Reference Broth Microdilution Methods Research Articles

1378. Evaluation of the In vitro Activity of Meropenem-Vaborbactam Against Carbapenem-Resistant Enterobacteriaceae, Including Isolates Resistant to Ceftazidime–Avibactam

BackgroundMeropenem-vaborbactam (M-V) is a novel antibiotic for treatment of carbapenem-resistant Enterobacteriaceae (CRE) infections. Our objective was to determine the in vitro activity of meropenem-vaborbactam against genetically-diverse CRE isolates, including those that have developed resistance to Ceftazidime–Avibactam (C-A).MethodsMinimum inhibitory concentrations (MICs) were determined for meropenem (MER), M-V, and C-A by reference broth microdilution (BMD) methods in triplicate. Vaborbactam and avibactam were tested at fixed concentrations of 8 and 4 µg/mL, respectively. Quality control strains were used and within expected ranges. Polymerase chain reaction (PCR) with DNA sequencing was used to detect resistance determinants, including Klebsiella pneumoniae carbapenemase (KPC) subtypes and porin mutations.ResultsA total of 117 CRE isolates were tested, including K. pneumoniae (Kp; n = 83), E. cloacae (n = 17), E. coli (n = 10), and E. aerogenes (n = 7). Seventy-nine percent harbored blaKPC. KPC subtypes included KPC-2 (n = 32), KPC-3 (n = 41), KPC-3 variants (n = 16), and KPC [not typed] (n = 4, all E. coli). Among 74 K. pneumoniae, 95% had a premature stop codon in ompk35 and ompK36 genotypes included wild type (n = 48), IS5 insertion (n = 13), 135–136 DG duplication (n = 9), and other mutations (n = 4). The median (range) MICs for MER, C-A, and M-V were 8 (0.06 to ≥128), 1 (0.25 to ≥512), and 0.03 (0.015––16), respectively. Corresponding rates of susceptibility were 23, 84, and 98%, respectively. Fifty-three percent and 95% of C-A-resistant isolates were susceptible to MER and M-V, respectively. Among Kp, C-A MICs did not vary by KPC subtype or porin genotype. On the other hand, median M-V MICs were higher among KPC-2 than KPC-3 Kp (0.12 vs. 0.03; P = 0.002), and among Kp with ompK36 porin mutations compared with wild type (0.25 vs. 0.03; P < 0.001). Among Kp with KPC-3 variants (n = 16), the median M-V MIC was 0.03 (0.015––2); 100% were M-V susceptible. Median M-V MICs did not vary by CRE species. Only two isolates were M-V resistant, both were E. cloacae that did not harbor blaKPC.ConclusionM-V demonstrates high rates of in vitro susceptibility against diverse CRE isolates, including those that are resistant to C-A. As this agent is introduced into the clinic, it will be important to identify K. pneumoniae isolates harboring KPC-2 with ompK36 porin mutations that demonstrate higher MICs.Disclosures M. H. Nguyen, Merck: Grant Investigator, Research grant. Astellas: Grant Investigator, Research grant.

2076. Expanded Antibiotic Menu Demonstration for Novel Rapid Phenotypic Antimicrobial Susceptibility Testing Platform

BackgroundFast transitions to targeted therapies for infectious disease patients are paramount for optimal patient care and antibiotic stewardship. A next-generation phenotypic antimicrobial susceptibility test (AST) system that provides rapid results for broad menus of >30 antibiotics per patient sample is required. SeLux has developed a rapid, phenotypic AST platform that utilizes standard 384-well microplates. These consumables provide sufficient wells for simultaneous testing of newly approved antibiotics and broad selections of conventional antibiotics. Here, we demonstrate the platform’s ability to produce fast, accurate results with newly approved and not-yet-approved antibiotics.MethodsThe core of SeLux’s technology is a novel assay for bacterial surface area, which enables delineation of truly resistant bacteria from organisms that filament or swell in antibiotic concentrations above the MIC. AST was performed with the SeLux platform and compared with the CLSI broth microdilution reference method. Testing of 20 representative conventional antibiotics was performed on 1,191 isolates, including 323 FDA-CDC “challenge” strains and comprising 20 species of nonfastidious bacteria. Testing of newly developed antibiotics, generous gifts from the manufacturers, was performed with ~20 to 50 isolates with representative MICs throughout the dilution series and encompassing the breakpoint region.ResultsTesting of conventional antibiotics showed essential agreements (EA) and categorical agreements (CA) ≥90% with the CLSI reference method for all combinations tested (Figures 1 and 2). The platform returned results within 6.5 hours for >98% of the isolates tested to date. The SeLux platform’s EA was ≥90% for all newly developed antibiotics tested to date (Figure 3). For newly approved antibiotics, CAs were similarly ≥90% with no very major errors (Vmj).Figure 1Figure 2Figure 3ConclusionThe SeLux platform’s compatibility with 384-well microplates should transform the rate with which newly approved antibiotics gain use. By speeding the reporting of AST results, SeLux’s platform will further enable hospitals to simultaneously improve patient care, decrease lengths-of-stay, and meet antibiotic stewardship goals.Disclosures E. Stern, SeLux Diagnostics: Board Member, Employee and Shareholder, Salary. A. Vacic, SeLux Diagnostics: Employee and Shareholder, Salary. K. Flentie, SeLux Diagnostics: Employee and Shareholder, Salary. B. Spears, SeLux Diagnostics: Employee and Shareholder, Salary. N. Purmort, SeLux Diagnostics: Employee and Shareholder, Salary. F. Giok, SeLux Diagnostics: Employee and Shareholder, Salary. K. DaPonte, SeLux Diagnostics: Employee and Shareholder, Salary. S. Scott, SeLux Diagnostics: Employee and Shareholder, Salary. D. Puff, SeLux Diagnostics: Employee and Shareholder, Salary. F. Floyd Jr., SeLux Diagnostics: Employee and Shareholder, Salary. Z. Zhang, SeLux Diagnostics: Employee and Shareholder, Salary. P. Reilly, SeLux Diagnostics: Employee, Salary. J. Liu, SeLux Diagnostics: Employee, Salary. E. Viveiros, SeLux Diagnostics: Employee, Salary. N. Phelan, SeLux Diagnostics: Employee, Salary. C. Krebill, SeLux Diagnostics: Employee, Salary. A. Flyer, SeLux Diagnostics: Consultant, Scientific Advisor and Shareholder, Consulting fee. D. Smalley, SeLux Diagnostics: Scientific Advisor and Shareholder, Consulting fee. D. C. Hooper, SeLux Diagnostics: Scientific Advisor, Consulting fee. M. J. Ferraro, SeLux Diagnostics: Scientific Advisor and Shareholder, Consulting fee.

1373. Activity of Ceftriaxone–Sulbactam–EDTA Against Multi-Drug-resistant A. baumannii, P. aeruginosa and Enterobacteriaceae Isolates (WHO Critical Priority Pathogens) Collected from Various Hospitals in India

BackgroundCeftriaxone–Sulbactam–EDTA (CSE) is the first cephalosporin–β-lactamase inhibitor combination with an antibiotic resistance breaker–disodium edetate, recently evaluated in a Phase 3 clinical trial for treatment of adults with complicated urinary tract infections (NCT03477422). The addition of Sulbactam and EDTA expands the spectrum of activity of Ceftriaxone to include extended-spectrum-β-lactamase (ESBL) and metallo-β-lactamase (MBL) producing bacteria. This study evaluated the in vitro activity of CSE against 3,150 isolates (716 (22.73%) E. coli; 435 (13.81%) K. pneumoniae; 1,075 (34.13%) A. baumannii; 924 (29.33%) P. aeruginosa) collected from 22 hospitals in India during 2013–2016.MethodsA total of 3,150 nonduplicate Gram-negative clinical isolates were collected, and susceptibility testing was conducted using reference broth microdilution method for CSE and comparators. CLSI defined phenotypic methods were used for ESBL and MBL detection, and thereafter, all isolates were further characterized genotypically using single PCRs and a panel of primers for detection of most β-lactamase enzymes, including blaTEM, blaSHV, blaCTX-M, blaAmpC, blaOXA, blaKPC, blaVIM, blaNDM, and blaIMP.ResultsOf the 3,150 isolates, 2,717 (86.25%) were β-lactamase producers, of which, 851 (31.32%) tested positive for ESBL, 1,591 (58.56%) tested positive for MBL, while 275 (10.12%) tested positive for both ESBL and MBL production during phenotypic evaluation. Once the genotype data were available, isolates were re-characterized as per the functional classification of β-lactamases into four distinct categories, including ESBL, AmpC, Carbapenemase and MBL. An astonishing 1,866 (59.23%) isolates harbored at least one MBL gene, of which, the prevalence was the highest in A. baumannii (78.6%), followed by K. pneumoniae (63%), P. aeruginosa (46.6%) and E. coli (44.1%). A summary of the results of susceptibility testing is shown in Figures 1, 2, and 3. ConclusionCSE showed a high overall susceptibility in ESBL- and MBL-producing bacteria and could provide a useful alternative to carbapenems and colistin in clinical settings.Disclosures R. Girotra, Venus Medicine Research Centre: Employee, Salary. A. Pyasi, Venus Medicine Research Centre: Employee, Salary. M. Chaudhary, Venus Medicine Research Centre: Board Member and Shareholder, Salary. M. A. Mir, Venus Medicine Research Centre: Employee, Salary. S. Chaudhary, Venus Medicine Research Centre: Employee and Shareholder, Salary. P. Mandale, Venus Medicine Research Centre: Employee, Salary.

Antimicrobial activity of ceftaroline and comparator agents tested against organisms isolated from patients with community-acquired bacterial pneumonia in Europe, Asia, and Latin America

ObjectiveTo evaluate the potency and spectrum of ceftaroline and comparator agents tested against contemporary bacteria isolated from patients with community-acquired bacterial pneumonia (CABP) in Europe (EUR), Asia-Pacific (APAC), and Latin America (LATAM). MethodsA total of 4321 bacterial isolates were collected consecutively by the SENTRY Antimicrobial Surveillance Program in 2015–2017 from 65 medical centers located in Western Europe (W-EUR; 21 centers in 10 nations), Eastern Europe and the Mediterranean region (E-EUR; 15 centers in 11 nations), APAC (18 centers in nine nations), and LATAM (11 centers in nine nations). Isolates were collected from lower respiratory tract specimens, and an isolate obtained from an outpatient or earlier than 48h after hospitalization was considered community-acquired. Organisms were tested for susceptibility by reference broth microdilution methods in a central laboratory. ResultsAmong Streptococcus pneumoniae (n=1736), 99.9% of isolates were ceftaroline-susceptible (MIC50/90, 0.008/0.12mg/l), and ceftriaxone susceptibility (≤1mg/l) ranged from 97.4% in W-EUR to 85.3% in the APAC region. Ceftaroline was also active against Haemophilus influenzae (n=1172; MIC50/90, 0.008/0.03mg/l; 99.8%/93.9% susceptible per CLSI/EUCAST criteria) and Staphylococcus aureus (n=777; MIC50/90, 0.25/1mg/l; 97.4% susceptible). Oxacillin resistance ranged from 31.9% in the APAC region to 15.0% in E-EUR. Ceftaroline also demonstrated potent activity against Moraxella catarrhalis (n=613; MIC50/90, 0.06/0.25mg/l) and Haemophilus parainfluenzae (n=23; MIC50/90, 0.015/0.03mg/l). ConclusionsSusceptibility rates varied widely by geographic region. Ceftaroline was active against the vast majority of bacterial organisms isolated from patients with CABP.

In Vitro Activity of Imipenem-Relebactam against Clinical Isolates of Gram-Negative Bacilli Isolated in Hospital Laboratories in the United States as Part of the SMART 2016 Program.

Relebactam is a non-β-lactam, bicyclic diazabicyclooctane β-lactamase inhibitor of class A and class C β-lactamases, including Klebsiella pneumoniae carbapenemases (KPCs). It is in phase 3 clinical development in combination with imipenem/cilastatin. The in vitro activities of imipenem-relebactam, imipenem, and comparators were determined using the Clinical and Laboratory Standards Institute (CLSI) reference broth microdilution method for isolates of Enterobacteriaceae (n = 3,419) and Pseudomonas aeruginosa (n = 896) collected in 2016 by 21 U.S. hospital laboratories participating in the SMART (Study for Monitoring Antimicrobial Resistance Trends) global surveillance program. Relebactam was tested at a fixed concentration of 4 μg/ml. Imipenem-relebactam MICs were interpreted using CLSI breakpoints for imipenem. Rates of susceptibility to imipenem-relebactam and imipenem for non-ProteeaeEnterobacteriaceae (n = 3,143) and P. aeruginosa were 99.1% (3,115/3,143) and 95.9% (3,013/3,143) and were 94.4% (846/896) and 74.7% (669/896), respectively. Relebactam restored imipenem susceptibility to 78.5% (102/130) of imipenem-nonsusceptible non-ProteeaeEnterobacteriaceae and to 78.0% (177/227) of imipenem-nonsusceptible P. aeruginosa isolates. Susceptibility to imipenem-relebactam was 98.2% (444/452) and 82.2% (217/264) for multidrug-resistant (MDR) non-ProteeaeEnterobacteriaceae and MDR P. aeruginosa, respectively. Given the ability of relebactam to restore susceptibility to imipenem in nonsusceptible isolates of both non-ProteeaeEnterobacteriaceae and P. aeruginosa and to demonstrate potent activity against current MDR isolates of both non-ProteeaeEnterobacteriaceae and P. aeruginosa, further development of imipenem-relebactam appears warranted.

Murepavadin activity tested against contemporary (2016-17) clinical isolates of XDR Pseudomonas aeruginosa.

Murepavadin (POL7080) represents the first member of a novel class of outer membrane protein-targeting antibiotics. Murepavadin acts by binding to LPS transport protein D and is being developed for the treatment of hospital-acquired and ventilator-associated pneumonia caused by Pseudomonas aeruginosa. To evaluate the antimicrobial activity of murepavadin against XDR P. aeruginosa. A total of 785 clinical isolates of XDR P. aeruginosa were collected in 2016-17 through the SENTRY Antimicrobial Surveillance Program from 34 medical centres in 21 European nations (n = 353) and 75 medical centres in North America (n = 432). Isolates were categorized as XDR when susceptible (CLSI) to ≤2 of the following antimicrobial classes: antipseudomonal cephalosporins, carbapenems, broad-spectrum penicillin/β-lactamase inhibitor combinations, fluoroquinolones, aminoglycosides and polymyxins. Susceptibility testing was performed by the reference broth microdilution method and EUCAST and CLSI interpretative criteria were applied. Murepavadin (MIC50/90, 0.12/0.25 mg/L) inhibited 96.7% of isolates at ≤0.5 mg/L and was 8-fold more active than colistin (MIC50/90, 1/2 mg/L). Only seven isolates (0.9%) exhibited murepavadin MIC values >4 mg/L. Colistin (MIC50/90, 1/2 mg/L; 93.6% susceptible) was the most active comparator, followed by ceftolozane/tazobactam (MIC50/90, 2/>32 mg/L; 70.6% susceptible) and tobramycin (MIC50/90, 8/>8 mg/L; 47.5% susceptible). Murepavadin remained active against isolates that were non-susceptible to colistin (n = 50; MIC50/90, 0.25/0.25 mg/L), ceftolozane/tazobactam (n = 231; MIC50/90, 0.12/0.25 mg/L) and/or tobramycin (n = 412; MIC50/90, 0.12/0.25 mg/L). Murepavadin exhibited potent activity against a large collection of clinical XDR P. aeruginosa isolates from Europe and North America, including isolates that were non-susceptible to colistin, ceftolozane/tazobactam and/or tobramycin.

Ceftazidime-Avibactam Antimicrobial Activity and Spectrum When Tested Against Gram-negative Organisms From Pediatric Patients: Results From the INFORM Surveillance Program (United States, 2011-2015).

Ceftazidime-avibactam was approved by the US Food and Drug Administration in 2015 to treat complicated intra-abdominal and urinary tract infections in adults and is under clinical development for treating pediatric patients. Among 53,381 Gram-negative organisms (1 per patient) collected in 2011-2015, 8461 (15.9%) were from pediatric (≤17 years old) patients. The isolates were collected from 82 US medical centers and susceptibility tested against ceftazidime-avibactam (avibactam at fixed 4 μg/mL) and comparators by reference broth microdilution methods. Susceptibility results were stratified by patient age: ≤1 (3671 isolates); 2-5 (1900); 6-12 (1644) and 13-17 years old (1246). Enterobacteriaceae with an extended-spectrum β-lactamase (ESBL) screening-positive phenotype were evaluated by microarray-based assay for genes encoding ESBLs, KPC, NDM and transferable AmpC enzymes. Ceftazidime-avibactam inhibited >99.9% of all Enterobacteriaceae at the ≤8 μg/mL susceptible break point and was highly active against ESBL screening-positive phenotype Escherichia coli and Klebsiella pneumoniae. Overall, 83.6% of ESBL screening-positive phenotype K. pneumoniae were susceptible to meropenem. Only 1 of 4724 Enterobacteriaceae (0.02%) isolates was nonsusceptible to ceftazidime-avibactam, an Enterobacter aerogenes with a ceftazidime-avibactam minimum inhibitory concentration (MIC) value of 16 μg/mL and negative results for all β-lactamase tested. Ceftazidime-avibactam was very active against Pseudomonas aeruginosa (MIC50/90, 1/4 μg/mL; 99.1% susceptible), including isolates nonsusceptible to meropenem (94.0% susceptible to ceftazidime-avibactam) or piperacillin-tazobactam (91.7% susceptible) or ceftazidime (89.6% susceptible). Ceftazidime-avibactam activity against P. aeruginosa did not vary substantially among age groups (98.8%-99.3% susceptible) or year of isolation (98.5%-100.0% susceptible). Ceftazidime-avibactam was very active against a large collection of Gram-negative bacilli isolated from pediatric patients, including P. aeruginosa and Enterobacteriaceae with an ESBL screening-positive phenotype and resistant to carbapenems.

In vitro activity of meropenem/vaborbactam and characterisation of carbapenem resistance mechanisms among carbapenem-resistant Enterobacteriaceae from the 2015 meropenem/vaborbactam surveillance programme

The activity of meropenem/vaborbactam was evaluated against 11 559 Enterobacteriaceae isolates, including 330 carbapenem-resistant phenotypes (CRE) and carbapenemase genotypes collected worldwide during 2015. Antimicrobial susceptibility testing for meropenem/vaborbactam (inhibitor at 8 mg/L) and comparators was performed by the reference broth microdilution method. CRE isolates were screened for the presence of genes encoding carbapenemases, and 292 (88.5%) of the CRE isolates carried these resistance genes. A total of 209 isolates (63.3% of the CRE; 1.8% of the overall Enterobacteriaceae population) carried blaKPC, including genes encoding KPC-2 (90 isolates), KPC-3 (117 isolates) and KPC-17 (2 isolates). Overall, meropenem/vaborbactam (vaborbactam at 8 mg/L) inhibited 99.3% of all Enterobacteriaceae isolates at the US-FDA susceptibility breakpoint of ≤4/8 mg/L. Meropenem alone inhibited 96.9% of the isolates at the current CLSI susceptibility breakpoint of ≤2 mg/L. Susceptibility rates for comparator antimicrobial agents tested against Enterobacteriaceae isolates ranged from 82.1-98.2% applying the CLSI breakpoints. Against CRE isolates, meropenem/vaborbactam displayed MIC50/90 values at 0.5/32 mg/L, whereas meropenem MIC50/90 values were 16/>32 mg/L. Meropenem/vaborbactam was very active against KPC-producers, and 99.5% of these isolates were inhibited by ≤4/8 mg/L. The single resistant isolate was shown to harbour an outer membrane porin alteration. Meropenem/vaborbactam had limited activity against MBL-producing isolates (including 49 NDM-, 1 IMP-64- and 2 VIM-producers) and/or oxacillinases (47 OXA-48/-232) that were detected mainly in European countries. Meropenem/vaborbactam was active against contemporary CRE and wild-type Enterobacteriaceae collected worldwide and this combination demonstrated enhanced activity compared with meropenem and most comparator agents against CRE and KPC-producers.

Activity of Ceftolozane-Tazobactam against Pseudomonas aeruginosa and Enterobacteriaceae Isolates Collected from Respiratory Tract Specimens of Hospitalized Patients in the United States during 2013 to 2015.

The activities of ceftolozane-tazobactam and comparator agents against organisms deemed to be the cause of pneumonia among patients hospitalized in the United States during 2013 to 2015 were evaluated. Organisms included 1,576 Pseudomonas aeruginosa and 2,362 Enterobacteriaceae isolates susceptibility tested using reference broth microdilution methods. Ceftolozane-tazobactam, cefepime, ceftazidime, meropenem, and piperacillin-tazobactam inhibited 96.3%, 84.8%, 83.5%, 80.0%, and 78.6%, respectively, of the P. aeruginosa isolates. Ceftolozane-tazobactam inhibited 77.5 to 85.1% of isolates nonsusceptible to antipseudomonal β-lactams and 86.6% and 71.0% of the 372 (23.6% overall) multidrug- and 155 (9.8%) extensively drug-resistant isolates tested. The activity of this combination was greater than those of other β-lactams evaluated against P. aeruginosa groups across all U.S. census divisions. Ceftolozane-tazobactam was active against 90.6% of the Enterobacteriaceae, being less active than only meropenem (95.6% susceptible) among the β-lactams evaluated. Against 145 Escherichia coli and Klebsiella pneumoniae isolates carrying extended-spectrum-β-lactamase (ESBL)-encoding genes without carbapenemases, ceftolozane-tazobactam inhibited 82.8% of these isolates and was more active than cefepime and piperacillin-tazobactam (15.2% and 74.3% susceptible, respectively). ESBL genes included in this analysis were mainly blaCTX-M-15-like (89 isolates) and blaCTX-M-14-like (22) genes but also blaSHV (31) and blaTEM (3). Ceftolozane-tazobactam also displayed activity (84.6% susceptible) against 13 isolates harboring acquired AmpC genes. All β-lactams displayed limited activity against blaKPC-carrying isolates. Ceftolozane-tazobactam was the most active β-lactam tested against P. aeruginosa isolates from isolates that were the probable cause of pneumonia and displayed in vitro activity against Enterobacteriaceae, including isolates resistant to cephalosporins and carrying ESBL genes.

Surveillance of tigecycline activity tested against clinical isolates from a global (North America, Europe, Latin America and Asia-Pacific) collection (2016)

Tigecycline and comparators were tested by the reference broth microdilution method against 33 348 non-duplicate bacterial isolates collected prospectively in 2016 from medical centres in the Asia-Pacific (3443 isolates), Europe (13 530 isolates), Latin America (3327 isolates) and the USA (13 048 isolates). Among 7098 Staphylococcus aureus isolates tested, >99.9% were inhibited by ≤0.5 mg/L tigecycline (MIC50/90, 0.06/0.12 mg/L), including >99.9% of methicillin-resistant S. aureus and 100.0% of methicillin-susceptible S. aureus. Tigecycline was slightly more active against Enterococcus faecium (MIC50/90, 0.03/0.06 mg/L) compared with Enterococcus faecalis (MIC50/90, 0.06/0.12 mg/L) and its activity was not adversely affected by vancomycin resistance when tested against these organisms. Tigecycline potency was comparable for Streptococcus pneumoniae (MIC50/90, 0.03/0.06 mg/L), viridans group streptococci (MIC50/90, 0.03/0.06 mg/L) and β-haemolytic streptococci (MIC50/90, 0.06/0.06 mg/L) regardless of species and penicillin susceptibility. Tigecycline was active against Enterobacteriaceae (MIC50/90, 0.25/1 mg/L; 97.8% inhibited at ≤2 mg/L) but was slightly less active against Enterobacteriaceae isolates expressing resistant phenotypes: carbapenem-resistant Enterobacteriaceae (MIC50/90, 0.5/2 mg/L; 98.0% susceptible); multidrug-resistant (MIC50/90, 0.5/2 mg/L; 93.1% susceptible); and extensively drug-resistant (MIC50/90, 0.5/4 mg/L; 87.8% susceptible). Tigecycline inhibited 74.4% of 888 Acinetobacter baumannii isolates at ≤2 mg/L (MIC50/90, 2/4 mg/L) and demonstrated good in vitro activity against Stenotrophomonas maltophilia (MIC50/90, 1/2 mg/L; 90.6% inhibited at ≤2 mg/L) Tigecycline was active against Haemophilus influenzae (MIC50/90, 0.12/0.25 mg/L) regardless of β-lactamase status. Tigecycline represents an important treatment option for resistant Gram-negative and Gram-positive bacterial infections.

322 Correlation Between Etest and Broth Microdilution for Colistin Antimicrobial Susceptibility Testing of Enterobacteriaceae

Colistin is increasingly being used to treat serious infections caused by multidrug-resistant Gram-negative bacteria. With the recent emergence of plasmid-mediated colistin resistance (mcr genes), screening for isolates with phenotypic colistin resistance has become clinically and epidemiologically relevant. However, antimicrobial susceptibility testing (AST) of colistin is associated with several technical challenges. Disk diffusion does not work because of poor diffusion of the large colistin molecule, Etest has been reported to underestimate minimum inhibitory concentration (MIC) values, and most clinical microbiology laboratories lack the capacity to perform broth microdilution (BMD). In this study, we compared the Etest method with the reference BMD method described by the Clinical and Laboratory Standards Institute (CLSI) for determining colistin MICs among Enterobacteriaceae. A total of 114 isolates of Enterobacteriaceae (51 carbapenem-resistant [CRE] and 63 carbapenem-susceptible [CSE]) were selected for colistin AST. One CRE isolate and 13 CSE isolates harbored a mcr gene. BMD testing was performed according to CLSI guidelines using a frozen reference panel with colistin concentrations ranging from 0.25 to 8 µg/mL. The Etest (BioMérieux, Durham, NC) method was performed on Mueller-Hinton agar (BBL, Becton Dickinson, Sparks, MD) according to the manufacturer’s instructions. Interpretive criteria based on CLSI epidemiological cutoff values (non-wild-type ≥4 µg/mL; wild-type ≤2 µg/mL) were used to define resistance and susceptibility to colistin, respectively. By BMD, colistin MICs ranged from ≤0.25 to >8 µg/mL and 27% were determined to be colistin-resistant. The categorical agreement between Etest and BMD was 92%. All 14 mcr-positive isolates were categorized as colistin-resistant by BMD, while Etest classified 13/14 as resistant. In conclusion, clinical microbiology laboratories lacking capacity to perform BMD could implement the Etest method with BBL Mueller-Hinton agar for epidemiological purposes to help identify candidate isolates for mcr screening. To determine whether colistin can be used clinically, BMD would still be required.

In Vitro Comparison of Ceftolozane-Tazobactam to Traditional Beta-Lactams and Ceftolozane-Tazobactam as an Alternative to Combination Antimicrobial Therapy for Pseudomonas aeruginosa

Guidelines for the treatment of sepsis, febrile neutropenia, and hospital-acquired pneumonia caused by Pseudomonas aeruginosa include empirical regimens incorporating two antibiotics from different classes with activity against P. aeruginosa for select at-risk patients to increase the likelihood that the organism will be susceptible to at least one agent. The activity against P. aeruginosa and the rates of cross-resistance of ceftolozane-tazobactam were compared to those of the β-lactam comparators cefepime, ceftazidime, piperacillin-tazobactam, and meropenem alone and cumulatively with ciprofloxacin or tobramycin. Nonurine P. aeruginosa isolates were collected from adult inpatients at 44 geographically diverse U.S. hospitals. MICs were determined using reference broth microdilution methods. Of the 1,257 isolates collected, 29% were from patients in intensive care units and 39% were from respiratory sites. The overall rate of susceptibility to ceftolozane-tazobactam was high at 97%, whereas it was 72 to 76% for cefepime, ceftazidime, piperacillin-tazobactam, and meropenem. The rate of nonsusceptibility to all four comparator β-lactams was 11%; of the isolates nonsusceptible to the four comparator β-lactams, 80% remained susceptible to ceftolozane-tazobactam. Among the isolates nonsusceptible to the tested β-lactam comparators, less than half were susceptible to ciprofloxacin. By comparison, approximately 80% of the β-lactam-nonsusceptible isolates were susceptible to tobramycin, for overall cumulative susceptibility rates of 94 to 95%, nearly 10% higher than that of the ciprofloxacin-β-lactam combinations and approaching that of ceftolozane-tazobactam as a single agent. The rates of susceptibility to ceftolozane-tazobactam were consistently high, with little observable cross-resistance. Ceftolozane-tazobactam monotherapy performed at or above the level of commonly utilized combination therapies on the basis of in vitro susceptibilities. Ceftolozane-tazobactam should be considered for use in patients at high risk for resistant P. aeruginosa infection and as an alternative to empirical combination therapy, especially for patients unable to tolerate aminoglycosides.

In Vitro Activity of the Novel Lactone Ketolide Nafithromycin (WCK 4873) against Contemporary Clinical Bacteria from a Global Surveillance Program.

Nafithromycin (WCK 4873), a novel antimicrobial agent of the lactone ketolide class, is currently in phase 2 development for treatment of community-acquired bacterial pneumonia (CABP). A total of 4,739 nonduplicate isolates were selected from a 2014 global surveillance program at medical institutions located in 43 countries within the United States, Europe, Latin America, and the Asia-Pacific region. Nafithromycin and comparator agents were used for susceptibility testing by reference broth microdilution methods. Nafithromycin was active against Staphylococcus aureus (MIC50/90, 0.06/>2 μg/ml), including erythromycin-resistant strains exhibiting an inducible clindamycin resistance phenotype (MIC50/90, 0.06/0.06 μg/ml) and telithromycin-susceptible strains (MIC50/90, 0.06/0.06 μg/ml), but it exhibited limited activity against most telithromycin-resistant and clindamycin-resistant isolates that were constitutively resistant to macrolides (MIC50/90, >2/>2 μg/ml). Nafithromycin was very active (MIC50/90, 0.015/0.06 μg/ml) against 1,911 Streptococcus pneumoniae strains, inhibiting all strains, with MIC values of ≤0.25 μg/ml. Telithromycin susceptibility was 99.9% for Streptococcus pneumoniae strains, and nafithromycin was up to 8-fold more potent than telithromycin. Overall, 37.9% of S. pneumoniae strains were resistant to erythromycin, and 19.7% were resistant to clindamycin. Nafithromycin was highly active against 606 Streptococcus pyogenes strains (MIC50/90, 0.015/0.015 μg/ml), inhibiting 100.0% of isolates at ≤0.5 μg/ml, and MIC50/90 values (0.015/0.015 to 0.03 μg/ml) were similar for the 4 geographic regions. Nafithromycin and telithromycin demonstrated comparable in vitro activities against 1,002 Haemophilus influenzae isolates and 504 Moraxella catarrhalis isolates. Overall, nafithromycin showed potent in vitro activity against a broad range of contemporary (2014) global pathogens. These results support the continued clinical development of nafithromycin for treatment of CABP.

Antimicrobial Activity of Ceftazidime-Avibactam Tested against Multidrug-Resistant Enterobacteriaceae and Pseudomonas aeruginosa Isolates from U.S. Medical Centers, 2013 to 2016.

The in vitro activity of ceftazidime-avibactam and many comparator agents was determined against various resistant subsets of organisms selected among 36,380 Enterobacteriaceae and 7,868 Pseudomonas aeruginosa isolates. The isolates were consecutively collected from 94 U.S. hospitals, and all isolates were tested for susceptibility by reference broth microdilution methods in a central monitoring laboratory (JMI Laboratories). Enterobacteriaceae isolates resistant to carbapenems (CRE) and/or ceftazidime-avibactam (MIC ≥ 16 μg/ml) were evaluated for the presence of genes encoding extended-spectrum β-lactamases and carbapenemases. Ceftazidime-avibactam inhibited >99.9% of all Enterobacteriaceae at the susceptible breakpoint of ≤8 μg/ml and was active against multidrug-resistant (MDR; n = 2,953; MIC50/90, 0.25/1 μg/ml; 99.2% susceptible), extensively drug-resistant (XDR; n = 448; MIC50/90, 0.5/2 μg/ml; 97.8% susceptible), and CRE (n = 513; MIC50/90, 0.5/2 μg/ml; 97.5% susceptible) isolates. Only 82.2% of MDR Enterobacteriaceae (n = 2,953) and 64.2% of ceftriaxone-nonsusceptible Klebsiella pneumoniae (n = 1,063) isolates were meropenem susceptible. Among Enterobacter cloacae (22.2% ceftazidime nonsusceptible), 99.8% of the isolates, including 99.3% of the ceftazidime-nonsusceptible isolates, were ceftazidime-avibactam susceptible. Only 23 of 36,380 Enterobacteriaceae (0.06%) isolates were ceftazidime-avibactam nonsusceptible, including 9 metallo-β-lactamase producers and 2 KPC-producing strains with porin alteration; the remaining 12 strains showed negative results for all β-lactamases tested. Ceftazidime-avibactam showed potent activity against P. aeruginosa (MIC50/90, 2/4 μg/ml; 97.1% susceptible), including MDR (MIC50/90, 4/16 μg/ml; 86.5% susceptible) isolates, and inhibited 71.8% of isolates nonsusceptible to meropenem, piperacillin-tazobactam, and ceftazidime (n = 628). In summary, ceftazidime-avibactam demonstrated potent activity against a large collection (n = 44,248) of contemporary Gram-negative bacilli isolated from U.S. patients, including organisms resistant to most currently available agents, such as CRE and meropenem-nonsusceptible P. aeruginosa.

Corrigendum.

[This corrects the article DOI: 10.1177/2049936117690501.].

In Vitro Activity of Omadacycline and Comparator Compounds Against Gram-positive Isolates Collected in the USA During 2016 as Part of a Global Surveillance Program

BackgroundOmadacycline (OMC) is a broad spectrum aminomethylcycline antibacterial in late stage clinical development (PO and IV formulations) for treatment of acute bacterial skin and skin structure infections (ABSSSI) and community-acquired bacterial pneumonia (CABP). OMC has potent in vitro activity against gram-positive (GP) pathogens expressing common resistance mechanisms to penicillins, tetracyclines, fluoroquinolones and macrolides.MethodsA total of 4,122 GP isolates were collected in 2016 from 30 USA medical centers and included 2,366 staphylococci, 1,252 streptococci and 504 enterococci. A single isolate/patient/infection episode was included. Identifications were confirmed by matrix-assisted laser desorption/ionization mass spectrometry and susceptibility (S) testing was performed using reference broth microdilution methods.ResultsOMC was equally active against methicillin-susceptible (55.1% MSSA) and -resistant (44.9% MRSA) Staphylococcus aureus (SA; MIC50/90, 0.12/0.25 µg/mL). All SA were S to daptomycin (DAP), linezolid (LZD) and vancomycin (VAN). In MRSA, S was lower for levofloxacin (LEV; 28.2%), clindamycin (CLI; 69.9%), and erythromycin (ERY; 10.9%). OMC (MIC50/90, 0.12/0.5 µg/mL) and tigecycline (TGC; MIC50/90, 0.06/12 µg/mL) were the most active agents against coagulase-negative staphylococci (CoNS) and methicillin-R CoNS. S. pneumoniae (including penicillin- [12.8% resistant], ceftriaxone- and ERY-resistant strains), viridans group streptococci (VGS) and β-hemolytic streptococci (including ERY and tetracycline resistant strains) were inhibited by low levels of OMC (MIC50/90 0.06/0.06–0.12 µg/mL) and TGC (MIC50/90 0.03–0.06/0.06–0.12 µg/mL). OMC was highly potent against enterococci (MIC50/90 0.12/0.25 µg/mL) including vancomycin-R isolates. Vancomycin resistance rates were 4.3% and 66.5% in E. faecalis and E. faecium, respectively.ConclusionOMC demonstrated potent activity against susceptible and resistant GP pathogens often associated with ABSSSI and CABP including staphylococci, S. pneumoniae, β-hemolytic streptococci, VGS and enterococci. These data support further omadacycline clinical studies, especially in infections where resistant GP isolates occur.Disclosures M. D. Huband, Paratek Pharma, LLC: Research Contractor, Research grant; M. A. Pfaller, Paratek Pharma, LLC: Research Contractor, Research grant; H. S. Sader, Paratek Pharma, LLC: Research Contractor, Research grant R. K. Flamm, Paratek Pharma, LLC: Research Contractor, Research grant

Ceftobiprole Activity When Tested Against Contemporary Bacteria Causing Bloodstream Infections in the US (2016)

BackgroundCeftobiprole medocaril (prodrug of ceftobiprole) is an advanced cephalosporin, approved for adults in multiple European countries for the treatment of hospital-acquired pneumonia (excluding ventilator-associated pneumonia) or community-acquired pneumonia. It is not approved in the US; however, it has achieved qualified infectious disease product status and two phase 3 studies supported by BARDA are planned to begin in the US in 2017.MethodsA total of 2,787 Gram-positive (GP) and -negative (GN) isolates from bloodstream infections (BSI) from 30 medical centers in the SENTRY Antimicrobial Surveillance Program were evaluated. Isolates were collected in the US during 2016. Susceptibility (S) testing was performed by reference broth microdilution method against ceftobiprole and comparators. Isolates included 693 Staphylococcus aureus (SA), 216 coagulase-negative staphylococci (CoNS), 244 enterococci, 63 Streptococcus pneumoniae (SPN), 74 viridans group streptococci (VGS), 138 β-haemolytic streptococci (BHS), 1,105 Enterobacteriaceae (ENT), 129 Pseudomonas aeruginosa (PSA), 41 Acinetobacter spp. (ASP), 30 Stenotrophomonas maltophila, 19 Haemophilus spp. and 35 miscellaneous bacteria.ResultsMethicillin-resistant S. aureus (MRSA) S rates were lower than for methicillin-susceptible S. aureus (MSSA) for most agents. For levofloxacin (LEV) and erythromycin (ERY), the S rates were LEV: MRSA, 23.2%; MSSA, 86.1%; ERY: MRSA, 9.0%; MSSA, 69.3%. All MSSA and 99.0% of MRSA were S to ceftobiprole, while all MSSA and 96.5% of MRSA were S to ceftaroline (CPT). For CoNS, 98.1% of ceftobiprole MIC values were ≤2mg/L. Ceftobiprole was active against Enterococcus faecalis (96.1% ≤2mg/L) and not against E. faecium (18.9% ≤2mg/L). Against ENT, ceftobiprole (85.0%S) was similar in activity to ceftazidime (CAZ, 87.2%S) and cefepime (FEP, 88.9%S). The MIC50/90 values for ceftobiprole, FEP, and CAZ against PSA were identical at 2/16 mg/L.ConclusionCeftobiprole exhibited potent in vitro activity against GP and GN isolates from contemporary BSI in the US. These results support further clinical evaluation of ceftobiprole for the treatment of BSI.Disclosures R. K. Flamm, Basilea Pharmaceutica International Ltd.: Research Contractor, Research grant; L. R. Duncan, Basilea Pharmaceutica International Ltd.: Research Contractor, Research grant; D. Shortridge, Basilea Pharmaceutica International Ltd.: Research Contractor, Research grant; J. I. Smart, Basilea Pharmaceutica International Ltd.: Employee, Salary; K. Hamed, Basilea Pharmaceutica International Ltd.: Employee, Salary; R. E. Mendes, Basilea Pharmaceutica International Ltd.: Research Contractor, Research grant; H. S. Sader, Basilea Pharmaceutica International Ltd.: Research Contractor, Research grant

Antimicrobial Activity of Ceftazidime-Avibactam Tested against Multidrug-Resistant Enterobacteriaceae and Pseudomonas aeruginosa Isolates from United States (US) Medical Centers (2013–2016)

BackgroundThe in vitro activity of ceftazidime-avibactam (CAZ-AVI) and many comparator agents were tested against various resistant subsets of organisms selected among 36,380 Enterobacteriaceae and 7,868 P. aeruginosa isolates.MethodsIsolates were consecutively collected from 94 US hospitals in 2013–2016 and tested for susceptibility by reference broth microdilution methods in a central monitoring laboratory (JMI Laboratories) as part of the International Network for Optimal Resistance Monitoring (INFORM) program. Enterobacteriaceae strains with elevated CAZ-AVI MIC values (≥16 μg/mL) were evaluated for the presence of genes encoding extended-spectrum β-lactamases, KPC, NDM, and transferable AmpC enzymes.ResultsCAZ-AVI inhibited >99.9% of all Enterobacteriaceae at the susceptible (S) breakpoint of ≤8 μg/mL and was active against multidrug-resistant (MDR; n = 2,953; MIC50/90, 0.25/1 μg/mL; 99.2%S, extensively drug-resistant (XDR; n = 448; MIC50/90, 0.5/2 μg/mL; 97.8%S), and carbapenem-resistant isolates (CRE; n = 513; MIC50/90, 0.5/2 μg/mL; 97.5%S). Only 82.2% of MDR Enterobacteriaceae and 64.2% of ceftriaxone-nonsusceptible (NS) Klebsiella pneumoniae (n = 1,063) were meropenem-S. Among Enterobacter cloacae (n = 3,740; 22.2% ceftazidime-NS), 99.8% of isolates, including 99.3% of ceftazidime-NS isolates, were CAZ-AVI-S. Only 22 of 36,380 Enterobacteriaceae (0.06%) isolates were CAZ-AVI-NS, including 8 MBL-producers (0.02%) and 2 KPC-producing strains with porin alteration; the remaining 12 strains showed negative results for all β-lactamases tested. CAZ-AVI showed potent activity against P. aeruginosa (n = 7,868; MIC50/90, 2/4 μg/mL; 97.1% S), including meropenem-NS (n = 1,471; MIC50/90, 4/16 μg/mL; 87.2%S) and MDR (n = 1,562; MIC50/90, 4/16 μg/mL; 86.5%S) isolates, and inhibited 71.8% of isolates NS to meropenem, piperacillin-tazobactam, and ceftazidime (n = 628).ConclusionCAZ-AVI demonstrated potent activity against a large US collection (n = 44,248) of contemporary gram-negative bacilli, including organisms resistant to most currently available agents, such as CRE and meropenem-NS P. aeruginosa.Disclosures H. S. Sader, Allergan: Research Contractor, Research grant; M. Castanheira, Allergan: Research Contractor, Research grant; D. Shortridge, Allergan: Research Contractor, Research grant; R. E. Mendes, Allergan: Research Contractor, Research grant; R. K. Flamm, Allergan: Research Contractor, Research grant

Cefepime-Zidebactam (WCK 5222) Activity Tested against Gram-negative Organisms Causing Bloodstream Infections Worldwide

BackgroundZidebactam (ZID), a bicyclo-acyl hydrazide, is a β-lactam enhancer with a dual mechanism of action involving selective and high binding affinity to Gram-negative (GN) PBP2 and β-lactamase inhibition. We evaluated the in vitro activity of cefepime (FEP) combined with ZID against GN organisms causing bloodstream infections (BSI) in hospitals worldwide.MethodsA total of 2,094 isolates from 105 medical centers were evaluated. Isolates were collected from Europe (1,050), USA (331), Latin America (LA; 200) and the Asia-Pacific region (AP; 393) in 2015, and China (120) in 2013 by the SENTRY Program. Susceptibility (S) testing was performed by reference broth microdilution method against FEP-ZID (1:1 ratio) and comparators. The collection included 1,809 Enterobacteriaceae (ENT), 170 P. aeruginosa (PSA) and 115 Acinetobacter spp. (ASP).ResultsFEP-ZID was very active against ENT (MIC50/90 of ≤0.03/0.12 μg/mL) with 99.9 and 100.0% of isolates inhibited at ≤4/4 and ≤8/8 μg/mL, respectively, and retained potent activity against carbapenem-resistant (CRE; n = 44; MIC50/90, 1/4 μg/mL), multidrug-resistant (MDR), and extensively drug-resistant (XDR) isolates (Table). Amikacin (AMK; MIC50/90, 2/4 μg/mL; 97.7% S) was also very active against ENT, and colistin (COL; MIC50/90, 0.12/>8 μg/mL) inhibited only 87.3% of isolates at ≤2 μg/mL. FEP-ZID was highly active against PSA, including isolates resistant to other antipseudomonal β-lactams, MDR (MIC50/90, 4/8 μg/mL) and XDR (MIC50/90, 4/8 μg/mL) isolates. Among the comparators, COL (MIC50/90 of ≤0.5/1 μg/mL; 100.0% S) and AMK (MIC50/90, 4/16 μg/mL; 91.2% S) were the most active agents against PSA. FEP-ZID (MIC50/90, 16/32 μg/mL) was 4-fold more active than FEP against ASP.ConclusionFEP-ZID (WCK 5222) exhibited potent in vitro activity against a large worldwide collection of GN isolates from BSI, including MDR and XDR isolates. These results support further clinical development of WCK 5222 for treating BSI.Disclosures H. S. Sader, Wockhardt Bio Ag: Research Contractor, Research grant; M. Castanheira, Wockhardt Bio Ag: Research Contractor, Research grant; J. M. Streit, Wockhardt Bio Ag: Research Contractor, Research grant; L. R. Duncan, Wockhardt Bio Ag: Research Contractor, Research grant; R. K. Flamm, Wock: Research Contractor, Research support

Antimicrobial Activity of Aztreonam-Avibactam and Comparator Agents Tested against Contemporary (2016) Clinical Enterobacteriaceae Isolates

BackgroundAvibactam (AVI) is a non-β-lactam β-lactamase (BL) inhibitor that inhibits Ambler class A, C, and some class D enzymes (eg, ESBL, KPC, and AmpC), and aztreonam (ATM) is a monobactam stable to hydrolysis by metallo-β-lactamases (MBL).MethodsA total of 10,451 Enterobacteriaceae(ENT) consecutively collected from 84 United States (US) medical centers and 250 carbapenem-resistant ENT (CRE) collected from 38 centers in 25 other countries (ex-US) were tested for susceptibility (S) by reference broth microdilution methods in a central monitoring laboratory (JMI Laboratories). CRE strains were screened for the presence of carbapenemase (CBP)-encoding genes using whole genome sequencing analysis.ResultsAll ENT isolates from US (MIC50/90, ≤0.03/0.12 μg/mL), except for 1 Escherichia coli strain with an ATM-AVI MIC of 8 μg/mL, and all ex-US CRE isolates (MIC50/90, 0.25/0.5 μg/mL) were inhibited at ATM-AVI MIC of ≤4 µg/mL (CLSI S breakpoint for ATM). Among US isolates, ATM-AVI was also very active against CRE (n = 120; MIC50/90, 0.12/0.5 μg/mL; highest MIC, 4 μg/mL), multidrug-resistant (MDR; n = 876; MIC50/90, 0.06/0.25 μg/mL), extensively drug-resistant (XDR; n = 111; MIC50/90, 0.12/0.5 μg/mL), pan-drug resistant (n = 2; MICs ≤0.03 and 0.12 μg/mL), and ceftazidime-non-S Enterobacter cloacae (MIC50/90, 0.25/1 μg/mL) isolates. Meropenem was very active against US ENT overall (MIC50/90, 0.03/0.06 μg/mL; 98.8%S per CLSI), but showed limited activity against MDR (86.2%S) and XDR (30.6%S) isolates. Amikacin and colistin were active against 74.2% and 81.7% of US CRE, 93.4% and 58.3% US MDR, 65.8% and 57.7% of US XDR, and 58.0% and 79.2% of ex-US CRE isolates, respectively. A total of 106 CBPs were detected in 106 US CRE isolates, including 102 KPC-like, 2 SME-4, 1 NDM-1, and 1 IMP-27. Also, 248 CBPs were identified on 241 ex-US CRE isolates, including 124 KPC-like, 64 OXA-like, 50 NDM-like, 7 VIM-1, 2 IMP-4, and 1 SME-4. All CRE isolates, including all CBP-producing ENT (US and ex-US), were inhibited at ATM-AVI MIC of ≤4 μg/mL.ConclusionATM-AVI demonstrated potent in vitro activity against a large collection of contemporary (2016) ENT isolated from patients in US hospitals and CRE isolates collected worldwide, including NDM, KPC, OXA, VIM, and SME producers.Disclosures H. S. Sader, Allergan: Research Contractor, Research grant; R. E. Mendes, Allergan: Research Contractor, Research grant; D. Shortridge, Allergan: Research Contractor, Research grant; R. K. Flamm, Allergan: Research Contractor, Research grant; M. Castanheira, Allergan: Research Contractor, Research grant