The OprF porin as a potential target for the restoration of carbapenem susceptibility in Pseudomonas aeruginosa expressing acquired carbapenemases.

Carbapenem resistance in Pseudomonas aeruginosa is primarily due to the acquisition of carbapenemases and is often associated with a diminution of the membrane permeability. The outer membrane protein, OprD, is a well-known route, by which carbapenems, predominantly imipenem, can enter the cell, and its loss has been associated with reduced susceptibility to imipenem. In this study, we investigated the antibiotic susceptibility patterns of isogenic P. aeruginosa mutants containing various acquired ß-lactamases, including carbapenemases, in a porin-depleted background. We identified that the deletion of oprF was associated with some recovery of susceptibility to carbapenems.

KSA-1, a naturally occurring Ambler class A extended spectrum ß-lactamase from the enterobacterial species Kosakonia sacchari.

BACKGROUND: Several bacterial species belonging to the Gammaproteobacteria possess intrinsic class A ß-lactamase genes that may represent a source of further dissemination and acquisition to other Gram-negative species. Here we characterised KSA-1 class A ß-lactamase, the gene of which was identified within the chromosome of an environmental Enterobacterales species, namely Kosakonia sacchari, which was also recently identified as the progenitor of an MCR-like colistin-resistance determinant. METHODS: In silico analysis using the GenBank database identified a class A ß-lactamase gene within the chromosome of K. sacchari SP1 (GenBank accession no. WP_017456759). The corresponding protein KSA-1 shared 63% amino acid identity with the intrinsic CKO-1 from Citrobacter koseri and 53% with TEM-1. Using the K. sacchari DSM 100203 reference strain as a template, blaKSA-1 was amplified, cloned into the plasmid pUCp24 and expressed in Escherchia coli TOP10. Minimal inhibitory concentrations and kinetic parameters were obtained from the purified enzyme. RESULTS: K. sacchari strain SP1 conferred resistance to amino-, carboxy- and ureido-penicillins only. Once produced within E. coli, KSA-1 showed a typical clavulanic acid-inhibited extended spectrum ß-lactamase associated with a peculiar temocillin resistance profile. Kinetic assays were performed using a purified extract of KSA-1 and demonstrated a high hydrolysis rate for benzylpenicillin and piperacillin, as well as weakly extended spectrum cephalosporins. Determination of inhibitory constants showed 50% inhibitory concentration values of 2.2, 3 and 1.8 nM for clavulanic acid, tazobactam and avibactam, respectively. Analysis of sequences surrounding the blaKSA-1 gene did not reveal any mobile element that could have been involved in the acquisition of this ß-lactamase gene in that species. CONCLUSION: KSA-1 is a class A extended spectrum ß-lactamase distantly related to known extended spectrum or broad-spectrum Ambler class A ß-lactamases, which is highly resistant to temocillin. The blaKSA-1 gene could be considered as intrinsic within the species.

Rapid detection of imipenem/relebactam susceptibility/resistance in Pseudomonas aeruginosa.

OBJECTIVES: Imipenem-relebactam (IPR) has been reported to exhibit a good activity against non-metallo-ß-lactamase carbapenem-resistant Pseudomonas aeruginosa (CRPA), and the rapid detection of susceptibility/resistance to this new therapeutic alternative may be crucial. Therefore, the Rapid IPR Pseudomonas NP test was developed to quickly identify IPR susceptibility/resistance among multidrug-resistant P. aeruginosa. METHODS: The principle of the Rapid IPR Pseudomonas NP test is based on visually detecting glucose metabolization by observing (or not) a color change from yellow to red or orange of the red phenol pH indicator in the presence of imipenem at 2 mg/L and relebactam at 4 mg/L A total of 80 clinical Pseudomonas aeruginosa isolates were analyzed, among which 42 isolates were IPR resistant according to EUCAST guidelines (MICs, susceptible ≤2 mg/L, resistant >2 mg/L). Results obtained with the Rapid IPR Pseudomonas NP test were compared with the reference broth microdilution (BMD). RESULTS: The sensitivity, specificity and accuracy of the test were found to be 100 %, 89.5 % and 95 %, respectively, using the BMD reference method as a comparator. Moreover, five out of the IPR-susceptible isolates (n = 38) exhibiting an MIC of IPR close to the breakpoint (MIC = 1 mg/L, n = 2; MIC = 2 mg/L, n = 3) yielded to a major error result, namely a positive result with the rapid IPR Pseudomonas NP test (resistance). By contrast, all IPR-resistant isolates (n = 42) were all correctly categorized. CONCLUSIONS: The Rapid IPR Pseudomonas NP test is sensitive, specific, and easy to perform and interpret. Therefore, it is suitable for implementation in routine clinical laboratories.

Complete genome sequence of the OXA-48-producing Klebsiella pneumoniae strain 11978.

We announce the complete genome sequence of Klebsiella pneumoniae strain 11978 isolated from a patient hospitalized in Turkey in 2001. The genome belongs to sequence type 14 and includes three plasmids. Notably, it presents an IncL plasmid carrying blaOXA-48, which demonstrated global success in terms of dissemination.

Impact of mutations in the mtrR, rpdlVD and rrl genes on azithromycin resistance in Neisseria gonorrhoeae.

INTRODUCTION: Bacterial sexually transmitted infections (STIs) pose a major public health problem. The emergence of antibiotic-resistant strains of Neisseria gonorrhoeae represents a serious threat to successful treatment and epidemiological control. The first extensively drug-resistant (XDR) strains (ceftriaxone-resistant and high-level azithromycin-resistant [HLR AZY]) have been reported. AIMS: To identify molecular mechanisms implicated in azithromycin resistance in strains isolated from patients over a three-year period in a university hospital in Switzerland. MATERIAL AND METHODS: From January 2020 to December 2022, 34 isolates (one per patient) were recovered from samples analyzed at the University Hospital of Lausanne. Eight genes involved in azithromycin resistance were sequenced: mtrR repressor (mtrCDE operon repressor) and his promotor mtrR-pr, rplD gene (L4 ribosomal protein), rplV gene (L22 ribosomal protein) and the four alleles of the rrl gene (23S rRNA). RESULTS: With a cutoff value of 1 mg/L, 15 isolates were considered as being resistant to azithromycin, whereas the remaining 19 were susceptible. The C2597T mutation in 3 or 4 of the rrl allele confer a medium-level resistance to azithromycin (MIC = 16 mg/L, N = 2). The following mutations were significantly associated with MIC values ≥1 mg/L: the three mutations V125A, A147G, R157Q in the rplD gene (N = 10) and a substitution A->C in the mtrR promotor (N = 9). Specific mutations in the mtrR repressor and its promotor were observed in both susceptible and resistant isolates. CONCLUSIONS: Resistance to azithromycin was explained by the presence of mutations in many different copies of 23S RNA ribosomal genes and their regulatory genes. Other mutations, previously reported to be associated with azithromycin resistance, were documented in both susceptible and resistant isolates, suggesting they play little role, if any, in azithromycin resistance.

Detection of volatile organic compounds as new paradigm to accelerate antimicrobial susceptibility testing: performance evaluation of VITEK® REVEAL™.

OBJECTIVES: The measurement of VOCs release in the headspace of a bacterial culture represents a new approach to rapidly assess antimicrobial susceptibility. Herein, we evaluated the diagnostic performance of the VITEK® REVEAL™ system directly from a collection of Gram-negative positive blood cultures. MATERIALS AND METHODS: One hundred and twenty-eight positive blood cultures were included in the analysis (Enterobacterales, n = 95; Pseudomonas aeruginosa, n = 21; Acinetobacter baumannii complex, n = 12). Samples were processed using VITEK® REVEAL™ according to the manufacturer's recommendations, and MICs of 22 antimicrobials were compared with those obtained using reference methods. Categorical agreement (CA), essential agreement (EA) and categorical errors were calculated. RESULTS: Overall, 2220 strain/antibiotic pair combinations were analysed. Of these, most were classified as resistant by reference antimicrobial susceptibility testing (1091/2220; 48.7%). The overall CA and EA were 97.6% and 97.7%, respectively. CA ranged from 97.5% in Enterobacterales to 97.9% in both P. aeruginosa and A. baumannii complex. The overall number of categorical discrepancies were: 18 very major errors (1.6%), 13 major errors (1.2%) and 22 minor errors (2.4%). EA ranged from 95.2% in P. aeruginosa to 98.1% in Enterobacterales. Screening test for ESBL phenotype was positive, indeterminate and negative in 13.7%, 32.6% and 27.4% of Enterobacterales isolates tested by both VITEK® REVEAL™ and the reference method, showing 100% CA. CONCLUSIONS: VITEK® REVEAL™ represents a reliable tool to obtain antimicrobial susceptibility results of the main Gram-negative species directly from positive blood cultures with time to results of less than 8 h.

Impact of extended-spectrum chromosomal AmpC (ESAC) of Escherichia coli on susceptibility to cefiderocol.

The impact of chromosomally encoded wild-type or extended-spectrum (ESAC) AmpC ß-lactamases of Escherichia coli on susceptibility to ceftazidime, cefepime, and cefiderocol was evaluated in different genetic backgrounds, including wild-type, PBP3-modified, and porin-deficient E. coli strains. Recombinant E. coli strains possessing the different backgrounds and producing variable ESACs were evaluated. Although ESAC enzymes conferred resistance to ceftazidime and decreased susceptibility to cefepime as expected, we showed here that cefiderocol was also a substrate of ESAC enzymes. IMPORTANCE: We showed here that chromosomally encoded intrinsic extended-spectrum cephalosporinases of Escherichia coli may impact susceptibility not only to ceftazidime and cefepime but also to cefiderocol.

MultiRapid ATB NP test for detecting concomitant susceptibility and resistance of last-resort novel antibiotics available to treat multidrug-resistant Enterobacterales infections.

BACKGROUND: Recently developed therapeutics against Gram-negative bacteria include the ß-lactam-ß-lactamase inhibitor combinations ceftazidime-avibactam (CZA), meropenem-vaborbactam (MEV), and imipenem-relebatam (IPR), and the siderophore cephalosporin cefiderocol (FDC). The aim of this study was to develop a test for rapid identification of susceptibility/resistance to CZA, MEV, IPR, and FDC for Enterobacterales in a single test for rapid clinical decision making. METHODS: The MultiRapid ATB NP test is based on the detection of glucose metabolism occurring after bacterial growth in the presence of defined concentrations of CZA, MEV, IPR, and FDC, followed by visual detection of colour change of the pH indicator red phenol (red to yellow) generated by the acidification of the medium upon bacterial growth. This test is performed in 96-well microplates. The MultiRapid ATB NP test was evaluated using 78 Enterobacterales isolates and compared to the reference method broth microdilution. RESULTS: The MultiRapid ATB NP test displayed 97.0% (confidence interval [CI] 92.6-98.8) sensitivity, 97.7% (CI 94.3-99.1) specificity, and 97.4% (CI 95.0-98.7) accuracy. The results were obtained after 3 h of incubation at 35 °C ± 2 °C, representing at least a 15-h gain-of-time compared with currently used antimicrobial susceptibility testing methods. CONCLUSION: The MultiRapid ATB NP test provided accurate results for the concomitant detection of susceptibility/resistance to CZA, MEV, IPR, and FDC in Enterobacterales, independent of the resistance mechanism. This test may be suitable for implementation in any microbiology routine laboratory.

Characterisation of a novel AmpC beta-lactamase, DHA-33, resistant to inhibition by cloxacillin.

Plasmid-encoded DHA-type AmpCs have been extensively reported in Enterobacterales. The expression of the genes encoding these plasmid-mediated enzymes are inducible and these enzymes are capable of conferring resistance to a wide spectrum of beta-lactams including penicillins and broad-spectrum cephalosporins. The identification of infections caused by AmpC-producing bacteria is a necessity, both for infection control/epidemiology purposes and to inform treatment choices. A common testing method for AmpC production in the clinical laboratory setting is to supplement Mueller-Hinton agar plates used for antibiotic disk diffusion with cloxacillin, a potent inhibitor of AmpC enzymes. Here we describe a novel DHA variant, produced by a clinical Escherichia coli isolate, which is resistant to cloxacillin inhibition.

Relative inhibitory activities of the broad-spectrum ß-lactamase inhibitor xeruborbactam in comparison with taniborbactam against metallo-ß-lactamases produced in Escherichia coli and Pseudomonas aeruginosa.

Xeruborbactam is a newly developed ß-lactamase inhibitor designed for metallo-ß-lactamases (MBLs). This study assessed the relative inhibitory properties of this novel inhibitor in comparison with another MBL inhibitor, namely taniborbactam (TAN), against a wide range of acquired MBL produced either in Escherichia coli or Pseudomonas aeruginosa. As observed with taniborbactam, the combination of xeruborbactam (XER) with ß-lactams, namely, ceftazidime, cefepime and meropenem, led to significantly decreased MIC values for a wide range of B1-type MBL-producing E. coli, including most recombinant strains producing NDM, VIM, IMP, GIM-1, and DIM-1 enzymes. Noteworthily, while TAN-based combinations significantly reduced MIC values of ß-lactams for MBL-producing P. aeruginosa recombinant strains, those with XER were much less effective. We showed that this latter feature was related to the MexAB-OprM efflux pump significantly impacting MIC values when testing XER-based combinations in P. aeruginosa. The relative inhibitory concentrations (IC50 values) were similar for XER and TAN against NDM and VIM enzymes. Noteworthily, XER was effective against NDM-9, NDM-30, VIM-83, and most of IMP enzymes, although those latter enzymes were considered resistant to TAN. However, no significant inhibition was observed with XER against IMP-10, SPM-1, and SIM-1 as well as the representative subclass B2 and B3 enzymes, PFM-1 and AIM-1. The determination of the constant inhibition (Ki) of XER revealed a much higher value against IMP-10 than against NDM-1, VIM-2, and IMP-1. Hence, IMP-10 that differs from IMP-1 by a single amino-acid substitution (Val67Phe) can, therefore, be considered resistant to XER.

Inter-phylum circulation of a beta-lactamase-encoding gene: a rare but observable event.

Beta-lactamase-mediated degradation of beta-lactams is the most common mechanism of beta-lactam resistance in Gram-negative bacteria. Beta-lactamase-encoding genes can be transferred between closely related bacteria, but spontaneous inter-phylum transfers (between distantly related bacteria) have never been reported. Here, we describe an extended-spectrum beta-lactamase (ESBL)-encoding gene (blaMUN-1) shared between the Pseudomonadota and Bacteroidota phyla. An Escherichia coli strain was isolated from a patient in Münster (Germany). Its genome was sequenced. The ESBL-encoding gene (named blaMUN-1) was cloned, and the corresponding enzyme was characterized. The distribution of the gene among bacteria was investigated using the RefSeq Genomes database. The frequency and relative abundance of its closest homolog in the global microbial gene catalog (GMGC) were analyzed. The E. coli strain exhibited two distinct morphotypes. Each morphotype possessed two chromosomal copies of the blaMUN-1 gene, with one morphotype having two additional copies located on a phage-plasmid p0111. Each copy was located within a 7.6-kb genomic island associated with mobility. blaMUN-1 encoded for an extended-spectrum Ambler subclass A2 beta-lactamase with 43.0% amino acid identity to TLA-1. blaMUN-1 was found in species among the Bacteroidales order and in Sutterella wadsworthensis (Pseudomonadota). Its closest homolog in GMGC was detected frequently in human fecal samples. This is, to our knowledge, the first reported instance of inter-phylum transfer of an ESBL-encoding gene, between the Bacteroidota and Pseudomonadota phyla. Although the gene was frequently detected in the human gut, inter-phylum transfer was rare, indicating that inter-phylum barriers are effective in impeding the spread of ESBL-encoding genes, but not entirely impenetrable.

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

PURPOSE: Carbapenemase-producing Enterobacterales are a growing threat, and very few therapeutic options remain active against those multidrug resistant bacteria. Aztreonam is the molecule of choice against metallo-beta-lactamases (MBL) producers since it is not hydrolyzed by those enzymes, but the co-production of acquired plasmidic cephalosporinases or extended-spectrum ß-lactamases leading to aztreonam resistance may reduce the efficacy of this molecule. Hence, the development of the aztreonam-avibactam (AZA) combination provides an interesting therapeutic alternative since avibactam inhibits the activity of both cephalosporinases and extended-spectrum ß-lactamases. However, structural modifications of penicillin binding protein PBP3, the target of aztreonam, may lead to reduced susceptibility to aztreonam-avibactam. METHODS: Here the impact of various plasmid-encoded AmpC-type ß-lactamases (ACC-1, ACT-7, ACT-17, CMY-2, CMY-42, DHA-1, FOX-1, and FOX-5) on susceptibility to aztreonam-avibactam was evaluated using isogenic E. coli MG1655 strains harboring insertions in PBP3 (YRIN and YRIK). The inhibitory activity of various ß-lactamase inhibitors (clavulanic acid, tazobactam, avibactam, relebactam, and vaborbactam) were also compared against these enzymes. RESULTS: Hence, we showed that reduced susceptibility to AZA was due to the combined effect of both AmpC production and amino acid insertions in PBP3. The highest resistance level was achieved in strains possessing the insertions in PBP3 in association with the production of ACT-7, ACC-1, or CMY-42. CONCLUSION: Although none of the recombinant strains tested displayed clinical resistance to aztreonam-avibactam, our data emphasize that the occurrence of such profile might be of clinical relevance for MBL-producing strains.

Effect of modification of penicillin-binding protein 3 on susceptibility to ceftazidime-avibactam, imipenem-relebactam, meropenem-vaborbactam, aztreonam-avibactam, cefepime-taniborbactam, and cefiderocol of Escherichia coli strains producing broad-spectrum ß-lactamases.

The impact of penicillin-binding protein 3 (PBP3) modifications that may be identified in Escherichia coli was evaluated with respect to susceptibility to ß-lactam/ß-lactamase inhibitor combinations including ceftazidime-avibactam, imipenem-relebactam, meropenem-vaborbactam, aztreonam-avibactam, cefepime-taniborbactam, and to cefiderocol. A large series of E. coli recombinant strains producing broad-spectrum ß-lactamases was evaluated. While imipenem-relebactam showed a similar activity regardless of the PBP3 background, susceptibility to other molecules tested was affected at various levels. This was particularly the case for ceftazidime-avibactam, aztreonam-avibactam, and cefepime-taniborbactam.

Rapid Detection of Ceftazidime/Avibactam Susceptibility/Resistance in Enterobacterales by Rapid CAZ/AVI NP Test.

We developed a novel culture-based test, the Rapid CAZ/AVI NP test, for rapid identification of ceftazidime/avibactam susceptibility/resistance in Enterobacterales. This test is based on glucose metabolization upon bacterial growth in the presence of a defined concentration of ceftazidime/avibactam (128/53 µg/mL). Bacterial growth is visually detectable by a red to yellow color change of red phenol, a pH indicator. A total of 101 well characterized enterobacterial isolates were used to evaluate the test performance. This test showed positive percent agreement of 100% and negative percent agreement of 98.5% with overall percent agreement of 99%, by comparison with the MIC gradient strip test (Etest) taken as the reference standard method. The Rapid CAZ/AVI NP test had only 1.5% major errors and 0% extremely major errors. This test is rapid (result within 2 hours 45 minutes), reliable, affordable, easily interpretable, and easy to implement in clinical microbiology laboratories without requiring any specific equipment.

Molecular analysis of metallo-beta-lactamase-producing Pseudomonas aeruginosa in Switzerland 2022-2023.

OBJECTIVES: The occurrence of metallo-beta-lactamase-producing Pseudomonas aeruginosa (MBL-PA) isolates is increasing globally, including in Switzerland. The aim of this study was to characterise, phenotypically and genotypically, the MBL-PA isolates submitted to the Swiss National Reference Center for Emerging Antibiotic Resistance (NARA) reference laboratory over a 12-month period from July 2022 to July 2023. METHODS: Thirty-nine non-duplicate MBL-PA Isolates were submitted to NARA over the study period from across Switzerland. Susceptibility was determined by broth microdilution according to EUCAST methodology. Whole-genome sequencing was performed on 34 isolates. Sequence types (STs) and resistance genes were ascertained using the Centre for Genomic Epidemiology platform. MBL genes, blaNDM-1, blaIMP-1, and blaVIM-2, were cloned into vector pUCP24 and transformed into P. aeruginosa PA14. RESULTS: The most prevalent MBL types identified in this study were VIM (21/39; 53.8%) followed by NDM (11/39; 28.2%), IMP (6/39; 15.4%), and a single isolate produced both VIM and NDM enzymes. WGS identified 13 different STs types among the 39 isolates. They all exhibited resistance to cephalosporins, carbapenems, and the beta-lactam-beta-lactamase inhibitor combinations, ceftolozane-tazobactam, ceftazidime-avibactam, imipenem-relebactam, and meropenem-vaborbactam, and 8 isolates were cefiderocol (FDC) resistant. Recombinant P. aeruginosa strains producing blaNDM-1, blaIMP-1, and blaVIM-2 exhibited FDC MICs of 16, 8, and 1 mg/L, respectively. CONCLUSIONS: This study showed that the MBL-PA in Switzerland could be attributed to the wide dissemination of high-risk clones that accounted for most isolates in this study. Although FDC resistance was only found in 8 isolates, MBL carriage was shown to be a major contributor to this phenotype.

Relative inhibitory activities of the broad-spectrum ß-lactamase inhibitor taniborbactam against metallo-ß-lactamases.

Taniborbactam (TAN) is a novel broad-spectrum ß-lactamase inhibitor with significant activity against subclass B1 metallo-ß-lactamases (MBLs). Here, we showed that TAN exhibited an overall excellent activity against B1 MBLs including most NDM- and VIM-like as well as SPM-1, GIM-1, and DIM-1 enzymes, but not against SIM-1. Noteworthy, VIM-1-like enzymes (particularly VIM-83) were less inhibited by TAN than VIM-2-like. Like NDM-9, NDM-30 (also differing from NDM-1 by a single amino acid substitution) was resistant to TAN.

Pharmacokinetic/pharmacodynamic evaluation of tigecycline dosing in a hollow fiber infection model against clinical bla-KPC producing Klebsiella Pneumoniae isolates.

The FDA announced a boxed warning for tigecycline due to progression of infections caused by Gram-negative bacteria and increased risk of mortality during treatment. Plasma exposure of tigecycline might not prevent bacteraemia in these cases from the focuses. Hence, we evaluated intensified dosing regimens and breakpoints that might suppress bloodstream infections, caused by progression of infection by e.g., Gram-negatives. A pharmacometric model was built from tigecycline concentrations (100-600 mg daily doses) against clinical Klebsiella pneumoniae isolates (MIC 0.125-0.5 mg/L). Regrowth occurred at clinically used doses and stasis was only achieved with 100 mg q8h for the strain with the lowest studied MIC of 0.125 mg/L. Stasis at 24 h was related to fAUC/MIC of 38.5. Our study indicates that even intensified dosing regimens might prevent bloodstream infections only for MIC values ≤0.125 mg/L for tigecycline. This indicates an overly optimistic breakpoint of 1 mg/L for Enterobacterales, which are deemed to respond to the tigecycline high dose regimen (EUCAST Guidance Document on Tigecycline Dosing 2022).

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

PURPOSE: To evaluate the different present and future therapeutic ß-lactam/ß-lactamase inhibitor (BL/BLI) alternatives, namely aztreonam-avibactam, imipenem-relebactam, meropenem-vaborbactam, cefepime-zidebactam, cefepime-taniborbactam, meropenem-nacubactam, and sulbactam-durlobactam against clinical isolates showing reduced susceptibility or resistance to cefiderocol in Enterobacterales, Acinetobacter baumannii, and Pseudomonas aeruginosa. METHODS: MIC values of aztreonam, aztreonam-avibactam, cefepime, cefepime-taniborbactam, cefepime-zidebactam, imipenem, imipenem-relebactam, meropenem, meropenem-vaborbactam, meropenem-nacubactam, sulbactam-durlobactam, and cefiderocol combined with a BLI were determined for 67, 9, and 11 clinical Enterobacterales, P. aeruginosa or A. baumannii isolates, respectively, showing MIC values of cefiderocol being ≥1 mg/L. If unavailable, the respective ß-lactam breakpoints according to EUCAST were used for BL/BLI combinations. RESULTS: For Enterobacterales, the susceptibility rates for aztreonam, cefepime, imipenem, and meropenem were 7.5%, 0%, 10.4%, and 10.4%, respectively, while they were much higher for cefepime-zidebactam (91%), cefiderocol-zidebactam (91%), meropenem-nacubactam (71.6%), cefiderocol-nacubactam (74.6%), and cefiderocol-taniborbactam (76.1%), as expected. For P. aeruginosa isolates, the higher susceptibility rates were observed for imipenem-relebactam, cefiderocol-zidebactam, and meropenem-vaborbactam (56% for all combinations). For A. baumannii isolates, lower susceptibility rates were observed with commercially or under development BL/BLI combos; however, a high susceptibility rate (70%) was found for sulbactam-durlobactam and when cefiderocol was associated to some BLIs. CONCLUSIONS: Zidebactam- and nacubactam-containing combinations showed a significant in vitro activity against multidrug-resistant Enterobacterales clinical isolates with reduced susceptibility to cefiderocol. On the other hand, imipenem-relebactam and meropenem-vaborbactam showed the highest susceptibility rates against P. aeruginosa isolates. Finally, sulbactam-durlobactam and cefiderocol combined with a BLI were the only effective options against A. baumannii tested isolates.