>We were unable to obtain mutants of S. aureus or M. tuberculosis resistant to teixobactin even when plating on media with a low dose (4 X MIC [minimal inhibitory concentration]) of the compound. Serial passage of S.aureus in the presence of sub-MIC levels of teixobactin over a period of 27 days failed to produce resistant mutants as well (Fig. 2d, Supplementary Discussion)
From the Supplementary information:
>Cells were added to teixobactin present at 0.25xMIC, 0.5xMIC,1xMIC,2xMIC and 224xMIC. At 24 hour intervals, the cultures were checked for growth. Cultures from the second highest concentrations that allowed growth (OD600≥2) were diluted 1:100 into fresh media containing 0.25xMIC, 0.5xMIC,1xMIC,2xMIC and 4xMIC of teixobactin. This serial passaging was repeated daily for 30 days. Any cultures that grew at higher than the MIC levels were passaged on drug free MHA plates and the MIC was then determined by broth microdilution. No resistant mutants were obtained. This experiment was repeated, and produced the same negative result. In order to maximize the chance of obtaining a resistant mutant, we performed an additional experiment with very small incremental increases in the drug concentration. Cells were added to a series of tubes with small differences in the concentration of teixobactin (0.06xMIC, 0.25xMIC, 0.5xMIC, 0.75xMIC,1xMIC, 1.25xMIC, 1.5xMIC, and 2xMIC). At 24 hour intervals, cultures from the highest concentration that allowed growth to a minimum OD600 of 0.2 were diluted 1:100 into fresh medium containing 0.06xMIC, 0.25xMIC, 0.5xMIC, 0.75xMIC,351xMIC, 1.25xMIC, 1.5xMIC, and 2xMIC. This passaging was repeated for 27 days. Cultures that grew at levels higher than the MIC were passaged on drug free MHA plates, and the MIC was determined. For teixobactin, there were no mutants with an MIC greater than the parent S. aureus ATCC 29213.