Antimicrobial susceptibility testing of anaerobes − Challenges and potential solutions for identifying breakpoints using the disk-diffusion method, based on re-analysis of a large panel of anaerobes

A popular disk diffusion method was recently revisited by EUCAST (The European Committee on Antimicrobial Susceptibility Testing) and CA-SFM (Antibiogram committee of the French society of Microbiology), both of which encourage more laboratories to determine the susceptibility of anaerobes to antibiotics. Alternatives (gradient method or microdilution in liquid medium) to the reference method (dilution in agar) are proposed, but they are not without shortcomings. Despite significant progress, disk diffusion techniques remain a subject of debate. Our aims are to highlight the controversies concerning the choice of breakpoints as well as the major error and very major error rates observed using disk-diffusion methods. Based on the MIC distributions of five antibiotics obtained by the same reference method in Brucella agar, using a wide range of anaerobic species (which have been the subject of 12 publications), we re-interpreted the previous results according to the categorization criteria of the CLSI, CA-SFM (often close to each other), and also to the more recent EUCAST criteria. This enabled us to compare resistance rates based on the breakpoints proposed by these three institutions. When discrepancies in results appeared, modifications of some breakpoints were suggested.