High MICs Research Articles

Antimicrobial resistance of clinical Streptococcus pyogenes isolates in Russia: the results of multicenter epidemiological study «PEHASus 2014–2017»

Objective. To evaluate antimicrobial resistance patterns of clinical Streptococcus pyogenes isolates from different regions of Russia during 2014–2017. Materials and Methods. A total of 792 clinical S. pyogenes isolates from 14 Russian cities were included in the study. Susceptibility testing was performed using reference broth microdilution method (ISO 207761:2006). Susceptibility testing results were interpreted using EUCAST v.10.0 breakpoints. Results. Penicillin G was active against all tested isolates with the MIC50–90 values of 0.016 mg/l and the highest MIC value of 0.25 mg/l. No resistance to linezolid, tedizolid and vancomycin were observed. Respiratory fluoroquinolones, moxifloxacin and levofloxacin, showed a high activity with the only 0.3% and 0.8% of resistant isolates, respectively. Almost all isolates (99.9%) were susceptible to trimethoprim/ sulfamethoxazole. The resistance rates to 14- and 15-membered macrolides varied from 12.1% to 17.2%. The only 2.4% of isolates were resistant to clindamycin. Resistance to chloramphenicol was 6.1%, to tetracycline – 17.2%. Conclusions. The comparison of this study results with the previously published Russian data on antimicrobial resistance of S. pyogenes shows there were no significant changes in the susceptibility of this pathogen over the past two decades, with the exception of macrolides. The growing resistance to 14and 15-membered macrolides raises a question on the further use of this antibiotic class for the empiric treatment of streptococcal infections in Russia.

Susceptibility of Methicillin-Resistant Staphylococcus aureus to Five Quinazolinone Antibacterials.

The in vitro activities of five quinazolinone antibacterials, compounds Q1 to Q5, were tested against 210 strains of methicillin-resistant Staphylococcus aureus (MRSA). The MIC50/MIC90 values (in μg/ml) were as follows: Q1, 0.5/2; Q2, 1/4; Q3, 2/4; Q4, 0.06/0.25; and Q5, 0.125/0.5. Several strains with high MIC values (from 8 to >32 μg/ml) for some of these compounds exhibited amino acid changes in the penicillin-binding proteins, which are targeted by these antibacterials.

Diversity of amino acid substitutions in PmrCAB associated with colistin resistance in clinical isolates of Acinetobacter baumannii

This study aimed to investigate the mechanisms of colistin resistance in 64 Acinetobacter baumannii isolates obtained from patients with ventilator-associated pneumonia hospitalised in Greece, Italy and Spain. In total, 31 A. baumannii isolates were colistin-resistant. Several novel amino acid substitutions in PmrCAB were found in 27 colistin-resistant A. baumannii. Most substitutions were detected in PmrB, indicating the importance of the histidine kinase for colistin resistance. In two colistin-resistant isolates, 93 amino acid changes were observed in PmrCAB compared with A. baumannii ACICU, and homologous recombination across different clonal lineages was suggested. Analysis of gene expression revealed increased pmrC expression in isolates harbouring pmrCAB mutations. Complementation of A. baumannii ATCC 19606 and ATCC 17978 with a pmrAB variant revealed increased pmrC expression but unchanged colistin MICs, indicating additional unknown factors associated with colistin resistance. Moreover, a combination of PmrB and PmrC alterations was associated with very high colistin MICs, suggesting accumulation of mutations as the mechanism for high-level resistance. The pmrC homologue eptA was detected in 29 colistin-susceptible and 26 colistin-resistant isolates. ISAba1 was found upstream of eptA in eight colistin-susceptible and one colistin-resistant isolate and eptA was disrupted by ISAba125 in two colistin-resistant isolates. Whilst in most isolates an association of eptA with colistin resistance was excluded, in one isolate an amino acid substitution in EptA (R127L) combined with a point mutation in ISAba1 upstream of eptA contributed to elevated colistin MICs. This study helps to gain an insight into the diversity and complexity of colistin resistance in A. baumannii.

Clinical pharmacokinetics of flomoxef in prostate tissue and dosing considerations for prostatitis based on site-specific pharmacodynamic target attainment

Flomoxef is used to treat bacterial prostatitis; however, its prostatic pharmacokinetics have not been fully clarified. Flomoxef (500 or 1000 mg) was administered to patients with benign prostatic hypertrophy (n = 54). After a 0.5-h infusion, venous blood samples were drawn at time points of 0.5–5 h, and prostate tissue samples were collected at time points of 0.5–1.5 h during transurethral resection of the prostate. The drug concentrations in plasma and prostate tissue were analyzed pharmacokinetically and used for a stochastic simulation to predict the probability of attaining pharmacodynamic target in prostate tissue. Showing dose linearity in the prostatic pharmacokinetics, flomoxef rapidly penetrated into prostate tissue, with a prostate/plasma ratio of 0.48–0.50 (maximum drug concentration) and 0.42–0.55 (area under the drug concentration-time curve). Against the tested populations of Escherichia coli, Klebsiella and Proteus species isolates, 0.5-h infusion of 1000 mg three times daily achieved a ≥90% expected probability of attaining the bactericidal target (70% of the time above the minimum inhibitory concentration [MIC]) in prostate tissue. The site-specific pharmacodynamic-based breakpoint (the highest MIC at which the target-attainment probability in prostate tissue was >90%) values were 0.25 mg/L (MIC for 90th percentile of E. coli and Klebsiella species) for 500 mg four times daily and 0.5 mg/L (MIC90 of Proteus species) for 1000 mg four times daily. These results help to fully characterize the prostatic pharmacokinetics of flomoxef, while also helping to rationalize and optimize the dosing regimens for prostatitis based on site-specific pharmacodynamic target attainment.

NG-STAR genotypes are associated with MDR in Neisseria gonorrhoeae isolates collected in 2017 in Shanghai.

ObjectivesTo determine the association of Neisseria gonorrhoeae antimicrobial resistance and genotypes using N. gonorrhoeae sequence typing for antimicrobial resistance (NG-STAR). MethodsWe characterized 124 N. gonorrhoeae isolates for their antimicrobial susceptibility profiles and NG-STAR ST characteristics using the guidelines of CLSI and EUCAST. The NG-STAR STs of seven loci were analysed. N. gonorrhoeae multiantigen sequence typing (NG-MAST) and MLST analysis was conducted in isolates with specific NG-STAR STs.ResultsNG-STAR differentiated 124 N. gonorrhoeae isolates into 84 STs, of which 66 STs were novel to the NG-STAR database. NG-STAR ST-199, ST-348, ST-428, ST-497 and ST-1138 were the predominant STs. Three N. gonorrhoeae isolates with ceftriaxone and cefixime MICs ≥1.0 mg/L were grouped as NG-STAR ST-233. NG-STAR ST-202 isolates (n=4) were associated with high azithromycin MICs and had an identical NG-MAST ST. The NG-STAR ST-348 group (n=5) comprised more isolates with reduced susceptibility to cefixime (n=4) than cefixime-susceptible isolates (n=1).ConclusionsNG-STAR analysis differentiated N. gonorrhoeae isolates in settings with a high prevalence of antimicrobial resistance. Specific NG-STAR STs are associated with reduced susceptibility to ceftriaxone or cefixime and resistance to azithromycin in N. gonorrhoeae.

In vitro evaluation of the antibacterial activities of radezolid and linezolid for Streptococcus agalactiae

PurposeThis study aims to evaluate the antimicrobial activities of linezolid and radezolid against Streptococcus agalactiae in vitro and compared for genetic resistance factors. MethodNonduplicate S. agalactiae clinical isolates (n = 136) were collected and the minimal inhibitory concentrations of antimicrobials were determined by agar dilution methodology. The linezolid-resistant mechanism in the clinical linezolid-non-susceptible S. agalactiae isolates and that induced by linezolid pressure in vitro were analyzed by PCR and sequence alignment. Antimicrobial activities and resistance mechanism distinctions between linezolid and radezolid were further investigated in the clinical linezolid-non-susceptible S. agalactiae isolates and that induced by linezolid pressure in vitro. ResultsOur data indicated that 17 (13%) of the 136 clinical S. agalactiae isolates were not susceptible to linezolid. For individual S. agalactiae isolates, including linezolid-nonsusceptible isolates with 23S rRNA V domain mutations, radezolid MIC90 values were generally one-half to one-quarter of the linezolid MIC90 values. Radezolid MICs remained low relative to linezolid MICs among linezolid-resistant S. agalactiae isolates, but exhibited the synchronous increases with the increasing copy numbers of 23S rRNA V domain mutations. Overall, 13 optrA-carrying clinical S. agalactiae isolates were found in this study and their MICs all remained sensitive to both linezolid and radezolid. Clinical S. agalactiae isolates with high radezolid MICs showed clonality clustering to sequence type (ST)10. ConclusionRadezolid exhibits stronger potency against S. agalactiae than linezolid and there is a concerning presence of linezolid-nonsusceptible S. agalactiae in clinical samples.

Continuous versus intermittent infusion of cefotaxime in critically ill patients: a randomized controlled trial comparing plasma concentrations.

In critical care patients, reaching optimal β-lactam concentrations poses challenges, as infections are caused more often by microorganisms associated with higher MICs, and critically ill patients typically have an unpredictable pharmacokinetic/pharmacodynamic profile. Conventional intermittent dosing frequently yields inadequate drug concentrations, while continuous dosing might result in better target attainment. Few studies address cefotaxime concentrations in this population. To assess total and unbound serum levels of cefotaxime and an active metabolite, desacetylcefotaxime, in critically ill patients treated with either continuously or intermittently dosed cefotaxime. Adult critical care patients with indication for treatment with cefotaxime were randomized to treatment with either intermittent dosing (1 g every 6 h) or continuous dosing (4 g/24 h, after a loading dose of 1 g). We defined a preset target of reaching and maintaining a total cefotaxime concentration of 4 mg/L from 1 h after start of treatment. CCMO trial registration number NL50809.042.14, Clinicaltrials.gov NCT02560207. Twenty-nine and 30 patients, respectively, were included in the continuous dosing group and the intermittent dosing group. A total of 642 samples were available for analysis. In the continuous dosing arm, 89.3% met our preset target, compared with 50% in the intermittent dosing arm. Patients not reaching this target had a significantly higher creatinine clearance on the day of admission. These results support the application of a continuous dosing strategy of β-lactams in critical care patients and the practice of therapeutic drug monitoring in a subset of patients with higher renal clearance and need for prolonged treatment for further optimization, where using total cefotaxime concentrations should suffice.

Genetic diversity and antifungal susceptibility patterns of Aspergillus nidulans complex obtained from clinical and environmental sources.

The molecular epidemiology and antifungal susceptibility of Aspergillus nidulans species complex has not been well studied. To evaluate the genetic diversity and antifungal susceptibility patterns of clinical and environmental isolates of A.nidulans complex. Sixty clinical and environmental isolates of Aspergillus section Nidulantes were collected from five countries (Iran, The Netherlands, Spain, Portugal and Greece). The species were molecularly identified by sequencing of β-tubulin gene. The genetic diversity of Anidulans complex isolates (n=54) was determined with a microsatellite genotyping assay. Antifungal susceptibility profile was determined using EUCAST method. The isolates were classified as Anidulans (46.7%), Aspinulosporus (26.6%), Aquadrilineatus (10%), Apachycristatus (3.3%), Arugulosus (3.3%), Aunguis (5%), Acreber, (1.7%), Aolivicola (1.7%) and Asydowii (1.7%). Thirty-four sequence types (STs) were identified among the 54 Anidulans complex isolates. A high level of genetic diversity was found among Anidulans sensu stricto strains but low diversity was found among Aspinulosporus strains. Amphotericin B showed high MICs to all species. The most active azole was posaconazole (GM=0.64mg/L), while itraconazole showed the highest MICs among azoles (GM=2.95mg/L). Aspinulosporus showed higher MICs than Anidulans sensu stricto for all antifungals except for micafungin and anidulafungin. Interspecies variations may result in differences in antifungal susceptibility patterns and challenge antifungal therapy in infections caused by Anidulans. Differences in the distribution of STs or persistence of multiple STs might be related to the sources of isolation and niche specialisation.

Voriconazole efficacy against Candida glabrata and Candida krusei: preclinical data using a validated in vitro pharmacokinetic/pharmacodynamic model.

Voriconazole exhibits in vitro activity against Candida glabrata and Candida krusei (EUCAST/CLSI epidemiological cut-off values 1/0.25 and 1/0.5 mg/L, respectively). Yet, EUCAST found insufficient evidence to set breakpoints for these species. We explored voriconazole pharmacodynamics (PD) in an in vitro dynamic model simulating human pharmacokinetics (PK). Four C. glabrata and three C. krusei isolates (voriconazole EUCAST and CLSI MICs of 0.03-2 mg/L) were tested in the PK/PD model simulating voriconazole exposures (t½ ∼6 h q12h dosing for 3 days). PK/PD breakpoints were determined calculating the PTA for exposure indices fAUC0-24/MIC associated with half-maximal activity (EI50) using Monte Carlo simulation analysis. Fungal load increased from 3.60±0.35 to 8.41±0.24 log10 cfu/mL in the drug-free control, with a maximum effect of ∼1 log10 kill of C. glabrata and C. krusei isolates with MICs of 0.06 and 0.25 mg/L, respectively, at high drug exposures. The 72 h log10 cfu/mL change versus fAUC0-24/MIC relationship followed a sigmoid curve for C. glabrata (R2=0.85-0.87) and C. krusei (R2=0.56-0.76) with EI50 of 49 (32-76) and 52 (33-78) fAUC/MIC for EUCAST and 55 (31-96) and 80 (42-152) fAUC/MIC for CLSI, respectively. The PTAs for C. glabrata and C. krusei isolates with EUCAST/CLSI MICs ≤0.125/≤0.06 mg/L were >95%. Isolates with EUCAST/CLSI MICs of 0.25-1/0.125-0.5 would require trough levels 1-4 mg/L; isolates with higher MICs would not attain the corresponding PK/PD targets without reaching toxicity. The in vitro PK/PD breakpoints for C. glabrata and C. krusei for EUCAST (0.125 mg/L) and CLSI (0.06 mg/L) bisected the WT populations. Trough levels of >4 mg/L, which are not clinically feasible, are necessary for efficacy against WT isolates.

Chromosomal and plasmid-mediated fluoroquinolone resistance in human Salmonella enterica infection in Ghana

BackgroundSalmonella infection poses significant public health threat globally, especially in resource-limited countries. Emergence and spread of antibiotic resistant strains to fluoroquinolones have led to treatment failures and increased mortality in Salmonella infection. However, there is dearth of information regarding mechanisms of resistance to fluoroquinolones in Ghana. This study therefore sought to identify chromosomal mutations and plasmid-mediated resistance as possible mechanisms of fluoroquinolone resistance from clinical isolates in Ghana.MethodsThis was a retrospective study of archived isolates biobanked at Kumasi Centre for Collaborative Research in Tropical Medicine, Ghana. Isolates were obtained from blood, stool and oropharynx samples at two hospitals, between May, 2016 and January, 2018. Salmonella identification was done using standard microbiological protocols and antibiotic susceptibility testing performed by Kirby-Bauer disc diffusion method. Isolates with intermediate susceptibility and/or resistance to nalidixic acid and/or ciprofloxacin were selected and examined for chromosomal mutations by Sanger sequencing and plasmid-mediated resistance by PCR.ResultsOf 133 biobanked isolates cultured, 68 (51.1%) and 16 (12%) were identified as Salmonella Typhi and non-typhoidal Salmonella (NTS), respectively. Sequence analysis of gyrA gene revealed the presence of 5 different nonsynonymous mutations, with the most frequent mutation (Ile203Ser) occurring in 12 out of 13 isolates tested. Gyrase B (gyrB) gene had 1 nonsynonymous mutation in 3 out of 13 isolates, substituting phenylalanine with leucine at codon 601 (Phe601Leu). No mutation was observed in parC and parE genes. Two NTS isolates were found to harbour qnrS plasmid-mediated resistant gene of molecular size 550 bp with high ciprofloxacin MIC of 0.5 μg/ml.ConclusionThis study reports for the first time in Ghana plasmid-mediated fluoroquinolone resistant gene qnrS in Salmonella clinical isolates. Nonsynonymous mutations of gyrA and gyrB genes likely to confer Salmonella reduced susceptibility to ciprofloxacin were also reported.

1598. Antimicrobial Activity of the Novel β-Lactam Enhancer Combination Cefepime–Zidebactam (WCK-5222) Tested Against Gram-Negative Bacteria Isolated in United States Medical Centers from Patients with Bloodstream Infections

BackgroundZidebactam (ZID) is a β-lactam enhancer antibiotic with a dual mechanism of action: high binding affinity to gram-negative PBP2 and β-lactamase (BL) inhibition. We evaluated the activity of cefepime (FEP) combined with ZID against contemporary clinical isolates of gram-negative bacilli (GNB) causing bloodstream infections (BSIs) in the US hospitals.Methods1,239 GNB were consecutively collected (1/patient) from 34 US medical centers in 2018. Susceptibility (S) testing against FEP-ZID (1:1 ratio) and comparators were performed by reference broth microdilution method in a central laboratory. The FEP S breakpoint of ≤ 8 mg/L (CLSI, high dose) was applied to FEP-ZID for comparison purposes. An FEP-ZID S breakpoint of ≤ 64 mg/L has been proposed for non-fermentative GNB based on pharmacokinetic/pharmacodynamic target attainment and was also applied. Selected Enterobacterales (ENT) isolates were evaluated by whole-genome sequencing.ResultsFEP-ZID was highly active against ENT (MIC50/MIC90, 0.03/0.12 mg/L; highest MIC, 4 mg/L; Table), including multidrug-resistant (MDR, MIC50/MIC90, 0.12/0.25 mg/L) and carbapenem-resistant isolates (n = 7; MIC50, 0.5 mg/L). The highest FEP-ZID MIC values among E. coli, K. pneumoniae, and E. cloacae were 1, 2, and 0.25 mg/L, respectively. The most active comparators tested against MDR ENT were ceftazidime–avibactam (CAZ-AVI; MIC50/MIC90, 0.25/1 mg/L; 98.0%S), meropenem (MEM; MIC50/MIC90, 0.03/0.12 mg/L; 93.1%S) and amikacin (AMK; MIC50/MIC90, 4/16 mg/L; 92.1%S). The most active agents tested against P. aeruginosa were FEP-ZID (MIC50/MIC90, 1/4 mg/L; highest MIC, 8 mg/L), colistin (MIC50/MIC90, 0.5/1 mg/L; 100.0%S), and AMK (MIC50/MIC90, 4/8 mg/L; 99.2%S); whereas CAZ-AVI and ceftolozane–tazobactam were active against 96.5–96.7% of isolates. FEP-ZID exhibited good activity against Acinetobacter spp. (MIC50/MIC90, 2/8 mg/L) and S. maltophilia (MIC50/MIC90, 4/32 mg/L). S. maltophilia displayed low S rates to most comparators.ConclusionFEP-ZID demonstrated potent activity against a large collection GNB from BSI, including isolates resistant to other BL inhibitor combinations and/or carbapenems. These results support further clinical development of FEP-ZID. Disclosures All authors: No reported disclosures.

1445. Antimicrobial Activity of Novel β-Lactamase Inhibitor Combinations Tested against Organisms Causing Complicated Urinary Tract Infections in United States Medical Centers

BackgroundHigh-dose extended-infusion cefepime-tazobactam (FEP-TAZ) is in clinical development at 2g/2g q8 hours administered over 90 minutes. We evaluated the potency and spectrum of activity of FEP-TAZ, ceftolozane–tazobactam (C-T), ceftazidime–avibactam (CAZ-AVI), and comparators tested against gram-negative bacilli (GNB) causing complicated urinary tract infections (cUTIs) in United States (US) hospitals.MethodsIn 2018, 3,023 GNBisolates (1/patient) were consecutively collected and susceptibility tested against FEP-TAZ (TAZ at fixed 8 mg/L) and comparators by reference broth microdilution methods. Percentage of isolates inhibited at ≤8 mg/L (CLSI, cefepime high dose) and at ≤16 mg/L (pharmacokinetic/pharmacodynamic [PK/PD] susceptible [S] breakpoint based on extended infusion and high dosage) were evaluated.ResultsFEP-TAZ (99.9% inhibited at ≤16 mg/L; Table), CAZ-AVI (99.9%S), and meropenem (MEM; 99.5%S) were the most active agents against Enterobacterales (ENT). An ESBL phenotype (CLSI criteria) was observed in 12.5%, 12.9%, and 3.6% of E. coli (EC), K. pneumoniae (KPN), and P. mirabilis (PM), respectively. FEP-TAZ and CAZ-AVI exhibited complete activity against EC, whereas C-T and piperacillin–tazobactam (PIP-TAZ) were active against 91.5% and 88.1% of ESBL-phenotype EC isolates, respectively. The most active agents against KPN were FEP-TAZ (99.6% inhibited at ≤16 mg/L), CAZ-AVI (100.0%S), and amikacin (AMK; 99.4%S). All PM isolates were S to FEP-TAZ (highest MIC, 0.12 mg/L), C-T, CAZ-AVI, MEM, PIP-TAZ and AMK. FEP-TAZ was highly active against E. cloacae (n = 94; MIC90, 0.5 mg/L; 98.9% inhibited at ≤16 mg/L) and Citrobacter spp. (n = 91; MIC90, 0.12 mg/L; highest MIC, 0.5 mg/L). Against P. aeruginosa (PSA), FEP-TAZ inhibited 97.6% of isolates at ≤16 mg/L, with spectrum of activity similar to CAZ-AVI (96.4%S), C-T (99.4%S) and AMK (97.6%S), and greater than MEM (85.5%S) and PIP-TAZ (87.3%S).ConclusionFEP-TAZ showed potent activity against ENT and PSA isolated in US hospitals in 2018, with overall spectrum (ENT + PSA) similar to CAZ-AVI and greater than C-T, PIP-TAZ, and MEM when FEP-TAZ proposed PK/PD S breakpoint of ≤16 mg/L was applied. FEP-TAZ may represent a valuable option for treating cUTIs caused by resistant GNB. Disclosures All authors: No reported disclosures.

2214. Comparison of Cefepime–Zidebactam (WCK 5222), Ceftazidime–Avibactam, and Ceftolozane–Tazobactam Tested Against Gram-Negative Organisms Causing Pneumonia in United States Hospitals in 2018

BackgroundZidebactam (ZID) is a bicyclo-acyl hydrazide antibiotic with a dual mechanism of action: selective Gram-negative PBP2 binding and β-lactamase inhibition. We evaluated the frequency and antimicrobial susceptibility (S) of Gram-negative bacilli (GNB) isolated from patients with pneumonia in US hospitals.MethodsAll 3,086 clinical isolates were consecutively collected from patients hospitalized with pneumonia (1/patient) in 29 US medical centers in 2018, and the GNB (n = 2,171) were S tested against cefepime (FEP)-ZID (1:1 ratio) and comparators by reference broth microdilution methods. The FEP S breakpoint of ≤8 mg/L (CLSI, high dose) was applied to FEP-ZID for comparison purposes. An FEP-ZID S breakpoint of ≤64 mg/L has been proposed for non-fermentative GNB based on pharmacokinetic/pharmacodynamic target attainment and was applied. Enterobacterales (ENT) isolateswere screened for β-lactamase genes by whole-genome sequencing.ResultsGNB represented 70.3% of the collection, and the most common GNB were P. aeruginosa (PSA; 34.9% of GNB), K. pneumoniae (10.9%), E. coli (9.7%), S. marcescens (7.7%), and S. maltophilia (XM; 6.4%). FEP-ZID was highly active against PSA (MIC50/90, 2/8 mg/L; 98.8% and 99.9% inhibited at ≤8 and ≤16 mg/L, respectively; highest MIC, 32 mg/L), including resistant subsets (table). Among comparators, colistin (99.6%S), ceftazidime–avibactam (CAZ-AVI; 95.2%S), and ceftolozane–tazobactam (C-T; 94.5%S) were the most active compounds against PSA. FEP-ZID inhibited all ENT at ≤4 mg/L, including ESBL-producers (MIC90, 0.25 mg/L) and carbapenem-resistant ENT (MIC90, 4 mg/L). The most active comparators against ENT were CAZ-AVI (99.9%S), amikacin (98.5%S), and meropenem (MEM; 98.3%S). FEP-ZID inhibited 75.0% and 97.9% of XM isolates at ≤8 and ≤16 mg/L, respectively (highest MIC, 64 mg/L). The only other compounds active against XM were co-trimoxazole (MIC50/90, ≤0.12/2 mg/L; 95.7%S) and levofloxacin (MIC50/90, 1/2 mg/L; 70.7%S). FEP-ZID inhibited 71.0% and 98.9% of A. baumannii isolates at ≤8 and ≤64 mg/L,, respectively.ConclusionFEP-ZID showed potent in vitro activity against GNB causing pneumonia in US hospitals and may represent a valuable therapeutic option for these difficult-to-treat infections Disclosures All authors: No reported disclosures.

1535. Pharmacokinetic (PK) and Pharmacodynamic (PD) Evaluation of Cefepime (CPM) in Obese and Non-Obese Patients

BackgroundAppropriate application of antimicrobial PK/PD properties is crucial to optimizing patient outcomes. Although β-lactams are among the most utilized and effective antibiotics, optimal dosing strategies in obese populations are largely unknown. The objective of this study was to compare PK/PD of CPM in non-obese (NO, weight 80–100 kg) and obese (O, weight > 100 kg) patients.MethodsA prospective comparative PK/PD analysis was conducted in NO and O patients receiving CPM. Blood samples were obtained at 30, 60, 120, 240, 360, and 480 minutes after CPM infusion. CPM concentrations were determined by reversed-phase high-performance liquid chromatography. Non-compartmental PK analyses were performed, followed by Monte Carlo simulations (Oracle Crystal Ball®, 5,000 simulated patients) to estimate probability of target attainment (PTA) against common Gram-negative pathogens. The desired PD target for CPM was % time above MIC of unbound drug (%fT > MIC) ≥ 60%. Chi-squared and Mann–Whitney U tests were used for analysis.ResultsSeventeen patients were enrolled and most (94%) received CPM 2 g q8h. A significant difference in actual body weight and body mass index was observed (P < 0.001). There were no differences in other baseline or PK characteristics between the two groups. Utilizing CPM 2 g q8h, PTA ≥ 90% was not observed for organisms with an MIC of 8 μg/mL, the current CLSI breakpoint for P. aeruginosa and A. baumannii (PTA = 88% vs. 81% in NO and O groups, respectively). With a 6 g continuous infusion (CI), however, ≥ 90% PTA was achieved in both groups (PTA = 100%) for organisms with an MIC of 8 μg/mL, while a regimen of 2 g q8h (infused over 3 hours [EI]) also provided PTA of ≥ 90% in both groups (PTA = 98% vs. 92% in NO and O groups, respectively). Goal PTA was not obtained in either group for organisms with an MIC of 4 μg/mL with CPM 1 g q8h or 2 g q12h (i.e., CLSI recommended dosing for organisms with MICs of 4 μg/mL).ConclusionOptimizing PK/PD parameters through novel dosing strategies are essential in both the NO and O populations for optimal CPM exposure in susceptible pathogens with higher MICs. CPM 6 grams/day by either CI or EI provides more optimal PK/PD characteristics in obese patients for pathogens with MICs at or near the current CLSI-recommended breakpoint. Disclosures All authors: No reported disclosures.

723. Synergistic Effect of Cefiderocol Combined With Other Antibiotics Against Cefiderocol High MIC Isolates From the Multi-National SIDERO-WT Studies

BackgroundCefiderocol (CFDC) is a novel siderophore cephalosporin, which demonstrated potent activity at MICs of ≤4 µg/mL against ≥99% of the Gram-negative clinical isolates, including carbapenem-resistant Enterobacteriaceae spp. and nonfermenters in a multi-national SIDERO-WT study. In this study, we evaluated the synergistic effects of CFDC combined with other antibiotics against isolates with high CFDC MIC (i.e., ≥8 µg/mL).MethodsThe combination effects of CFDC and other antibiotics were evaluated by several Methods. (1) broth microdilution method in the presence of β-lactamase inhibitors avibactam and/or picolinic acid, (2) checkerboard method or time-kill assays in the presence of amikacin, meropenem, colistin, ceftazidime/avibactam or ceftolozane/tazobactam, (3) chemostat model reproducing humanized antibiotic exposures. Iron-depleted cation-adjusted Mueller–Hinton broth was used as the standard medium for CFDC as recommended by the Clinical Laboratory and Standard Institute (CLSI).ResultsA total of 39 CFDC nonsusceptible (NS) isolates were found among 9,205 isolates in the SIDERO-WT-2014 study. Among 28 CFDC-NS A. baumannii, 25 isolates were PER producers mainly from Russia and Turkey. Among 9 CFDC-NS Enterobacteriaceae, 5 isolates were NDM-producers from Turkey. Against all 34 non-NDM-producing isolates, the addition of avibactam significantly reduced CFDC MIC. The combination of CFDC with ceftazidime/avibactam and ceftolozane/tazobactam also showed strong synergy against these isolates, presumably due to the effect of β-lactamase inhibition. In addition, CFDC had synergistic effects with meropenem against 42.9% (9 of 21) of carbapenem NS isolates. Against the CFDC-NS isolates, including PER-producing A. baumannii, the combination of CFDC and meropenem under humanized drug exposures (chemostat model) showed synergistic effect leading to sustained eradication of bacteria over 24 hours.ConclusionEven against the non-NDM-producing CFDC-NS isolates, including PER-producing A. baumannii, the combinations of CFDC with ceftazidime/avibactam, ceftolozane/tazobactam, or meropenem were effective due to synergistic effects. The mechanism of CFDC-meropenem synergy against A. baumannii is not known.Disclosures All authors: No reported disclosures.

704. Incidence and Patient Outcomes of S. aureus Isolates from Acute Bacterial Skin and Skin Structure Infections (ABSSSI) with High Iclaprim MIC values in Phase 3 REVIVE Trials

BackgroundThe incidence and outcomes of patients with S. aureus isolates with an iclaprim MIC >8 µg/mL, a concentration that is not systemically achievable, were determined among patients from two Phase 3 studies for the treatment of ABSSSI, REVIVE-1 and -2.MethodsREVIVE-1 and REVIVE-2 studies were 600-patient, double-blinded, randomized (1:1), active-controlled trials among patients with ABSSSI that compared the safety and efficacy of iclaprim 80 mg fixed dose with vancomycin 15 mg/kg (adjusted for renal function), both administered intravenously over 2 hours every 12 hours for 5–14 days. Patients had a bacterial skin infection suspected or confirmed to be due to a Gram-positive pathogen with a lesion size ≥75 cm2. An early clinical response (ECR) was defined as a ≥20% reduction in lesion size compared with baseline at the early time point (ETP) 48–72 hours after the start of administration of the study drug in the intent-to-treat (ITT) population. A clinical cure, defined as complete resolution of all signs and symptoms of ABSSSI was measured at the end of therapy (EOT) and test of cure (TOC) visit, 7–14 days after the last dose of study drug. At baseline, EOT and TOC visits, ABSSSIs were sampled for microbiological culture and broth microdilution susceptibility testing conducted in accordance with CLSI M7.ResultsThe incidence of culture confirmed S. aureus isolates among patients with ABSSSI with an iclaprim MIC >8 µg/mL was 2.0% (16/790). Six were MSSA and 10 were MRSA. The clinical outcomes of these infections included ECR of 63% (10/16), EOT response of 81.3% (13/16) and the TOC response of 75% (12/16). For microbiological outcomes of these infections, the end of therapy response was 92.9% (13/14) and the test of cure response was 92.3% (12/13). In comparison, there was less variation in vancomycin MICs among the S. aureus isolates. For patients who were randomized to vancomycin and had a pathogen identified from their ABSSSI, the pooled ECR was 82.6% (242 of 293) at a vancomycin MIC of 0.5–1 µg/mL and one isolate from a patient with ECR had a MIC of 2 µg/mL.ConclusionPatients receiving iclaprim had good clinical and microbiological responses against S. aureus isolates with an iclaprim MIC >8 µg/mL, which are uncommon (2.0%).Disclosures All authors: No reported disclosures.

675. Efficacy of Human-Simulated Bronchopulmonary Exposures of Cefepime and Zidebactam (WCK 5222) Against Multidrug-Resistant (MDR) Pseudomonas aeruginosa (PSA) in a Neutropenic Murine Pneumonia Model

BackgroundWCK 5222 combines cefepime (FEP) with zidebactam (ZID), a bicycloacyl hydrazide β-lactam enhancer which binds PBP2 in PSA and inhibits class A and C β-lactamases. The in vivo efficacy of human-simulated bronchopulmonary exposures of WCK 5222 against MDR PSA, a recalcitrant pneumonia-causing pathogen with few treatment options, was investigated in a neutropenic murine pneumonia model.MethodsThirteen clinical isolates of MDR PSA with FEP MIC ≥64 mg/L were studied in neutropenic CD-1 mice. FEP, ZID, and WCK 5222 MICs were measured by broth microdilution in triplicate. For in vivo experiments, lungs were intranasally inoculated with 107–108 CFU/mL bacterial suspensions. Human-simulated regimens (HSR) of FEP and ZID alone and in combination which achieved epithelial lining fluid (ELF) exposures in mice approximating human ELF exposures after doses of 2 g FEP/1 g ZID as a 1 hour infusion at steady state were developed. For each regimen, groups of 6 mice were dosed subcutaneously 2 hours after inoculation for 24 hours, then sacrificed. Vehicle-dosed control mice were sacrificed at the start (0 hour) and end (24 hours) of the dosing period. Lungs were aseptically harvested and bacterial CFU/lungs were determined.ResultsFEP MIC was >64 mg/L for all isolates, while ZID and WCK 5222 MICs ranged from 4–512 and 4–32 mg/L, respectively. Mean bacterial growth for all isolates at 0 hour was 6.68 log10 CFU/lungs. Mean changes ± SD in bacterial density at 24 hours compared with 0 hour controls for 12 isolates with WCK5222 MIC ≤16 mg/L were 2.08 ± 1.09, 1.09 ± 0.98, −0.92 ± 1.45, and −2.13 ± 0.75, for control, FEP, ZID, and WCK5222, respectively. Against these isolates, ZID yielded >1 log10 CFU/lungs reduction in 7/12, while activity was enhanced with WCK5222, producing >1 log10 CFU/lungs reduction in 11/12 and >2 log10 CFU/lungs reduction in 9/12. All isolates showed growth or stasis on FEP.ConclusionHuman-simulated bronchopulmonary exposures of WCK5222 is effective against MDR PSA at MIC up to 16 mg/L in a neutropenic murine model. These data support the clinical development of WCK5222 for the treatment of pseudomonal lung infections, but further studies of PSA with high WCK5222 MIC are necessary to delineate the susceptibility breakpoint.Disclosures All authors: No reported disclosures.

A nationwide study of molecular epidemiology and antimicrobial susceptibility of Clostridioides difficile in South Korea

The molecular epidemiology and antimicrobial resistance of Clostridioides difficile were studied in South Korea in 2017 as part of a National Surveillance System. From February to May 2017, all non-duplicate isolates of C. difficile were recovered from patients who were suspected to have C. difficile infection and collected from 6 referral hospitals representing the 6 regions in South Korea. We performed PCRs for the toxin gene, PCR ribotyping, multilocus sequence typing (MLST), antimicrobial susceptibility testing by agar dilution according to the recommendations of the CLSI and detection of antimicrobial resistance genes such as ermB, catD, tetM, vanZ and nimR by PCR. Of 331 C. difficile isolates, 257 (77.6%) were toxigenic and the prevalence of strains producing binary toxin (CDT) was 5.1% (13/257). A total of 52 different ribotype (RT) patterns were found. RT018 was the most common (25.1% of all isolates), and RT014/020, RT002 and RT012 were also common. RT010 was most common non-toxigenic strain. MLST analysis of randomly selected 72 C. difficile isolates identified 46 sequence types (STs), of which three were new and not in the PubMLST library. There was a good correlation between MLST and RT as following: ST1 (RT027), ST8 (RT002), ST11 (RT078), ST17 (RT018), ST35 (RT046), ST37 (RT017), ST42 (RT106), ST53 (RT103), ST81 (RT369), and ST99 (RT070). All toxigenic isolates were susceptible to metronidazole and vancomycin (MIC ≤ 2 mg/L). For rifaximin, 24% of toxigenic isolates were resistant. Of randomly selected 106 toxigenic isolates, resistance rates for ampicillin, cefotetan, clindamycin, imipenem, chloramphenicol, tetracycline, and moxifloxacin were 48%, 46%, 64%, 54%, 0%, 6% and 52% respectively and frequencies of various resistance genes were 62.3% for ermB, 0.9% catD and 10.4% tetM. RTs018, 002, 017 and 369 showed high MICs to various antimicrobial agents and multi-drug resistance was common also.

2372. PCR Ribotype and Antimicrobial Susceptibility of Clostridioides (Formerly Clostridium) difficile in Korea

Background Clostridioides difficile infection is a leading cause of healthcare-associated diarrhea. The epidemiology and characteristics of C. difficile vary geographically. We performed toxin enzyme immunoassay (EIA), toxigenic gene analysis, antimicrobial susceptibility tests (AST), and PCR ribotyping to elucidate the characteristics of C. difficile in Korea.MethodsBetween July 2017 and June 2018, C. difficile was prospectively isolated in 128 specimens from the culture of 1,182 unduplicated specimens. Seventy-five stool specimens with a positive toxin EIA between July 2016 and June 2017 were also included. We performed PCR for the tcdA and tcdB genes on these isolates, and AST and PCR ribotyping on the isolates with a positive toxin EIA.ResultsOlder patients tended to have a higher rate of positive toxin EIA and positive cultures than did younger patients. Ribotype 018 was predominantly identified (48.6%), followed by ribotype 014/020 (9.9%), and ribotype 002 (8.3%). All of A-B+ isolates were either ribotype 017 or B-2. Ribotypes 017, 018, and B-2 showed high resistance to various antibiotics. In contrast, ribotypes 002, 014/020 and C-4 demonstrated low resistance rates, except that to moxifloxacin in ribotype 002. Clindamycin and erythromycin showed a positive correlation. Most of the isolates resistant to rifampicin or tetracycline showed a high MIC to both erythromycin and clindamycin.ConclusionRibotype 018, which is highly transmissible and resistant to various antimicrobial agents, is predominant in Korea. Ribotype 002 has also been increasing in prevalence in Korea.Disclosures All authors: No reported disclosures.

Cyp51A Mutations, Extrolite Profiles, and Antifungal Susceptibility in Clinical and Environmental Isolates of the Aspergillus viridinutans Species Complex.

The past decade has seen an increase in aspergillosis in humans and animals due to Aspergillus viridinutans species complex members. Azole resistance is common to these infections, carrying a poor prognosis. cyp51A gene mutations are the main cause of acquired azole resistance in Aspergillus fumigatus This study aimed to determine if the azole-resistant phenotype in A. viridinutans complex members is associated with cyp51A mutations or extrolite profiles. The cyp51A gene of clinical and environmental isolates was amplified using novel primers, antifungal susceptibility was tested using the Clinical and Laboratory Standards Institute methodology, and extrolite profiling was performed using agar plug extraction. Very high azole MICs were detected in 84% of the isolates (31/37). The MICs of the newer antifungals luliconazole and olorofim (F901318) were low for all isolates. cyp51A sequences revealed 113 nonsynonymous mutations compared to the sequence of wild-type A. fumigatus M172A/V and D255G, previously associated with A. fumigatus azole resistance, were common among all isolates but were not correlated with azole MICs. Two environmental isolates with nonsusceptibility to itraconazole and high MICs of voriconazole and isavuconazole harbored G138C, previously associated with azole-resistant A. fumigatus Some novel mutations were identified only among isolates with high azole MICs. However, cyp51A homology modeling did not cause a significant protein structure change for these mutations. There was no correlation between extrolite patterns and susceptibility. For A. viridinutans complex isolates, cyp51A mutations and the extrolites that they produced were not major causes of antifungal resistance. Luliconazole and olorofim show promise for treating azole-resistant infections caused by these cryptic species.