Figure 2. . Analyses of the amplification of multidrug-resistant Mycobacterium tuberculosis during treatment of a drug-sensitive (S) strain in a mixed infection (i.e., infection with drug-resistant [R] and S strains). In the presence of isoniazid (INH), the faster growing S strain lost its competitive advantage, and the R strain became more prevalent. A–C) Data are means of 3 independent replicates with SE bars. A) Single strain growth analyses of S (black circles) and R (gray triangles) M. tuberculosis strains. Growth was measured by optical density at 600 nm (OD600). *p<0.05. B) Competitive growth analyses of mixed strains alone (black squares) and with 0.2 μg/mL INH (gray triangles). Growth, in triplicate, in 7H9 broth plus OADC (oleic acid, albumin, dextrose, and catalase growth supplement), glycerol, and Tween 80 was measured by optical density at OD600. Statistical analyses were performed on triplicate samples by using 2-way analysis of variance. *p<0.05. C) Identification of the predominant strain in mixed cultures with and without 0.2 μg/mL INH (INH/plate+ and INH/plate−, respectively). Strains were identified on day 7 by plating a 10-fold dilution series of co-cultures onto 7H10 agar, plus OADC and glycerol, with or without 0.2 μg/mL INH (0/plate+ and 0/plate−, respectively) and incubating for 2 weeks at 37°C. Statistical analyses were conducted on triplicate samples by using a 2-tailed t-test. *p<0.02. CFU, colony-forming units.