Search Results (175)

Introduction: The combination of ceftazidime−avibactam (CAZ-AVI) with aztreonam (ATM) may be an option for the treatment of infections due to metallo-β-lactamases (MBLs) producing bacteria, as recommended by current guidelines. MBLs protect the pathogen from any available β-lactam/β-lactamase inhibitor (BL/BLI). Moreover, in vitro and clinical data suggest that double carbapenem therapy (DCT) may be an option for such infections. Materials and Methods: This retrospective study was conducted in two mixed intensive care units (ICUs) at the University Hospital of Larissa, Thessaly, Greece, and the General Hospital of Larissa, Thessaly, Greece, during a three-year period (2022−2024). Mechanically ventilated patients with bloodstream infection (BSI) caused by K. pneumoniae resistant to all BL/BLI combinations were studied. Patients were divided into three groups: in the first, patients were treated with CAZ-AVI + ATM; in the second, with DCT; and in the third, with antibiotics other than BL/BLIs that presented in vitro susceptibility. The primary outcome of the study was the change in Sequential Organ Failure Assessment (SOFA) score between the onset of infection and the fourth day of antibiotic treatment. Secondary outcomes were SOFA score evolution during the treatment period, total duration of mechanical ventilation (MV), ICU length of stay (LOS), and ICU mortality. Results: A total of 95 patients were recruited. Among them, 23 patients received CAZ-AVI + AZT, 22 received DCT, and 50 patients received another antibiotic regimen which was in vitro active against the pathogen. The baseline characteristics were similar. The mean (SE) overall age was 63.2 (1.3) years. Mean (SE) Acute Physiology and Chronic Health Evaluation II (APACHE II) and SOFA scores were 16.3 (0.6) and 7.6 (0.3), respectively. The Charlson Index was similar between groups. The control group presented a statistically lower SOFA score on day 4 compared to the other two groups [mean (SE) 8.9 (1) vs. 7.4 (0.9) vs. 6.4 (0.5) for CAZ-AVI + ATM, DCT and control group, respectively (p = 0.045)]. The duration of mechanical ventilation, ICU LOS, and mortality were similar between the groups (p > 0.05). Comparison between survivors and non-survivors revealed that survivors had a lower SOFA score on the day of BSI, higher PaO₂/FiO₂ ratio, higher platelet counts, and lower lactate levels (p < 0.05). Septic shock was more frequent among non-survivors (60.3%) in comparison to survivors (27%) (p = 0.0015). Independent factors for mortality were PaO₂/FiO₂ ratio and lactate levels (p < 0.05). None of the antibiotic regimens received by the patients was independently associated with survival. Conclusions: Treatment with CAZ-AVI + ATM or DCT may offer similar clinical outcomes for patients suffering from BSI caused by K. pneumoniae strains resistant to all available BL/BLIs. However, larger studies are required to confirm the findings. Full article

Cefepime-enmetazobactam is a novel β-lactam/β-lactamase inhibitor combination showing good activity against multidrug-resistant (MDR) Gram-negative bacteria producing a variety of β-lactamases. In this systematic review, we aimed to evaluate the available data on resistance to this drug. We performed a thorough search of four databases (Embase, PubMed, Scopus, and Web of Science), as well as backward citation searching, to identify studies containing data on resistance to cefepime-enmetazobactam. The data were extracted and analyzed according to the breakpoints established by the European Committee on Antimicrobial Susceptibility Testing (EUCAST) and the Food and Drug Administration (FDA), or the specific breakpoints reported by the authors of the respective studies. Analysis based on the type of lactamases produced by the isolates was also performed. Ten studies reported in vitro susceptibility testing and mechanisms of antimicrobial resistance. The total number of isolates was 15,408. The activity of cefepime-enmetazobactam against β-lactamase-producing isolates was variable. The resistance of the studied extended-spectrum β-lactamase (ESBL)-producing and ampicillin C β-lactamase (AmpC)-producing isolates was low (0–2.8% and 0%, respectively). The resistance was higher among oxacillinase-48 β-lactamase (OXA-48)-producing and Klebsiella pneumoniae carbapenemase (KPC)-producing isolates (3.4–13.2% and 36.7–57.8%, respectively). High resistance was noted among metallo-β-^lactamase (MBL)-producing isolates (reaching 87.5% in one study), especially those producing New Delhi metallo-β-lactamase (NDM) and Verona integron-encoded metallo-β-lactamase (VIM), which had the highest rates of resistance. The high activity of cefepime-enmetazobactam against Enterobacterales and selected lactose non-fermenting Gram-negative pathogens, including ESBL-producing and AmpC-producing isolates, makes it a potential carbapenem-sparing agent. The drug should be used after in vitro antimicrobial susceptibility testing in patients with infections caused by OXA-48, KPC, and MBL-producing isolates. Full article

Ceftazidime–avibactam (CAZ-AVI) is a second-generation intravenous β-lactam/β-lactamase inhibitor combination. In recent years, substantial evidence has emerged regarding the efficacy and safety of CAZ-AVI. However, data on its use in critically ill patients remain limited. Background/Objectives: This multicenter, retrospective, observational cohort study was conducted across four Intensive Care Units (ICUs) in three hospitals in the Marche region of Italy. The primary objective was to evaluate the 30-day clinical outcomes and identify risk factors associated with 30-day clinical failure—defined as death, microbiological recurrence, or persistence within 30 days after discontinuation of therapy—in critically ill patients treated with CAZ-AVI. Methods: The study included all adult critically ill patients admitted to the participating ICUs between January 2020 and September 2023 who received CAZ-AVI for at least 72 h for the treatment of a confirmed or suspected Gram-negative bacterial (GNB) infection. Results: Among the 161 patients included in the study, CAZ-AVI treatment resulted in a positive clinical outcome (i.e., clinical improvement and 30-day survival) in 58% of cases (n = 93/161), while the overall mortality rate was 24% (n = 38/161). Relapse or persistent infection occurred in a substantial proportion of patients (25%, n = 41/161). Notably, acquired resistance to CAZ-AVI was observed in 26% of these cases, likely due to suboptimal use of the drug in relation to its pharmacokinetic/pharmacodynamic (PK/PD) properties in critically ill patients. Furthermore, treatment failure was more frequent among immunosuppressed individuals, particularly liver transplant recipients. Conclusions: This study demonstrates that the mortality rate among ICU patients treated with this novel antimicrobial combination is consistent with findings from other studies involving heterogeneous populations. However, the rapid emergence of resistance underscores the need for vigilant surveillance and the implementation of robust antimicrobial stewardship strategies. Full article

Class D β-lactamases (DBLs) represent a major threat to antibiotic efficacy by hydrolyzing β-lactam drugs, including last-resort carbapenems, thereby driving antimicrobial resistance in Gram-negative bacteria. The enzymes share a structurally conserved two-domain α/β architecture with seven active-site motifs and three flexible extended loops (the P-loop, Ω-loop, and newly designated B-loop) that surround the active site. While each of these loops is known to influence enzyme function, their coordinated roles have not been fully elucidated. To investigate the significance of their interplay, we compared the sequences and crystal structures of 40 DBLs from clinically relevant Gram-negative pathogens and performed molecular dynamics simulations on selected representatives. Combined structural and dynamical analyses revealed a strong correlation between B-loop architecture and carbapenemase activity in the pathogens Klebsiella and Acinetobacter, particularly regarding loop length and spatial organization. These findings emphasize the B-loop’s critical contribution, in concert with the P- and Ω-loops, in tuning active site versatility, substrate recognition, catalytic activity, and structural stability. A deeper understanding of how these motifs and loops govern DBL function may inform the development of novel antibiotics and inhibitors targeting this class of enzymes. Full article

Background/objectives: Proteus mirabilis is a frequent causative agent of urinary and wound infections in both community and hospital settings. It develops resistance to expanded-spectrum cephalosporins (ESCs) due to the production of extended-spectrum β-lactamases (ESBLs) or plasmid-mediated AmpC β-lactamases (p-AmpCs). Recently, carbapenem-resistant isolates of P. mirabilis emerged due to the production of carbapenemases, mostly belonging to Ambler classes B and D. Here, we report an outbreak of infections due to carbapenem-resistant P. mirabilis that were observed in a psychiatric hospital in Zagreb, Croatia. The characteristics of ESBL and carbapenemase-producing P. mirabilis isolates, associated with an outbreak, were analyzed. Materials and methods: The antibiotic susceptibility testing was performed by the disk-diffusion and broth dilution methods. The double-disk synergy test (DDST) and inhibitor-based test with clavulanic and phenylboronic acid were applied to screen for ESBLs and p-AmpCs, respectively. Carbapenemases were screened by the modified Hodge test (MHT), while carbapenem hydrolysis was investigated by the carbapenem inactivation method (CIM) and EDTA-carbapenem-inactivation method (eCIM). The nature of the ESBLs, carbapenemases, and fluoroquinolone-resistance determinants was investigated by PCR. Plasmids were characterized by PCR-based replicon typing (PBRT). Selected isolates were subjected to molecular characterization of the resistome by an Inter-Array Genotyping Kit CarbaResisit and whole-genome sequencing (WGS). Results: In total, 20 isolates were collected and analyzed. All isolates exhibited resistance to amoxicillin alone and when combined with clavulanic acid, cefuroxime, cefotaxime, ceftriaxone, cefepime, imipenem, ceftazidime–avibactam, ceftolozane–tazobactam, gentamicin, amikacin, and ciprofloxacin. There was uniform susceptibility to ertapenem, meropenem, and cefiderocol. The DDST and combined disk test with clavulanic acid were positive, indicating the production of an ESBL. The MHT was negative in all except one isolate, while the CIM showed moderate sensitivity, but only with imipenem as the indicator disk. Furthermore, eCIM tested positive in all of the CIM-positive isolates, consistent with a metallo-β-lactamase (MBL). PCR and sequencing of the selected amplicons identified VIM-1 and VIM-4. The Inter-Array Genotyping Kit CarbaResist and WGS identified β-lactam resistance genes bla_VIM, bla_CTX-M-15, and bla_TEM genes; aminoglycoside resistance genes aac(3)-IId, aph(6)-Id, aph(3″)-Ib, aadA1, armA, and aac(6′)-IIc; as well as resistance genes for sulphonamides sul1 and sul2, trimethoprim dfr1, chloramphenicol cat, and tetracycline tet(J). Conclusions: This study revealed an epidemic spread of carbapenemase-producing P. mirabilis in two wards in a psychiatric hospital. Due to the extensively resistant phenotype (XDR), therapeutic options were limited. This is the first report of carbapenemase-producing P. mirabilis in Croatia. Full article

Background: Multidrug-resistant (MDR) Gram-negative infections, particularly those caused by carbapenem-resistant Enterobacterales (CRE) and difficult-to-treat Pseudomonas aeruginosa (DTR-Pa), present a growing global healthcare challenge, especially in critically ill populations. Imipenem–relebactam (I/R), a novel β-lactam/β-lactamase inhibitor combination, has shown efficacy in clinical trials, but real-world data remain limited. Methods: We conducted a multicenter, retrospective–prospective observational study across tertiary-care hospitals in Italy between January 2020 and May 2025. Adult patients (≥18 years) treated with I/R for ≥48 h for suspected or confirmed MDR Gram-negative infections were included. Primary endpoints were clinical success at the end of therapy and 30-day all-cause mortality. Secondary endpoints included microbiological eradication, recurrence, safety, and predictors of treatment failure. Statistical analysis involved descriptive methods and correlation analysis for mortality predictors. Results: Twenty-nine patients were included (median age 66 years; 58.6% ICU admission; 71.4% mechanical ventilation). Clinical success was achieved in 22/29 patients (75.9%), while 30-day mortality was 24.1% (7/29). The most common pathogen was Klebsiella pneumoniae (62.1%), with 41.4% of infections being polymicrobial. Microbiological eradication was confirmed in all the BSIs. Parenteral nutrition (p = 0.016), sepsis at presentation (p = 0.04), candidemia (p = 0.036), and arterial catheter use (p = 0.029) were significantly more frequent in non-survivors. Survivors showed significant reductions in CRP, PCT, and bilirubin at 48 h, while non-survivors did not. Parenteral nutrition (rho = 0.427, p = 0.023), sepsis (rho = 0.378, p = 0.043), and arterial catheter use (rho = 0.384, p = 0.04) were significantly correlated with mortality. Conclusions: In this Italian multicenter cohort of critically ill patients, imipenem–relebactam demonstrated high clinical success and acceptable mortality rates in the treatment of severe MDR Gram-negative infections, particularly those caused by KPC-producing K. pneumoniae. Early biomarker dynamics may aid in monitoring treatment response. Larger prospective studies are needed to confirm these findings and define optimal treatment strategies. Full article

Background/Objectives: Carbapenem-resistant Enterobacterales (CRE) and carbapenem-resistant Pseudomonas aeruginosa (CRPA) are challenging multidrug-resistant pathogens. This study evaluated the in vitro susceptibility of CRE and CRPA blood isolates from Korea to novel β-lactam/β-lactamase inhibitor combinations: ceftolozane/tazobactam (C/T), ceftazidime/avibactam (CZA), imipenem/cilastatin/relebactam (IMR), and meropenem/vaborbactam (MEV). Methods: Blood isolates of CRE (n = 55) and CRPA (n = 65) collected between September 2017 and September 2022 in a Korean tertiary hospital were included. Carbapenemase production was determined using phenotypic and molecular methods. In vitro susceptibility to C/T, CZA, IMR, and MEV was determined primarily by broth microdilution using current CLSI/EUCAST breakpoints. Clinical characteristics and in-hospital mortality were retrospectively reviewed. Results: Among non-carbapenemase-producing (non-CP) CRPA isolates (n = 47), susceptibility rates were 83.0% to C/T and 70.2% to CZA. For KPC-producing CRE isolates (n = 28), susceptibility rates were high to CZA (92.9%), IMR (82.1%), and MEV (96.4%). However, non-CP CRE isolates (n = 22) showed low susceptibility to C/T (18.2%) but high susceptibility to CZA (100%), IMR (81.8%), and MEV (95.5%). CRE infections were associated with higher rates of hematologic malignancy, immunosuppression, and in-hospital mortality (63.6% vs. 18.5% for CRPA, p < 0.001). Conclusions: The susceptibility of CRE and CRPA to novel β-lactam/β-lactamase inhibitors varies significantly by species and carbapenemase production. CZA, IMR, and MEV showed promising activity against KPC-producing CRE. These findings can inform empirical therapy and stewardship efforts in Korea. Full article

Background: Urinary tract infections (UTIs) caused by the multidrug resistance (MDR) phenotype termed extended-spectrum beta lactamase (ESBL)-producing E. coli is a significant and growing global health concern. In response to the rising prevalence, the novel Beta Lactam-Beta Lactamase inhibitor (BL/BLI) combinations have been introduced in recent years. While these agents have shown efficacy, their clinical utility is constrained by high cost, limited availability, and emerging resistance mechanisms. The rational of this study was to test the in vitro activity of a cost-effective alternative to currently available BL–BLI combinations against ESBL-producing E. coli isolated from urinary tract infections (UTIs). Objective: This study investigates the in vitro antimicrobial activity of cefaclor (CFC), both as monotherapy and in combination with the β-lactamase inhibitors clavulanic acid (CA) and sulbactam (SUL), against 52 ESBL-producing E. coli isolates derived from urine cultures of patients diagnosed with UTIs. Methods: The susceptibility ranges were measured by disk diffusion and minimal inhibitory concentration (MIC) methods. In addition, the Time kill assay and disk approximation method were performed to measure the synergistic and bactericidal activity of the approached combination. Results: The MIC50 and MIC90 for CFC were improved from more than 128 µg/mL to 8/4 µg/mL when CFC was combined with either CA or SUL. The triple combination format of CFC/CA/SUL showed MIC50 and MIC90 values at 8/4/4 µg/mL and 64/32/32 µg/mL, respectively. The recovered susceptibility percentages were 54%, 54%, and 58% for CFC/CA, CFC/SUL, and CFC/CA/SUL combinations, respectively. Disk approximation and time–kill assay results revealed synergy and bactericidal effects when CFC combined with CA or SUL for isolates that showed susceptibility restorations of CFC when coupled with CA or SUL by the disk diffusion and MIC method. Conclusions: This study proposes a cost-effective combination that could mitigate resistance development and offer a sparing option to last resort treatment choices including carbapenems. However, testing efficacy in a clinical setting is crucial. Full article

Nowadays, antimicrobial resistance (AMR) is a growing global health threat, with carbapenem-resistant Enterobacteriaceae (CRE) posing particular concern due to limited treatment options. In fact, CRE have been classified as a critical priority by the World Health Organization (WHO). Carbapenem resistance results from complex mechanisms, often combining the production of hydrolytic enzymes such as β-lactamases with reduced membrane permeability and efflux system induction. The Ambler classification is an effective tool for differentiating the characteristics of serine-β-lactamases (SβLs) and metallo-β-lactamases (MβLs), including ESβLs (different from carbapenemases), KPC, NDM, VIM, IMP, AmpC (different from carbapenemases), and OXA-48. Recently approved inhibitor drugs, such as diazabicyclooctanones and boronic acid derivatives, only partially address this problem, not least because of their ineffectiveness against MβLs. However, compared with taniborbactam, xeruborbactam is the first bicyclic boronate in clinical development with a pan-β-lactamase inhibition spectrum, including the IMP subfamily. Recent studies explore strategies such as chemical optimization of β-lactamase inhibitor scaffolds, novel β-lactam/β-lactamase inhibitor combinations, and siderophore–antibiotic conjugates to enhance bacterial uptake. A deeper understanding of the mechanistic properties of the active sites enables rational drug design principles to be established for inhibitors targeting both SβLs and MβLs. This review aims to provide a comprehensive overview of current therapeutic strategies and future perspectives for the development of carbapenemase inhibitor drug candidates. Full article

Background/Objectives: As a monocyclic β-lactam antibiotic, aztreonam has regained attention recently because combining it with β-lactamase inhibitors helps fight drug-resistant bacteria. This study aimed to systematically characterize the plasma and tissue concentration-time profiles of aztreonam in rats, mice, dogs, monkeys, and humans by developing a multi-species, physiologically based pharmacokinetic (PBPK) model. Methods: A rat PBPK model was optimized and validated using plasma concentration-time curves determined by liquid chromatography–tandem mass spectrometry (LC-MS/MS) following intravenous administration, with reliability confirmed through another dose experiment. The rat model characteristics, modeling experience, ADMET Predictor (11.0) software prediction results, and allometric scaling were used to extrapolate to mouse, human, dog, and monkey models. The tissue-to-plasma partition coefficients (K_p values) were predicted using GastroPlus (9.0) software, and the sensitivity analyses of key parameters were evaluated. Finally, the cross-species validation was performed using the average fold error (AFE) and absolute relative error (ARE). Results: The cross-species validation showed that the model predictions were highly consistent with the experimental data (AFE < 2, ARE < 30%), but the deviation of the volume of distribution (V_ss) in dogs and monkeys suggested the need to supplement the species-specific parameters to optimize the prediction accuracy. The K_p values revealed a high distribution of aztreonam in the kidneys (K_p = 2.0–3.0), which was consistent with its clearance mechanism dominated by renal excretion. Conclusions: The PBPK model developed in this study can be used to predict aztreonam pharmacokinetics across species, elucidating its renal-targeted distribution and providing key theoretical support for the clinical dose optimization of aztreonam, the assessment of target tissue exposure in drug-resistant bacterial infections, and the development of combination therapy strategies. Full article

The introduction of new β-lactam–β-lactamase inhibitors (BLBLIs), such as ceftazidime/avibactam, meropenem/vaborbactam, and imipenem/cilastatin/relebactam, expands our therapeutic options against carbapenem-resistant Gram-negative bacteria, including those pathogens for which therapeutic options are limited. These new combinations are active against ESBL-, AmpC-, and KPC-producing Enterobacterales, with the exception of ceftazidime/avibactam, which is active in vitro against OXA-48. However, one drawback that must be taken seriously by the clinician is that they are ineffective against metallo-β-lactamases as well as Acinetobacter baumannii. The recent introduction of aztreonam/avibactam marks a significant advancement in our therapeutic armamentarium against metallo-β-lactamase-producing pathogens. The question to be answered is whether there is a preferred, newer BLBLI combination for the treatment of KPC-producing Enterobacterales infections. This review provides a thorough analysis of the similarities and differences between these new combinations to identify the most effective treatment options. The present review aims to provide clinicians with a detailed understanding of each BLBLI treatment option to guide the optimal use of these new agents for the effective treatment of difficult infections caused by carbapenemase-producing Enterobacterales infections. This review is based on literature retrieved from PubMed, Scopus, Web of Science, and the Cochrane Library. Full article

Pseudomonas aeruginosa is a significant threat to public health as an aggressive, opportunistic pathogen. The use of β-lactam antibiotics such as penicillins, cephalosporins, monobactams, and carbapenems remains a front-line treatment against P. aeruginosa. However, the widespread use of β-lactams has led to the emergence of β-lactam-resistant isolates that significantly increase the economic burden and risk of mortality in patients. With the declining productivity of the antibiotic discovery pipeline, research has investigated synergistic agents to revive the use of β-lactam antibiotics against β-lactam-resistant P. aeruginosa. In this review, we summarize the mechanism of β-lactam antibiotics and provide an overview of major mechanisms associated with β-lactam resistance in P. aeruginosa. We then describe the background and use of three promising classes of agents that have shown extensive beneficial effects with β-lactam antibiotics against P. aeruginosa, namely β-lactamase inhibitors, bacteriophages, and antimicrobial peptides. The current understanding of the mechanisms of these synergistic agents is discussed. Lastly, we provide an overview of the current barriers impeding antibiotic development, and offer a glimpse into recent advances of artificial intelligence-based discovery that may serve as a new foundation for antimicrobial discovery and treatment. Full article

This review examines the latest progress of β-lactam antibiotics, focusing on penems. Penems are distinguished by their unique structural characteristics and remarkable antibacterial activity. The structural characteristics of the class that differentiate it from carbapenems or monobactams are addressed. Notable representatives such as sulopenem and faropenem are discussed. Faropenem’s stability and efficacy against resistant bacterial strains emphasize the therapeutic potential of penems. The review highlights sulopenem’s recent FDA approval, marking a key point in treating uncomplicated urinary tract infections caused by specific bacteria in adult women with minimal or no other options for oral antibiotic treatment. The review covers sulopenem’s structural considerations, physicochemical properties, mechanisms of action, antibacterial activity, and clinical pharmacology. The development of penem derivatives is also addressed, emphasizing their potential in combating resistant bacterial infections. Despite having few approved representatives, penems show promising prospects for future design and may significantly contribute to the fight against bacterial resistance. The review also highlights the challenges and future possibilities in penem research, including the need for improved oral bioavailability and the potential for combination therapies with β-lactamase inhibitors. Overall, penems are valuable antibacterial agents in the antimicrobial arsenal, offering hope in the ongoing battle against multidrug-resistant pathogens. Full article

Background: Mycobacterium abscessus (MABS) is an opportunistic pathogen that causes chronic, difficult-to-treat pulmonary infections, particularly in people with cystic fibrosis (PwCF), leading to rapid lung function decline and increased morbidity and mortality. Treatment is particularly challenging due to the pathogen’s resistance mechanisms and the need for prolonged multidrug therapy, which is characterized by poor clinical outcomes and highlights the urgent need for novel therapeutic strategies. Imipenem/relebactam, a novel β-lactam-β-lactamase inhibitor combination, demonstrates in vitro activity against resistant MABS strains and effective pulmonary penetration. Prior research indicates synergistic activity of imipenem with various antibiotics against M. abscessus. Objectives: This study aims to evaluate the in vitro activity of imipenem/relebactam, alone and in combination with various antibiotics, against MABS clinical isolates from PwCF (n = 28). Methods: Susceptibility and synergy were assessed using broth microdilution and checkerboard assays. Extracellular time-kill assays were performed to evaluate the bactericidal activity of synergistic three-drug combinations containing imipenem/relebactam. Results: Imipenem/relebactam demonstrated potent in vitro activity against clinical MABS isolates, exhibiting substantial synergy with cefuroxime, cefdinir, amoxicillin, and cefoxitin. Rifabutin, azithromycin, moxifloxacin, clofazimine, and minocycline also demonstrated additive effects with imipenem/relebactam. Extracellular time-kill assays identified imipenem/relebactam + cefoxitin + rifabutin and imipenem/relebactam + cefoxitin + moxifloxacin as the most effective combinations. Conclusions: These findings suggest that imipenem/relebactam may offer a significant advancement in the management of MABS infections in PwCF. The promising efficacy of multidrug regimens combining imipenem/relebactam with agents like cefoxitin, azithromycin, moxifloxacin, clofazimine, and rifabutin highlights potential therapeutic strategies. Full article

Background/Objectives: Proteus mirabilis is a frequent causative agent of urinary tract and wound infections in community and hospital settings. It develops resistance to expanded-spectrum cephalosporins (ESC) due to the production of extended-spectrum β-lactamases (ESBLs) or plasmid-mediated AmpC β-lactamases (p-AmpC). Here, we report the characteristics of ESBLs and p-AmpC β-lactamases encountered among hospital and community isolates of P. mirabilis in two hospitals and the community settings in Zagreb, Croatia. Methods: Antibiotic susceptibility testing was performed using disk-diffusion and broth dilution methods. The double-disk-synergy test (DDST) and inhibitor-based test with clavulanic and cloxacillin were applied to screen for ESBLs and p-AmpC, respectively. PCR investigated the nature of ESBL, carbapenemases, and fluoroquinolone resistance determinants. Selected strains were subjected to molecular analysis of resistance traits by the Inter-Array CarbaResist Kit and whole-genome sequencing (WGS). Results: In total, 39 isolates were analyzed. Twenty-two isolates phenotypically tested positive for p-AmpC and seventeen for ESBLs. AmpC-producing organisms exhibited uniform resistance to amoxicillin-clavulanate, ESC, ciprofloxacin, and sulphamethoxazole-trimethoprim, and uniform susceptibility to carbapenems and piperacillin-tazobactam and all harbored bla_CMY-16 genes. ESBL-positive isolates demonstrated resistance to amoxicillin-clavulanate, cefuroxime, cefotaxime, ceftriaxone, and ciprofloxacin but variable susceptibility to cefepime and aminoglycosides. They possessed bla_CTX-M genes that belong to cluster 1 (n = 5) or 9 (n = 12), with CTX-M-14 and CTX-M-65 as the dominant allelic variants. Conclusions: The study demonstrated the presence of CTX-M ESBL and CMY-16 p-AmpC among hospital and community-acquired isolates. AmpC-producing isolates showed uniform resistance patterns, whereas ESBL-positive strains had variable degrees of susceptibility/resistance to non-β-lactam antibiotics, resulting in more diverse susceptibility patterns. The study found an accumulation of various resistance determinants among hospital and outpatient isolates, mandating improvement in detecting β-lactamases during routine laboratory work. Full article