ABSTRACT The KPC family of serine β-lactamases comprises more than 260 members. Some variants are associated with ceftazidime-avibactam resistance in clinical isolates, often linked to substitutions and/or insertions/deletions (i.e., INDELs) in three distinct loops of the KPC sequence: (i) the 164–179 loop (i.e., the Ω loop); (ii) the 237–243 loop; and (iii) the 267–275 loop. Inconsistencies in residue numbering across published reports, however, complicate the accurate annotation of KPC variants. We retrieved 267 KPC variant sequences from the Beta-Lactamase Database (BLDB) in September 2025 and analyzed sequence differences between variants using combined nucleotide and structure-guided alignment algorithms, supported by AlphaFold3 modeling in ambiguous cases. Variants were classified into four groups to comprehensively review sequence changes across the KPC family: substitutions only ( n = 126), deletions only ( n = 17), insertions only ( n = 66), and variants with two or more types of amino acid changes ( n = 57). Comparisons with previous reports indicate that many annotation errors stem from overlooking the absent residues 58 and 253 in the KPC consensus sequence, and that most inconsistencies in annotation and residue assignment occur in INDEL variants. To address these issues, we propose a standardized annotation scheme for substitutions, deletions, and insertions for the KPC family, based on the Ambler numbering system, and supported by structural information. This systematic scheme will help to standardize the description of newly emerging KPC variants and prevent discrepancies in future reports in the context of antimicrobial resistance.

Patients with complicated urinary tract infections (cUTIs) caused by multidrug-resistant (MDR) uropathogens often require hospitalization and intravenous (IV) antibiotic treatment. This real-world cohort study aimed to describe clinical complications in patients with cUTI. Adults (≥ 18 years) who received IV-carbapenem (IVC) treatment within 7 days of a primary or admitting UTI diagnosis between January 1, 2018 and September 30, 2023 were identified using Optum’s de-identified Electronic Health Record dataset. These patients had cUTI infection (pyelonephritis, complicated cystitis or urosepsis), with the index date defined as the date of IVC treatment initiation. IV complications were evaluated from index to the earlier of 30 days following index or death. Complications related to extended length of stay (LOS) were evaluated from hospital admission to the earlier of discharge or death among a subgroup of hospitalized patients admitted for cUTI. Clinical complications were assessed overall and by LOS (1–3 vs 4+ days). In total, 16,144 patients treated with IVC for UTI were included: median age 71.0 years, 63.8% female, 41.9% obese, 30.5% uncontrolled and complicated diabetes, 42.2% severe renal dysfunction, and median (interquartile range [IQR]) Quan-Charlson comorbidity index of 2.0 (0.0, 4.0; Table). The incidence of any IV-related complication was 23.0% (n=3708), with a median (IQR) time to event of 3.0 days (1.0, 11.0; Figure 1). Among the subgroup of hospitalized patients (n=10,090), median (IQR) LOS was 6.0 days (4.0, 8.0). The incidence of any non-IV-related complications related to hospital LOS was 51.1% (n=5153); the incidence of any complication among patients with a LOS of 1–3 and 4+ days was 30.8% and 55.0%, respectively. The most common complication related to extended LOS was falls (overall, 30.7%; 1–3 days, 16.9%; 4+ days, 33.3%; Figure 2). Over half of cUTI patients treated with IV carbapenems experienced at least one complication, with higher incidence among patients with extended LOS. These findings highlight the clinical burden of patients treated with IVC and the potential value of new oral treatment options in cUTI. Funding: GSK study 221138. Myriam Drysdale, PhD, GSK: Employee|GSK: Stocks/Bonds (Public Company) Rose Chang, ScD, Analysis Group: Employee|GSK: Grant/Research Support Louise Yu, MS, Analysis Group: Employee|GSK: Grant/Research Support Maral DerSarkissian, PhD, Analysis Group: Employee|GSK: Grant/Research Support Megan Pinaire, MPH, Analysis Group: Employee|GSK: Grant/Research Support Zhuo Chen, MPH, Analysis Group: Employee|GSK: Grant/Research Support Mei Sheng Duh, MPH, ScD, Analysis Group: Employee|GSK: Grant/Research Support Alanna Farrell-Foster, GSK: Employee|GSK: Stocks/Bonds (Public Company) Fanny S. Mitrani-Gold, MPH, GSK: Employee|GSK: Stocks/Bonds (Public Company)

This editorial explores the impact and interpretation of desirability of outcome ranking (DOOR) analyses compared to traditional endpoints in recent infectious diseases pharmacotherapy literature.

ABSTRACT Klebsiella pneumoniae (KP) isolates belonging to multi-locus sequence type 258 (ST258) are a frequent cause of hospital-associated outbreaks and display extensive multidrug resistance. The KP ST258 lineage consists of two genetically distinct clades, called Clade 1 and Clade 2. These two clades are genetically related to one another, but are historically distinguished by having different capsular polysaccharide types. While bacteria belonging to both clades are isolated from clinical infections, Clade 2 is isolated more frequently compared to Clade 1. To investigate drivers of this difference in clade prevalence, we collected 172 clinical KP ST258 isolates from patients at a single medical center. Clinical review showed that patients infected with Clade 2 isolates were more acutely ill than Clade 1-infected patients, despite having fewer comorbidities. We also found that Clade 2 isolates were more resistant to killing by human serum, despite binding more complement protein C3 than Clade 1 isolates. Additionally, mice infected with a Clade 2 isolate had increased bacterial dissemination from the lungs to the liver and spleen than mice infected with a Clade 1 isolate, and this dissemination required an intact capsule locus. Increased dissemination in mice was not due to differential serum killing, as mouse serum was unable to kill isolates of either clade, but dissemination was associated with decreased macrophage uptake of the Clade 2 isolate. Taken together, these data suggest that KP ST258 Clade 2 is more virulent than Clade 1, although the specific mechanisms at play appear to differ between mice and humans. IMPORTANCE KP ST258 is an epidemic lineage of multidrug-resistant gram-negative bacteria that has caused numerous outbreaks in hospitals around the world. The KP ST258 population is divided into two genetically related but distinct clades, which differ primarily in their capsule type. In this study, we found that patients infected with one of the KP ST258 clades were more acutely ill than patients infected with the other clade. We also observed clade-specific differences both in killing by human serum and in bacterial dissemination in a mouse model of pneumonia. Finally, we identified important limitations in the use of mouse models to study host defenses against multidrug-resistant KP infection. Overall, this work underscores the importance of capsule composition in KP ST258 virulence, identifies differences in the host response to KP infection between mice and humans, and highlights a potential role for complement-targeting immunotherapeutics in the treatment of KP infections.

We describe the first case of cefepime-zidebactam treatment-emergent resistance associated with clinical failure in a patient with Pseudomonas aeruginosa pneumonia. During treatment, cefepime-zidebactam minimum inhibitory concentrations increased from 8/8 to 32/32 mg/L. Whole-genome sequence analysis showed new mutations in the MexAB-OprM efflux operon that were associated with cross-resistance to ceftazidime-avibactam and imipenem-relebactam.

Imipenem/relebactam (I/R) demonstrates potent in vitro activity against multidrug-resistant (MDR) Pseudomonas aeruginosa. The objective of this study was to evaluate the effectiveness of I/R for treatment of MDR P. aeruginosa infections across the U.S.Table 1.Detailed inclusion and exclusion criteria for patients receiving I/R1 Pneumonia was defined as the presence of a new or progressive infiltrate with at least one of the following: purulent tracheal secretions, worsening cough or dyspnea, PaO2/FiO2 < 200 with PEEP ≥5 cm H2O, fever (≥38°C) or hypothermia (≤35°C), leukocytosis (≥10,000 white blood cells per µL), or tachypnea (respiratory rate >30 beats per minute).2 MDR was defined as non-susceptibility to at least one agent in three or more antibiotic classes.Table 2.Patient demographics, underlying diseases, severity of illness, and treatment characteristics of patients treated with I/R for pneumonia or bacteremia.*Other immunocompromising conditions included bone-marrow transplant, chronic steroid use, neutropenia, and AIDS.Abbreviations: IQR = interquartile range This was a retrospective, multicenter, observational study of I/R for MDR P. aeruginosa pneumonia and bacteremia. Patients were included if they received I/R for >48h initiated within 7 days of the index MDR P. aeruginosa culture (Table 1). Clinical success was defined as survival, resolution of signs and symptoms of infection, completion of the intended treatment course, and the absence of a recurrent infection due to MDR P. aeruginosa. I/R susceptibility was determined by site-level microbiology labs; non-susceptibility was defined by the Clinical and Laboratory Standards Institute (CLSI) criteria.Table 3.Real-world characteristics of I/R use in pneumonia and bloodstream infections.1 I/R treatment was discontinued in one patient with acute interstitial nephritisTable 4.Clinical outcomes of patients treated with I/R for MDR P. aeruginosa pneumonia or bacteremia1 Non-susceptibility was defined as a categorical change from susceptible to non-susceptible as defined by CLSI interpretive criteria. Among the 16 cases meeting this criteria, non-susceptibility was identified by gradient strip testing and broth microdilution in 25% and 75%, respectively. The median I/R MICs for isolates categorized as susceptible and non-susceptible were 2 and 8 mg/L, respectively. 64 patients from 10 centers were included (Table 2); patients from 6 additional centers were screened and did not meet inclusion criteria. The overall cohort was critically-ill; 80%, 75%, and 48% were in the intensive care unit, receiving mechanical ventilation, and on vasopressors, respectively. The median (interquartile range; IQR) SOFA score was 7 (5 – 12). 53% received treatment with another new β-lactam for MDR P. aeruginosa infections prior to I/R. The median time to I/R initiation was 67 hours. I/R treatment was primarily prescribed based on susceptibility results in 75% of patients, including resistance to other novel β-lactam agents (Table 3). 63% of patients completed the intended I/R treatment course as planned. At day 7 and 30, 80% and 55% met criteria for clinical success, respectively (Table 4). The overall 30- and 90-day mortality rates were 17% and 30%, respectively. Recurrent infections were documented in 38% of patients within 90 days. In this critically-ill patient population we found that I/R was often used following treatment with other novel β-lactams. Clinical outcomes were generally comparable to those previously reported in similar real-world studies for other novel β-lactam agents suggesting that I/R plays a role in treatment of MDR P. aeruginosa infections, particularly when other agents are not available or test resistant. jason M. Pogue, PharmD, Entasis: Advisor/Consultant|Entasis: Grant/Research Support|GlaxoSmithKline: Advisor/Consultant|Melinta: Grant/Research Support|Merck: Advisor/Consultant|Merck: Grant/Research Support|Shionogi: Advisor/Consultant|Shionogi: Grant/Research Support Alexander J. Lepak, MD, FIDSA, BioMerieux: Grant/Research Support William R. Miller, M.D., Merck: Grant/Research Support|UpToDate: Royalties, topic author Jeffrey C. Pearson, PharmD, InflaRx Pharmaceuticals, Inc.: Advisor/Consultant Emre Yucel, PhD, Merck & Co., Ltd: Stocks/Bonds (Public Company)

Background Oral antibiotics are effective in treating uncomplicated bloodstream infections (BSIs), but they are underused. The objective of this study was to evaluate the impact of a standardized antimicrobial stewardship intervention for early transition to oral antibiotics ("oral transition") for patients with uncomplicated BSIs.Methods A quasi-experimental study was conducted before and after implementation of a standardized stewardship intervention for patients with uncomplicated non-staphylococcal BSI over a 4-month period. Rates of oral transition and clinical outcomes were compared before and after intervention. The primary outcome was the rate of oral transition. Clinical failure (relapsed bacteremia, infection-attributable death, or new deep-seated infection with the same BSI organism) was a secondary outcome.Results A total of 187 and 177 BSIs were evaluated before and after the intervention, respectively. Overall, 44% (82 of 187) and 43% (76 of 177) met pre-specified criteria for oral transition. Overall oral transition rates increased from 59% (48 of 82) before to 93% (71 of 76) after intervention (P < .01). Baseline characteristics, including severity of illness, underlying diseases, and the BSI source, were similar between groups. Clinical failure occurred within 90 days in 7% (6 of 82) and 7% (5 of 76) of patients in the pre- and post-intervention groups, respectively. The total mean antibiotic duration was shorter in the post-intervention group (10.76 vs 12.15 in the pre-intervention group; P = .02). Secondary outcomes, including length of stay, 30-day readmission rate, and antibiotic adverse events, were similar between groups.Conclusions The implementation of a real-time antimicrobial stewardship intervention resulted in an increased rate of transition to oral antibiotics and fewer overall days of therapy, without an observed difference in clinical or safety outcomes.

Background: Pseudomonas aeruginosa is a ubiquitous opportunistic bacterial pathogen associated with nosocomial infections and is a leading cause of infection in persons with cystic fibrosis (pwCF). The front-line treatment for multidrug-resistant P. aeruginosa infections is ceftolozane-tazobactam (C/T). While previous research has characterized clinical P. aeruginosa isolates that evolved resistance to C/T, the collateral effect of evolved resistance on susceptibility to bacteriophages has not been explored. Methods: We collected paired P. aeruginosa clinical isolates from 10 pwCF and 18 non-pwCF who developed treatment-emergent C/T resistance. We compared genetic relatedness, acute and chronic virulence phenotypes, and antibiotic and phage susceptibilities between each pair of susceptible baseline and treatment-emergent C/T-resistant isolates. Results: Treatment-emergent C/T-resistant isolates were genetically closely related to baseline isolates in all patients. Virulence phenotypes did not differ between pre- and post-C/T exposure isolates, but isolates from pwCF demonstrated differences in protease production, twitching motility, and amino acid auxotrophy compared to isolates from non-pwCF. Treatment-emergent C/T resistance was associated with increased resistance to ceftazidime and ceftazidime/avibactam, but no other trends in antibiotic or phage susceptibility were detected. Conclusions: Treatment-emergent resistance to C/T does not cause predictable alterations in phage susceptibility across genotypically and phenotypically diverse multidrug-resistant P. aeruginosa clinical isolates.

OBJECTIVES:Carbapenem-resistant Enterobacterales (CRE) are classified as carbapenemase- and non-carbapenemase-producing (non-CP) CRE. The purpose of this study was to identify factors associated with meropenem non-susceptible non-CP CRE to help stratify treatment approaches and to report rates of treatment-emergent resistance among patients with non-CP CRE bacteraemia. METHODS:This was a retrospective, single-centre study of patients hospitalized between January 2015 and September 2025 with bacteraemia due to Enterobacterales resistant to ertapenem and confirmed to be non-CP. Ertapenem resistance was determined by broth microdilution susceptibility testing. Carbapenemase testing was performed by modified carbapenem-inactivation method, polymerase chain reaction or whole-genome sequencing. RESULTS:Among 60 patients who met the inclusion criteria, the median (IQR) age was 63 years (57-73), 33% (20/60) were female and 18% (11/60) were organ transplant recipients. Patients infected with meropenem non-susceptible non-CP CRE were more likely to have prior meropenem exposure [60% (10/15) versus 13% (8/45), P < 0.001] and have breakthrough bacteraemia [60% (9/15) versus 29% (13/45), P = 0.031]. Treatment-emergent resistance developed in 16% (4/24) and 0% (0/15) of patients treated with meropenem and ceftazidime-avibactam, respectively. CONCLUSIONS:These exploratory data provide new insights into clinical factors associated with meropenem non-susceptible non-CP CRE and support future investigations into the comparative-effectiveness of meropenem versus beta-lactam/beta-lactamase inhibitors for the treatment of meropenem-susceptible non-CP CRE.

BACKGROUND:Hypoalbuminaemia in patients with MSSA bacteraemia is associated with a delayed time to blood culture sterilization and increased mortality. Although this phenomenon has been postulated to be due to suboptimal antimicrobial exposure, the albumin thresholds that predispose patients to worse outcomes remain unclear. METHODS:This was a multicentre, retrospective study of adult patients with MSSA-positive blood cultures for at least 48 h despite treatment with in vitro active antimicrobials. To determine the precise cut-off for defining hypoalbuminaemia, we applied a bootstrap resampling approach in combination with Cox proportional hazards models. A range of candidate albumin thresholds was evaluated to identify the optimal value that maximized the partial log-likelihood of the Cox model, with time from target therapy to negative blood cultures as the outcome. RESULTS:Among the 461 patients screened, 285 patients met the inclusion criteria; 34% (97/285) were people who inject drugs and 45% (130/285) had definite endocarditis. The median (IQR) albumin level was 2.4 (2-2.9) g/dL. The albumin threshold that corresponded to the highest mean bootstrapped log-likelihood was ≤3.0 g/dL. After propensity score weighting, patients with an albumin level of ≤3.0 g/dL had a significantly slower time to blood culture sterilization compared with patients with albumin levels >3 g/dL (HR = 0.6; 95% CI: 0.44-0.81; P < 0.01). CONCLUSIONS:In the largest study to evaluate the impact of hypoalbuminaemia on the outcomes of patients with MSSA bacteraemia, we identified a cut-off value of albumin ≤3 g/dL to be associated with an increased risk for a delayed blood culture sterilization.

Objectives:This study aimed to characterize susceptibility profiles of both currently approved and investigational β-lactam agents against difficult-to-treat resistant Pseudomonas aeruginosa (DTR P. aeruginosa) clinical isolates, including carbapenemase-producing strains. Patients and methods:A total of 502 consecutive DTR P. aeruginosa clinical isolates were collected from unique hospitalized patients between 2022 and 2024 across three tertiary care centres. Isolates originated from bloodstream, respiratory, urinary and intra-abdominal infections. Minimum inhibitory concentrations (MICs) for seven β-lactam agents were determined using triplicate reference broth microdilution. Susceptibility interpretations were based on CLSI criteria or, where applicable, investigational breakpoints. Whole-genome sequencing was performed to detect β-lactamase genes. Results:Among the 502 isolates, cefepime-zidebactam exhibited the highest in vitro activity (100% susceptibility), followed by cefiderocol-xeruborbactam (98%), cefiderocol (95%), ceftolozane-tazobactam (90%), cefepime-taniborbactam (86%), ceftazidime-avibactam (85%), and imipenem-relebactam (41%). Notably, only 35% of isolates were susceptible to all seven agents. Carbapenemase genes were identified in 4% of isolates, including bla GES (n = 13), bla VIM (n = 7), bla KPC (n = 1) and bla IMP (n = 1). Among carbapenemase-producing isolates, cefepime-zidebactam retained activity against all isolates (100%), exceeding that of other approved β-lactam/β-lactamase inhibitor combinations (0%-24% susceptible), cefiderocol (76%), cefiderocol-xeruborbactam (86%) and cefepime-taniborbactam (81%). Conclusions:While currently approved β-lactam agents retained activity against many DTR P. aeruginosa isolates, their performance was markedly compromised in the presence of carbapenemases, with the notable exceptions of cefepime-zidebactam and cefiderocol. Investigational β-lactam/β-lactamase inhibitor combinations exhibited consistently robust activity across resistance phenotypes, including carbapenemase-producing isolates, representing promising options for DTR P. aeruginosa infections.

BACKGROUND:Population pharmacokinetic (PK) models can be combined with Bayesian estimation to optimize dosing regimens. The impact of sample collection time on the accuracy and precision of Bayesian predictions was evaluated. METHODS:Data from adult and pediatric patients were used to develop a cefepime population PK model for Bayesian prior use. Holdout data were used for model evaluation. Clinical dosing regimens in the latter cohort were used to conduct optimal sample-time analysis. The accuracy and precision of the Bayesian predictions were assessed as a function of infusion duration and the differences between the observed and optimal sampling times. Analyses were conducted using Pmetrics for R. RESULTS:An allometrically scaled 2-compartment model was fitted (n = 71 patients, 685 observations). In the holdout group (n = 116 patients, 203 observations), the posterior Bayesian fit was acceptable (R2 = 0.923; relative bias -3%; median absolute error, 11.2%; F20, 72%; and F30, 86%). Mid-interval sampling was the optimal 1-sample design for 11/16 regimens. In the 2-sample design, a peak (8/16 regimens) and trough (9/16 regimens) approach was frequently optimal. The 2-sample design yields a lower Bayesian risk of misclassification. For 0.5-hour infusions, Bayesian predictions were similarly accurate but significantly more imprecise when samples were collected >2 hour away from the optimal time versus within ±1 hour of the optimal time (ΔRMSE: 8.98 mg/L, 95% CI: 3.61-15.7 mg/L). For 3 hours infusions, no significant differences in the accuracy or imprecision of the Bayesian predictions were noted. CONCLUSIONS:The nonparametric cefepime population PK model fit as a Bayesian prior in the holdout group. The optimal timing of PK sample collection varied according to regimen type and infusion duration. The precision of Bayesian estimates was lower for 0.5-hour infusions when samples were collected further from the model-predicted regimen-specific optimal collection times.

In vitro datiderocol and newly developed β-lactam–β-lactamase inhibitors (BL–BLIs). The PROVE study enrolled patients with serious Gram-negative bacterial infections treated with cefiderocol. We compared patient characteristics, pathogens, and clinical outcomes by susceptibility status to BL–BLIs. PROVE was an observational medical chart review study (November 2020–July 2024). Data from hospitalized patients with confirmed Gram-negative bacterial infections and known BL–BLI susceptibility who received cefiderocol for ≥72 hours were included. Susceptible bacteria were susceptible to all BL–BLIs tested (S); non-susceptible bacteria were resistant or intermediate to at least one BL–BLIs tested (NS): ceftazidime-avibactam, ceftolozane-tazobactam, and imipenem-relebactam. Baseline demographics, clinical characteristics, and clinical outcomes were assessed. Among 504 patients, those infected by NS (N=382) vs S (N=122) bacteria were older (median age 62.0 vs 56.0 years, respectively; Table 1). Proportionally, fewer patients with NS vs S bacteria had indicators of more severe disease at cefiderocol initiation (intensive care unit stay: 52.6% vs 67.2%; organ support: 39.8% vs 51.6%). Patients with S vs NS pathogens more frequently had respiratory tract infections (71.3% vs 49.2%; Table 1) and were more likely to have ≥2 risk factors for acquired CR Gram-negative bacteria (66.4% vs 58.9%; Table 2). Polymicrobial infections were more common in patients with S vs NS bacteria (45.1% vs 25.1%, respectively). Nearly all NS bacteria were carbapenem resistant. Clinical cure rates were similar in patients with NS and S bacteria (70.2% vs 70.5%, respectively). 30-day all-cause mortality was numerically lower for patients with S vs NS bacteria (18.9% vs 23.6%, respectively) (Table 2). Clinical cure rates were similar in patients with BL–BLI-S and NS pathogens, but differences in baseline severity limit comparability. Further analyses are needed to clarify the role of cefiderocol in infections caused by NS pathogens. Mathias W. Pletz, MD, GSK: Advisor/Consultant|GSK: Honoraria|MSD: Advisor/Consultant|MSD: Honoraria|Pfizer: Advisor/Consultant|Pfizer: Grant/Research Support|Pfizer: Honoraria|Shionogi: Advisor/Consultant|Shionogi: Honoraria Maria Cruz Soriano Cuesta, MD, Gilead: Advisor/Consultant|Gilead: Honoraria|MSD: Advisor/Consultant|MSD: Honoraria|Mundipharma: Advisor/Consultant|Mundipharma: Honoraria|Pfizer: Advisor/Consultant|Pfizer: Honoraria|Shionogi: Advisor/Consultant|Shionogi: Honoraria|Viatris: Advisor/Consultant|Viatris: Honoraria Stefano Verardi, MD, Shionogi BV: Employee Karan Gill, Master of Science, Shionogi BV: Employee Anne Santerre Henriksen, PHD, Shionogi BV: Advisor/Consultant Sean T. Nguyen, PharmD, Shionogi Inc: Employee

Colonization of the gastrointestinal (GI) tract by vancomycin-resistant Enterococcus faecium (VREfm) often precedes bloodstream infection and serves as a reservoir for onward patient transmission in healthcare settings. Routine clonal isolate-based sequencing often underestimates within-patient diversity and can miss transmission involving low-abundance and co-colonizing strains. Here, we applied culture-enriched metagenomic sequencing to matched GI tract and blood VREfm populations collected ≤14 days apart from 35 patients with positive VREfm blood cultures obtained between 2020 and 2025 at a single hospital. GI tract populations exhibited greater within-patient diversity than bloodstream populations, including multi-strain colonization in five patients. Among single-strain populations, variant analysis suggested distinct environment-specific pressures between the GI tract and bloodstream environments. To assess transmission using culture-enriched metagenomic sequencing, we compared all 70 VREfm populations against 470 contemporary clinical VREfm isolate genomes collected from the same hospital and identified 19 putative transmission clusters including 6 clusters involving multi-strain populations. Together, these results demonstrate how culture-enriched metagenomic sequencing improves resolution for assessing within-patient VREfm diversity and enhances the detection of transmission events that could be missed by clonal isolate-based surveillance.

OBJECTIVES:We report on our initial clinical experience and identify factors associated with subtherapeutic cefazolin levels among patients with severe infections. METHODS:This was an observational study of patients treated with cefazolin guided by therapeutic drug monitoring (TDM) for severe infections. Cefazolin trough levels were collected and considered subtherapeutic when total drug concentrations were <40 mg/L. Hypoalbuminemia was defined as an albumin level of <3 g/dL within 72 h of cefazolin TDM. Trough levels were drawn within 90 min of the next scheduled dose for intermittent regimens. Steady state levels were drawn after at least 8 h of cefazolin via continuous infusion. RESULTS:Seventy patients were included. Overall, 76% (53/70) of patients had bloodstream infections, of whom 60% (32/53) had definitive endocarditis. MSSA was the underlying pathogen in 86% (60/70) of cases. Subtherapeutic levels were observed in 39% (27/70) of patients. The total median (IQR) daily cefazolin dose was similar among patients who received dosing by continuous and intermittent infusions [6 g (6-8) versus 6 g (6-6), P = 0.271]; however, those who received continuous infusions had a higher median (IQR) cefazolin level [61 mcg/mL) (38-80) versus 38 (19-71), P = 0.012]. Among those with measured albumin levels (n = 55), hypoalbuminemia was associated with endocarditis [OR = 11.9; 95% CI: 2.4-59.1; P = 0.002] and a lower cefazolin concentration [OR = 0.97; 95%CI: 0.95-0.99; P = 0.026]. CONCLUSIONS:Despite standard dosing with cefazolin, subtherapeutic levels were commonly observed. Factors associated with subtherapeutic levels included hypoalbuminemia and administration by intermittent infusion. Future studies are imperative to validate or refute current therapeutic targets for cefazolin.

ABSTRACT Twelve pairs of baseline and post-exposure Pseudomonas aeruginosa isolates from patients treated with ceftazidime-avibactam were evaluated to define mechanisms of treatment-emergent resistance. Resistance was associated with amino acid substitutions in ampC and OXA β-lactamases, or mutations in regulatory genes conferring hyper-production of AmpC and MexAB-OprM efflux pumps. Cross-resistance was common between ceftazidime-avibactam and ceftolozane-tazobactam, less common for imipenem-relebactam and cefepime-zidebactam, and lowest for cefiderocol. These findings have important implications for sequential treatment of P. aeruginosa infections.

Introduction: Infections attributed to multidrug-resistant organisms have resulted in a significant clinical burden, high mortality, and excessive costs. Identifying the most appropriate and efficacious treatments will aid in reducing these burdens. Imipenem/cilastatin + relebactam (I/R) is used against multidrug-resistant infections providing an alternative option which may support patients where traditional treatments are no longer effective. Objective: The objective was to evaluate the efficacy of I/R for complicated urinary tract infections, complicated intra-abdominal infections, hospital-acquired bacterial pneumonia, and ventilator-associated bacterial pneumonia, based on data aggregated from randomized controlled trials. Method: Two systematic literature reviews were conducted to include randomized controlled trials which aligned with the inclusion criteria reporting on the efficacy of I/R against placebo or other comparators such as piperacillin/tazobactam or colistin. The outcomes of interest were mortality, clinical response, and microbiological response. Results: The results found reduced mortality and comparable clinical and microbiological response with I/R versus its comparators. I/R displayed the largest favorable clinical and microbiological responses within high-risk populations, including those with severe renal impairment when compared with piperacillin/tazobactam. Conclusions: These findings support the efficacy of I/R for key Gram-negative infections, particularly within vulnerable patient populations. Despite the favorable outcomes reported, there is a need for further real-world evidence generation to support the efficacy of I/R to aid in standardizing treatment guidelines and reducing the clinical and economic burden associated with multidrug-resistant bacterial infections.

Multidrug-resistant (MDR) Pseudomonas aeruginosa infections pose a major challenge to effective treatment. Understanding genomic adaptations during antimicrobial therapy in patients infected with this pathogen is crucial for preventing therapeutic failure. Here, we investigated the population diversity and evolution of P. aeruginosa collected longitudinally from six patients who evolved multidrug-resistant infections. Serial clinical P. aeruginosa single-colony isolates (n = 63) and culture-enriched metagenomic population samples (n = 39) were collected and subjected to whole-genome sequencing. The resulting data were used to characterize and compare the species composition, multi-locus sequence types (STs), and resistance-associated mutations present within each sample type. Single-colony isolate sequencing showed that each patient was infected with a single P. aeruginosa strain that accumulated mutations and became increasingly more antibiotic-resistant over time. Mutations in genes associated with β-lactam resistance, including ampC, ftsI, and mexR, arose over time and corresponded with changes in antimicrobial susceptibility in single-colony isolates. Species profiling of culture-enriched metagenomic populations revealed that all samples contained P. aeruginosa, but also additional gram-negative pathogens. Metagenomic analysis of culture-enriched populations identified resistance-associated mutations at low frequency, many of which were not identified in single-colony isolates from the same sample. In some cases, resistance-associated mutations initially detected at low frequency rose to fixation after antimicrobial treatment. Overall, this study shows that population-based metagenomic sequencing effectively captures the within-patient genomic diversity of P. aeruginosa during antimicrobial therapy and could aid the detection and interpretation of resistance-associated mutations in this pathogen. IMPORTANCE:Pseudomonas aeruginosa infections are notoriously difficult to treat and are associated with high rates of morbidity and mortality. While the genetic basis of resistance in P. aeruginosa is well documented in vitro, less is known about how resistance evolves within patients during antibiotic therapy. Standard approaches based on analysis of clonal isolates may miss within-patient diversity, potentially overlooking low-frequency mutations that contribute to treatment failure. In this study, we compared single-colony isolate whole-genome sequencing with culture-enriched metagenomic sequencing to monitor the evolution of P. aeruginosa populations in patients receiving antibiotic therapy. The culture-enriched metagenomic approach enabled the detection of emerging resistance mutations, such as low-frequency variants in ampC and ftsI, before these variants rose to fixation. It also revealed genetically resistant subpopulations missed by isolate sequencing alone. Overall, our findings highlight the value of population-based metagenomic sequencing in capturing bacterial adaptation during infection and underscore its potential to improve resistance surveillance and guide personalized antimicrobial therapy.

OBJECTIVE:To investigate the epidemiology and outcomes of patients with non-Serratia Enterobacterales endocarditis. METHODS:Adult patients were identified at 14 hospitals between January 2000 and December 2024 with definite non-Serratia Enterobacterales endocarditis. Combination therapy was defined as receipt of ≥2 antimicrobial agents with documented in vitro activity against the primary pathogen for ≥72 hours. Clinical failure was defined as a composite of all-cause 42-day mortality or treatment-emergent resistance. RESULTS:Seventy-five patients were included. The median (IQR) age was 67 (51-78) years, and 16% (12/75) were patients who inject drugs. The most common pathogens were E. coli [41% (31/75)] and Klebsiella spp. [37% (28/75)]. Klebsiella spp. were most commonly associated with septic emboli. Patients who experienced clinical failure were more likely to have a higher median (IQR) age [71 (66-79) versus 62 (43-76); P = 0.031], higher median (IQR) Charlson comorbidity index [4 (2-7) versus 3 (1-4); P = 0.026] and were less likely to receive surgery despite an indication [59% (10/17) versus 21% (10/47); P = 0.007]. After propensity score weighting, patients with valvular endocarditis treated with a combination regimen had a trend towards a lower 90-day mortality (HR = 0.39; 95% CI: 0.13-1.19; P = 0.098). However, patients who received combination therapy also had a proportionally higher rate of adverse events [33% (7/21) versus 13% (7/47); P = 0.054]. CONCLUSIONS:Use of combination treatment may improve mortality in patients with non-Serratia Enterobacterales endocarditis; however, larger studies are required. Adverse effects among patients who received combination therapy were common.