Cefiderocol exhibits excellent in vitro activity against Pseudomonas aeruginosa; however, resistance can emerge. We investigated the molecular mechanisms underlying cefiderocol resistance (minimal inhibitory concentration [MIC] >2 mg/L) in 103 clinical strains collected from 61 hospitals (2021-2024). MICs ranged from 4 to >128 mg/L, with 39.8% of strains showing MICs > 8 mg/L. Although 37.8% were classified as difficult-to-treat resistant (DTR), acquired β-lactamases were detected in 72.8% of strains, including carbapenemases (39.8%), mainly NDM-1 (29.1%), and extended-spectrum β-lactamases (ESBLs) (38.8%). Cloning of 11 β-lactamases into pUCP24, including the acquired cephalosporinase PAC-1 and ESBLs (VEB-1 and VEB-9), resulted in marked increases in cefiderocol MICs (up to 128-fold). Introduction of six mutations in the PDC enzyme into a PAO1ΔblaPDC-1 background increased MICs up to 4 mg/L and conferred cross-resistance to ceftolozane/tazobactam, notably T70I, F121L, G157D, and E219K. Alterations in siderophore transporters or regulators were identified in 38.8% of strains, most frequently a PirR frameshift (R132fs), consistent with PirR inactivation, which was confirmed in the PAO1 strain to contribute to cefiderocol resistance. Overall, cefiderocol resistance in clinical strains is multifactorial, mainly involving acquired β-lactamases (ESBLs and carbapenemases) and impaired siderophore uptake (PiuA/PiuD, PirA, and PiuC), leading to high-level resistance (>8 mg/L). The polyclonal distribution and diversity of mechanisms highlight the need for routine susceptibility testing and surveillance. Detection of NDM producers is critical, as cefiderocol should be used with caution in this context.

In France, the use of essential oils to treat bacterial infections is common, with approximately 40% of cystic fibrosis patients regularly using these natural products to control infections caused by Pseudomonas aeruginosa often in combination with their antibiotic treatments. Previous research has demonstrated that prolonged exposure of P. aeruginosa PA14 to cinnamaldehyde (CNA), the main component of cinnamon essential oil, can select for mutants resistant to β-lactams through overproduction of the MexAB-OprM efflux pump, some of which are also hypersusceptible to aminoglycosides and colistin. We showed here that this hypersusceptibility is not due to an efflux defect, as the deletion of MexXY(OprM)-the specific efflux pump for aminoglycosides-still results in decreased minimum inhibitory concentrations of aminoglycosides. Genome sequencing of hypersusceptible mutants revealed mutations in the ATP synthase operon or its promoter (atpIBEFHAGDC). Surprisingly, although mutations in the atp operon reduced bacterial growth and ATP production, they are not uncommon in clinical strains. We found that ATP synthase alterations modified the respiratory chain and led to inner membrane hyperpolarization, likely enhancing positively charged antibiotic (aminoglycosides and colistin) uptake and susceptibility to these molecules. In addition, the modified respiratory chain increased the proton motive force, allowing the overproduction of the MexAB-OpM efflux pump, which protects bacteria from CNA and from the clinically relevant β-lactam antibiotics. Altogether, these results indicate a trade-off between CNA resistance and aminoglycoside/colistin susceptibility, a reaction that may question the survival of P. aeruginosa in the lung of CF patients possibly submitted to these therapeutic molecules.IMPORTANCEIn France, essential oils are widely used by cystic fibrosis patients (40%), often alongside antibiotic therapies, to help control Pseudomonas aeruginosa infections. Cinnamaldehyde from cinnamon essential oil appears to select for P. aeruginosa mutants that are resistant to β-lactam antibiotics due to the overproduction of the MexAB-OprM efflux pump and hypersusceptible to aminoglycosides and colistin. This increased susceptibility is associated with mutations in ATP synthase, which elevate the proton motive force (PMF) and facilitate both (i) increased uptake of positively charged antibiotics (aminoglycosides, colistin) and (ii) more efficient efflux of β-lactams via MexAB-OpM. Thus, the use of cinnamaldehyde may drive a trade-off in P. aeruginosa between β-lactam resistance and aminoglycosides/polymyxins susceptibility, potentially compromising bacterial persistence in the lung of patients.

Exposure of Pseudomonas aeruginosa to cinnamaldehyde (CNA), a natural electrophilic antimicrobial often used as self-medication to treat mild infections, triggers overproduction of the MexAB-OprM efflux system, leading to multidrug resistance. In this study, we demonstrate that CNA exposure induces expression of genes regulated by the two-component system AmgRS. AmgRS activates MexAB-OprM production, independent of repressors MexR and NalD. In addition to the essential role played by the NalC-ArmR pathway in this adaptive process, AmgRS is critical for the survival of P. aeruginosa challenged with CNA. Altogether, these data suggest that efflux-dependent and -independent mechanisms are activated in the early phase of CNA exposure, allowing for progressive enzymatic reduction of the biocide to non-toxic cinnamic alcohol.IMPORTANCEExposure of Pseudomonas aeruginosa to cinnamaldehyde (CNA), an antimicrobial used in self-medication, induces overproduction of the MexAB-OprM efflux system, leading to multidrug resistance. Our study demonstrates that the AmgRS two-component system aids in the survival of P. aeruginosa strain PA14 under CNA exposure through both MexAB-OprM-dependent and -independent mechanisms until the enzymatic reduction of CNA into the less toxic cinnamic alcohol. This discovery highlights the pivotal role of AmgRS in mediating defense against aldehyde biocides, emphasizing its significance in the persistence of P. aeruginosa, a pathogen associated with hospital-acquired infections and cystic fibrosis, and underscores the potential impact on clinical treatment strategies.

Persistent bacterial infections evade host immunity and resist antibiotic treatments through various mechanisms that are difficult to evaluate in a living host. Pseudomonas aeruginosa is a main cause of chronic infections in patients with cystic fibrosis (CF) and wounds. Here, by immersing wounded zebrafish embryos in a suspension of P. aeruginosa isolates from CF patients, we established a model of persistent infection that mimics a murine chronic skin infection model. Live and electron microscopy revealed persisting aggregated P. aeruginosa inside zebrafish cells, including macrophages, at unprecedented resolution. Persistent P. aeruginosa exhibited adaptive resistance to several antibiotics, host cell permeable drugs being the most efficient. Moreover, persistent bacteria could be partly re-sensitized to antibiotics upon addition of anti-biofilm molecules that dispersed the bacterial aggregates in vivo. Collectively, this study demonstrates that an intracellular location protects persistent P. aeruginosa in vivo in wounded zebrafish embryos from host innate immunity and antibiotics, and provides new insights into efficient treatments against chronic infections.

The ongoing rise of bacterial resistance against antibiotics is an incentive to develop radically new antibacterial molecules. Innovative organo-gold(I) antibacterial compounds were synthesized by click chemistry with triethylphosphine‑gold(I) azides and alkyne-functionalized metabolites like amino acids, nicotinic acid and biotin. The resulting organo‑gold(I) compounds exhibited remarkable activities against a panel of Gram-positive pathogens (Staphyloccocus sp., Enterococcus sp. and Clostridium difficile) but failed to inhibit the growth of Gram-negative bacilli (e.g., Escherichia coli) under the conditions used (8 μg. mL-1 maximum).

The decline of antibiotics efficacy worldwide has recently reached a critical point urging for the development of new strategies to regain upper hand on multidrug resistant bacterial strains. In this context, the raise of photodynamic therapy (PDT), initially based on organic photosensitizers (PS) and more recently on organometallic PS, offers promising perspectives. Many PS exert their biological effects through the generation of reactive oxygen species (ROS) able to freely diffuse into and to kill surrounding bacteria. Hijacking of the bacterial iron-uptake systems with siderophore-PS conjugates would specifically target pathogens. Here, we report the synthesis of unprecedented conjugates between the siderophore desferrioxamine B (DFOB) and an antibacterial iridium(III) PS. Redox properties of the new conjugates have been determined at excited states and compared to that of an antibacterial iridium PS previously reported by our groups. Tested on nosocomial pathogen Pseudomonas aeruginosa and other bacteria, these conjugates demonstrated significant inhibitory activity when activated with blue LED light. Ir(III) conjugate and iridium free DFOB-2,2'-dipyridylamine ligands were crystallized in complex with FoxA, the outer membrane transporter involved in DFOB uptake in P. aeruginosa and revealed details of the binding mode of these unprecedented conjugates.

BACKGROUND:Carbapenem-resistant strains of Pseudomonas aeruginosa (CRPA) have become a major health care concern in many countries, against which anti-infective strategies are limited and which require adequate infection control interventions. Knowing the different modes of transmission of CRPA in intensive care units (ICUs) would be helpful to adapt the means of prevention. METHODS:The aim of this retrospective case-control study was conducted between January 1, 2017 and February 28, 2022 to identify the risk factors for the acquisition of CRPA in ICUs. RESULTS:During the study period, 147 patients were included (49 cases and 98 controls). Among the 49 patients, 31 (63%) acquired CRPA in clusters and 18 (37%) sporadically. A univariate analysis showed that 4 variables were associated with CRPA acquisition, including (1) prior antibiotic prescriptions, (2) admission to rooms 203 and 207, (3) severity of illness at admission, and (4) use of mechanical ventilation. Multivariate analysis identified 3 factors of CRPA acquisition, including admission to room 203 (odds ratio [OR] = 29.5 [3.52-247.09]), previous antibiotic therapy (OR = 3.44 [1.02-11.76]), and severity of condition at admission (OR = 1.02 [1-1.04]). CONCLUSIONS:Our study suggests the role of a contaminated environment in the acquisition of CRPA in the ICU, along with antibiotic use.

Innovative therapeutic strategies are more than ever needed to counter the rise of antibiotic‐resistant bacterial pathogens worldwide. The use of light, and especially photodynamic therapy (PDT) appears as a promising alternative or complement to antibiotic treatments, fostered by the development of new photosensitizers. In this study, eight luminescent Ir(III) complexes were synthesized and evaluated for their photoactivation properties and capacity to generate radical species under blue (452 nm), green (525 nm), and red (631 nm) LED light, respectively. Their antibacterial properties were assessed on Pseudomonas aeruginosa, Acinetobacter baumannii, Escherichia coli, and Staphylococcus aureus with most of these complexes exhibiting potentially useful activities upon light irradiation, at concentrations below 10 mg/L. A complex of Ir(III) cyclometallated to thiophenyl‐isoquinoline (tiq) and bearing 2,2’‐bipyridine (bipy) as ancillary ligand was further investigated. This latter showed a concentration‐ and light intensity‐dependent bactericidal activity on P. aeruginosa when irradiated under blue to red lights, proving that such complexes would be suitable candidates for PDT. Importantly, this lead complex remained active against antibiotic resistant clinical strains and was unaffected by active efflux systems. These data open interesting perspectives for the development of new treatments to tackle antibiotic resistant Gram‐negative bacteria.

Innovative therapeutic strategies are more than ever needed to counter the rise of antibiotic-resistant bacterial pathogens worldwide. The use of light, and especially photodynamic therapy (PDT) appears as a promising alternative or complement to antibiotic treatments, fostered by the development of new photosensitizers. In this study, eight Ir(III) complexes were synthesized and evaluated for their photoactivation properties and capacity to generate radical species under blue (452 nm), green (525 nm), and red (631 nm) LED light, respectively. Their antibacterial properties were assessed on Pseudomonas aeruginosa, Acinetobacter baumannii, Escherichia coli, and Staphylococcus aureus with most of these complexes exhibiting potentially useful activities upon light irradiation, at concentrations below 10 mg/L. A complex of Ir(III) cyclometallated to thiophenyl-isoquinoline (tiq) and bearing 2,2'-bipyridine (bipy) as ancillary ligand was further investigated. This latter showed a concentration- and light intensity-dependent bactericidal activity on P. aeruginosa when irradiated under blue to red lights, proving that such complexes would be suitable candidates for PDT. Importantly, this lead complex remained active against antibiotic resistant clinical strains and was unaffected by active efflux systems. These data open interesting perspectives for the development of new treatments to tackle antibiotic resistant Gram-negative bacteria.

Murepavadin is a peptidomimetic exhibiting specific inhibitory activity against Pseudomonas species. In the present study, its in vitro activity was assessed on 230 cystic fibrosis (CF) strains of Pseudomonas aeruginosa isolated from 12 French hospitals, in comparison with 12 other antipseudomonal antibiotics. Although murepavadin is still in preclinical stage of development, 9.1% (n = 21) of strains had a minimum inhibitory concentration (MIC) >4 mg/L, a level at least 128-fold higher than the modal MIC value of the whole collection (<= 0.06 mg/L). Whole-genome sequencing of these 21 strains along with more susceptible isogenic counterparts coexisting in the same patients revealed diverse mutations in genes involved in the synthesis (lpxL1 and lpxL2) or transport of lipopolysaccharides (bamA, lptD, and msbA), or encoding histidine kinases of two-component systems (pmrB and cbrA). Allelic replacement experiments with wild-type reference strain PAO1 confirmed that alteration of genes lpxL1, bamA, and/or pmrB can decrease the murepavadin susceptibility from 8- to 32-fold. Furthermore, we found that specific amino acid substitutions in histidine kinase PmrB (G188D, Q105P, and D45E) reduce the susceptibility of P. aeruginosa to murepavadin, colistin, and tobramycin, three antibiotics used or intended to be used (murepavadin) in aerosols to treat colonized CF patients. Whether colistin or tobramycin may select mutants resistant to murepavadin or the opposite needs to be addressed by clinical studies.

Objectives: Cefiderocol has an excellent in vitro activity on clinical strains of Pseudomonas aeruginosa ( P. aeruginosa ). However, the resistance of some isolates has been associated with the production of some ,B-lactamases. Whether some acquired extended-spectrum oxacillinases (ES-OXA) common in this species may compromise the susceptibility of P. aeruginosa to cefiderocol has not been evaluated so far. Methods: Eighteen genes encoding OXA belonging to the major subgroups identified in P. aeruginosa OXA-1 (n = 3); - 2 (n = 5); - 10 (n = 8), and - 46 (n = 2) were cloned into pUCP24 shuttle vector and transferred into reference strain PAO1. Results: Although production of the OXA-1 subgroup enzymes did not alter cefiderocol MICs, the ,B- lactamases of OXA-2, OXA-46, and four variants of the OXA-10 subgroup resulted in an 8-fold to 32-fold decrease in susceptibility in the PAO1 background. Interestingly, point mutations Ala149Pro and Asp150Gly in OXA-2 subgroup, Trp154Cys and Gly157Asp in OXA-10 subgroup (all located in the Q loop), and the duplication of a Thr206 and a Gly207 in the ,B5 -,B6 loop of OXA-10 subgroup were related to de-creased susceptibility to cefiderocol. We also showed that some ES-OXA, including the most frequent ES-OXA in P. aeruginosa strains, OXA-19 (derived from OXA-10 subgroup), significantly compromised activity of cefiderocol in addition to ceftazidime, ceftolozane/tazobactam, and ceftazidime/avibactam in clinical strains. Conclusion: This work shows that several ES-OXA have a significant effect on cefiderocol susceptibility. Of concern are the Trp154Cys and Gly157Asp mutations that occur in some of these ,B-lactamases, as they are associated with a decreased activity of the most recent cephalosporins introduced to combat P. aeruginosa infections. & COPY; 2023 Published by Elsevier Ltd.

The Acinetobacter baumannii clonal lineage ST25 has been identified in humans and animals and found associated with outbreaks globally. To highlight possible similarities among ST25 A. baumannii of animal and human origins and to gather clues on the dissemination and evolution of the ST25 lineage, we conducted a phylogenetic analysis on n = 106 human and n = 35 animal A. baumannii ST25 genomes, including 44 sequenced for this study. Resistance genes and their genetic background were analyzed, as well. ST25 genomes are clustered into four clades: two are widespread in South America, while the other two are largely distributed in Europe, Asia and America. One particular clade was found to include the most recent strains and the highest number of acquired antibiotic resistance genes. OXA-23-type carbapenemase was the most common. Other resistance genes such as blaNDM-1, blaPER-7, and armA were found embedded in complex chromosomal regions present in human isolates. Genomic similarity among multidrug resistant ST25 isolates of either animal or human origin was revealed, suggesting cross-contaminations between the two sectors. Tracking the clonal complex ST25 between humans and animals should provide new insights into the mode of dissemination of these bacteria, and should help defining strategies for preserving global health.

Objectives: Pseudomonas aeruginosa is a main cause of chronic infections in patients with cystic fibrosis (CF). Our aim was to set up P. aeruginosa persistent infection in zebrafish (Danio rerio), a non-mammalian vertebrate model suitable for real-time imaging of infection and assessment of treatment efficacy. Methods: Our novel model is based on the infection of clinical P. aeruginosa isolates from CF patients in zebrafish embryos (using transgenic fish lines with fluorescent phagocytic cells). Bacteria were counted in infected embryos up to 3 days after infection and visualized by confocal microscopy to address their interaction with innate immune cells. The efficacy of known antibiotics, as well as novel molecules, was evaluated at different times post-infection. Experiments can be conducted in normal context or in CF embryos (using cftr morphans or a CFTR-deficient fish line). Results: Quantification of the bacterial load of infected embryos allowed to identify clinical isolates with a persistent phenotype. We focused on a particular persistent isolate, showing aggregated bacteria at the infection site, which are often located inside macrophages. The persistence of this isolate is associated with a decreased susceptibility to several antibiotics classically used in clinics. The efficiency of anti-biofilm molecules, in combination or not with antibiotics, to eliminate persistent bacteria is currently tested. Studies will be conducted both in normal and CF embryos. Conclusion: We established for the first time a persistent model of P. aeruginosa infection in zebrafish. The persistence of a P. aeruginosa clinical isolate from CF patient in the zebrafish embryo is associated with tolerance to antibiotics, which mimics the failure of antibiotic treatment during chronic clinical infections. We thus propose a novel model relevant to study the efficacy of innovative treatments in the context of a P. aeruginosa persistent infection.

Here, we characterized the first French NDM-9-producing Acinetobacter baumannii isolate. A. baumannii 13A297, which belonged to the STPas25 (international clone IC7), was highly resistant to β-lactams including cefiderocol (MIC >32 mg/L). Whole genome sequencing (WGS) using both Illumina and Oxford Nanopore technologies revealed a 166-kb non-conjugative plasmid harboring a blaNDM-9 gene embedded in a Tn125 composite transposon. Complementation of E. coli DH5α and A. baumannii CIP70.10 strains with the pABEC plasmid carrying the blaNDM-1 or blaNDM-9 gene, respectively, resulted in a significant increase in cefiderocol MIC values (16 to >256-fold), particularly in the NDM-9 transformants. Interestingly, steady-state kinetic parameters, measured using purified NDM-1 and NDM-9 (Glu152Lys) enzymes, revealed that the affinity for cefiderocol was 3-fold higher for NDM-9 (Km = 53 μM) than for NDM-1 (Km = 161 μM), leading to a 2-fold increase in catalytic efficiency for NDM-9 (0.13 and 0.069 μM−1.s−1, for NDM-9 and NDM-1, respectively). Finally, we showed by molecular docking experiments that the residue 152 of NDM-like enzymes plays a key role in cefiderocol binding and resistance, by allowing a strong ionic interaction between the Lys152 residue of NDM-9 with both the Asp223 residue of NDM-9 and the carboxylate group of the R1 substituent of cefiderocol.

AbstractMurepavadin is a peptidomimetic exhibiting specific inhibitory activity againstPseudomonasspecies. In the present study, itsin vitroactivity was assessed on 230 cystic fibrosis (CF) strains ofP. aeruginosaisolated from twelve French hospitals, in comparison with twelve other antipseudomonal antibiotics. Although murepavadin is still in pre-clinical stage of development, 9.1% (n=21) of the strains displayed a resistance superior to 4 mg/L, a level at least 128-fold higher than the modal MIC value of the whole collection (≤ 0.06 mg/L). Whole-genome sequencing of these 21 strains along with more susceptible isogenic counterparts coexisting in the same patients revealed diverse mutations in genes involved in the synthesis (lpxL1andlpxL2) or transport of lipopolysaccharides (bamA, lptD, andmsbA), or encoding histidine kinases of two-component systems (pmrBandcbrA). Allelic replacement experiments with wild-type reference strain PAO1 confirmed that alteration of geneslpxL1, bamAand/orpmrBcan increase murepavadin resistance from 8- to 32-fold. Furthermore, we found that specific amino-acid substitutions in histidine kinase PmrB (G188D, Q105P, and D45E) reduce the susceptibility ofP. aeruginosato murepavadin, colistin and tobramycin, three antibiotics used or intended to be used (murepavadin) in aerosols to treat colonized CF patients. Whether colistin or tobramycin may select mutants resistant to murepavadin or the opposite needs to be addressed by clinical studies.

OBJECTIVES:To evaluate the performance of commercially available tests to determine the susceptibility of multidrug-resistant (MDR) clinical Pseudomonas aeruginosa strains to cefiderocol. METHODS:A collection of 150 clinical strains of P. aeruginosa resistant to ceftazidime, (MIC, Minimal Inhibitory Concentration, MIC > 8 mg/L) imipenem (MIC> 4 mg/L) and ceftolozane/tazobactam (MIC> 4/4 mg/L), isolated from 2015 to 2022 was selected. Cefiderocol susceptibility was determined in parallel (a) by disc diffusion using Mast, Oxoid and Liofilchem discs deposited on Mueller-Hinton agar batches from Bio-Rad, BioMérieux, Mast, Becton Dickinson, I2A and Oxoid; (b) by MIC gradient test strips (MTS) (Liofilchem); and (c) by EUMDROXF Sensititre microplates. MICs and inhibition zones were compared with the broth microdilution reference method (BMD) MICs. RESULTS:The MIC50 and MIC90 of cefiderocol were 1 mg/L and 8 mg/L by BMD, respectively, including 21.3% (32/150) resistant strains. None of the methods tested fulfilled acceptable criteria (essential agreement [EA] ≥ 90%; bias = ± 30%). Although the Sensititre EUMDROXF microplates overestimated MIC values (categorical agreement [CA] = 86.7% [130/150, 95% CI 80.3-91.2]; EA = 69.3% [104/150, 95% CI 61.6-76.2]; bias = 68.2%), MTS strips underestimated the MIC values for many strains (CA = 86.7%, 130/150, 95% CI 80.3-91.2; EA = 69.3%, 104/150, 95% CI 61.6-76.2; bias = -30.4%), classifying properly only 50% (16/32) of resistant strains. Finally, many cefiderocol-resistant strains were not identified by the disc method, although the CA ranged from 78.0% (117/150, 95% CI 70.7-83.0) to 89.3% (134/150, 95% IC 83.4-93.3) according to Mueller-Hinton agar batches. CONCLUSION:Determination of cefiderocol susceptibility in MDR P. aeruginosa clinical strains by Sensititre EUMDROXF microplates is an alternative to the reference BMD method. However, MIC values ± 1 dilution apart from the breakpoint (2 mg/L) should be controlled by BMD whereas the use of MTS gradient strips is discouraged. Disc diffusion might be useful for screening, unfortunately many cefiderocol-resistant strains are not detected.

The genes that encode CTX-M-type ESBLs have diffused extensively among Enterobacterales over the past decades.1 Despite this epidemic success at the global level, few of these have been detected (blaCTX-M-1,-2,-3,-14,-15and-43) in the distantly related opportunistic pathogen Pseudomonas aeruginosa.1–4 The narrow host range of plasmids carrying the blaCTX-M determinants in Enterobacterales (mainly of incompatibility group IncF) plausibly accounts for the low prevalence of these resistance genes in P. aeruginosa.5 On the other hand, little is known about the mobile elements that enable the acquisition of CTX-M enzymes by clinical P. aeruginosa strains. One plasmid close to broad-host-range incompatibility group IncP2 has been found to carry the gene blaCTX-M-2, while another one belonging to IncQ harboured blaCTX-M-3.6,7 The present report describes the structure of a large conjugative plasmid determining a novel CTX-M variant, named CTX-M-206, and the genetic environment of a chromosomally located blaCTX-M-15 gene in a P. aeruginosa isolate present in a second strain.

OBJECTIVES:To characterize Acinetobacter baumannii strains co-producing the ESBL CTX-M-115 and carbapenem-hydrolysing class D β-lactamases (CHDLs), and to assess the potential diffusion of their resistance genes by horizontal transfer.METHODS:Nineteen CTX-M-115/CHDL-positive A. baumannii were collected between 2015 and 2019 from patients hospitalized in France. Their whole-genome sequences were determined on Illumina and Oxford Nanopore platforms and were compared through core-genome MLST (cgMLST) and SNP analyses. Transferability of resistance genes was investigated by natural transformation assays.RESULTS:Eighteen strains were found to harbour CHDL OXA-72, and another one CHDL OXA-23, in addition to CTX-M-115, narrow-spectrum β-lactamases and aminoglycoside resistance determinants including ArmA. cgMLST typing, as well as Oxford Scheme ST and K locus typing, confirmed that 17 out of the 18 CTX-M-115/OXA-72 isolates belonged to new subclades within clonal complex 78 (CC78). The chromosomal region carrying the blaCTX-M-115 gene appeared to vary greatly both in gene content and in length (from 20 to 79 kb) among the strains, likely because of IS26-mediated DNA rearrangements. The blaOXA-72 gene was localized on closely related plasmids showing structural variations that occurred between pdif sites. Transfer of all the β-lactamase genes, as well as aminoglycoside resistance determinants to a drug-susceptible A. baumannii recipient, was easily obtained in vitro by natural transformation.CONCLUSIONS:This work highlights the propensity of CC78 isolates to collect multiple antibiotic resistance genes, to rearrange and to pass them to other A. baumannii strains via natural transformation. This process, along with mobile genetic elements, likely contributes to the considerable genomic plasticity of clinical strains, and to the diversity of molecular mechanisms sustaining their multidrug resistance.