Harris, Harley T.

ABSTRACT Metallo-beta-lactamase (MBL)-producing Enterobacterales are a global public health concern. Infections caused by these pathogens are often hard to treat, with few effective agents currently available. While aztreonam-avibactam only recently gained U.S. and European regulatory approval, laboratories have implemented laboratory-developed tests (LDTs) using existing aztreonam and ceftazidime-avibactam reagents. We sought to compare the performance of four phenotypic methods, up to three reagent manufacturers, and two media manufacturers. One hundred two MBL-producing Enterobacterales isolates were evaluated at two sites (Centers for Disease Control and Prevention [CDC] and Johns Hopkins University [JHU] School of Medicine). The performance of four methods was evaluated: broth disk elution (BDE), disk stacking (DS), gradient diffusion strip stacking (SS), and a strip-to-disk proximity (strip-disk) method, which were compared to reference broth microdilution (rBMD) tested in parallel. Respective category agreement for CDC and JHU was 92.2% and 96.1% for BDE, 51%–76.5% and 31%–49% for DS, 71.6%–86.3% and 83.3%–86.3% for SS, and 82.4%–87.3% and 80%–89.2% for the strip-disk method. The SS essential agreement ranged from 46.1% to 62.7% for JHU and 54.9% to 92.2% for CDC. Interlaboratory reproducibility was 93.1% for rBMD, 92.2% for BDE, 47.1%–53.1% for DS, 76.5%–80% for SS, and 87.3%–88.9% for strip-disk. BDE was the only method that exceeded acceptable performance of >90% accuracy and reproducibility. DS had the lowest and most variable performance between sites and among the different disk manufacturers used. LDTs for combination testing can be highly variable, and further work is needed to improve standardization and harmonization to achieve accurate and reliable results for clinical purposes. IMPORTANCE Carbapenem-resistant Enterobacterales are classified as an urgent public health threat in the Centers for Disease Control and Prevention’s 2019 Antibiotic Resistance Threats Report. Among the most concerning resistance mechanisms are carbapenemase enzymes encoded on mobile genetic elements, which can spread rapidly between bacteria. Of the various carbapenemases, metallo-beta-lactamases are particularly challenging, as they confer resistance to nearly all beta-lactams, and current treatment options remain limited. While aztreonam-avibactam has been U.S. Food and Drug Administration (FDA)-approved for treating certain infections with limited or no alternative treatment options, no FDA-cleared susceptibility test is yet available. In this context, laboratory-developed methods, such as combination testing of aztreonam and ceftazidime-avibactam, play a critical role in guiding therapy. Our study evaluates several of the more common laboratory-developed tests compared to reference broth microdilution and found that broth disk elution was the most accurate; in contrast, other methods had variable performance, some demonstrating limited reliability and being prone to overcalling resistance.

ABSTRACT The purpose of this study was to establish tentative Clinical and Laboratory Standards Institute (CLSI) M45 MIC and disk diffusion (DD) breakpoints (BPs) for Pseudomonas other than Pseudomonas aeruginosa (POPA). Mechanisms of antimicrobial resistance (AMR) and the modified carbapenem inactivation method (mCIM) to detect carbapenemase production were also evaluated. MIC data from P. aeruginosa and POPA from 2013 to 2022 were evaluated to compare the MIC distributions and modal MICs relative to the CLSI M100 P. aeruginosa BPs. A disk-to-MIC correlation study with 83 isolates was completed by testing reference broth microdilution and DD from the same inoculum, and the error-rate bounded method was used to establish DD BPs. For most antimicrobials, the modal MICs between P. aeruginosa and POPA were within 1-doubling dilution and lower than the M100 P. aeruginosa- susceptible BP. For amikacin, the modal MIC for POPA was 2-doubling dilutions lower than P. aeruginosa and was evaluated relative to the Enterobacterales BP. For aztreonam and trimethoprim-sulfamethoxazole, the modal MICs were elevated, and no BPs were set. New DD correlates were established for most antimicrobial agents, except for fluoroquinolones, where the P. aeruginosa correlates were suitable. AMR genes conferring resistance to multiple antimicrobial classes were identified by WGS. Beta-lactamase genes were identified in 30 (36.1%) isolates, with metallo-beta-lactamases (90.6%) predominating. The mCIM had a sensitivity and specificity of 100%. Upon review, the CLSI M45 committee proposed tentative MIC and DD BPs for expanded-spectrum cephalosporins (ceftazidime and cefepime), carbapenems (meropenem and imipenem), fluoroquinolones (ciprofloxacin and levofloxacin), and the aminoglycosides (amikacin and tobramycin). IMPORTANCE Pseudomonas species other than Pseudomonas aeruginosa (POPA) can cause opportunistic infections which may be difficult to treat due to a variety of antimicrobial resistance mechanisms. Antimicrobial susceptibility testing is a critical component of patient management for these infections. Currently, the Clinical and Laboratory Standards Institute (CLSI) M100 non-Enterobacterales breakpoints and methodology are utilized for POPA by US clinical laboratories and likely do not accurately predict susceptibility results. The purpose of this study was to establish tentative CLSI M45 MIC and disk diffusion breakpoints for POPA. Mechanisms of antimicrobial resistance and the modified carbapenem inactivation method to detect carbapenemase production were also evaluated. We present the data used by the volunteers tasked by CLSI to develop POPA breakpoints in the M45 guidelines.

ABSTRACT Achromobacter species can cause opportunistic infections which are difficult to treat due to a variety of antimicrobial resistance (AMR) mechanisms. As such, antimicrobial susceptibility testing (AST) is a cornerstone to successful treatment for these organisms. Currently, there are no specific Clinical and Laboratory Standards Institute (CLSI) AST guidelines for Achromobacter species. The purpose of this study was to (i) establish tentative CLSI M45 MIC and disk diffusion (DD) breakpoints (BPs), (ii) evaluate the modified carbapenem inactivation method (mCIM), and (iii) understand the mechanisms mediating AMR among Achromobacter species. Contemporary MIC data from multiple sources were collated to define the tentative epidemiological cutoff values (tECVs), and the MIC distributions were used to inform the establishment of MIC BPs for various agents. A disk-to-MIC correlate study with 91 isolates from across the United States was completed by testing reference broth microdilution and DD from the same inoculum and applying the dBETS software to establish DD BPs. Lastly, the mCIM and whole-genome sequencing (WGS) were pursued. The tECVs for piperacillin-tazobactam, imipenem, and meropenem were 1, 2, and 0.5 µg/mL, respectively. Disk correlates met CLSI M23 acceptance criteria with a few exceptions related to a small number of isolates, resulting in high minor errors. WGS revealed that 82 (90.1%) isolates harbored a bla OXA variant with bla OXA-114 predominating (90.2%). Nineteen isolates harbored acquired beta-lactamase genes, including 16 bla AXC , 2 bla VIM-4 , and 1 bla AZM-1. The mCIM had a sensitivity of 100% and specificity of 87%. Upon review, the CLSI M45 committee set tentative MIC and DD BPs for piperacillin-tazobactam, imipenem, meropenem, and trimethoprim-sulfamethoxazole. IMPORTANCE Achromobacter species can cause opportunistic infections which may be difficult to treat due to a variety of antimicrobial resistance mechanisms. Antimicrobial susceptibility testing is a critical component of patient treatment for these infections. Currently, the Clinical and Laboratory Standards Institute M100 non-Enterobacterales susceptibility test interpretive criteria and methodology are utilized for Achromobacter by clinical laboratories and likely do not accurately predict susceptibility results for Achromobacter species. This study was designed to establish tentative MIC and disk diffusion breakpoints specific to Achromobacter species.