Comparison of Etest to Broth Microdilution for Testing of Susceptibility of Pseudomonas aeruginosa to Ceftolozane-Tazobactam

Evaluation of three commercial methods of susceptibility testing for ceftolozane/tazobactam against carbapenem-resistant Pseudomonas aeruginosa

INTRODUCTION

Ceftolozane/tazobactam has shown excellent activity against Pseudomonas aeruginosa, but this drug is not always included in commercial panels. The aim of the study was to evaluate the performance of 2 gradient strips (BioMérieux and Liofilchem) and a commercial microdilution panel (Sensititre, EURGNCOL panel) using this combination against carbapenem-resistant P. aeruginosa isolates.

METHODS

Three commercial methods were tested with 41 metallo-beta-lactamase-producing and 59 non-carbapenemase-producing P. aeruginosa isolates. Broth microdilution was used as reference.

RESULTS

All carbapenemase-producing isolates and only one non-producing isolate were resistant to this antibiotic. Both essential agreement and bias were outside the acceptance intervals since MIC values were higher than reference values for all three methods. The Kappa index indicated poor or weak agreement. Changes in clinical categories were observed in 3 isolates.

CONCLUSIONS

The three methods yielded poor agreement with the reference. Despite the differences in MIC values, fewer than 3% involved category changes.

Evaluation of in vitro activity of ceftolozane-tazobactam in combination with other classes of antibacterial agents against Enterobacterales and Pseudomonas aeruginosa-the EM200 study

Ceftolozane-tazobactam is a cephalosporin/β-lactamase inhibitor combination developed for use against some β-lactam- and multidrug-resistant Gram-negative organisms. This study aimed to evaluate the in vitro activity of ceftolozane-tazobactam against clinical bacterial isolates at the University Hospital of Marrakech. This is a descriptive and analytical prospective study. A total of 143 Enterobacterales and 48 Pseudomonas aeruginosa isolates were collected from January 2018 to December 2018 from patients with respiratory, urinary and intra-abdominal infections. The identification was made by Phoenix automated system (BioMérieux). MIC50/90 were tested by broth microdilution for ceftolozane-tazobactam, and other drugs using dried panels. Antimicrobial susceptibility results were interpreted according to CLSI guidelines. Ceftolozane-tazobactam inhibited 98% of Escherichia coli (MIC_50/90; 0.25/0.5 μg/mL). The susceptibility rate of Klebsiella pneumoniae to ceftolozane-tazobactam was 68.8% (MIC_50/90, 0.5/>32 μg/mL); other Enterobacterales have shown susceptibility rates of 80.4% (MIC_50/90; 0.5/8 μg/mL). In carbapenemase-producing K. pneumoniae, the bla _OXA-48 mutation was found in two isolates. Susceptibility of P. aeruginosa to ceftolozane-tazobactam was 91.7% (MIC_50/90, 0.5/>32 μg/mL). In non-carbapenemase-producing P. aeruginosa, AmpC mutations were found in all isolates. Ceftolozane-tazobactam was satisfactorily active against a wide range of tested isolates and offers clinicians a potential therapeutic option even against resistant strains in patients with intra-abdominal infections, urinary tract infections and nosocomial pneumonia.

Activity of ceftolozane/tazobactam against a collection of Pseudomonas aeruginosa isolates from bloodstream infections in Australia

Pseudomonas aeruginosa is a common pathogen causing nosocomial infection. In particular, bloodstream infection (BSI) is associated with a high rate of morbidity and mortality. Ceftolozane/tazobactam is a new β-lactam/β-lactamase antimicrobial with activity against P. aeruginosa as well as multidrug resistant (MDR) Gram negative Enterobacteriaceae. Ceftolozane/tazobactam has frequently been used in salvage therapy for MDR P. aeruginosa infections. The aim of this study was to determine the activity of ceftolozane/tazobactam against P. aeruginosa isolates from BSIs collected from three clinical microbiology laboratories in Queensland, Australia, with a high proportion of isolates demonstrating β-lactam resistance. Antimicrobial susceptibility testing was performed by broth microdilution using custom made sensititre plates sourced from ThermoFisher Scientific. In addition to ceftolozane/tazobactam, we also tested piperacillin/tazobactam, ceftazidime, cefepime, meropenem, doripenem, imipenem, aztreonam, ciprofloxacin, levofloxacin, gentamicin, amikacin, tobramycin and colistin. Overall, ceftolozane/tazobactam was the most active agent tested [(MIC_50/90 = 1/2 μg/mL, 96% susceptible (S)]. Against 44 isolates with resistance to at least one other β-lactam agent, 40 were susceptible to ceftolozane/tazobactam. Three ceftolozane/tazobactam resistant isolates were susceptible to colistin, with one of those isolates also susceptible to levofloxacin but not to any other antimicrobials tested. One ceftolozane/tazobactam resistant isolate was susceptible only to meropenem and doripenem but was non-susceptible to imipenem. An association was found between fluoroquinolone resistance and aminoglycoside resistance but not with β-lactam resistance. In summary, ceftolozane/tazobactam was active against most strains tested, including those resistant to other β-lactams. Laboratories should consider testing P. aeruginosa against ceftolozane/tazobactam in suspected MDR or extensively drug resistant (XDR) infections.

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.

The Continued Value of Disk Diffusion for Assessing Antimicrobial Susceptibility in Clinical Laboratories: Report from the Clinical and Laboratory Standards Institute Methods Development and Standardization Working Group

Expedited pathways to antimicrobial agent approval by the U.S. Food and Drug Administration (FDA) have led to increased delays between drug approval and the availability of FDA-cleared antimicrobial susceptibility testing (AST) devices. Antimicrobial disks for use with disk diffusion testing are among the first AST devices available to clinical laboratories. However, many laboratories are reluctant to implement disk diffusion testing for a variety of reasons, including dwindling proficiency with this method, interruptions of the laboratory workflow, uncertainty surrounding the quality and reliability of disk diffusion tests, and a perceived need to report MIC values to clinicians. This minireview provides a report from the Clinical and Laboratory Standards Institute Methods Development and Standardization Working Group on the current standards and clinical utility of disk diffusion testing.

Susceptibility of MDR Pseudomonas aeruginosa to ceftolozane/tazobactam and comparison of different susceptibility testing methods

Background

Infections caused by MDR Pseudomonas aeruginosa are on the rise, particularly in critically ill patients. Therefore, there is a need to evaluate new antimicrobial regimens. The objectives of this study were to investigate the ceftolozane/tazobactam resistance rates of MDR and XDR P. aeruginosa, the underlying resistance genes, the clonal structure and different antimicrobial susceptibility testing (AST) methods regarding their accuracy for ceftolozane/tazobactam testing.

Methods

In total, 112 MDR and XDR P. aeruginosa (from infection and colonization) from one German tertiary care hospital were included (2013-16). AST was done using broth microdilution (BMD), gradient diffusion test strips and disc diffusion. Resistance genes were screened by PCR. A randomly selected subset of 77 isolates was subjected to WGS to assess the clonal structure.

Results

In total, 38 isolates (33.9%) were resistant to ceftolozane/tazobactam according to the BMD reference method. Resistance was significantly lower in MDR P. aeruginosa (4.8%) compared with XDR P. aeruginosa (50%, P < 0.0001). The underlying mechanism in carbapenemase-positive ceftolozane/tazobactam-resistant isolates (n = 38) was blaIMP (n = 25), blaVIM (n = 4) and blaGES (n = 1). The resistance mechanism of the remaining eight ceftolozane/tazobactam-resistant isolates remained unclear. Although our strain collection was diverse, resistance to ceftolozane/tazobactam was almost exclusively associated with MLST ST235. The disc diffusion method was accurate for ceftolozane/tazobactam AST (no false-susceptible results, categorical agreement = 92.9%).

Conclusions

Ceftolozane/tazobactam resistance was low in MDR P. aeruginosa, but higher in XDR P. aeruginosa. The disc diffusion method showed an acceptable accuracy for ceftolozane/tazobactam AST.

Performance of Ceftolozane-Tazobactam Etest, MIC Test Strips, and Disk Diffusion Compared to Reference Broth Microdilution for β-Lactam-Resistant Pseudomonas aeruginosa Isolates

The performance characteristics of the ceftolozane-tazobactam (C-T) Etest (bioMérieux, Marcy l'Etoile, France), MIC test strips (MTS; Liofilchem, Italy), and disk diffusion (Hardy, Santa Ana, CA) were evaluated for a collection of 308 beta-lactam-resistant isolates of Pseudomonas aeruginosa recovered from three institutions in Los Angeles, CA. Reference testing was performed by the reference broth microdilution (rBMD) method. MIC and disk results were interpreted using Clinical and Laboratory Standards Institute breakpoints. Overall, 72.5% of the isolates were susceptible to C-T by rBMD. Etest and disk diffusion demonstrated acceptable performance, whereas MTS yielded a greater than acceptable percentage of minor errors. Categorical agreement was 96.8% for Etest, 87.0% for MTS, and 92.9% for disk diffusion. No very major errors were observed by any test, and no major errors (ME) were observed by Etest or disk diffusion. Two ME (0.9% of susceptible isolates) were observed by MTS. The incidence of minor errors was 3.2%, 12.3%, and 7.1% for Etest, MTS, and disk diffusion, respectively. Essential agreement (EA) for Etest was excellent, at 97.7%, whereas the MICs obtained by MTS tended to be 1 to 2 dilutions higher than those obtained by rBMD, with an EA of 87.0%.

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.

Development of MAST: A Microscopy-Based Antimicrobial Susceptibility Testing Platform

Antibiotic resistance is compromising our ability to treat bacterial infections. Clinical microbiology laboratories guide appropriate treatment through antimicrobial susceptibility testing (AST) of patient bacterial isolates. However, increasingly, pathogens are developing resistance to a broad range of antimicrobials, requiring AST of alternative agents for which no commercially available testing methods are available. Therefore, there exists a significant AST testing gap in which current methodologies cannot adequately address the need for rapid results in the face of unpredictable susceptibility profiles. To address this gap, we developed a multicomponent, microscopy-based AST (MAST) platform capable of AST determinations after only a 2 h incubation. MAST consists of a solid-phase microwell growth surface in a 384-well plate format, inkjet printing-based application of both antimicrobials and bacteria at any desired concentrations, automated microscopic imaging of bacterial replication, and a deep learning approach for automated image classification and determination of antimicrobial minimal inhibitory concentrations (MICs). In evaluating a susceptible strain set, 95.8% were within ±1 and 99.4% were within ±2, twofold dilutions, respectively, of reference broth microdilution MIC values. Most (98.3%) of the results were in categorical agreement. We conclude that MAST offers promise for rapid, accurate, and flexible AST to help address the antimicrobial testing gap.