Daptomycin MICs Research Articles

Addition of daptomycin to levofloxacin increased the efficacy of levofloxacin monotherapy against a methicillin-susceptible Staphylococcus aureus strain in experimental meningitis and prevented development of resistance in vitro.

Introduction. Daptomycin and levofloxacin were tested as monotherapies and in combination against the antibiotic-susceptible S. aureus strain MSSA 1112 in a rabbit meningitis model and the effect of the combination on induction of resistance was determined in vitro. Hypothesis/Gap Statement. Treatment combination might be more efficacious than the monotherapies regarding antibacterial efficacy in vivo and development of resistance. Aim. To demonstrate a synergy of the antibiotic combination daptomycin and levofloxacin in experimental meningitis, and also its ability to reduce the emergence of resistance against the antibiotics in vitro. Methodology. Changes of the susceptibility to fluoroquinolones and daptomycin were determined by the measurement of the MIC and mutations were detected by whole genome sequence comparison of the mutants with the parent strain MSSA 1112. Meningitis was induced by intracisternal inoculation of 105 c.f.u. (colony forming unit) of MSSA 1112 and treatment was started 10 h later by injection of daptomycin (15 mg kg−1) and levofloxacin (10 mg kg−1) standard doses. Cerebrospinal fluid (CSF) samples were repeatedly collected during therapy in order to determine killing rates and results of bactericidal activity were expressed in Δlog10c.f.u. ml-1 over 8 h. Results. The combination of daptomycin with levofloxacin was significantly (P<0.001) superior to levofloxacin monotherapy and increased the antibacterial activity of daptomycin. In vitro, MSSA 1112 was cycled over 6 days with either increasing concentrations of levofloxacin or daptomycin or with a combination of levofloxacin with half of the MIC of daptomycin or daptomycin with half of the MIC of levofloxacin leading to mutations in target genes as identified by whole genome sequence analysis. Addition of low concentration of daptomycin (0.25 mg l−1) reduced levofloxacin-induced resistance in vitro. Addition of levofloxacin in low concentration (0.125 mg l−1) did not influence daptomycin-induced resistance. Conclusion. These findings highlight the lack of reciprocal interference of antibiotics in combination with regard to the development of resistance.

Reporting antimicrobial susceptibilities and phenotypes of resistance to vancomycin in vancomycin-resistant Enterococcus spp. clinical isolates: A nationwide proficiency study

IntroductionThe ability of Spanish microbiology laboratories to (a) determine antimicrobial susceptibility (AS), and (b) correctly detect the vancomycin resistance (VR) phenotype in vancomycin-resistant Enterococcus spp. (VRE) was evaluated. MethodsThree VRE isolates representing the VanA (E. faecium), VanB (E. faecium) and VanC (E. gallinarum) VR phenotypes were sent to 52 laboratories, which were asked for: (a) AS method used; (b) MICs of ampicillin, imipenem, vancomycin, teicoplanin, linezolid, daptomycin, ciprofloxacin, levofloxacin and quinupristin–dalfopristin, and high-level resistance to gentamicin and streptomycin; (c) VR phenotype. Results(a) The most frequently used system was MicroScan; (b) according to the system, the highest percentage of discrepant MICs was found with gradient strips (21.3%). By antimicrobial, the highest rates of discrepant MICs ranged 16.7% (imipenem) to 0.7% (linezolid). No discrepant MICs were obtained with daptomycin or levofloxacin. Mayor errors (MEs) occurred with linezolid (1.1%/EUCAST) and ciprofloxacin (5.0%/CLSI), and very major errors (VMEs) with vancomycin (27.1%/EUCAST and 33.3%/CLSI) and teicoplanin (5.7%/EUCAST and 2.3%/CLSI). For linezolid, ciprofloxacin, and vancomycin, discrepant MICs were responsible for these errors, while for teicoplanin, errors were due to a misassignment of the clinical category. An unacceptable high percentage of VMEs was obtained using gradient strips (14.8%), especially with vancomycin, teicoplanin and daptomycin; (c) 86.4% of the centers identified VanA and VanB phenotypes correctly, and 95.0% the VanC phenotype. ConclusionMost Spanish microbiology laboratories can reliably determine AS in VRE, but there is a significant percentage of inadequate interpretations (warning of false susceptibility) for teicoplanin in isolates with the VanB phenotype.

Activity of Oritavancin against Gram-Positive Pathogens Causing Bloodstream Infections in the United States over 10 Years: Focus on Drug-Resistant Enterococcal Subsets (2010-2019).

ABSTRACTOritavancin displayed potent and stable activity (MIC90 range of 0.06 to 0.5 mg/L) over a 10-year period (2010 to 2019) against Gram-positive pathogens that cause bloodstream infections (BSI), including methicillin-resistant Staphylococcus aureus (MRSA) and resistant subsets of Enterococcus spp. Daptomycin and linezolid were also active against methicillin-resistant S. aureus and vancomycin-resistant Enterococcus (VRE). Only oritavancin and linezolid remained active against Enterococcus faecium isolates displaying an elevated daptomycin MIC (i.e., 2 to 4 mg/L). Proportions of methicillin-resistant S. aureus and vancomycin-resistant Enterococcus within the respective S. aureus and enterococcal populations decreased over this period.

In vitro activity of dalbavancin and other anti-staphylococcal agents against infecting isolates of methicillin-resistant coagulase-negative staphylococci.

Introduction. Dalbavancin was approved in Europe in 2015 for skin and soft tissue infections.Hypothesis/Gap Statement. Data on methicillin-resistant coagulase-negative staphylococci (MR-CNS) dalbavancin susceptibility are scarce.Aim. To assess the susceptibility of MR-CNS to dalbavancin and other anti-staphylococcal agents.Methodology. A total of 443 MR-CNS clinical isolates from patients hospitalized in a Greek university hospital during a 2.5-year period (January 2018 to June 2020) were included. The MICs for vancomycin, teicoplanin, linezolid and daptomycin were investigated by Etest and the MIC for dalbavancin was determined according to European Committee on Antimicrobial Susceptibility Testing (EUCAST) guidelines in 196 isolates. The consumption of the aforementioned antimicrobials was calculated.Results. In total, 51 isolates were resistant to teicoplanin (11.5 %) and 211 (47.6 %) to linezolid; all were susceptible to vancomycin and daptomycin. Among 196 isolates tested, 32 (16.3 %) were resistant to dalbavancin. A significant increase of MIC during the study period was found for vancomycin, teicoplanin and daptomycin, while a decrease in linezolid's MIC was observed. Dalbavancin's MIC remained stable. No difference in consumption was observed among the studied anti-staphylococcal agents.Conclusion. An increase of vancomycin, teicoplanin and daptomycin MICs among MR-CNS was observed, whereas 47.6 % of isolates were non-susceptible to linezolid. Dalbavancin retains excellent potency against MR-CNS, even in the presence of non-susceptibility to other anti-staphylococcal antibiotics.

1264. Assessment of In Vivo Efficacy of CF-296 in addition to Vancomycin (VAN) and Daptomycin (DAP) against Staphylococcus aureus in the Neutropenic Murine Thigh Infection Model

BackgroundCF-296 is a novel lysin in pre-clinical development for the treatment of methicillin-susceptible and methicillin-resistant Staphylococcus aureus infections, used in addition to standard of care antibiotics including VAN and DAP. We evaluated the in vivo efficacy of CF-296 alone and in addition to VAN and DAP against S. aureus.MethodsEight isolates (1 MSSA and 7 MRSA) were studied. Murine ICR MIC (100% serum) and human MIC (100% serum) for CF-296 ranged from 32-256 mg/L to 0.5-1 mg/L respectively. Broth microdilution MICs for DAP ranged from 0.5-1 mg/L while all isolates exhibited a VAN MIC of 1 mg/L. Neutropenic ICR mice were thigh inoculated with bacterial suspensions (107 CFU/mL). Mice were administered three monotherapy regimens subcutaneously (SC) or intravenously (IV): i) sub-therapeutic VAN, SC (i.e., a dose that yielded bacteria stasis or growth in order to evaluate further bacterial killing), ii) sub-therapeutic DAP, SC, or iii) CF-296 50 mg/kg, IV. Combination of sub-therapeutic VAN or DAP in addition to 5 escalating CF-296 doses ranging from 0.5 to 50 mg/kg were also examined. Control mice were vehicle-dosed. Efficacy was measured as the change in mean thigh bacterial density at 24h relative to 0h controls.ResultsRelative to starting inoculum (5.71 ± 0.27 at 0h), bacterial density in controls increased by +2.49 ± 0.98 log10 CFU/thigh across all 8 strains. On average, VAN, DAP, and CF-296 monotherapy resulted in +0.90 ± 1.21, +1.47 ± 0.80, and +0.87 ± 1.39 log10 CFU/thigh bacteria growth, respectively. In addition to VAN, escalating CF-296 exposures (0.5 – 50 mg/kg) resulted in an augmented dose-response, ranging from bacterial reduction of -0.26 ± 1.10 (with addition of CF-296 0.5 mg/kg) to -1.01 ± 0.41 log10 CFU/thigh (with addition of CF-296 50 mg/kg). Similarly, escalating CF-296 exposures in addition to DAP resulted in an augmented dose-response, ranging from bacterial density of +0.80 ± 1.19 to -0.72 ± 0.59 log10 CFU/thigh.ConclusionCompared with 24h control, VAN, DAP, and CF-296 alone displayed modest CFU reduction while CF-296 synergized with VAN and DAP to cause further bacterial killing highlighting a potential role for CF-296 adjunctive therapy against MSSA and MRSA isolates.DisclosuresCara Cassino, MD, ContraFect Corporation (Employee)ContraFect Corporation (Employee) Dario Lehoux, PhD, ContraFect Corporation (Consultant) Raymond Schuch, PhD, ContraFect Corporation (Employee) David P. Nicolau, PharmD, Cepheid (Other Financial or Material Support, Consultant, speaker bureau member or has received research support.)Merck & Co., Inc. (Consultant, Grant/Research Support, Speaker’s Bureau)Wockhardt (Grant/Research Support)

1323. Substantial Doses of Daptomycin and Rifampin Eradicate S. epidermidis Biofilm in an In Vitro Pharmacodynamic (IVPD) Model

BackgroundThe concentration of antibiotics at the site of action needed to eradicate biofilm is currently unknown. Studies have previously suggested that bacteria in biofilms are 1000-fold more resistant to antibiotics than free-floating planktonic bacteria. We sought to describe concentrations of daptomycin alone and in combination with rifampin in relation to the pharmacodynamic exposures for biofilm eradication.MethodsWe utilized a methicillin-resistant high biofilm-forming S. epidermidis strain RP62a (ATCC® 35984) over a 48-hour in vitro PD biofilm model. The Centers for Disease Control (CDC) Biofilm Reactor model was used with chromium cobalt materials to simulate an orthopedic device infection. The reactor was inoculated and underwent a 24-hr growth phase and 16-hr conditioning phase to form biofilm on the chromium cobalt coupons, and then a 48-hr PK-PD phase was run. The daptomycin MIC for RP62a was 0.5 mg/L and the rifampin MIC was 0.015 mg/L. We modeled a growth control of the isolate alone, a 12 mg/kg regimen of daptomycin (fCmax: 14.7 mg/L, Ke 0.09), a daptomycin concentration of 1000 mg/L (2000x MIC), and a combination model of daptomycin 1000 mg/L with rifampin 15 mg/L (1000x MIC). Coupons with bacteria embedded in biofilm were sonicated, vortexed, and plated on Tryptic Soy Agar for colony counts read at 24 hrs. Bactericidal activity was defined as ≥ 3-log10 CFU/mL reduction from the initial inoculum.ResultsThe simulated humanized dosing regimen of daptomycin 12 mg/kg (fAUC0-24/MIC: 204) was similar to the growth control model. Bactericidal kill was demonstrated at 24hr and 48hr in the daptomycin 1000 mg/L model (fAUC0-24/MIC: 20,248) but did not fall beneath the limit of detection. The daptomycin and rifampin combination model demonstrated bactericidal kill at 24hr and 48hr and went below the limit of detection.ConclusionThis study demonstrated that significantly higher concentrations of antibiotics are needed at the site of action to eradicate biofilm than what maximum systemic dosing can provide. Identifying these concentrations provides a foundation for localized antibiotic therapy and further studies are needed to elucidate these concentrations for a variety of antibiotics and biofilm-forming organisms.Disclosures Kerry LaPlante, PharmD, Merck (Advisor or Review Panel member, Research Grant or Support)Ocean Spray Cranberries, Inc. (Research Grant or Support)Pfizer Pharmaceuticals (Research Grant or Support)Shionogi, Inc. (Research Grant or Support)

1573. Daptomycin Resistant Enterococcus faecium: Combination Therapy Screening

Background Enterococcus faecium infections are difficult to treat and there is a growing concern regarding the rising occurrence of daptomycin resistance. We have previously demonstrated that only daptomycin plus ampicillin combination was effective against DAP-R E. faecium R497. The efficacy and systematic screening DAP plus β-lactams and DAP plus other combinations against daptomycin-resistant strains of E. faecium has not been investigated. Here, we evaluated 40 selected single, dual and triple combinations of antibacterial regimens against two clinical isolates of DAP-R E. faecium (R497 and R496 (with daptomycin MIC of 16 and 32 µg/ml, respectively).Methods E.faecium R497 and R496 were tested against an array of antibacterial agents including daptomycin, tigecycline, linezolid, ertapenem, ceftaroline and ceftriaxone using MIC susceptibility tests and 24h time-kill curves (TKC). All susceptibility tests and TKCs were performed in MHB broth containing 50 mg/L calcium. TKCs were performed at half MIC or free peak concentration of each antibacterial (whichever was lower). Synergy was defined as >2 log10 CFU/ml decrease compared to the most potent antibacterial agent.ResultsSusceptibility tests demonstrated resistance to all listed β-lactams for both organisms. TKCs demonstrated that combination of daptomycin-ertapenem, daptomycin-ceftriaxone and daptomycin-ceftaroline was not effective against R497. However, addition of ceftriaxone or linezolid to either daptomycin-ertapenem or daptomycin-ceftaroline combinations resulted in synergy against this organism. Combinations of daptomycin-ertapenem and daptomycin-ceftaroline were synergistic against R496. Addition of linezolid, ceftriaxone or tigecycline to either daptomycin-ceftaroline or daptomycin-ertapenem combination did not increase killing activity against R496.ConclusionDifferential affinity of β-lactams to specific PBP isotypes seems to be a key parameter for the success of daptomycin- β-lactam combinations against multi-drug resistant E. faecium . The optimized use of double β-lactam therapy in addition to daptomycin can potentially lead to improved patient outcomes and preserving antibiotic therapy for serious enterococcus infections.Disclosures Cesar A. Arias, MD, MSc, PhD, FIDSA, Entasis Therapeutics (Scientific Research Study Investigator)MeMed (Scientific Research Study Investigator)Merck (Grant/Research Support) Michael J. Rybak, PharmD, MPH, PhD, Paratek (Grant/Research Support)

1629. Vancomycin Resistance in Enterococcus faecium Clinical Isolates Responsible for Bloodstream Infections in US Hospitals Over Ten Years (2010-2019) and Activity of Oritavancin

Background Enterococcus faecium (EFM) causes difficult-to-treat infections due to its intrinsic resistance (R) and ability to acquire R to many antimicrobials. This study evaluated the vancomycin (VAN)-R rates over time and the activity of oritavancin (ORI) against a collection of EFM causing bloodstream infections (BSI).MethodsA total of 1,081 BSI EFM isolates collected from 36 US hospitals in a prevalence mode design during 2010-2019 were evaluated. Bacterial identification was confirmed by MALDI-TOF MS. Susceptibility testing was performed by reference broth microdilution. For comparison, the ORI breakpoint for VAN-susceptible E. faecalis was applied to EFM. Isolates were characterized as VanA or VanB phenotypes based on their susceptibility (S) to VAN and teicoplanin (TEC). The VanB phenotype was confirmed by PCR and/or whole genome sequencing.ResultsOverall, 72.3% (782/1,081) of EFM were VAN-R (Table). VanA was the most common phenotype (97.7%; 764/782). The yearly VAN-R rates decreased from 81.8% in 2010 to 58.7% in 2019. A total of 18 (2.3%) isolates exhibited a VanB phenotype (TEC MIC, 0.5-8 mg/L); however, the vanB gene only was confirmed in 9 EFM isolates (TEC MIC, 0.5-1 mg/L), which were all collected in 2010-2012. The remaining 9 (50.0%) VanB phenotype EFM isolates carried a vanA gene (TEC MIC, 4-8 mg/L). ORI was very active against VAN-susceptible EFM (MIC50/90, ≤ 0.008/≤0.008/mg/L), VanA (MIC50/90, 0.03/0.12 mg/L; MIC100, 0.5 mg/L), and VanB (MIC50/90, ≤ 0.008/0.015 mg/L; MIC100, 0.03 mg/L) subsets. Only linezolid (LZD) and ORI (MIC, ≤ 0.12 mg/L) showed > 95.0%S against EFM and VAN-R subsets. Daptomycin (DAP)-R rarely was observed (0.8%), but it was more frequently found in the last 5 years. However, 49.9% of EFM isolates showed elevated DAP MICs (2 and 4 mg/L). ORI inhibited 77.8%, and 100.0% of DAP-R and LZD-nonsusceptible EFM isolates at ≤ 0.12 mg/L, respectively.ConclusionVAN-R rates among EFM causing BSI in the US decreased during 2010-2019. VanA remains the most common phenotype, whereas vanB-carrying isolates became rarer in later years. Interestingly, half of VanB-phenotype isolates carried a vanA gene. ORI was very active against EFM causing BSI, including isolates R to VAN, DAP, and/or nonsusceptible to LZD.Table 1 DisclosuresCecilia G. Carvalhaes, MD, PhD, A. Menarini Industrie Farmaceutiche Riunite S.R.L. (Research Grant or Support)Allergan (Research Grant or Support)Cidara Therapeutics (Research Grant or Support)Cipla Ltd. (Research Grant or Support)Fox Chase Chemical Diversity Center (Research Grant or Support)Melinta Therapeutics, Inc. (Research Grant or Support)Merck (Research Grant or Support)Merck (Research Grant or Support)Merck & Co, Inc. (Research Grant or Support)Pfizer (Research Grant or Support) Helio S. Sader, MD, PhD, A. Menarini Industrie Farmaceutiche Riunite S.R.L. (Research Grant or Support)Allergan (Research Grant or Support)Allergan (Research Grant or Support)Allergan (Research Grant or Support)Cipla Ltd. (Research Grant or Support)Cipla Ltd. (Research Grant or Support)Melinta (Research Grant or Support)Merck (Research Grant or Support)Merck (Research Grant or Support)Paratek Pharma, LLC (Research Grant or Support)Pfizer (Research Grant or Support) Jennifer M. Streit, BS, A. Menarini Industrie Farmaceutiche Riunite S.R.L. (Research Grant or Support)A. Menarini Industrie Farmaceutiche Riunite S.R.L. (Research Grant or Support)Allergan (Research Grant or Support)Melinta Therapeutics, Inc. (Research Grant or Support)Melinta Therapeutics, Inc. (Research Grant or Support)Melinta Therapeutics, Inc. (Research Grant or Support)Merck (Research Grant or Support)Paratek Pharma, LLC (Research Grant or Support) Mariana Castanheira, PhD, 1928 Diagnostics (Research Grant or Support)A. Menarini Industrie Farmaceutiche Riunite S.R.L. (Research Grant or Support)Allergan (Research Grant or Support)Allergan (Research Grant or Support)Amplyx Pharmaceuticals (Research Grant or Support)Cidara Therapeutics (Research Grant or Support)Cidara Therapeutics (Research Grant or Support)Cipla Ltd. (Research Grant or Support)Cipla Ltd. (Research Grant or Support)Fox Chase Chemical Diversity Center (Research Grant or Support)GlaxoSmithKline (Research Grant or Support)Melinta Therapeutics, Inc. (Research Grant or Support)Melinta Therapeutics, Inc. (Research Grant or Support)Melinta Therapeutics, Inc. (Research Grant or Support)Merck (Research Grant or Support)Merck (Research Grant or Support)Merck & Co, Inc. (Research Grant or Support)Merck & Co, Inc. (Research Grant or Support)Paratek Pharma, LLC (Research Grant or Support)Pfizer (Research Grant or Support)Qpex Biopharma (Research Grant or Support) Rodrigo E. Mendes, PhD, A. Menarini Industrie Farmaceutiche Riunite S.R.L. (Research Grant or Support)Allergan (Research Grant or Support)Allergan (Research Grant or Support)Basilea Pharmaceutica International, Ltd (Research Grant or Support)Cipla Ltd. (Research Grant or Support)Department of Health and Human Services (Research Grant or Support)GlaxoSmithKline (Research Grant or Support)Melinta Therapeutics, Inc. (Research Grant or Support)Merck (Research Grant or Support)Merck (Research Grant or Support)Pfizer (Research Grant or Support)

Dalbavancin exposure in vitro selects for dalbavancin-non-susceptible and vancomycin-intermediate strains of methicillin-resistant Staphylococcus aureus

ObjectivesDalbavancin is a lipoglycopeptide active against methicillin-resistant Staphylococcus aureus (MRSA). Its long half-life (8.5–16 days) allows for once-weekly or single-dose treatments but could prolong the mutant selection window, promoting resistance and cross-resistance to related antimicrobials such as vancomycin. The objective of this study was to evaluate the capacity of post-distributional pharmacokinetic exposures of dalbavancin to select for resistance and cross-resistance in MRSA. MethodsWe simulated average, post-distributional exposures of single-dose (1500 mg) dalbavancin (fCmax 9.9 μg/mL, β-elimination t1/2 204 h) in an in vitro pharmacokinetic/pharmacodynamic (PK/PD) model for 28 days (672 h) against five MRSA strains and one methicillin-susceptible strain (MSSA). Samples were collected at least daily, and surviving colonies were enumerated and screened for resistance on drug-free and dalbavancin-supplemented medium respectively. Isolates from resistance screening plates were subjected to whole-genome sequencing (WGS) and susceptibly testing against dalbavancin, vancomycin, daptomycin, and six β-lactams with varying penicillin-binding protein (PBP) affinities. ResultsDalbavancin was bactericidal against most strains for days 1–4 before regrowth of less susceptible subpopulations occurred. Isolates with eight-fold increases in dalbavancin MIC were detected as early as day 4 but increased 64–128-fold in all models by day 28. Vancomycin and daptomycin MICs increased 4–16-fold, exceeding the susceptibly breakpoints for both antibiotics; β-lactam MICs generally decreased by two-to eight-fold, suggesting a dalbavancin–β-lactam seesaw effect, but increased by eight-fold or more in certain isolates. Resistant isolates carried mutations in a variety of genes, most commonly walKR, apt, stp1, and atl. ConclusionsIn our in vitro system, post-distributional dalbavancin exposures selected for stable mutants with reduced susceptibility to dalbavancin, vancomycin, and daptomycin, and generally increased susceptibility to β-lactams in all strains of MRSA tested. The clinical significance of these findings remains unclear, but created an opportunity to genotype a unique collection of dalbavancin-resistant strains for the first time. Mutations involved genes previously associated with vancomycin intermediate susceptibility and daptomycin non-susceptibility, most commonly walKR-associated genes.

Cloxacillin or fosfomycin plus daptomycin combinations are more active than cloxacillin monotherapy or combined with gentamicin against MSSA in a rabbit model of experimental endocarditis.

In vitro and in vivo activity of daptomycin alone or plus either cloxacillin or fosfomycin compared with cloxacillin alone and cloxacillin plus gentamicin were evaluated in a rabbit model of MSSA experimental endocarditis (EE). Five MSSA strains were used in the in vitro time-kill studies at standard (105-106 cfu/mL) and high (108 cfu/mL) inocula. In the in vivo EE model, the following antibiotic combinations were evaluated: cloxacillin (2 g/4 h) alone or combined with gentamicin (1 mg/kg/8 h) or daptomycin (6 mg/kg once daily); and daptomycin (6 mg/kg/day) alone or combined with fosfomycin (2 g/6 h). At standard and high inocula, daptomycin plus fosfomycin or cloxacillin were bactericidal against 4/5 and 5/5 strains, respectively, while cloxacillin plus gentamicin was bactericidal against 3/5 strains at standard inocula but against none at high inocula. Fosfomycin, cloxacillin, gentamicin and daptomycin MIC/MBCs of the MSSA-678 strain used in the EE model were: 8/64, 0.25/0.5, 0.25/0.5 and 1/8 mg/L, respectively. Adding gentamicin to cloxacillin significantly reduced bacterial density in vegetations compared with cloxacillin monotherapy (P = 0.026). Adding fosfomycin or cloxacillin to daptomycin [10/11 (93%) and 8/11 (73%), respectively] significantly improved the efficacy of daptomycin in sterilizing vegetations [0/11 (0%), P < 0.001 for both combinations] and showed better activity than cloxacillin alone [0/10 (0%), P < 0.001 for both combinations] and cloxacillin plus gentamicin [3/10 (30%), P = 0.086 for cloxacillin plus daptomycin and P = 0.008 for fosfomycin plus daptomycin]. No recovered isolates showed increased daptomycin MIC. The addition of cloxacillin or fosfomycin to daptomycin is synergistic and rapidly bactericidal, showing better activity than cloxacillin plus gentamicin for treating MSSA EE, supporting their clinical use.

Verification of a Novel Approach to Predicting Effects of Antibiotic Combinations: In Vitro Dynamic Model Study with Daptomycin and Gentamicin against Staphylococcus aureus.

To explore whether susceptibility testing with antibiotic combinations at pharmacokinetically derived concentration ratios is predictive of the antimicrobial effect, a Staphylococcus aureus strain was exposed to daptomycin and gentamicin alone or in combination in multiple dosing experiments. The susceptibility of the S. aureus strain to daptomycin and gentamicin in combination was tested at concentration ratios equal to the ratios of 24 h areas under the concentration–time curve (AUC24s) of antibiotics simulated in an in vitro dynamic model in five-day treatments. The MICs of daptomycin and gentamicin decreased in the presence of each other; this led to an increase in the antibiotic AUC24/MIC ratios and the antibacterial effects. Effects of single and combined treatments were plotted against the AUC24/MIC ratios of daptomycin or gentamicin, and a significant sigmoid relationship was obtained. Similarly, when the effects of single and combined treatments were related to the total exposure of both drugs (the sum of AUC24/MIC ratios (∑AUC24/MIC)), a significant sigmoid relationship was obtained. These findings suggest that (1) the effects of antibiotic combinations can be predicted by AUC24/MICs using MICs of each antibacterial determined at pharmacokinetically derived concentration ratios; (2) ∑AUC24/MIC is a reliable predictor of the antibacterial effects of antibiotic combinations.

Assessment of Tedizolid In Vitro Activity and Resistance Mechanisms against a Collection of Enterococcus spp. Causing Invasive Infections, Including Isolates Requiring an Optimized Dosing Strategy for Daptomycin from U.S. and European Medical Centers, 2016 to 2018.

High-level aminoglycoside resistance was noted in 30.0% of Enterococcus faecalis and 25.2% of Enterococcus faecium isolates. Only 3.3% and 2.1% of E. faecalis isolates had elevated daptomycin MIC (≥2 mg/liter) and vancomycin resistance, respectively. In contrast, 37.4% to 40.3% of E. faecium isolates exhibited these phenotypes. Tedizolid inhibited 98.9% to 100.0% of enterococci causing serious invasive infections, including resistant subsets. Oxazolidinone resistance was mainly driven by G2576T; however, optrA and poxtA genes were also detected, including poxtA in the United States and Turkey.

601. TelA and XpaC Are Novel Mediators of Daptomycin Resistance in Enterococcus faecium

BackgroundThe YycFG system is an essential two-component regulatory system involved in cell wall homeostasis associated with the development of daptomycin (DAP) resistance in E. faecium. Importantly, the standard combination of DAP plus β-lactam is ineffective against strains harboring mutations in yycFG. Transcriptional profiling identified a cluster of two genes (xpaC and telA) that is upregulated in the presence of a YycGS333L substitution. xpaC and telA are annotated as 5-bromo-4-chloroindolyl phosphate hydrolysis and tellurite resistance proteins, respectively. Here, we aimed to determine the contribution of xpaC and telA in DAP resistance.MethodsNon-polar in-frame deletions of xpaC/telA and complementation of xpaC were performed in clinical strain E. faecium R446RIF. All mutants were characterized by PFGE and sequencing of the open reading frames to confirm the deletion. DAP MIC determination was performed by Etest on Mueller–Hinton agar. Binding of DAP was evaluated using BODIPY-labeled DAP (BDP-DAP). Cell membrane phospholipid microdomains were visualized using 10-N-nonyl acridine orange. All assays were compared with a DAP-susceptible clinical E. faecium strain S447.ResultsR446RIFΔ telA and R446RIFΔ xpaCtelA did not alter DAP MICs in R446RIF (24–32 μg/mL). However, deletion of xpaC alone (R446RIFΔ xpaC) markedly decreased DAP MIC 8 fold (to 4 μg/mL). R446RIFΔ telA and R446RIFΔ xpaCtelA exhibited similar binding of BDP-DAP compared with parental R446RIF. In contrast, R446RIFΔ xpaC exhibited increased binding of the antibiotic molecule to the cell membrane, similar to that of DAP-susceptible S447. Complementation of xpaC restored DAP MIC to 32–48 µg/mL and decrease binding of DAP. NAO staining of S447, R446RIF, R446RIFΔ telA, R446RIFΔ xpaCtelA, and R446RIFΔ xpaC:: xpaC displayed septal and polar distribution. In stark contrast, R446RIFΔ xpaC showed a redistribution of phospholipid microdomains away from the septa.ConclusionXpaC is a key contributor to DAP binding and phospholipid architecture of E. faecium but only in the presence of an intact TelA. The xpaC and telA gene cluster is a novel mediator of DAP-resistance in E. faecium via theYycFG system and independent of the LiaFSR system Disclosures All authors: No reported disclosures.

2249. Impact of Minimum Inhibitory Concentration on Clinical Outcomes of Daptomycin for VRE Bloodstream Infection Among Neutropenic Oncology Patients

BackgroundVancomycin-resistant Enterococcus (VRE) bloodstream infection (BSI) is a significant cause of morbidity and mortality in immunocompromised patients. This study aimed to assess the impact of daptomycin (DAP) MIC on outcomes of treatment for VRE BSI in neutropenic oncology patients.MethodsThis was a retrospective, observational, single-center, cohort study at an academic medical center. Included: age ≥ 18, neutropenia, admitted to oncology unit, and DAP for VRE BSI. Excluded: death within 24 hours after initiation of DAP, polymicrobial BSI, and linezolid use for > 48 hours before DAP initiation. Patients with VRE BSI 2008–2018 were identified using a report from the micro lab. Data were collected by electronic medical record review. The primary outcome of the study was clinical success, defined as culture sterilization, hypotension resolution, defervescence, and no need to change DAP due to persistent signs/symptoms of infection. Patients were analyzed according to DAP MIC ≤ 2 vs. ≥ 4 mg/L. Multivariable logistic regression analysis was performed to identify factors associated with clinical success.Results44 patients met study criteria (MIC ≤ 2, n = 26; MIC ≥ 4, n = 18). Mean age was 58 years, 59% were male, and median ANC was 0. Median Charlson Comorbidity Index Score and Pitt Bacteremia Score (Pitt) were 5 and 1, respectively. 34% required ICU admission. More patients achieved clinical success with MIC ≤ 2 (88% vs. 56%; P = 0.03). Time to success (2.4 vs. 4 days, P = 0.02) and time to culture sterilization (2.2 vs. 2.9 days, P = 0.24) were shorter with MIC ≤ 2. Mortality was similar between groups (31% vs. 33%). Time to culture sterilization (P = 0.008), neutropenia resolution (P = 0.02), MIC group (P = 0.096), and Pitt (P = 0.52) were included in the multivariable model.ConclusionDAP MIC should be considered when choosing therapy for VRE BSI among neutropenic oncology patients, particularly those expected to have prolonged neutropenia and those with persistently positive cultures. Disclosures All authors: No reported disclosures.

1546. Efficacy of Daptomycin Combinations Against Daptomycin-Resistant Enterococcus faecium Differs by β-lactam

BackgroundWe have previously demonstrated that daptomycin (DAP) combinations with β-lactams offer enhanced bactericidal activity and prevent the emergence of resistance in Enterococcus faecium infections. Although the mechanisms of DAP resistance in enterococci are not fully comprehended, they are associated with alterations in cell envelope phospholipids assembly which leads to either repulsion of the drug from cell exterior or diversion from the cell septum. In this context, we sought to evaluate combinations of DAP with a panel of β-lactams including ampicillin (AMP), amoxicillin (AMX), ceftaroline (CPT), ceftriaxone (CRO) and ertapenem (ERT).Methods E. faecium R497 harboring liaSFR mutations (DAP MIC of 16 mg/L) was evaluated in a simulated endocardial vegetation (SEV) pharmacokinetic and pharmacodynamic model over 336 h at a starting inoculum of 109 log10 CFU/g. DAP 10 mg/kg/day combinations with AMP, AMX (2 g continuous infusion), CPT 600 mg q 12 h, CRO 2g q 24 h or ERT 1 g q 24 h were evaluated. The emergence of DAP resistance was determined daily over the course of treatment.ResultsDAP alone was not bactericidal and high-level DAP resistance was observed (MIC increase from 16 to 64 µg/mL). Combination of DAP+AMP offered a significant reduction in log10CFU/g amounts (Up to 7 log10 CFU/g and to detection limits) in 24h in with no emergence of DAP resistance. DAP 10+ AMX caused 6–6.5 log10 CFU/g reduction and counts were maintained around the detection limit while demonstrating no increased resistance. Dose de-escalation with AMP indicated that even DAP 4 mg/kg/d with AMP (2g) combination, reached detection limit at 168 h with no further resistance. None of the CPT, CRO or ERT regimens in combination with DAP was effective against R497 and elevated DAP MICs (>64 µg/mL) was observed during the 14-day model.ConclusionCombination of DAP+AMP offered the most encouraging results against E. faecium R497, while DAP+AMX caused enhanced reduction. The reason for this discrepancy in various β-lactam activity may be related to diverse β-lactam targeting or affinity toward PBP proteins. Further dissection of our observations is warranted to understand the optimized DAP-β-lactam combination and consequently improve patient outcomes and prevention of resistance. Disclosures All authors: No reported disclosures.

The Potential Role of Sulbactam and Cephalosporins Plus Daptomycin Against Daptomycin-Nonsusceptible VISA and H-VISA Isolates: An in Vitro Study.

This study assesses the synergistic effect of the combination of cephalosporins and sulbactam with daptomycin against daptomycin-nonsusceptible, vancomycin-intermediate resistant Staphylococcus aureus (VISA) or heterogeneous vancomycin-intermediate S. aureus (h-VISA) isolates. The in vitro activity of daptomycin against daptomycin-nonsusceptible VISA/h-VISA isolates after adding cephalosporins with or without sulbactam was evaluated. The MIC of daptomycin against the VISA/h-VISA isolates was reduced after adding cephalosporins to daptomycin. Except for one VISA and two h-VISA isolates, the other VISA/h-VISA isolates became daptomycin-susceptible (MICs ≤ 1 mg/L). After adding sulbactam to each daptomycin/cephalosporin combination, the MIC of daptomycin against the VISA/h-VISA isolates decreased for 5 (33.3%), 6 (40.0%), 6 (40.0%), and 6 (40.0%) isolates with the cefazolin, cefmetazole, cefotaxime, and cefepime combinations, respectively. Synergism using the checkerboard method was noted in 100% of cefazolin and cefotaxime combinations and 87% and 80% of cefmetazole and cefepime combinations for all the VISA and h-VISA isolates. With the addition of sulbactam, synergism was noted in 100% of cefazolin, cefmetazole, and cefotaxime combinations and 93% of the cefepime combinations for all the VISA and h-VISA isolates. Almost all the FICs for the three-drug combinations were lower than those for the two-drug combinations. Using time-killing methods, a synergistic effect against five h-VISA isolates was observed. A synergistic effect of daptomycin, sulbactam, and each cephalosporin was observed for all VISA isolates. In conclusion, the activity of daptomycin against daptomycin-nonsusceptible VISA/h-VISA isolates can be enhanced by adding cephalosporins, and partially further promoted by sulbactam.

Rapid Emergence of Daptomycin Resistance in Nonstriatum Corynebacterium Species: A Multicenter Study

Abstract Members of the genus Corynebacterium are increasingly recognized as causes of opportunistic infection; some species can be multidrug resistant, posing a treatment challenge. Daptomycin is frequently used as therapy of last resort in this setting, but previous work from our group demonstrated the ability of C striatum clinical isolates to rapidly develop high-level resistance to daptomycin, both in vivo and in vitro. Here, our objective was to expand this investigation into a multicenter study evaluating multiple Corynebacterium species. Corynebacterium strains from three tertiary-care academic medical centers (total, n = 76; site 1, n = 44; site 2, n = 15; site 3, n = 17) were evaluated, representing 16 species. Isolates were identified during routine clinical testing and reported to species level in accordance with each laboratory’s standard operating procedures. Identification of each species was confirmed using both VITEK MS and Bruker BioTyper MALDI-TOF MS. MICs to daptomycin (Etest), vancomycin (Etest), and telavancin (Liofilchem) at baseline were determined using gradient diffusion methods on Mueller-Hinton agar with blood (Hardy Diagnostics). Each isolate was then inoculated in duplicate to 5 mL Tryptic Soy Broth. A daptomycin Etest was submerged in one tube from each pair, and growth was observed after 24-hour incubation. If turbidity was observed in the tube with daptomycin, MICs for each of the 3 antimicrobials were reassessed. High-level daptomycin resistance emerged in 24 strains: C aurimucosum (1/1 isolate tested), C bovis (1/2), C jeikeium (2/11), C macginleyi (3/3), C resistens (1/1), C simulans (1/1), C striatum (14/14 isolates), and C ulcerans (1/1). The majority of these isolates had MIC values &gt;256 µg/mL following exposure to daptomycin. Forty-eight other isolates remained susceptible to daptomycin: C afermentans (1/1), C amycolatum (19/20), C diphtheriae (1/1), C jeikeium (7/11), C kroppenstedtii (2/2), C propinquum (3/3), C pseudodiphtheriticum (6/6), C tuberculostearicum (0/6), and C urealyticum (0/3). Many of these isolates did not undergo MIC testing postdaptomycin exposure in broth due to complete lack of growth. Among those that did (n = 19), the median daptomycin MIC was 0.38 µg/mL (mean 0.42 µg/mL; range 0.023-1.0 µg/mL). One isolate of C bovis and two isolates of C jeikeium yielded variable susceptibility to daptomycin; a subset of resistant colonies grew adjacent to the gradient diffusion strip. Upon isolation and further MIC testing, these colonies maintained high-level resistance. In addition, one isolate of C amycolatum exhibited high-level daptomycin resistance (MIC &gt;256 µg/mL) prior to in vitro exposure. All isolates in the cohort were susceptible to vancomycin and telavancin, both before and after daptomycin exposure. Our findings suggest that multiple Corynebacterium species can rapidly develop high-level daptomycin resistance after a short period of exposure to this antimicrobial. This finding has important clinical implications, especially in the treatment of invasive infections or infections of indwelling medical devices.

Impact of an Antimicrobial Stewardship Intervention on Within- and Between-Patient Daptomycin Resistance Evolution in Vancomycin-Resistant Enterococcus faecium.

Vancomycin-resistant Enterococcus (VRE) is a leading cause of hospital-acquired infection, with limited treatment options. Resistance to one of the few remaining drugs, daptomycin, is a growing clinical problem and has previously been described in this hospital. In response to increasing resistance, an antimicrobial stewardship intervention was implemented to reduce hospital-wide use of daptomycin. To assess the impact of the intervention, daptomycin prescribing patterns and clinically reported culture results from vancomycin-resistant Enterococcus faecium (VREfm) bloodstream infections (BSIs) from 2011 through 2017 were retrospectively extracted and the impact of the intervention was estimated using interrupted time series analysis (ITS). We corrected for a change in MIC determination methodology by retesting 262 isolates using Etest and broth microdilution. Hospital-wide and within-patient resistance patterns of corrected daptomycin MICs are reported. Our data show that daptomycin prescriptions decreased from an average of 287 days of therapy/month preintervention to 151 days of therapy/month postintervention. Concurrently, the proportion of patients experiencing an increase in daptomycin MIC during an infection declined from 14.6% (7/48 patients) in 2014 to 1.9% (1/54 patients) in 2017. Hospital-wide resistance to daptomycin also decreased in the postintervention period, but this was not maintained. This study shows that an antimicrobial stewardship-guided intervention reduced daptomycin use and improved individual level outcomes but had only transient impact on the hospital-level trend.

Antimicrobial resistance monitoring in commensal enterococci from healthy cattle, pigs and chickens across Europe during 2004-14 (EASSA Study).

The European Antimicrobial Susceptibility Surveillance in Animals (EASSA) programme collects zoonotic and commensal bacteria from healthy food-producing animals at slaughter and tracks their susceptibility to medically important antibiotics. Results for enterococci, collected over three time periods, are presented. Intestinal contents from cattle, pigs and chickens were randomly sampled (five or six countries/host; at least four abattoirs/country; one sample/animal/farm) for isolation of enterococci; antimicrobial susceptibilities were centrally assessed by CLSI agar dilution. Clinical breakpoints (CLSI) and epidemiological cut-off values (EUCAST) were applied for data interpretation. In total, 2435 Enterococcus faecium and 1389 Enterococcus faecalis strains were recovered. Seven E. faecium/faecalis strains were linezolid resistant. One E. faecium strain was non-WT (NWT), with a daptomycin MIC of 8 mg/L. Clinical vancomycin resistance was very low or absent; eight strains had decreased susceptibility (MICs of 8 mg/L). Two strains were clinically resistant to tigecycline. Little resistance to ampicillin or gentamicin was observed. Clinical resistance of E. faecium to quinupristin/dalfopristin was slightly higher (2.2%-33.6%) and 38.5%-83.2% of the strains were classified NWT. Very high resistance to tetracycline (67.4%-79.1%) and erythromycin (27.1%-57.0%) was noted for E. faecium and E. faecalis in pigs and chickens. For both of these compounds, similar NWT results were observed for Enterococcus hirae (n = 935), Enterococcus durans (n = 286) and Enterococcus casseliflavus (n = 154) whereas the percentage of NWT for linezolid, tigecycline and vancomycin was generally zero or low. In this pan-EU survey of commensal enterococci, antibiotic susceptibility varied widely between antibiotics, animal species, countries and enterococcal species. Clinical resistance to antibiotics that are critically important for human medicine was absent or low, except for erythromycin.

1225. High Rate of Linezolid (LZD) Nonsusceptibility (LNS) Among Enteric Vancomycin-Resistant Enterococci (VRE) Recovered From Hospitalized Patients Actively Screened for VRE Rectal Colonization (VREC)

BackgroundSelect hospitalized patients are actively screened for VREC but VRE isolates may not undergo antibiotic susceptibility testing. We sought to identify predictors of daptomycin (DAP) nonsusceptibility (DNS, MIC > 4) and LNS (MIC > 2) among enteric VRE isolates recovered from patients actively screened for VREC for which antibiotic susceptibility testing was not preformed.MethodsThis was a retrospective study of consecutive adults admitted to a surgical intensive care unit (ICU) or associated medical unit between June 1, 2017 and March 1, 2018 who had a VRE isolate from active screening. Only index isolates were included. DAP and LZD MICs were determined by Etest. Patient- and antimicrobial-level data, including ambulatory prescriptions, dating back to January 1, 2016 were collected. Multivariable logistic regression models were used to determine predictors of DNS and LNS VRE.ResultsIn total, 64 patients’ VRE rectal isolates were included. Fifty-nine (92.2%) were E. faecium and 50 (78.1%) were from ICU patients. Thirty-seven patients (57.8%) were female and the mean age ± SD was 60 ± 13 years. Five (7.8%) and 20 (31.3%) patients had previous abdominal transplant and VRE infection, respectively. DAP and LZD MIC distributions are shown in the table below. Forty-one (64.1%) VRE isolates were LNS, including five LZD-resistant isolates. Only one (1.6%) isolate was DNS precluding an analysis of DNS predictors; 12 (18.8%) isolates had a DAP MIC > 2 mg/L. Common antimicrobial exposures prior to index VRE isolate included: vancomycin (62.5%), ceftriaxone (64.1%), cefepime (53.1%), metronidazole (50%), and ciprofloxacin (50%). Previous LZD (17.2%) and DAP (15.6%) exposure were less common. In a multivariable model, number of previous cefazolin doses (adjusted odds ratio (aOR) 0.74 95% confidence interval (CI) 0.55–0.95), and previous tobramycin exposure (aOR 0.15, 95% CI 0.02–0.81) were inversely associated with LNS. Previous LZD exposure was not associated with LNS.MIC50MIC90DAP1.5 mg/L3 mg/LLZD4 mg/L4 mg/LConclusionLNS was common amongst VRE isolates in this cohort. Previous LZD exposure was infrequent and not associated with LNS. LZD susceptibility testing among VRE isolates recovered from patients actively screened for VREC warrants clinical consideration.Disclosures All authors: No reported disclosures.