Antimicrobial resistance and genomic characterization of Escherichia coli isolated from mink in northern China.

Poultry Proteus mirabilis in selected areas of Hunan, China: Resistance profiles and virulence genes.

The increasing presence of pharmaceutical contaminants in aquatic environments raises serious concerns regarding water quality and public health. In this study, activated carbon derived from snake fruit seeds was developed and applied for the adsorption of two β-lactam antibiotics, doripenem (DOR) and meropenem (MER), in both single and binary aqueous systems. The adsorbent was characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and nitrogen adsorption-desorption isotherms. The results confirmed the development of a rough and porous surface morphology, and a mesoporous network with high surface area (1260.61 m2/g), which are favorable for adsorption. Based on single experimental data, the maximum adsorption capacities are193 mg/g for DOR and 171mg/g for MER. These performances were reduced when a second adsorbate (MER or DOR) is present in solution, reflecting a competition on the same adsorbent site. The model for binary solutions contains number of molecules per site (n1, n2) explaining the antagonist effect between both adsorbates when interacting for the same activated carbon receptor sites (ACRS). Comparatively, it was demonstrated that our adsorbent is an efficient material for various pharmaceuticals removal. In summary, the activated carbon indicated promising performance to remove various pharmaceuticals including MER and DOR.

Carbapenem-resistant Klebsiella pneumoniae (CRKP) and Acinetobacter baumannii (CRAB) pose significant therapeutic challenges due to their high resistance and global spread. Combination therapy with colistin (COL) and meropenem (MEM) was used to enhance antimicrobial activity. This study evaluated the COL-MEM combination against CRKP and CRAB isolates with a high resistance profile. A total of 58 carbapenem-resistant clinical isolates (31 CRKP and 27 CRAB), including extensively resistant and pandrug-resistant strains, were collected over a period of 12 months. Synergy between COL and MEM was assessed by microdilution checkerboard (MCB) and time-kill (TKA) assays. Carbapenemase genes were detected using molecular methods. The results showed that the COL-MEM combination yielded synergy (35.5% and 40.7%, respectively) and additive effects (35.5% and 37.0%, respectively), while no antagonism was observed. TKA confirmed bactericidal activity, especially at doubled MCB-detected concentrations, indicating dose-dependent activity. The significant reduction in the minimum inhibitory concentration in the combination indicated its potential for dose optimization, minimizing COL-associated toxicities. Genotypic profiling showed that the expression of blaNDM and blaOxa-48 can reduce synergy. These findings, obtained with isolates of high resistance, support the efficacy of this combination therapy and could reduce the dose-related side effects of COL. However, they also highlight genotype-specific variations and COL resistance mechanisms as limiting variables.

Antimicrobial resistance in Klebsiella pneumoniae (KP) has become a major global public health threat. Allicin, a natural antimicrobial agent, has demonstrated antibacterial activity against KP, but its effect on drug resistance remains unclear. To investigate whether allicin can weaken KP's resistance to meropenem (MEM) and reveal its synergistic mechanism, this study treated KP strains with gradient concentrations of MEM. Resistant strains were identified using the broth microdilution method and viable bacterial counting. Checkerboard assay, time-kill kinetics and isobologram analyses were used to analyse the antibacterial effect of different concentrations of allicin and MEM combined treatment. Molecular docking and cellular thermal shift assay (CETSA) were employed to evaluate the binding of allicin to carbapenemase and efflux pump-related proteins. A mouse model of KP pneumonia was established to investigate the therapeutic effect of allicin combined with MEM. Compared with MEM monotherapy, allicin + MEM acted synergistically, lowering the effective MEM dose by targeting carbapenemase and efflux pumps and mitigating KP-induced pneumonia. This study suggests that the combination of allicin and MEM may be a potential method for reducing bacterial resistance and exerting antibacterial efficacy.

An ID-HPLC-MS/MS based candidate reference measurement procedure for the quantification of imipenem, meropenem and ertapenem in human plasma.

Fosfomycin-based combinations against carbapenemase-producing Klebsiella pneumoniae bloodstream infections: Correlating in vitro synergy with clinical outcomes.

Objective: Septicemia, a dangerous blood disease, is a life-threatening condition that can affect infants and young children, especially in less developed countries. Methods: This study involves a bacteriological investigation to correlate the pattern of resistance of pathogenic bacteria isolates against antibiotic resistance patterns in septicemia cases of children under 5 years old admitted to Kerkuk Gynecology and Children Hospital in Iraq for the period extended from July 2024 to Jan. 2025. A total of 120 children (72 Female and 48 Male) aged 1–60 months old, compared with 40 similar age healthy children. The isolated pathogenic bacteria were also subject to biochemical tests and antibiotic susceptibility profiling. Results: Babies and infants ages ranged between 1-18 months were the most susceptible ages in comparison with older children. The most common bacteria isolated from blood samples were Staphylococcus aureus (31.6%) and Escherichia coli (25.5%). Septicemia isolates exhibited significant resistance to antibiotics, i.e., Azithromycin (AZM) and Cephalexin (CEV), while Meropenem (MRP) and Vancomycin (VAN) were effective against most isolates. Novelty: The specific cell culture is deemed mandatory as a quick check-up for antibiotic resistance prior to administration for babies. The applications of the results can help enhance the handling of sepsis in infants and younger children, especially in poorer regions of Kirkuk.

Abstract Background Traditional antimicrobial stewardship initiatives aimed at reducing drug acquisition expenditures may fail to capture overall healthcare costs including increased hospital length of stay (LOS). Meropenem (MEM) had historically been utilized at our institution for treatment of extended-spectrum beta-lactamase producing Enterobacterales (ESBL-E) bloodstream infections (BSI) due to the high cost of ertapenem (ETP). Drugs requiring multiple daily infusions like MEM are associated with additional expenses for nurse time, intravenous tubing, admixture fluids, syringes and waste disposal compared to once-daily ETP and have also been associated with increased LOS. We aimed to quantify differences in LOS and other associated healthcare costs for patients with ESBL-E BSI treated with MEM versus ETP following de-restriction of ETP for Infectious Diseases providers in the inpatient setting.Baseline patient characteristics.Abbreviations: BMI = body mass index; BMT = bone marrow transplant within the previous 12 months; CCI = Charlson Comorbidity Index; ETP = ertapenem; HIV = human immunodeficiency virus; ICU = intensive care unit; ID = infectious diseases; IQR = interquartile range; MEM = meropenem; NS = non-significant; SOT = any history of solid organ transplantLength of stay, antimicrobial use, and patient outcomes.Abbreviations: CDI = C. difficile infection; DOT = days of therapy; ETP = ertapenem; ICU = intensive care unit; IQR = interquartile range; LOS = length of stay; MEM = meropenem; NS = non-significant Methods This retrospective study included patients with ceftriaxone-resistant E. coli or K. pneumoniae BSIs between February 1, 2023 and March 31, 2025. Patients were included if they received at least 72 hours of therapy with ETP or MEM. Patients with polymicrobial bacteremia, those receiving combination therapy for > 72 hours, or infections without documented source control were excluded.Raincloud plots of LOS for (a) overall and (b) non-ICU cohorts.a) LOS was significantly decreased in the ETP group for the overall study cohort (median 7 [IQR: 6-10] versus 10 days [IQR: 7-18], p <0.004). b) LOS was significantly decreased in the ETP group for the subset of patients not admitted to the ICU (median 7 [IQR: 6-10] versus 10 days [IQR: 6-17], p <0.037). Abbreviations: ETP = ertapenem; ICU = intensive care unit; IQR = interquartile range; LOS = length of stay; MEM = meropenemAssociated costs of treatment, dollars.a) Associated costs per day of MEM and ETP administration for a patient with normal renal function and BMI <30; MEM is administered as 1 gram IV q8h over 3h and ETP 1 gram IV q24h over 30 minutes per institutional protocol. b) Total costs of MEM and ETP for all study patients based on dosing frequency and length of definitive therapy. Abbreviations: ETP = ertapenem; IV = intravenous; MEM = meropenem Results Of the 161 blood cultures reviewed, 82 patients met criteria for study inclusion, 46 in the MEM group and 36 in the ETP group. Baseline characteristics were comparable in both groups, except for more patients in the MEM group admitted to the ICU at the time of index culture (Figure 1). More than 63% of patients included had an immunosuppressive condition. LOS was decreased in the ETP group compared to MEM in the overall study population (7 versus 10 days, p < 0.004) and in the subset of patients not admitted to the ICU (p < 0.037) (Figures 2 and 3). Associated costs of administration were also decreased (Figure 4). Conclusion A change to de-restriction of inpatient ETP use for ESBL E.coli and K. pneumoniae BSI resulted in a significant decrease in patient LOS. Associated healthcare costs were also decreased despite the increased acquisition cost of ETP. Antimicrobial stewardship programs should look beyond traditional drug cost reduction initiatives to optimize overall healthcare savings and patient outcomes. Disclosures Lucy S. Witt, MD, MPH, Merck & Co: Grant/Research Support

BackgroundInstitutional implementation of beta-lactam therapeutic drug monitoring (TDM) requires aligned infrastructure, physician and pharmacist buy-in, and coordinated collaboration. At our institution, TDM is managed by pharmacists who are granted institutional authority to dose, order, and assess levels and labs as clinically indicated. Following validation of in-house assays for cefepime (CFP), meropenem (MER), and piperacillin (PIP), pharmacist-driven TDM for these three agents was piloted in the intensive care unit (ICU) and select medical wards.MethodsThis single-center, prospective study evaluated the impact of pharmacist-driven TDM in adult inpatients admitted to an ICU or medical ward at UK HealthCare receiving CFP, MER, or PIP and had two steady-state concentrations available for analysis. In-house assays provided results within 24 hours with estimation of free drug concentration based on protein binding (PIP: 30%, CFP: 20%, MER: 0%). Pharmacokinetic (PK) parameters were calculated using first-order equations, and the pharmacodynamic (PD) target was 100% fT >1–4×MIC. Pharmacists used individualized PK to ensure PD target attainment and all interventions were recorded.ResultsTwenty-seven patients were evaluated (ICU: n=22, non-ICU: n=5). Median (IQR) age was 61 years (50, 69), weight 82.5 kg (72.8, 102), BMI 28.2 kg/m² (25.8, 30.8), and 67% were male. Each drug was administered to 9 patients. Fifteen patients had an identified organism: P. aeruginosa (n=10), ESBL-producing organisms (n=2), S. marcescens (n=2), and other (n=1); 12 received empiric therapy. PK parameters for each agent are outlined in Table 1. Across all agents, 48% (n=13) required dose reductions, 7% (n=2) had doses increases (PIP only), only 37% (n=10) were maintained, and 7% (n=2) were discontinued (Table 2).ConclusionPharmacist-driven beta-lactam TDM identified substantial PK variability and led to individualized dose optimization in the majority of cases. This approach improved target attainment and highlights the clinical and operational value of integrating real-time TDM into antimicrobial stewardship strategies. As institutions strive to align stewardship with precision medicine, beta-lactam TDM may no longer be a luxury, but a necessity.DisclosuresAll Authors: No reported disclosures

Abstract Background Tebipenem pivoxil hydrobromide (TBP) (formerly SPR994) is in clinical development as the potential first oral broad-spectrum carbapenem agent in the US for the treatment of complicated urinary tract infections (cUTI) and acute pyelonephritis (AP). This study reports on the in vitro activity of TBP and comparator agents against molecularly characterized Enterobacterales isolates recovered from UTI and bloodstream infections (BSI) in the US, including ESBL and carbapenemase (CP) producing isolates. Methods A total of 3,523 Enterobacterales isolates collected from the US in 2023 were included. (UTI, 74.2% (2,614), BSI, 25.8% (909)). Isolates were tested for susceptibility (S) by CLSI reference broth microdilution method. E. coli and K. pneumoniae with aztreonam (ATM), ceftazidime (CAZ), or ceftriaxone (CRO) MICs of ≥2 µg/mL, and P. mirabilis with cefpodoxime (CPD) or CAZ MICs of ≥2 µg/mL were classified as ESBL phenotype. Isolates with MIC ≥2 µg/mL for imipenem (IMI) and/or meropenem (MER), or ≥1 µg/mL for ertapenem (ERT), were categorized as carbapenem-nonsusceptible (CNSE) phenotype. Isolates that met these criteria were screened for plasmid-mediated AmpC (pAmpC), ESBL, and CP genes. Results A total of 14.1% (496/3,523) of isolates were identified with an ESBL phenotype and 13.3% (471/3,523) were ESBL, CSE phenotype (Table). Of the latter, 91.7% (432/471) carried ESBL and/or pAmpC genes. TBP had MIC50/90 of 0.015/0.03 µg/mL against this subset, with those for IMI (MIC50/90, ≤0.12/0.5 µg/mL), MER (MIC50/90, 0.03/0.06 µg/mL) and ERT (MIC50/90, 0.03/0.12 µg/mL). The S to other comparators was below 81%. The CNSE phenotype accounted for only 1.6% (56/3,523) of isolates, and 39.3% (22/56) carried CP. TBP displayed MIC50/90 of 1/ >8 µg/mL. IMI (MIC50/90, 2/ >8 µg/mL), MER (MIC50/90, 1/ >32 µg/mL) and ERT (MIC50/90, 2/ >2 µg/mL) were active against 48%, 61% and 9% of the CNSE subset, respectively, while the S of oral comparators was ≤48%. Conclusion TBP displayed MICs similar to those for overall isolates against ESBL-producing Enterobacterales isolates from UTIs and BSIs in US medical centers. These results indicate that TBP has activity comparable to IV carbapenems and has the potential for use as oral treatment option for cUTI and AP. Disclosures Renuka Kapoor, PhD, GSK: Employee|GSK: Stocks/Bonds (Public Company) Mariana Castanheira, PhD, Melinta Therapeutics: Advisor/Consultant|Melinta Therapeutics: Grant/Research Support Didem Torumkuney, PhD, GSK: Stocks/Bonds (Public Company) Ian A. Critchley, PhD, Spero Therapeutics: Stocks/Bonds (Public Company)

BackgroundPharmacodynamic (PD) targets based on plasma exposure are commonly used for pneumonia (PNA). However, the exposure required in plasma may differ at the target site of infection when considering drug penetration. This study defined plasma and pulmonary epithelial lining fluid (ELF) PD targets for meropenem (MEM), cefiderocol (FDC), levofloxacin (LVX), and tobramycin (TOB) against a challenge set of Klebsiella pneumoniae (KP) in a preclinical PNA model, and assessed the predicted CFU/lung response of average human exposures relative to susceptible, intermediate, and resistant (SIR) MICs.Table 1.Phenotypic and genotypic information of Klebsiella pneumoniae isolatesFDC, cefiderocol; LVX, levofloxacin; MEM, meropenem; MIC, minimum inhibitory concentration; TOB, tobramycinTable 2.Plasma and ELF exposure targets in Klebsiella pneumoniaeMethodsTo provide a robust PD profile in both plasma and ELF, two dosing regimens per antibiotic were administered to neutropenic mice infected with 17 KPs (Table 1) following the COMBINE murine PNA protocol. Efficacy was evaluated as change in log10 CFU/lung at 24 h. Emax models were fitted to composite CFU data for each antibiotic, and plasma and ELF exposures required (%fT >MIC, fAUC/MIC, fCmax/MIC) for stasis, 1- and 2-log10 reduction were calculated using the Hill equation. The average human plasma and ELF exposures achieved with MEM 2 g q8h over 3 h infusion, FDC 2 g q8h over 3 h infusion, LVX 750 mg q24h, and TOB 7 mg/kg were assessed relative to SIR breakpoints and predicted CFU/lung efficacy.Figure 1.Plasma and ELF exposures of meropenem, cefiderocol, levofloxacin, and tobramycin versus CFU response against Klebsiella pneumoniaeGreen area: average free human exposures relative to susceptible MICs. Yellow area: average free human exposures relative to intermediate MICs. Red: average free human exposures relative to resistant MICs.ResultsThe ELF exposure required for stasis, 1-and 2-log10 CFU/lung reduction against KP were lower than in plasma for MEM, FDC, and TOB, but were higher than in plasma for LVX (Table 2/Figure 1). Notably, the plasma PD target for MEM was higher, and the magnitude of kill for FDC was lower relative to previously published data, as most KPs were carbapenemase producers. All Emax models had R2 > 0.85. At “S” MICs, the average human ELF exposure of all agents predicted a consistent >1 log10 CFU/lung reduction. The predicted CFU/lung responses at human ELF exposures were variable at “I” MICs, and limited at “R” MICs.ConclusionThe magnitude of free plasma and ELF exposures required to achieve PD targets were enumerated for four antibiotics of differing drug classes against KP. These data demonstrate the clinical translation of the model relative to established breakpoints and emphasize that exposures needed in plasma for efficacy may differ in magnitude in either direction from exposures needed at the target site of infection.DisclosuresAndrew J. Fratoni, PharmD, Qpex Biopharma, Inc.: Grant/Research Support David P. Nicolau, PharmD, CARB-X: Grant/Research Support|Innoviva: Advisor/Consultant|Innoviva: Grant/Research Support|Shionogi: Advisor/Consultant|Shionogi: Grant/Research Support

Abstract Background Treatment options for Acinetobacter baumannii are limited. The Infectious Diseases Society of America (IDSA) currently recommends the use of sulbactam-containing regimens whenever possible. We modeled serum concentrations of sulbactam-durlobactam (SUL-DUR), ampicillin-sulbactam (AMP-SUL), and meropenem (MER) to assist with combination treatment selection.Sulbactam-durlobactam and ampicillin-sulbactam dosing simulating humanized serum concentrations at steady state against carbapenem-resistant A. baumannii with a PBP3 mutationSulbactam-durlobactam, ampicillin-sulbactam, and meropenem dosing simulating humanized serum concentrations at steady state against carbapenem-susceptible A. baumannii Methods Two A. baumannii isolates were evaluated, one carbapenem-susceptible (AR300) and, one carbapenem-resistant (ARC5950) with a penicillin binding protein-3 (PBP3) mutation. We conducted one compartment in vitro pharmacodynamic models using a starting inoculum of 6-log10 CFU/mL, simulating monotherapies with SUL-DUR or AMP-SUL with inflow media set to the respective antibiotic half-life. We additionally ran MER treatment models against the carbapenem-susceptible strain. Models were run in duplicate, simulating humanized serum concentrations targets selected based on free drug concentrations achievable in patients using 2024 IDSA dosing recommendations. Models were sampled for CFU/mL counts at 0, 4, 6, 8, 24, 32, 48, and 72h. Susceptibility changes were evaluated by E-test every 24h and compared to 0h. Results Against AR300, all agents demonstrated initial bacteriostatic activity after 6h. Minimum inhibitory concentration (MIC) shifts were detected as early as 24h with AMP-SUL treatment and coincided with bacterial regrowth above the initial inoculum. There were no SUL-DUR MIC shifts seen for this isolate in any SUL-DUR or AMP-SUL treated models. Antibacterial activity was similar for both MER and SUL-DUR monotherapies through 72h. Against ARC5950, only SUL-DUR treatment produced a 1-log10 CFU reduction. Bacterial regrowth did occur with SUL-DUR treatment, accompanied by an increased MIC in one of the two models, and exceeded the initial inoculum after 48h of treatment. AMP-SUL monotherapy produced a slight initial reduction in colony count, however, after 6h, regrowth surpassed the starting inoculum. Treatment with AMP-SUL did not correspond with elevated SUL-DUR MICs. Conclusion SUL-DUR monotherapy was active against carbapenem-resistant A. baumannii with a PBP3 mutation and displayed similar activity to meropenem against carbapenem-susceptible A. baumannii with no MIC shifts. Disclosures Thomas Lavoie, PharmD, innoviva: Grant/Research Support|innoviva: Co-investogator|melinta: Grant/Research Support|pfizer: Grant/Research Support|shionogi: Grant/Research Support Kerry L. LaPlante, Pharm.D., FCCP, FIDSA, FIDP, Abbvie: Advisor/Consultant|Abbvie: Grant/Research Support|Innoviva: Advisor/Consultant|Innoviva: Grant/Research Support|Melinta: Grant/Research Support|Pfizer: Grant/Research Support|Shionogi: Advisor/Consultant|Shionogi: Grant/Research Support

Abstract Background Outpatient parenteral antimicrobial therapy (OPAT) is effective and cost-efficient, but 12%–63% of patients experience adverse events, requiring close monitoring. Therapeutic drug monitoring (TDM) is routinely used for vancomycin and aminoglycosides but is underutilized for beta-lactams (BLs) due to limited access, logistical barriers, and clinician inexperience. This study evaluated its use and clinical impact within a high-volume OPAT program. Methods This retrospective, single-center cohort included adult patients (≥18 years) enrolled in an OPAT program who received cefepime (CEF), piperacillin/tazobactam (TZP), or meropenem (MEN) and underwent BL TDM between April 2023 and April 2025. BL TDM was not protocolized within the OPAT program; however, institutional guidance supported its use in carefully selected patients. We aimed to describe TDM indications, lab trends, microbiological profiles, and the impact of TDM on treatment. Results During the study period, 25 OPAT patients underwent BL TDM. TDM was performed in patients receiving CEF (64%), TZP (20%), and MEN (16%). The most common indication for TDM was suspected beta-lactam toxicity (32%) (Figure 1). Post-TDM therapy adjustments were made in 52% (n = 13), including changes in dosing frequency (38.5%) and dosage strength (30.8%). Additional changes included infusion strategy modification or agent substitution (15.4% each). Subtherapeutic levels were the most common reason for adjustment (46.2%), followed by adverse effects, supratherapeutic levels, and changes in renal function (23.1% each). Skin, soft tissue, and musculoskeletal infections were most common (Figure 2). Positive cultures were present in 80%, with Pseudomonas spp most frequently isolated. Therapy was completed in 17 patients; 7 discontinued early, and 1 had treatment ongoing. Reasons for therapy discontinuation are detailed in Table 1. Conclusion BL TDM is contributing to a paradigm shift in OPAT practice by improving both efficacy and safety. While inpatient TDM typically targets efficacy, safety concerns more commonly motivate TDM in the OPAT setting. Although this review involved a small cohort, our experience adds to the growing body of evidence supporting BL TDM as a tool to enhance patient outcomes in the OPAT setting. Disclosures All Authors: No reported disclosures

BackgroundPseudomonas aeruginosa is a difficult Gram-negative pathogen due to its intrinsic and acquired resistance. As beta-lactams remain central to treatment, local susceptibility is vital. This study evaluated susceptibility to first-line agents and newer beta-lactam agents over a 4-year period at our academic medical center.MethodsNon-duplicate P. aeruginosa isolates from January 2021 through December 2024 were analyzed. Susceptibility was determined by BD Phoenix™ or E-test and interpreted according to CLSI breakpoints. Antimicrobials analyzed included aztreonam (AZT), cefepime (CFP), piperacillin-tazobactam (PTZ), meropenem (MEM), ceftolozane-tazobactam (C/T), ceftazidime-avibactam (C/A), ciprofloxacin (CIP) and levofloxacin (LEV). Resistance phenotypes including MDR, XDR, DTR, and PDR were defined per CDC criteria. Culture source and hospital location were also recorded.ResultsA total of 1,509 non-duplicate P. aeruginosa isolates were analyzed. The overall prevalence of resistance phenotypes was: MDR 18.2%, XDR 16.8%, DTR 5.0%, and PDR 0.9%. Respiratory specimens comprised the largest proportion of isolates (39.6%) compared to urine (23.7%), wound (12.9%), blood (12.6%), and other sources (9.1%).Overall susceptibility rates were as follows: C/T 98.5%, C/A 97.8%, MEM 84.7%, CFP 83.9%, PTZ 79.6%, AZT 75.9%, CIP 72.3%, and LEV 69.9%. Of all isolates, 63.0% were susceptible to all first-line beta-lactams, 30.8% were non-susceptible to at least one, and 6.2% were non-susceptible to all. Among isolates non-susceptible to at least one beta-lactam, 41.7% were resistant to a single agent, 19.4% to two agents, 17.6% to three, and 21.3% to four. Respiratory isolates consistently demonstrated the highest rates of non-susceptibility across all agents tested.ConclusionThis analysis highlights the persistent challenge of P. aeruginosa resistance, particularly among respiratory isolates. The high in vitro activity of newer beta-lactams such as C/T and C/A supports their role in treatment algorithms for resistant infections. Continued local surveillance, antimicrobial stewardship, and individualized therapy remain essential for preserving the efficacy of beta-lactams in the management of P. aeruginosa.DisclosuresAll Authors: No reported disclosures

BackgroundCarbapenem-resistant Acinetobacter baumannii (CRAB) presents a significant healthcare concern owing to restricted treatment alternatives and elevated mortality rates. This study assesses the in vitro synergistic effects of antimicrobial combinations to determine appropriate treatment alternatives.Table 1In Vitro Synergistic Activity of Antimicrobial CombinationsMethodsSixty non-duplicate CRAB isolates were identified and tested using the VITEK 2 system. Resistance mechanisms were determined via PCR for blaNDM and blaOXA-48. Synergistic activity of Polymyxin B (PB) + Sulbactam (SUL), Tigecycline (TGC) + SUL, Minocycline (MIN) + SUL, Sulbactam + Avibactam (SUL-AVI), and PB + Meropenem (MEM) + SUL was assessed using the Broth Disk Elution (BDE) method.ResultsA total of 60 clinical isolates were tested for in vitro synergy using different antibiotic combinations. Effectiveness was defined as no bacterial growth (-) in the presence of a drug combination. The most effective combinations were SUL+PB and SUL+PB+MEM, showing 100% synergy (no bacterial growth). SUL+TGC and SUL+PB were highly effective (98.33%, CI: 95.09%-101.57%), followed by SUL+MIN (91.67%, CI: 84.67%-98.66%). SUL+AVI was the least effective (65%, CI: 52.93%-77.07%) [Table 1]. Chi-square analysis showed no significant difference between SUL+TGC, SUL+PB, and SUL+PB+MEM (p > 0.05). Advanced pairwise Z-tests also found no statistically significant differences among combinations (all p-values > 0.05).ConclusionPolymyxin B-based regimens, particularly in combination with Meropenem and Sulbactum, appear highly promising for use in critically ill patients where rapid, broad-spectrum activity is paramount.DisclosuresAll Authors: No reported disclosures

BackgroundHypermutator strains with DNA mismatch repair (MMR) defects are well-recognized among E. coli but rarely described for other Enterobacterales.Figure 1.Heat map of 11 serial strains of CRKP recovered from a lung transplant recipient.Figure 2.Genotypic and phenotypic characteristics of the 11 CRKP strainsMethodsEleven CRKP (ST258) strains causing disease (D) or colonization (C) in a lung transplant recipient over ∼4 yrs underwent whole genome sequencing. Clinical and lab-created isogenic mutant strains were tested for hypermutator and other phenotypes.Figure 3.Characteristics of mutH mutantsFigure 4.Tissue burdens of WT and mutH V76G mutant strains.Results6 early (C1-C4, D1-D2) strains were from 0-2.5 mos post-transplant (Fig 1). 5 late strains were D3 (recurrent bacteremia, 40 mos) and C5, D4, C6 and D5 (41-43 mos). Early strains and D3 were meropenem (MEM)-resistant (R), ceftazidime-avibactam (C/A)-susceptible (S) KPC-3-carriers that differed by 2-7 and 12-15 single nucleotide polymorphisms (SNPs), respectively (Fig 1). C5, D4, C6 and D5 differed from early strains by 119-129, 151-161, 167-177 and 251-261 SNPs, respectively, and carried a mutHV76G (MMR gene) mutation. C5, D4 and C6 were MEM-S, C/A-R KPC-46-carriers (KPC-3-D179Y); D5 was a MEM-R, C/A-R KPC-61-carrier (S171P) (Fig 2). mutHV76G strains had significantly reduced mutH expression (RT-PCR), higher rifampin mutational frequencies (p-values< 0.0001), and greater MEM-vaborbactam (M/V) R during passage in vitro (Fig 3). Isogenic mutHΔ and mutHV76G mutants in D3 had significantly reduced mutH expression, higher rifampin mutational frequencies (p< 0.0001), greater C/A and M/V R during passage in vitro, and enhanced transfer and acceptance of IncX3 plasmid carrying NDM-5 to and from E. coli-074, respectively. In IV mouse infections (5x104 CFU/mouse), isogenic mutHV76G caused higher liver, kidney and spleen burdens than D3 (Fig 4). In mice treated with humanized M/V (0.3 mg/g intraperitoneal Q8hrs, starting 4hrs post-infection), mutHV76G and D3 tissue burdens were each reduced by ≥70%. However, mean mutational frequency (CFU/mL M/V plates/CFU/mL M/V-free plates) of mutHV76G was significantly greater than D3 (3x10-2 vs. < 1x10-8).ConclusionHypermutator CRKP emerged due to mutHV76G, which conferred increased propensity to SNPs, transference and acceptance of antibiotic-R-conferring plasmids, heightened virulence and β-lactam/β-lactamase R in vitro and in vivo. MMR-defective hypermutators merit further investigation among antibiotic-R Enterobacterales.DisclosuresCornelius J. Clancy, MD, Merck: Grant/Research Support|Shionogi: Advisor/Consultant M Hong Nguyen, MD, Basilea: Advisor/Consultant|BioMerieux: Grant/Research Support|Melinta: Grant/Research Support|Pulmocide: Advisor/Consultant|Pulmocide: Grant/Research Support

BackgroundPrevious studies have shown that patients with obesity are at risk for subtherapeutic beta-lactam concentrations, owing to the pharmacokinetic (PK) changes in factors such as volume of distribution and clearance. Beta-lactam therapeutic drug monitoring (TDM) has been utilized as a strategy to reduce PK variability and individualize dosing regimens, although data exploring the use of TDM in this population are limited. This study sought to evaluate the role of TDM in patients with obesity and its impact on pharmacokinetic/pharmacodynamic (PK/PD) target attainment of beta-lactams.MethodsThis single-center, retrospective, matched cohort study included adult patients with obesity (OB) and patients without (non-OB) who had routine TDM performed for cefepime (FEP), piperacillin-tazobactam (PTZ), or meropenem (MEM). Underweight patients were excluded. Patients were matched by drug, ICU status, renal function, and early (≤ 72 hours) or late TDM ( > 72 hours). The primary outcome compared PK/PD target attainment in OB vs non-OB. This study assessed two PK/PD targets (100% fT≥ MIC and 100% fT≥ 4xMIC). Secondary outcomes included treatment failure, length of stay, in-hospital mortality, adverse events, and 30-day readmission.ResultsOf 199 matched pairs of patients, 298 (75%) patients received FEP, 48 (12%) received PTZ, and 52 (13%) received MEM. There was no difference in attainment of 100% fT≥ MIC between OB vs non-OB at 84% (n=168) and 85% (n=170), respectively. Target attainments were similar, yet much lower for 100% fT≥ 4xMIC in OB vs non-OB at 53% (n=106) and 54% (n=108) respectively. OB and non-OB receiving PTZ had the lowest overall rates of attainment of 100% fT≥ MIC (69%) and 100% fT≥ 4xMIC (29%). More OB experienced any adverse effect compared to non-OB at 48 (24%) and 30 (15%), respectively (p = 0.02). Specifically, rates of nephrotoxicity were greater in OB vs non-OB (15% vs. 6%, p< 0.01). There were no differences in any other secondary outcomes.ConclusionPK/PD target attainment was comparable between OB and non-OB in this study, but lowest with PTZ, highlighting the need for targeted TDM in these patients. OB experienced significantly higher rates of adverse effects, including nephrotoxicity, emphasizing the importance of TDM in improving patient safety.DisclosuresVenugopalan Veena, PharmD, Merck: Grant/Research Support Kathryn DeSear, PharmD, Abbvie: Advisor/Consultant|Biomerieux: Advisor/Consultant|Cormedix: Speaking|GSK: Advisor/Consultant|Shionogi: Speaking

Abstract Background The 2024 IDSA Guidance on the Treatment of Antimicrobial-Resistant Gram-Negative Infections suggests selecting antibiotics according to susceptibility results to treat infections caused by S. marcescens, M. morganii, and Providencia spp. Although these organisms are thought to be at moderate to low risk of developing clinically significant AmpC expression, use of broad-spectrum beta-lactam antibiotics remains common even for urinary tract infections (UTIs). No study to date has compared the clinical outcomes for UTIs caused by these organisms in patients who initially received antibiotics susceptible to AmpC hydrolysis to those treated with AmpC-stable antibiotics.Table 1.Baseline CharacteristicsTable 2.Primary and Secondary Outcomes Methods This was a single-center retrospective study of non-pregnant patients &amp;gt; 18 years old with S. marcescens, M. morganii, or Providencia spp. isolated from a urine culture who received treatment with active antibiotics for at least 48 hours from October 2019 to June 2024. Patients with antibiotic use for other indications, polymicrobial urine cultures warranting broader antibiotic coverage, or isolates resistant to ceftriaxone were excluded. The primary outcome was recurrence of infection – defined as re-isolation of the same organism in a urine culture within 28 days of treatment. Results 112 unique patients met the inclusion criteria, Patients were divided into two cohorts—Group 1 consisted of 79 patients (70.5%) who received ceftriaxone (CRO), cefpodoxime (CPD), or piperacillin/tazobactam (TZP), and Group 2 had 33 patients (29.5%) receiving cefepime (FEP), meropenem (MEM), trimethoprim/sulfamethoxazole (SXT), or a fluoroquinolone (FQ). 90 patients (80.36%) met IDSA criteria for complicated UTI (Table 1). The primary outcome occurred in 2 patients (2.53%) in Group 1 and 3 patients (3.8%) in Group 2. The same organism was reisolated within a year of treatment in 5 patients from Group 1 and 4 patients from Group 2 – all of which had unchanged susceptibility patterns (Table 2). Conclusion In this population of patients with UTI caused by organisms with low risk of AmpC expression, the observed rate of UTI recurrence was similar after initial treatment with antibiotics susceptible to AmpC hydrolysis compared to antibiotics stable against AmpC hydrolysis. Disclosures All Authors: No reported disclosures

This study presents an eco-friendly method for synthesizing silver nanoparticles (AgNPs) using the supernatant of Candida albicans as a reducing agent. AgNPs were formed via redox reactions and characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and atomic force microscopy (AFM). XRD analysis confirmed a face-centered cubic structure. The antibacterial activity of AgNPs was evaluated against three Gram-negative bacteria—Escherichia coli, Klebsiella pneumoniae, and Proteus mirabilis—using the agar well diffusion method. Their efficacy was compared to four antibiotics: Imipenem (IPM), Amoxicillin/clavulanic acid (AMC), Meropenem (MRP), and Piperacillin/tazobactam (PIT). While antibiotics were effective against K. pneumoniae and P. mirabilis, E. coli showed resistance to PIT and AMC. In contrast, AgNPs exhibited concentration-dependent inhibition, with maximum zones of 16.00 ± 0.58 mm (K. pneumoniae), 27.00 ± 0.57 mm (P. mirabilis), and 22.33 ± 0.33 mm (E. coli). These findings suggest AgNPs as promising antibacterial agents.