The Clinical and Laboratory Standards Institute (CLSI) Subcommittee on Antimicrobial Susceptibility Testing (AST) develops and publishes standards and guidelines for AST methods and results interpretation in an annual update to the Performance Standards for Antimicrobial Susceptibility Testing (M100). This minireview will discuss changes to M100 for the 31st edition, including new and revised breakpoints and testing recommendations.

The Clinical and Laboratory Standards Institute Subcommittee on Antimicrobial Susceptibility Testing (AST) publishes annual updates to the M100 Performance Standards for Antimicrobial Susceptibility Testing . This important document contains key information critical to the laboratory’s performance of accurate and current AST. This minireview will highlight and provide supporting data for major changes in the M100 32nd and 33rd editions published in February 2022 and March 2023, respectively. New and revised breakpoints for gram-negative and gram-positive organisms are explained. Proper use of the restructured M100 Table 1 guidance for antimicrobial agents to be considered for testing and reporting is outlined. Reporting guidance for multidrug-resistant gram-negative bacilli is discussed. Other topics within this minireview include additional agents to test and report for direct blood disk diffusion of gram-negative bacilli and quality control range changes and troubleshooting. The minireview concludes with other issues under consideration by the AST Subcommittee.

Cefiderocol (CFDC) is a siderophore cephalosporin with activity against Gram-negative bacterial species that are resistant to carbapenems and other drugs. The MICs of CFDC were determined for 610 Gram-negative bacilli, including 302 multinational Enterobacterales isolates with characterized mechanisms of beta-lactam resistance, 180 clinical isolates from the Mayo Clinic and Mayo Clinic Laboratories not characterized for specific resistance mechanisms, and 128 isolates with CFDC MICs of ≥8 μg/ml obtained from International Health Management Associates, Inc.

Staphylococcus argenteus is a newly described species, formerly known as S. aureus clonal complex 75 (CC75). Here, we describe the largest collection of S. argenteus isolates in North America, highlighting identification challenges.