Ryan K. Shields

Objectives: To determine comparative effectiveness of piperacillin/tazobactam and cefepime for the treatment of bacteremia due to Enterobacterales at moderate risk for clinically significant AmpC production. Methods: A retrospective multicentre study was conducted among adults with bacteremia due to Enterobacter cloacae complex, Klebsiella aerogenes, or Citrobacter freundii complex from 2017 to 2025. Treatment-emergent resistance was defined as a ≥4-fold MIC increase or a categorical change in susceptibility within 90 days. Results: Seven hundred twenty-eight patients met inclusion criteria. The median (IQR) age was 67 years (55–74), and 35% (252/728) were immunocompromised. Cefepime was used for definitive therapy in 47% (342/728), followed by piperacillin/tazobactam in 18% (129/728) and carbapenems in 16% (115/728). Overall, 30-day mortality was 12% (88/728). Patients receiving piperacillin/tazobactam had a higher median (IQR) Pitt bacteremia score (2 [1–3] versus 1 [0–3], P=0.017) than those receiving cefepime, but rates of 30-day mortality (14% [18/129] versus 11% [37/342], P = 0.345) and relapsed infection between days 5–14 (3% [4/129] versus 2% [5/342], P = 0.266) were similar. After inverse probability of treatment weighting, 30-day mortality rates between patients treated with piperacillin/tazobactam and cefepime were not significantly different (HR 1.05, 95% CI 0.57–1.92). Treatment-emergent resistance to definitive therapy occurred in 1% (5/342), 2% (3/129), and 3% (4/115) of patients treated with cefepime, piperacillin/tazobactam, and carbapenems, respectively. Conclusions: In the largest study to date of bacteremia due to E. cloacae complex, K. aerogenes, or C. freundii complex, treatment-emergent resistance was uncommon, and outcomes did not differ significantly between patients treated with piperacillin/tazobactam and cefepime.

Bacteriophages (phages) are the viral predators of bacteria. Their ability to infect and kill bacterial strains has been harnessed for use in clinical practice for more than a century, but their development stalled for decades in many regions. Now, in the face of increasingly complex bacterial infections that fail conventional therapeutic options, and the growing global threat of antibiotic resistance, interest in phage therapy has resurged. Data evaluating the impact of phage therapy via clinical trials are steadily increasing, but remain sparse. Yet, an ever-growing number of case reports and case series highlights the potential of phage therapy to make a positive impact in the battle against antimicrobial resistance and difficult-to-treat bacterial infections. In this review, we summarize the available data describing the clinical utility of phage therapy, highlight infectious indications where phage therapy shows strong potential, and call attention to disease states where the benefits of phage therapy, as it is practiced today, are less certain. Furthermore, we note areas where there is a crucial need for additional research and propose how scientists and clinicians can work together to design and carry out the preclinical and clinical studies that will take phage therapy into its next century.

Infections caused by metallo-β-lactamase (MBL)-producing gram-negative pathogens represent a formidable and emerging threat to public health. MBLs compromise the activity of carbapenems and many newly developed β-lactam/β-lactamase inhibitor agents, leaving few treatment options. Remarkably, these enzymes have disseminated globally at an unprecedented rate. Therapeutic management of MBL-producing pathogens remains a major challenge because the enzymes are not inhibited by currently available β-lactamase inhibitors, and most pathogens that harbour MBLs are equipped with mechanisms of resistance to all available antibiotic classes. Recent advances have elucidated the biochemical and structural function of diverse MBLs thereby improving the development of future antibiotics. In this review, we summarize the contemporary global epidemiology of clinically relevant MBLs, describe the molecular mechanisms driving their dissemination and evaluate current and emerging therapeutic strategies for infections caused by MBL-producing gram-negative pathogens including Enterobacterales, Pseudomonas aeruginosa, Acinetobacter baumannii and Stenotrophomonas maltophilia.

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.

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.

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: 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 blaGES (n = 13), blaVIM (n = 7), blaKPC (n = 1) and blaIMP (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.

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.

Combination antimicrobial treatment for endocarditis secondary to Enterobacterales may be associated with clinical benefits, but this benefit comes with an increased risk of adverse drug reactions (ADRs). We hypothesized that short-course combination therapy would optimize clinical benefits while minimizing ADRs. This was a retrospective, multicenter study of adult patients with definitive endocarditis secondary to Enterobacterales. Short-course combination treatment was defined as receipt of combination therapy for ≤14 days. Seventy-three patients met the inclusion criteria. Serratia marcescens was the most common pathogen and was recovered in 62% (45/73) of patients. Patients who received short-course combination treatment were more likely to be admitted prior to 2016 (44% [12/27] vs 17% [8/46], P = 0.012) and to receive an aminoglycoside in combination regimens (63% [17/27] vs 28% [13/46], P = 0.013). Patients who received short-course combination treatment had a shorter post-infection length of stay (18 [13–25] days vs 29 [20–42], P = 0.021), but no significant difference in the rate of discontinuation of antimicrobial therapy due to an ADR (26% [7/27] vs 24% [11/46], P = 0.847). In the multivariate logistic regression model, short-course combination treatment was not associated with a higher risk of 90-day mortality (OR = 2.89; 95% CI: 0.63–13.35; P = 0.175). This is the first study to compare short- and long-duration combination therapy in patients with Enterobacterales endocarditis. Overall, shorter courses of combination therapy were commonly prescribed, which supports future studies that are designed to identify the optimal duration of combination therapy for patients with Enterobacterales endocarditis.

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