Establishing Clinical and Laboratory Standards Institute M45 antimicrobial susceptibility testing methods and breakpoints for Pseudomonas other than Pseudomonas aeruginosa

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).