Ceftazidime-avibactam (CZA) is an important therapeutic option for infections caused by Klebsiella pneumoniae carbapenemase (KPC)-producing Klebsiella pneumoniae. However, CZA exposure also selects for emergent KPC variants. Their in vivo evolutionary patterns, fitness consequences, and underlying molecular mechanisms remain unclear. We performed a longitudinal multiomics analysis of 35 clonally related ST11 KPC-producing K. pneumoniae isolates collected from eight hospitalized patients during clinical follow-up, most of whom had received CZA therapy. Whole-genome sequencing, antimicrobial susceptibility testing, in vitro competition assays, enzyme kinetic analysis, and transcriptomic sequencing were used to systematically characterize the within-host evolutionary dynamics of KPC variants and the fitness heterogeneity of KPC-33. Multiple KPC variants were identified during longitudinal follow-up, among which KPC-33 was the most frequently detected. Among the seven patients who received CZA treatment, KPC-33 was detected in longitudinal isolates from four patients. It was also identified in patient P3, who had not received CZA, whereas other variants were only sporadically identified. Biochemical analysis showed that KPC-33 exhibited an altered kinetic profile relative to KPC-2, characterized by reduced catalytic turnover and altered substrate affinity. KPC-33 did not exhibit a uniform and pronounced fitness defect but instead showed marked strain-dependent heterogeneity. Strains with higher competitive fitness generally showed only limited transcriptional changes, whereas those with lower fitness were accompanied by broader transcriptional remodeling. In this longitudinal cohort, KPC-33 was repeatedly detected, predominantly under CZA-associated selective conditions. Its fitness consequences were clearly strain background dependent and may be associated with the extent of transcriptional remodeling. These findings provide new evidence for understanding the in vivo evolution of CZA resistance.

Ceftolozane/tazobactam (C/T) and ceftazidime/avibactam (C/A) are frequently used for the treatment of multidrug-resistant Pseudomonas aeruginosa (MDR-PSA) pneumonia. However, comparative "real-world" effectiveness data are limited. We performed a retrospective cohort study within the U.S. Veterans Health Administration (VHA; 2015-2019, 2022-2024), including adults with microbiologically confirmed MDR-PSA pneumonia receiving C/T or C/A within ≤5 days of index culture. Inverse probability of treatment weighting (IPTW) was applied to balance baseline covariates. The primary outcome was 30-day all-cause mortality, and secondary outcomes were 60-day mortality, 90-day mortality, and 30-, 60-, and 90-day MDR-PSA recurrence and all-cause hospital readmission. A prespecified sensitivity analysis examined the effect of early treatment initiation ≤72 h. Among 260 patients (C/T, n = 132; C/A, n = 128), C/T recipients demonstrated more extensive prior antimicrobial exposure and substantially longer time-to-therapy (median, 56 versus 19 h; P < 0.001). After IPTW, balance improved across most prespecified covariates, although residual imbalance persisted for age and geographic region. No statistically significant between-group differences were observed in 30-day mortality (adjusted risk ratio [aRR], 1.02; 95% confidence interval [CI], 0.56-1.83; P = 0.944) or in other clinical outcomes. Among patients with early treatment within 72 h (n = 175), no significant differences in 30-day mortality (aRR, 1.18; 95% CI, 0.47-2.93; P = 0.724) or other mortality, recurrence, or readmission outcomes were observed between C/T and C/A. In this national VHA cohort, C/T and C/A demonstrated comparable clinical, microbiologic, and healthcare utilization outcomes. These findings support consideration of either C/T or C/A for MDR-PSA pneumonia when used at appropriate doses.

Commercial antimicrobial susceptibility testing (AST) systems for gram-negative bacteria provide operational advantages over reference broth microdilution (BMD) testing, but the results produced by commercial AST systems might not be as accurate as the reference method. Assessing and comparing the performance of AST systems can be informative for clinical microbiology practice. In total, 280 clinical isolates and 100 isolates from the U.S. Centers for Disease Control and Prevention Antimicrobial Resistance Isolate Bank, including Enterobacterales and Pseudomonas aeruginosa, were intended to be tested using disk diffusion (DD) and automated commercial AST systems, such as Selux AST System (Selux Diagnostics), MicroScan (Beckman Coulter), Phoenix (Becton, Dickinson and Company), and Vitek 2 (bioMérieux). These results were compared to BMD as the reference standard. In total, 372 isolates were assessed. For Enterobacterales, the greatest frequency of very major errors was identified with cefepime by Vitek 2 (29.0%) and ertapenem by DD (9.1%); essential agreement less than 90% was identified with the following combinations: aztreonam by MicroScan (86.4%), aztreonam by Vitek 2 (88.9%), cefepime by Selux (86.3%), cefepime by Vitek 2 (76.8%), ceftazidime by Vitek 2 (89.9%), and levofloxacin by MicroScan (83.4%). For P. aeruginosa, essential agreement less than 90% was identified with the following combinations: gentamicin by Selux (87.2%), tobramycin by Selux (86.6%), and ciprofloxacin by Phoenix (88.4%). Overall, antimicrobials that had low demonstrable bias across all automated commercial AST systems included ampicillin, amoxicillin-clavulanate, ceftazidime-avibactam, ertapenem, and trimethoprim-sulfamethoxazole. Comparing commercial testing systems head-to-head against a reference standard provides insight into the strengths and weaknesses of the performance of these test systems, and these insights can subsequently inform local decisions regarding further investigations and potential operational mitigation strategies.IMPORTANCEThis study assessed and compared the performance of multiple commercial antimicrobial susceptibility testing (AST) systems and disk diffusion methods. The accuracy of determining minimum inhibitory concentrations or zone diameters of antimicrobials and the associated interpretive categories were studied for 372 isolates of Enterobacterales and Pseudomonas aeruginosa. Performance of the evaluated systems revealed that multiple antimicrobial-organism combinations were below the standards typically required by regulatory agencies and clinical laboratories. These deficiencies varied across the commercial platforms and disk diffusion methods, highlighting the importance of benchmarking studies that compare the performance using identical isolates. These identified performance weaknesses may warrant further study and/or the implementation of mitigation strategies in clinical laboratory operations to prevent the reporting of errant results.

The approach to multidrug-resistant (MDR) Gram-negative bacilli (GNB) infections has changed following the introduction of avibactam-based β-lactam/β-lactamase inhibitor combinations. Ceftazidime/avibactam has become a cornerstone for the treatment of infections caused by KPC-producing Enterobacterales, whereas the recent availability of aztreonam/avibactam has provided the first targeted option against metallo-β-lactamase (MBL)-producing Enterobacterales. At the same time, the emergence of KPC variants, the increasing dissemination of MBLs and the growing complexity of resistance mechanisms have made both empirical and targeted therapeutic decisions increasingly challenging. This narrative review summarizes the current evidence regarding the microbiological activity, clinical efficacy and optimal place in therapy of avibactam-based combinations. Ceftazidime/avibactam is a cornerstore for the treatment of KPC infections, and significantly modified the outcome of patients with KPC infections during the last decade. Robust observational studies demonstrated its clinical efficacy and safety. It may also represent a carbapenem-sparing option for treating infections due to difficult-to-treat ESBL- and OXA-48-producing Enterobacterales. Moreover, its activity against MDR Pseudomonas aeruginosa makes it an attractive option for empirical therapy in patients with risk factors for this non-fermenting GNB. Aztreonam/avibactam represents the first-line option for the treatment of MBL-producing Enterobacterales and plays a pivotal role in the current era of increasing dissemination of multiple carbapenemases. Their optimal clinical positioning should rely on optimal use in empirical therapy in patients with rectal colonization with carbapenem-resistant Enterobacterales who develop sepsis or septic shock, antimicrobial stewardship principles and integration of local epidemiology, microbiological data and patient characteristics to maximize efficacy while preserving their long-term activity.

Management of severe infectious keratitis during pregnancy is challenging because rapid control of a sight-threatening infection must be balanced against potential fetal exposure to antimicrobial agents and surgical interventions. Photoactivated chromophore for keratitis-corneal cross-linking (PACK-CXL) has been proposed as an adjunctive treatment for infectious keratitis associated with progressive stromal melting. A 34-year-old woman at 19 weeks of gestation presented with a two-day history of painful visual deterioration in her left eye. She was a contact lens wearer. Slit-lamp examination revealed a 5.0 × 3.5-mm paracentral corneal ulcer with a wet, necrotic appearance, an associated 3.5 × 3.5-mm stromal infiltrate, and corneal melting. Marked anterior chamber inflammation and a 0.5-mm hypopyon were present. Best-corrected visual acuity was 20/20 in the right eye and 20/200 in the left eye. Cultures of corneal scrapings and the contact lenses yielded Pseudomonas aeruginosa, whereas polymerase chain reaction testing for fungi and Acanthamoeba was negative. Following consultation with the obstetric team, intensive topical amikacin and vancomycin, topical atropine, and intravenous ceftazidime were administered. Although the hypopyon resolved and anterior chamber inflammation substantially decreased, stromal infiltration and melting persisted after 14 days of treatment. Adjunctive PACK-CXL was therefore performed using 0.1% riboflavin and ultraviolet-A irradiation at 3.0 mW/cm[2] for 30 minutes. The ulcer and infiltrate subsequently regressed, with complete resolution of the corneal melting. At three months, the ulcer had healed completely, leaving a paracentral stromal scar that did not significantly involve the visual axis. Best-corrected visual acuity improved to 20/28. Therapeutic penetrating keratoplasty was avoided, and the pregnancy progressed to an uncomplicated delivery. Adjunctive PACK-CXL was associated with arrest of corneal melting and satisfactory anatomical and visual recovery in a pregnant patient with refractory P. aeruginosa keratitis. However, the reproductive safety of riboflavin-ultraviolet-A cross-linking has not been established. Its use during pregnancy should therefore be considered only in exceptional, sight-threatening circumstances following multidisciplinary assessment and informed evaluation of the potential maternal benefits and unknown fetal risks.

India bears one of the highest burdens of Gram-negative bacterial infections, yet their burden and associated mortality remain poorly characterised. We aimed to determine and compare the mortality and cost of therapy for susceptible and resistant Gram-negative bacterial infections from a multicentric hospital-based surveillance in India.

Objectives: This study aimed to compare the prevalence and resistance patterns of antimicrobial resistance (AMR) in Escherichia coli isolated from broiler chickens across Indonesia using the Clinical and Laboratory Standards Institute (CLSI) M100, 35[th] edition (2025), and the European Committee on Antimicrobial Susceptibility Testing (EUCAST) version 15.0 (2025) guidelines. Material and Methods: A total of 390 E. coli isolates were subjected to antimicrobial susceptibility testing using the broth microdilution method with the Sensititre EUVSEC3 panel. Resistance data were analyzed using the Chi-square test, Cohen's kappa coefficient, and percent agreement. Results: The highest resistance rates under both standards were observed for ampicillin (78.33% according to CLSI and 78.63% according to EUCAST), while all isolates remained susceptible to meropenem (100%) based on both references. Significant differences between CLSI and EUCAST interpretations were observed for colistin and ciprofloxacin (p < 0.05). Colistin demonstrated negligible agreement between the two standards (κ = -0.01; 95% CI: -0.05 to 0.03; 3% agreement). In contrast, interpretation of susceptibility results for ampicillin, cefotaxime, ceftazidime, gentamicin, and trimethoprim showed almost perfect concordance between CLSI and EUCAST (κ > 0.91). Conclusions: Variations in the interpretation of antimicrobial susceptibility testing results may influence AMR surveillance outcomes and antimicrobial use decision-making in the field. Strengthening and harmonizing AMR surveillance reporting based on both standards is essential to support evidence-based and effective AMR mitigation policies. This approach, within a One Health framework, is to safeguard animal health, public health, and food safety in Indonesia.