Figure 2
Activities of anti-TB TB drugs in the DAC system are not affected by inoculum size. (A) The MIC values for the first-line drugs according to various inoculum sizes. MTB H37Rv ATCC 27294 cells from ~104 to ~107 CFU/mL were inoculated in the DAC system and the MIC values were determined. The spots (circle, triangle and square) of each drug indicate the MICs values from three independently repeated experiments. The tested concentrations for each drug were a two-fold dilution scale. The breakpoints of the BACTEC 460 TB and MGIT 960 systems based on the Middlebrook 7H9 broth were adopted for the DAC system; 0.1 μg/mL for INH, 1.0 μg/mL for RIF, 1.0 μg/mL for STR, and 5.0 μg/mL for EMB. The red horizontal line indicates the breakpoints for each drug. All MIC values were determined under the breakpoints. (B) The comparison of an inoculum effect for the first-line drugs against H37Rv between the DAC system and two routine methods, the L-J method (solid) and MGIT 960 method (liquid). The various inoculum sizes from ~104 to ~107 CFU/mL were tested. The DST results were represented as resistant (R) or susceptible (S). The DST results were consistently “S” regardless of the various inoculum sizes in the DAC system, whereas they were changed from “S” to “R” or “Error” over 5 × 106 CFU/mL in the two routine methods. (C) The MICs values from clinical isolates in the various inoculum sizes. The MIC values from two pan-susceptible and two resistant strains were estimated for the first-line drugs. The inoculum sizes were from ~103 CFU/mL to ~108 CFU/mL for two drug susceptible strains and two drug resistant strains. There was no inoculum effect with the clinical isolates in the DAC system.
