Defining antimicrobial susceptibility testing methods and breakpoints among Achromobacter species

Area of Science:

• Clinical microbiology and infectious diseases
• Antimicrobial resistance mechanisms
• Bacterial genetics and genomics

Background:

• Achromobacter species are opportunistic pathogens causing difficult-to-treat infections due to diverse antimicrobial resistance (AMR) mechanisms.
• Current Clinical and Laboratory Standards Institute (CLSI) guidelines lack specific antimicrobial susceptibility testing (AST) breakpoints for Achromobacter species.
• Existing CLSI M100 criteria for non-Enterobacterales may inaccurately predict susceptibility in Achromobacter.

Purpose of the Study:

• To establish tentative CLSI M45 minimum inhibitory concentration (MIC) and disk diffusion (DD) breakpoints for Achromobacter species.
• To evaluate the performance of the modified carbapenem inactivation method (mCIM) for detecting carbapenemase production.
• To elucidate the AMR mechanisms prevalent in Achromobacter species.

Main Methods:

• Collation of contemporary MIC data to define tentative epidemiological cutoff values (tECVs).
• Disk-to-MIC correlate studies using broth microdilution and disk diffusion on 91 US isolates, analyzed with dBETS software.
• Application of mCIM and whole-genome sequencing (WGS) to characterize resistance genes and mechanisms.

Main Results:

• Tentative ECVs were established: piperacillin-tazobactam (1 µg/mL), imipenem (2 µg/mL), and meropenem (0.5 µg/mL).
• Disk diffusion breakpoints met CLSI M23 acceptance criteria with minor exceptions.
• WGS identified blaOXA variants in 90.1% of isolates (predominantly blaOXA-114) and acquired beta-lactamase genes in 19 isolates (including blaAXC, blaVIM-4, blaAZM-1). mCIM showed 100% sensitivity and 87% specificity.

Conclusions:

• Tentative MIC and disk diffusion breakpoints for key agents were established by the CLSI M45 committee.
• The mCIM is a sensitive method for detecting carbapenemase activity in Achromobacter.
• WGS provides valuable insights into the genetic basis of AMR in Achromobacter, guiding the development of specific AST guidelines.