Multicenter Evaluation of the Etest Gradient Diffusion Method for Ceftolozane-Tazobactam Susceptibility Testing of Enterobacteriaceae and Pseudomonas aeruginosa.

Ceftolozane-tazobactam (C/T) is a novel beta-lactam-beta-lactamase inhibitor combination antibiotic approved by the U.S. Food and Drug Administration in 2014 for the treatment of complicated intra-abdominal infections (in combination with metronidazole) and complicated urinary tract infections. In this study, we evaluated the performance of the C/T Etest, a gradient diffusion method. C/T Etest was compared to broth microdilution (BMD) for 51 Enterobacteriaceae challenge isolates and 39 Pseudomonas aeruginosa challenge isolates at three clinical sites. Essential agreement (EA) between the methods ranged from 47 to 49/51 (92.2 to 96.1%) for the Enterobacteriaceae, and categorical agreement (CA) ranged from 49 to 51/51 (96.1 to 100.0%). EA and CA for P. aeruginosa were 100% at all sites. The C/T Etest was also compared to BMD for susceptibility testing on 966 clinical isolates (793 Enterobacteriaceae, including 167 Klebsiella pneumoniae and 159 Escherichia coli isolates, in addition to 173 P. aeruginosa isolates) collected at four clinical sites. EA between Etest and BMD was 96.9% for Enterobacteriaceae isolates and 98.8% for P. aeruginosa isolates. Within the Enterobacteriaceae, isolates from each species examined had >96% CA. For the clinical isolates, no very major errors were identified but two major errors were found (one for K. pneumoniae and one for Providencia rettgeri). By BMD, 47.0% of Enterobacteriaceae and 46.2% of P. aeruginosa challenge strains were nonsusceptible to C/T by CLSI breakpoint criteria; 8.2% of clinical Enterobacteriaceae isolates and 12.1% of clinical P. aeruginosa isolates were nonsusceptible to C/T by CLSI breakpoint criteria. In conclusion, Etest is accurate and reproducible for C/T susceptibility testing of Enterobacteriaceae and P. aeruginosa.

Bailey AL ，Armstrong T ，Dwivedi HP ，Denys GA ，Hindler J ，Campeau S ，Traczewski M ，Humphries R ，Burnham CA ... -  《-》

Multicenter Evaluation of the New Etest Gradient Diffusion Method for Piperacillin-Tazobactam Susceptibility Testing of Enterobacterales, Pseudomonas aeruginosa, and Acinetobacter baumannii Complex.

Piperacillin-tazobactam (P/T) is a β-lactam-β-lactamase inhibitor combination frequently used in the hospital setting. Etest is a gradient diffusion method that represents an alternative to broth microdilution (BMD) for performing antimicrobial susceptibility testing. We conducted a multicenter evaluation of the performance of the new P/T Etest compared to that of BMD following U.S. Food and Drug Administration (FDA) and International Standards Organization (ISO) standard ISO 20776-2 criteria using Clinical and Laboratory Standards Institute (CLSI)-FDA and European Committee on Antimicrobial Susceptibility Testing (EUCAST) interpretive breakpoints, respectively. A total of 977 isolates (775 Enterobacterales isolates, 119 Pseudomonas aeruginosa isolates, and 83 Acinetobacter baumannii complex isolates) were tested. Overall essential agreement (EA) was 96.4% and 96.6% for Enterobacterales when FDA and ISO 20776-2 criteria, respectively, were followed. EA was 98.3% for P. aeruginosa and 91.6% for the A. baumannii complex when both the FDA and ISO criteria were followed. Applying CLSI-FDA breakpoints, categorical agreement (CA) reached 93.0%, 93.3%, and 89.2% for the Enterobacterales, P. aeruginosa, and the A. baumannii complex, respectively. Two very major errors (VMEs; 1.1%) were found among the Enterobacterales (for 2 Klebsiella pneumoniae isolates). No additional major errors (MEs) or VMEs were found. Applying EUCAST breakpoints, CA was 94.8% and 95.8% for Enterobacterales and P. aeruginosa, respectively (no breakpoints are currently available for the A. baumannii complex). No VMEs were observed among the Enterobacterales, but 2 (0.4%) MEs were found. Among the P. aeruginosa isolates, 2 (6.9%) VMEs and 3 (3.3%) MEs were observed. These errors resulted when P/T Etest MICs were 1 doubling dilution apart from the BMD MICs. In conclusion, the new P/T Etest represents an accurate tool for performing antimicrobial susceptibility testing of Enterobacterales, P. aeruginosa, and A. baumannii complex isolates with limited category errors.

García-Fernández S ，Bala Y ，Armstrong T ，García-Castillo M ，Burnham CA ，Wallace MA ，Hardy D ，Zambardi G ，Cantón R ... -  《-》

Verification of Ceftazidime-Avibactam and Ceftolozane-Tazobactam Susceptibility Testing Methods against Carbapenem-Resistant Enterobacteriaceae and Pseudomonas aeruginosa.

Ceftazidime-avibactam and ceftolozane-tazobactam are newly approved agents for the treatment of infections caused by multidrug-resistant Gram-negative bacteria. Resistance to both agents has been described clinically. Susceptibility testing on automated systems is unavailable for either agent. Our objective was to compare the disk diffusion and Etest methods to standard broth microdilution (BMD) methods for testing ceftazidime-avibactam and ceftolozane-tazobactam against a diverse collection of carbapenem-resistant Enterobacteriaceae (CRE) and carbapenem-resistant Pseudomonas aeruginosa (CRP) isolates, respectively. Among 74 ceftazidime-avibactam-susceptible and -resistant CRE isolates, BMD categorical agreement was higher with Etest (96%) than with disk diffusion (72%; P = 0.0003). Twenty-eight percent of ceftazidime-avibactam-susceptible CRE isolates were classified as resistant by disk diffusion. Results were comparable to those obtained with resistance defined genotypically. Among 72 ceftolozane-tazobactam-susceptible and -resistant CRP isolates, the levels of BMD categorical agreement with disk diffusion and Etest were 94% and 96%, respectively; the only errors identified were minor. Our findings demonstrate that Etest measurements of ceftazidime-avibactam and ceftolozane-tazobactam susceptibility correlate closely with standard BMD methods, suggesting a useful role clinically. On the other hand, disk diffusion measurements overcalled CRE resistance to ceftazidime-avibactam. A better understanding of ceftazidime-avibactam interpretive breakpoints is needed before disk diffusion is used routinely in the clinic. Until clinicians and microbiologists understand Etest and disk diffusion performance at their centers, test results should be interpreted cautiously.

Shields RK ，Clancy CJ ，Pasculle AW ，Press EG ，Haidar G ，Hao B ，Chen L ，Kreiswirth BN ，Nguyen MH ... -  《-》

Ceftolozane-tazobactam activity against drug-resistant Enterobacteriaceae and Pseudomonas aeruginosa causing healthcare-associated infections in Australia and New Zealand: Report from an Antimicrobial Surveillance Program (2013-2015).

To evaluate the in vitro activity of ceftolozane-tazobactam and comparator agents tested against isolates of Enterobacteriaceae and Pseudomonas aeruginosa from patients in Australia and New Zealand with healthcare-associated infection. A total of 1459 gram-negative organisms (440 P. aeruginosa and 1019 Enterobacteriaceae) were consecutively collected from 11 medical centers located in Australia and New Zealand. The organisms were tested for susceptibility by broth microdilution methods as described by the CLSI M07-A10 document and the results interpreted according to EUCAST and CLSI breakpoint criteria. Ceftolozane-tazobactam (MIC50/90, 0.25/0.5μg/mL; 97.7/95.9% susceptible [CLSI/EUCAST]), meropenem (MIC50/90, ≤0.06/≤0.06μg/mL; 99.8/99.9% susceptible [CLSI/EUCAST]) and amikacin (MIC50/90, 2/4μg/mL; 99.8/99.6% susceptible [CLSI/EUCAST]) were the most active compounds tested against Enterobacteriaceae. Enterobacteriaceae isolates displayed susceptibility rates to other β-lactam agents ranging from 95.3/94.4% for cefepime, 94.1/91.4% for piperacillin-tazobactam, and 93.3/91.5% for ceftazidime using CLSI/EUCAST breakpoints. Among the Enterobacteriaceae isolates tested, 0.1% were CRE and 6.6% exhibited an ESBL non-CRE phenotype. Whereas ceftolozane-tazobactam showed good activity against ESBL non-CRE phenotype strains of Enterobacteriaceae (MIC50/90, 0.5/2μg/mL), it lacked useful activity (MIC, >32μg/mL) against the single isolate with a CRE resistant phenotype. Ceftolozane-tazobactam was the most potent (MIC50/90, 0.5/2μg/mL) β-lactam agent tested against P. aeruginosa isolates, inhibiting 95.7% at an MIC of ≤4μg/mL. Ceftolozane-tazobactam was the most active β-lactam agent tested against P. aeruginosa and demonstrated higher in vitro activity than currently available cephalosporins and piperacillin-tazobactam when tested against Enterobacteriaceae.

Pfaller MA ，Shortridge D ，Sader HS ，Flamm RK ，Castanheira M ... -  《-》

Ceftolozane/tazobactam activity against drug-resistant Enterobacteriaceae and Pseudomonas aeruginosa causing healthcare-associated infections in the Asia-Pacific region (minus China, Australia and New Zealand): report from an Antimicrobial Surveillance Pr

The aim of this study was to evaluate the in vitro activity of ceftolozane/tazobactam and comparator agents tested against Enterobacteriaceae and Pseudomonas aeruginosa isolates from patients in the Asia-Pacific (APAC) region with healthcare-associated infections. Ceftolozane/tazobactam is an antipseudomonal cephalosporin combined with a well-established β-lactamase inhibitor. A total of 1963 Gram-negative organisms (489 P. aeruginosa and 1474 Enterobacteriaceae) were consecutively collected using a prevalence-based approach from 14 medical centres in the APAC region. Antimicrobial susceptibility testing was performed by broth microdilution method as described by the CLSI and the results were interpreted according to EUCAST and CLSI breakpoint criteria. Ceftolozane/tazobactam [MIC50/90, 0.25/4 µg/mL; 89.2/85.8% susceptible (CLSI/EUCAST)] and meropenem [MIC50/90, ≤0.06/≤0.06 µg/mL; 96.3/96.5% susceptible (CLSI/EUCAST)] were the most active compounds tested against Enterobacteriaceae. Isolates displayed susceptibility rates to other β-lactam agents ranging from 85.8/81.0% for piperacillin/tazobactam to 74.4/72.7% for cefepime and 72.8/68.1% for ceftazidime using CLSI/EUCAST breakpoints. Among the Enterobacteriaceae isolates, 3.6% were carbapenem-resistant Enterobacteriaceae (CRE) and 25.6% exhibited an extended-spectrum β-lactamase (ESBL) non-CRE phenotype. Ceftolozane/tazobactam showed good activity against ESBL non-CRE phenotype strains of Enterobacteriaceae (MIC50/90, 0.5/16 µg/mL), but not against isolates with a CRE phenotype (MIC50/90, >32/>32 µg/mL). Ceftolozane/tazobactam was the most potent (MIC50/90, 0.5/4 µg/mL) β-lactam agent tested against P. aeruginosa isolates, inhibiting 90.8% at an MIC of ≤4 µg/mL. Pseudomonas aeruginosa exhibited high rates of susceptibility to amikacin [91.2/89.4% (CLSI/EUCAST)] and colistin [98.4/100.0% (CLSI/EUCAST)]. Ceftolozane/tazobactam was the most active β-lactam agent tested against P. aeruginosa and demonstrated higher in vitro activity than available cephalosporins when tested against Enterobacteriaceae.

Pfaller MA ，Shortridge D ，Sader HS ，Castanheira M ，Flamm RK ... -  《-》