FIG 2.

Extended-range and high-resolution quantification of the inoculum effect. (A to C) Bacteria and antibiotics meropenem (A), cefepime (B), and ceftazidime-avibactam (C) were dispensed in orthogonal 2-fold dilution series. Each data point represents the log₂ difference between the MIC at the indicated inoculum and the corresponding MIC at the CLSI target inoculum averaged across all strains within each category indicated. Error bars represent 1 standard deviation from the mean. Dotted lines demarcate the CLSI-recommended inoculum range. Carbapenem-resistant strains showed a pronounced reduction in meropenem MIC at inocula below the CLSI-recommended inoculum. Cefepime-resistant and -susceptible dose-dependent (SDD) strains showed a rapid increase in cefepime MIC at inocula above the CLSI-recommended inoculum. In contrast, a pronounced inoculum effect was not observed for ceftazidime-avibactam. (D) Bacteria and meropenem were dispensed using inkjet technology in $\sqrt[7]{2}$ (≈1.10-fold) and $\sqrt[4]{2}$ (≈1.19-fold) orthogonal dilution series, respectively. Each data point represents the log₂ difference between the MIC determined at the indicated inoculum and the MIC at the CLSI target inoculum averaged across triplicate experiments performed on separate days using 8 CRE strains. Dotted lines demarcate the CLSI-recommended inoculum range.