutic potential of cefiderocol, and the mechanisms of resistance reported so far.

In vitro activities of omadacycline, eravacycline, cefiderocol, apramycin, and comparator antibiotics against Acinetobacter baumannii causing bloodstream infections in Greece, 2020-2021: a multicenter study

Resistance of Acinetobacter baumannii to multiple clinically important antimicrobials has increased to very high rates in Greece, rendering most of them obsolete. The aim of this study was to determine the molecular epidemiology and susceptibilities of A. baumannii isolates collected from different hospitals across Greece. Single-patient A. baumannii strains isolated from blood cultures (n = 271), from 19 hospitals, in a 6-month period (November 2020-April 2021) were subjected to minimum inhibitory concentration determination and molecular testing for carbapenemase, 16S rRNA methyltransferase and mcr gene detection and epidemiological evaluation. 98.9% of all isolates produced carbapenemase OXA-23. The vast majority (91.8%) of OXA-23 producers harbored the armA and were assigned mainly (94.3%) to sequence group G1, corresponding to IC II. Apramycin (EBL-1003) was the most active agent inhibiting 100% of the isolates at ≤16 mg/L, followed by cefiderocol which was active against at least 86% of them. Minocycline, colistin and ampicillin-sulbactam exhibited only sparse activity (S <19%), while eravacycline was 8- and 2-fold more active than minocycline and tigecycline respectively, by comparison of their MIC₅₀/₉₀ values. OXA-23-ArmA producing A. baumannii of international clone II appears to be the prevailing epidemiological type of this organism in Greece. Cefiderocol could provide a useful alternative for difficult to treat Gram-negative infections, while apramycin (EBL-1003), the structurally unique aminoglycoside currently in clinical development, may represent a highly promising agent against multi-drug resistant A. baumanni infections, due to its high susceptibility rates and low toxicity.

Navigating Available Treatment Options for Carbapenem-Resistant Acinetobacter baumannii-calcoaceticus Complex Infections

Carbapenem-resistant Acinetobacter baumannii-calcoaceticus complex (CRAB) is one of the top-priority pathogens for new antibiotic development. Unlike other antibiotic-resistant threats, none of the available therapies have been shown to consistently reduce mortality or improve patient outcomes in clinical trials. Antibiotic combination therapy is routinely used in clinical practice; however, the preferred combination has not been defined. This narrative review focuses on evidence-based solutions for the treatment of invasive CRAB infections. We dissect the promise and perils of traditional agents used in combination, such as colistin, sulbactam, and the tetracyclines, and offer clinical pearls based on our interpretation of the available data. Next, we investigate the merits of newly developed β-lactam agents like cefiderocol and sulbactam-durlobactam, which have demonstrated contrasting results in recent randomized clinical trials. The review concludes with the authors' perspective on the evolving treatment landscape for CRAB infections, which is complicated by limited clinical data, imperfect treatment options, and a need for future clinical trials. We propose that effective treatment for CRAB infections requires a personalized approach that incorporates host factors, the site of infection, pharmacokinetic-pharmacodynamic principles, local molecular epidemiology of CRAB isolates, and careful interpretation of antibiotic susceptibility testing results. In most clinical scenarios, a dose-optimized, sulbactam-based regimen is recommended with the addition of at least one other in vitro active agent. Should sulbactam-durlobactam receive regulatory approval, recommendations will need to be re-evaluated with the most recent evidence.

In Vitro Activity of Cefiderocol on Multiresistant Bacterial Strains and Genomic Analysis of Two Cefiderocol Resistant Strains

Cefiderocol is a new siderophore cephalosporin that is effective against multidrug-resistant Gram-negative bacteria, including carbapenem-resistant strains. The aim of this study was to evaluate the activity of this new antimicrobial agent against a collection of pathogens using broth microdilution assays and to analyze the possible mechanism of cefiderocol resistance in two resistant Klebsiella pneumoniae isolates. One hundred and ten isolates were tested, comprising 67 Enterobacterales, two Acinetobacter baumannii, one Achromobacter xylosoxidans, 33 Pseudomonas aeruginosa and seven Stenotrophomonas maltophilia. Cefiderocol showed good in vitro activity, with an MIC < 2 μg/mL, and was able to inhibit 94% of the tested isolates. We observed a resistance rate of 6%. The resistant isolates consisted of six Klebsiella pneumoniae and one Escherichia coli, leading to a resistance rate of 10.4% among the Enterobacterales. Whole-genome sequencing analysis was performed on two cefiderocol-resistant Klebsiella pneumoniae isolates to investigate the possible mutations responsible for the observed resistance. Both strains belonged to ST383 and harbored different resistant and virulence genes. The analysis of genes involved in iron uptake and transport showed the presence of different mutations located in fhuA, fepA, iutA, cirA, sitC, apbC, fepG, fepC, fetB, yicI, yicJ, and yicL. Furthermore, for the first time, to the best of our knowledge, we described two Klebsiella pneumoniae isolates that synthesize a truncated fecA protein due to the transition from G to A, leading to a premature stop codon in the amino acid position 569, and a TonB protein carrying a 4-amino acid insertion (PKPK) after Lysine 103. In conclusion, our data show that cefiderocol is an effective drug against multidrug-resistant Gram-negative bacteria. However, the higher resistance rate observed in Enterobacterales underlines the need for active surveillance to limit the spread of these pathogens and to avoid the risks associated with the emergence of resistance to new drugs.

Cefiderocol, a Siderophore Cephalosporin, as a Treatment Option for Infections Caused by Carbapenem-Resistant Enterobacterales

Carbapenem-resistant Enterobacterales (CRE) remain a significant public health threat, and, despite recent approvals, new antibiotics are needed. Severe infections caused by CRE, such as nosocomial pneumonia and bloodstream infections, are associated with a relatively high risk of morbidity and mortality. The recent approval of ceftazidime-avibactam, imipenem-relebactam, meropenem-vaborbactam, plazomicin, eravacycline and cefiderocol has broadened the armamentarium for the treatment of patients with CRE infections. Cefiderocol is a siderophore cephalosporin with overall potent in vitro activity against CRE. It is taken up via iron transport channels through active transport, with some entry into bacteria through traditional porin channels. Cefiderocol is relatively stable against hydrolysis by most serine- and metallo-beta-lactamases, including KPC, NDM, VIM, IMP and OXA carbapenemases-the most frequent carbapenemases detected in CRE. The efficacy and safety of cefiderocol has been demonstrated in three randomised, prospective, parallel group or controlled clinical studies in patients at risk of being infected by multidrug-resistant or carbapenem-resistant Gram-negative bacteria. This paper reviews the in vitro activity, emergence of resistance, preclinical effectiveness, and clinical experience for cefiderocol, and its role in the management of patients with CRE infections.

In vitro activity of cefiderocol against clinically important carbapenem non-susceptible Gram-negative bacteria from Saudi Arabia

OBJECTIVES

Cefiderocol is a novel catechol-substituted siderophore cephalosporin with broad-spectrum activity against Gram-negative pathogens. However, variation of its activity among carbapenemase producers from various regions and countries has been reported. Here, we checked the in vitro activity against Gram-negative carbapenem non-susceptible bacteria collected in Saudi Arabia.

METHODS

Cefiderocol MICs were determined using the iron-depleted cation-adjusted Mueller-Hinton broth and interpreted according to the Clinical and Laboratory Standards Institute guidelines. Isolates (n = 288) included carbapenemase-producing Escherichia coli (n = 46), Klebsiella pneumoniae (n = 98), Acinetobacter baumannii (n = 65), and Pseudomonas aeruginosa (n = 79) clinical isolates.

RESULTS

Cefiderocol inhibited 73.26% (211/288) of the isolates studied at concentrations of ≤ 4 mg/L. Cefiderocol inhibited all carbapenem-resistant A. baumannii isolates (65/65, 100%) producing OXA-23-like, OXA-24-like, and NDM, and nearly all P. aeruginosa isolates (75/79, 94.94%), including those producing VIM and NDM. In contrast, the carbapenemase-producing isolates from the Enterobacterales group demonstrated significantly higher MICs with only 53.06% (52/98) of K. pneumoniae and 41.3% (19/46) of E. coli isolates exhibiting MICs of ≤4 mg/L. Isolates showing elevated MICs (73/144, 50.69%) included NDM (20/29, 68.97%), NDM/OXA-48-like (34/59, 57.63%), OXA-48-like (18/52, 34.62%), and KPC (1/4, 25%) producers, thus showing no clear association with the production of serine-type or metallo-type carbapenemases. However, high cefiderocol MICs (≥ 32mg/L) were associated with isolates producing NDM, and in particular, among those coproducing the OXA-232-type enzyme.

CONCLUSIONS

Cefiderocol had excellent activity against multi-drug resistant non-fermenting Gram-negative pathogens. Reasons behind the high cefiderocol MICs in certain Enterobacterales isolates need further investigation.

Structural Basis of PER-1-Mediated Cefiderocol Resistance and Synergistic Inhibition of PER-1 by Cefiderocol in Combination with Avibactam or Durlobactam in Acinetobacter baumannii

Cefiderocol is a novel siderophore cephalosporin that displays activity against Gram-negative bacteria. To establish cefiderocol susceptibility levels of Acinetobacter baumannii strains from China, we performed susceptibility testing and genomic analyses on 131 clinical isolates. Cefiderocol shows high activity against the strains. The production of PER-1 is the key mechanism of cefiderocol resistance. In silico studies predicted that avibactam and durlobactam could inhibit cefiderocol hydrolysis by PER-1, which was confirmed by determining cefiderocol MICs in combination with inhibitors.

Cefiderocol against Multi-Drug and Extensively Drug-Resistant Escherichia coli: An In Vitro Study in Poland

Cefiderocol (CFDC) is a novel, broad-spectrum siderophore cephalosporin with potential activity against multi-drug (MDR) and extensively drug-resistant (XDR) Enterobacterales, including carbapenem-resistant strains. We assessed the in vitro susceptibility to CFDC of MDR, and XDR E. coli isolates derived from clinical samples of hospitalized patients. Disk diffusion (DD) and MIC (minimum inhibitory concentration) test strip (MTS) methods were used. The results were interpreted based on EUCAST (version 12.0 2022) recommendations. Among all E. coli isolates, 98 (94.2%) and 99 (95.2%) were susceptible to CFDC when the DD and MTS methods were used, respectively (MIC range: <0.016−4 µg/mL, MIC50: 0.19 µg/mL, MIC90: 0.75 µg/mL). With the DD and MTS methods, all (MIC range: 0.016−2 µg/mL, MIC50: 0.19 µg/mL, MIC90: 0.75 µg/mL) but three (96.6%) ESBL-positive isolates were susceptible to CFDC. Out of all the metallo-beta-lactamase-positive E. coli isolates (MIC range: 0.016−4 µg/mL, MIC50: 0.5 µg/mL, MIC90: 1.5 µg/mL), 16.7% were resistant to CFDC with the DD method, while 11.1% were resistant to CFDC when the MTS method was used. CFDC is a novel therapeutic option against MDR and XDR E. coli isolates and is promising in the treatment of carbapenem-resistant E. coli strains, also for those carrying Verona integron-encoded metallo-beta-lactamases, when new beta-lactam-beta-lactamase inhibitors cannot be used.

Synergistic Effect of Clinically Available Beta-Lactamase Inhibitors Combined with Cefiderocol against Carbapenemase-Producing Gram-Negative Organisms

The role of β-lactamases in reduced susceptibility or resistance to cefiderocol has been supported by recent reports. The purpose of this study was to investigate the in vitro impact of clinically available β-lactamase inhibitors on cefiderocol activity against characterized carbapenemase-producing Gram-negative isolates. A collection of 39 well-characterized Gram-negative isolates obtained from various clinical sources and countries were included. Cefiderocol antimicrobial susceptibility was evaluated via reference broth microdilution. The chequerboard microdilution method and time-kill assays were used to determine the synergy of tazobactam, avibactam, vaborbactam and relebactam in combination with cefiderocol. MICs of cefiderocol presented a 4- to 256-fold reduction against Klebsiella pneumoniae carbapenemase (KPC)-producing Gram-negative isolates (predominantly K. pneumoniae) when avibactam, vaborbactam and relebactam were combined individually. Notably, the KPC-inhibitors led to a 4- to 32-fold reduction in cefiderocol MICs in the four cefiderocol-resistant KPC-producing K. pneumoniae isolates, showing restoration of cefiderocol susceptibility (MIC ≤ 2 mg/L) in ten out of twelve cases. Tazobactam led to a 4- to 64-fold decrease in cefiderocol MICs only in K. pneumoniae strains harbouring bla_KPC-41, bla_KPC-31, bla_KPC-53 and bla_KPC-66. The synergistic effect of all serine-β-lactamase inhibitors on cefiderocol activity was also shown in OXA-48-like-producing Enterobacterales strains. Conversely, a combination of β-lactamases inhibitors with cefiderocol was not synergistic with all OXA-23-like-producing strains and most metallo-β-lactamases producers. In conclusion, the addition of clinically available serine β-lactamase inhibitors to cefiderocol might represent an important development in the formulation to increase its spectrum and therapeutic efficacy, and to limit in vivo resistance emergence.

Evaluation of the inoculum effect of new antibiotics against carbapenem-resistant enterobacterales

OBJECTIVES

New antibiotics have been developed to treat multidrug-resistant Enterobacterales. We evaluated the impact of the inoculum size on minimal inhibitory concentrations (MICs) of recently commercialized antibiotics.

METHODS

We focused on 40 clinical carbapenemase-producing Enterobacterales and evaluated the impact of the inoculum size on the MICs to cefiderocol and to new β-lactams/β-lactamase inhibitors (ceftolozane-tazobactam, ceftazidime-avibactam, imipenem-relebactam, and meropenem-vaborbactam) at usual and high inocula (105 and 107 CFU/mL, respectively).

RESULTS

At usual inoculum, 15% were resistant to cefiderocol (n = 6), 30% to meropenem-vaborbactam (n = 12), 42.5% to ceftazidime-avibactam (n = 17), 55% to imipenem-relebactam (n = 22), and 90% to ceftolozane-tazobactam (n = 36). At higher inoculum, a switch from susceptible to resistant category was observed for 88% (n = 30/34; CI, 71.6-96.2), 75% (n = 3/4; CI, 21.9-98.7), 72% (n = 13/18; CI, 46.4-89.3), 50% (n = 14/28; CI, 31.1-68.9), and 8.7% (n = 2/23; CI, 1.5-29.5) isolates regarding cefiderocol, ceftolozane-tazobactam, imipenem-relebactam, meropenem-vaborbactam, and ceftazidime-avibactam, respectively.

DISCUSSION

Cefiderocol and meropenem-vaborbactam were the most efficient against carbapenemase-producing Enterobacterales at usual inoculum. When increasing inoculum to 107 CFU/mL, all of the molecules were impacted, particularly cefiderocol and imipenem-relebactam, while others, such as ceftazidime-avibactam, remain mildly affected. Our in vitro results deserved to be confirmed in vivo.

Trends in Stenotrophomonas maltophilia antibiotic resistance rates in the United States Veterans Affairs Health System

Introduction. Stenotrophomonas maltophilia is an important multidrug-resistant Gram-negative pathogen. While largely a hospital-acquired pathogen, there have been increasing reports of the pathogen in the community.

Gap Statement. Trends in S. maltophilia prevalence and resistance rates that include outpatient isolates are unknown.

Aim. We described recent trends in prevalence and resistance of S. maltophilia in the national Veterans Affairs (VA) Healthcare system.

Methodology. The study identified positive S. maltophilia clinical cultures among VA adult patients from 2010 to 2018 across all VA hospitals, long-term care facilities/units, and outpatient settings. Annual S. maltophilia resistance rates were evaluated. Multidrug resistant (MDR) was defined as resistance to sulfamethoxazole/trimethoprim (SMX/TMP) and minocycline or levofloxacin. Time trends were assessed with regression analyses to estimate annual average percent changes (AAPC) with 95 % confidence intervals using Joinpoint software.

Results. Over the 9 year study period, 18 285  S . maltophilia cultures were identified (57 % hospital, 3 % long-term care, 40 % outpatient). The most common source of S. maltophilia cultures were respiratory cultures (34.6 %) followed by urine cultures (30.4 %). In VA hospitals and long-term care facilities, the number of S. maltophilia cultures decreased significantly (by 5.4% and 8.4 % per year respectively). Overall, 3.1 % of isolates were MDR which remained stable over the study period. Resistance to other antibiotics assessed mostly remained stable, except SMX/TMP resistance decreased significantly by 8.5 % (2010, 15 %; 2018, 6 %) per year in VA hospitals.

Conclusion. While previous work has recognized S. maltophilia as primarily a nosocomial pathogen, the present study found that 40 % of cultures collected were among outpatients. Between 2010 and 2018, the number of positive S. maltophilia cultures decreased significantly in the national VA Healthcare System. Resistance to SMX/TMP decreased over the study period in VA hospitals and now more closely reflects previously reported resistance rates worldwide (0–10 %). MDR S. maltophilia remained stable and low in the national VA Healthcare System.

Iron Acquisition Mechanisms and Their Role in the Virulence of Acinetobacter baumannii

Iron is an essential element for survival of most organisms. One mechanism of host defense is to tightly chelate iron to several proteins to limit its extracellular availability. This has forced pathogens such as Acinetobacter baumannii to adapt mechanisms for the acquisition and utilization of iron even in iron-limiting conditions. A. baumannii uses a variety of iron acquisition strategies to meet its iron requirements. It can lyse erythrocytes to harvest the heme molecules, use iron-chelating siderophores, and use outer membrane vesicles to acquire iron. Iron acquisition pathways, in general, have been seen to affect many other virulence factors such as cell adherence, cell motility, and biofilm formation. The knowledge gained from research on iron acquisition led to the synthesis of the antibiotic cefiderocol, which uses iron uptake pathways for entry into the cell with some success as a novel cephalosporin. Understanding the mechanisms of iron acquisition of A. baumannii allows for insight into clinical infections and offer potential targets for novel antibiotics or potentiators of current drugs.

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.

New antimicrobial treatment options for severe Gram-negative infections

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

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).

Empiric treatment of patients with sepsis and septic shock and place in therapy of cefiderocol: a systematic review and expert opinion statement

Carbapenem-resistant Gram-negative bacteria are frequent causes of sepsis and septic shock in intensive care unit (ICU) and thus considered a public health threat. Until now, the best available therapies consist of combinations of preexisting or new antibiotics with β-lactamase inhibitors (either new or preexisting). Several mechanisms of resistance, especially those mediated by metallo-β-lactamases (MBL), are responsible for the inefficacy of these treatments, leaving an unmet medical need. Intravenous cefiderocol has been recently approved by the American Food and Drug Administration (FDA) and European Medicines Agency (EMA) for the treatment of complicated urinary tract infections and nosocomial pneumonia due to Gram-negative, when limited therapeutical options are available. In addition, its ability to hijack bacterial iron uptake mechanisms makes cefiderocol stable against the whole Ambler β-lactamase inhibitors and increases the in vitro efficacy against Gram-negative pathogens (e.g., Enterobacterales spp., Pseudomonas aeruginosa, and Acinetobacter baumannii). Trials have already demonstrated their non-inferiority to comparators. In 2021, ESCMID guidelines released a conditional recommendation supporting the use of cefiderocol against metallo-β-lactamase-producing Enterobacterales and against Acinetobacter baumannii. This review provides the opinion of experts about the general management of empiric treatment of patients with sepsis and septic shock in the intensive care unit and detects the proper place in therapy of cefiderocol considering recent evidence sought through a systematic search.

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.

Clinical challenges treating Stenotrophomonas maltophilia infections: an update

Stenotrophomonas maltophilia is a non-fermenting, Gram-negative bacillus that has emerged as an opportunistic nosocomial pathogen. Its intrinsic multidrug resistance makes treating infections caused by S. maltophilia a great clinical challenge. Clinical management is further complicated by its molecular heterogeneity that is reflected in the uneven distribution of antibiotic resistance and virulence determinants among different strains, the shortcomings of available antimicrobial susceptibility tests and the lack of standardized breakpoints for the handful of antibiotics with in vitro activity against this microorganism. Herein, we provide an update on the most recent literature concerning these issues, emphasizing the impact they have on clinical management of S. maltophilia infections.

Cefiderocol: Systematic Review of Mechanisms of Resistance, Heteroresistance and In Vivo Emergence of Resistance

Cefiderocol appears promising, as it can overcome most β-lactam resistance mechanisms (including β-lactamases, porin mutations, and efflux pumps). Resistance is uncommon according to large multinational cohorts, including against isolates resistant to carbapenems, ceftazidime/avibactam, ceftolozane/tazobactam, and colistin. However, alarming proportions of resistance have been reported in some recent cohorts (up to 50%). A systematic review was conducted in PubMed and Scopus from inception to May 2022 to review mechanisms of resistance, prevalence of heteroresistance, and in vivo emergence of resistance to cefiderocol during treatment. A variety of mechanisms, typically acting in concert, have been reported to confer resistance to cefiderocol: β-lactamases (especially NDM, KPC and AmpC variants conferring resistance to ceftazidime/avibactam, OXA-427, and PER- and SHV-type ESBLs), porin mutations, and mutations affecting siderophore receptors, efflux pumps, and target (PBP-3) modifications. Coexpression of multiple β-lactamases, often in combination with permeability defects, appears to be the main mechanism of resistance. Heteroresistance is highly prevalent (especially in A. baumannii), but its clinical impact is unclear, considering that in vivo emergence of resistance appears to be low in clinical studies. Nevertheless, cases of in vivo emerging cefiderocol resistance are increasingly being reported. Continued surveillance of cefiderocol’s activity is important as this agent is introduced in clinical practice.

Antibiotic Therapy for Difficult-to-Treat Infections in Lung Transplant Recipients: A Practical Approach

Lung transplant recipients are at higher risk to develop infectious diseases due to multi-drug resistant pathogens, which often chronically colonize the respiratory tract before transplantation. The emergence of these difficult-to-treat infections is a therapeutic challenge, and it may represent a contraindication to lung transplantation. New antibiotic options are currently available, but data on their efficacy and safety in the transplant population are limited, and clinical evidence for choosing the most appropriate antibiotic therapy is often lacking. In this review, we provide a summary of the best evidence available in terms of choice of antibiotic and duration of therapy for MDR/XDR P. aeruginosa, Burkholderia cepacia complex, Mycobacterium abscessus complex and Nocardia spp. infections in lung transplant candidates and recipients.

Carbapenem-Resistant Acinetobacter baumannii in U.S. Hospitals: Diversification of Circulating Lineages and Antimicrobial Resistance

Carbapenem-resistant Acinetobacter baumannii (CRAb) is a major cause of health care-associated infections. CRAb is typically multidrug resistant, and infection is difficult to treat. Despite the urgent threat that CRAb poses, few systematic studies of CRAb clinical and molecular epidemiology have been conducted. The Study Network of Acinetobacter as a Carbapenem-Resistant Pathogen (SNAP) is designed to investigate the clinical characteristics and contemporary population structure of CRAb circulating in U.S. hospital systems using whole-genome sequencing (WGS). Analysis of the initial 120 SNAP patients from four U.S. centers revealed that CRAb remains a significant threat to hospitalized patients, affecting the most vulnerable patients and resulting in 24% all-cause 30-day mortality. The majority of currently circulating isolates belonged to ST2Pas, a part of clonal complex 2 (CC2), which is the dominant drug-resistant lineage in the United States and Europe. We identified three distinct sublineages within CC2, which differed in their antibiotic resistance phenotypes and geographic distribution. Most concerning, colistin resistance (38%) and cefiderocol resistance (10%) were common within CC2 sublineage C (CC2C), where the majority of isolates belonged to ST2Pas/ST281Ox. Additionally, we identified ST499Pas as the most common non-CC2 lineage in our study. Our findings suggest a shift within the CRAb population in the United States during the past 10 years and emphasize the importance of real-time surveillance and molecular epidemiology in studying CRAb dissemination and clinical impact. IMPORTANCE Carbapenem-resistant Acinetobacter baumannii (CRAb) constitutes a major threat to public health. To elucidate the molecular and clinical epidemiology of CRAb in the United States, clinical CRAb isolates were collected along with data on patient characteristics and outcomes, and bacterial isolates underwent whole-genome sequencing and antibiotic susceptibility phenotyping. Key findings included emergence of new sublineages within the globally predominant clonal complex 2 (CC2), increased colistin and cefiderocol resistance within one of the CC2 sublineages, and emergence of ST499Pas as the dominant non-CC2 CRAb lineage in U.S. hospitals.

Infectious Diseases Society of America 2022 Guidance on the Treatment of Extended-Spectrum β-lactamase Producing Enterobacterales (ESBL-E), Carbapenem-Resistant Enterobacterales (CRE), and Pseudomonas aeruginosa with Difficult-to-Treat Resistance (DTR-P. aeruginosa)

BACKGROUND

The Infectious Diseases Society of America (IDSA) is committed to providing up-to-date guidance on the treatment of antimicrobial-resistant infections. The initial guidance document on infections caused by extended-spectrum β-lactamase producing Enterobacterales (ESBL-E), carbapenem-resistant Enterobacterales (CRE), and Pseudomonas aeruginosa with difficult-to-treat resistance (DTR-P. aeruginosa) was published on 17 September 2020. Over the past year, there have been a number of important publications furthering our understanding of the management of ESBL-E, CRE, and DTR-P. aeruginosa infections, prompting a rereview of the literature and this updated guidance document.

METHODS

A panel of 6 infectious diseases specialists with expertise in managing antimicrobial-resistant infections reviewed, updated, and expanded previously developed questions and recommendations about the treatment of ESBL-E, CRE, and DTR-P. aeruginosa infections. Because of differences in the epidemiology of resistance and availability of specific anti-infectives internationally, this document focuses on the treatment of infections in the United States.

RESULTS

Preferred and alternative treatment recommendations are provided with accompanying rationales, assuming the causative organism has been identified and antibiotic susceptibility results are known. Approaches to empiric treatment, duration of therapy, and other management considerations are also discussed briefly. Recommendations apply for both adult and pediatric populations.

CONCLUSIONS

The field of antimicrobial resistance is highly dynamic. Consultation with an infectious diseases specialist is recommended for the treatment of antimicrobial-resistant infections. This document is current as of 24 October 2021. The most current versions of IDSA documents, including dates of publication, are available at www.idsociety.org/practice-guideline/amr-guidance/.

Cefiderocol for the Treatment of Multidrug-Resistant Gram-Negative Bacteria: A Systematic Review of Currently Available Evidence

Cefiderocol is a novel synthetic siderophore-conjugated antibiotic that hijacks the bacterial iron transport systems facilitating drug entry into cells, achieving high periplasmic concentrations. This systematic review analyzed the currently available literature on cefiderocol. It summarized in vitro susceptibility data, in vivo antimicrobial activity, pharmacokinetics/pharmacodynamics (PK/PD), clinical efficacy, safety and resistance mechanisms of cefiderocol. Cefiderocol has potent in vitro and in vivo activity against multidrug-resistant (MDR) Gram-negative bacteria, including carbapenem-resistant isolates. But New Delhi Metallo-β-lactamase (NDM)- positive isolates showed significantly higher MICs than other carbapenemase-producing Enterobacterales, with a susceptible rate of 83.4% for cefiderocol. Cefiderocol is well-tolerated, and the PK/PD target values can be achieved using a standard dose regimen or adjusted doses according to renal function. Clinical trials demonstrated that cefiderocol was non-inferiority to the comparator drugs in treating complicated urinary tract infection and nosocomial pneumonia. Case reports and series showed that cefiderocol was a promising therapeutic agent in carbapenem-resistant infections. However, resistant isolates and reduced susceptibility during treatment to cefiderocol have already been reported. In conclusion, cefiderocol is a promising powerful weapon for treating MDR recalcitrant infections.

Cefiderocol: A New Cephalosporin Stratagem Against Multidrug-Resistant Gram-Negative Bacteria

Cefiderocol is a novel injectable siderophore cephalosporin that hijacks the bacterial iron transport machinery to facilitate cell entry and achieve high periplasmic concentrations. It has broad in vitro activity against gram-negative bacteria, including multidrug-resistant (MDR) organisms such as carbapenem-resistant Enterobacterales, carbapenem-resistant Pseudomonas aeruginosa, and Acinetobacter baumannii. It was approved by the US Food and Drug Administration for the treatment of complicated urinary tract infections and nosocomial pneumonia based on clinical trials that demonstrated noninferiority to comparators. In this review, we summarize the available in vitro and clinical data, including recent evidence from 2 phase 3 clinical trials (APEKS-NP and CREDIBLE-CR), and discuss the place of cefiderocol in the clinician's armamentarium against MDR gram-negative infections.

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.

Urgent need for novel antibiotics in Republic of Korea to combat multidrug-resistant bacteria

Multidrug resistance in bacteria is an important issue and is increasing in frequency worldwide because of the limitations of therapeutic agents. From 2010 to 2019, 14 new systemic antibiotics received regulatory approval in the United States. However, few new antibiotics have been introduced in Republic of Korea to combat multidrug-resistant pathogens. Here, we introduce six novel antibiotics for Gram-positive bacteria and five for Gram-negative bacteria approved by the United States Food and Drug Administration and the European Medicines Agency from 2009 to October 2021, and recommend that they be approved for use in Republic of Korea at the earliest possible date.

Antimicrobial Treatment Options for Difficult-to-Treat Resistant Gram-Negative Bacteria Causing Cystitis, Pyelonephritis, and Prostatitis: A Narrative Review

Urinary tract infections, including cystitis, acute pyelonephritis, and prostatitis, are among the most common diagnoses prompting antibiotic prescribing. The rise in antimicrobial resistance over the past decades has led to the increasing challenge of urinary tract infections because of multidrug-resistant and "difficult-to-treat resistance" among Gram-negative bacteria. Recent advances in pharmacotherapy and medical microbiology are modernizing how these urinary tract infections are treated. Advances in pharmacotherapy have included not only the development and approval of novel antibiotics, such as ceftazidime/avibactam, meropenem/vaborbactam, imipenem/relebactam, ceftolozane/tazobactam, cefiderocol, plazomicin, and glycylcyclines, but also the re-examination of the potential role of legacy antibiotics, including older aminoglycosides and tetracyclines. Recent advances in medical microbiology allow phenotypic and molecular mechanism of resistance testing, and thus antibiotic prescribing can be tailored to the mechanism of resistance in the infecting pathogen. Here, we provide a narrative review on the clinical and pre-clinical studies of drugs that can be used for difficult-to-treat resistant Gram-negative bacteria, with a particular focus on data relevant to the urinary tract. We also offer a pragmatic framework for antibiotic selection when encountering urinary tract infections due to difficult-to-treat resistant Gram-negative bacteria based on the organism and its mechanism of resistance.