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Statistical Analysis The difference between different experimental groups was determined by analysis of variance (ANOVA) and Student��s t-test. The difference between the relapse rates was determined by Chi-square test. P-values Results RPF-Supernatant Resuscitates Persistent M. Tuberculosis Growth In Vitro In the in vitro hypoxic model (Wayne, 1976), exponential growth was observed up to 15 days, followed by growth stabilization until day 60, when a 2-log reduction of CFU RRAD counts occurred lasting up to day 200 (Figure ?Figure11). To determine the cause of this count reduction (cell death or viable but non-culturable state), log-phase and stationary phase culture were incubated with the supernatant of a late log-phase culture containing RPF or 7H9 (Mukamolova et al., 2010). These RPF-supernatant or 7H9 medium resuscitated cultures were quantified using the MPN method. Interestingly, at the time points selected for comparison (14, 35, 45, 60, 70, 80, 120, and 200 days), RPF-supernatant resuscitated cultures displayed progressively higher broth counts than the CFU counts (0.08, 0.23, 0. 30, 0.45, 1.28, 1.51, 1.99, and 2.03 logs, respectively; Figure ?Figure11) compared to those of cultures resuscitated by 7H9 (-0.08, 0.00, 0.13, 0.15, 0.72, 0.82, 0.99 and 0.96 logs, respectively; Figure ?Figure11). The 7H9 or RPF-dependent persisters were unable to form colonies during prolonged incubation periods. Significant differences were observed amongst CFU counts, 7H9 broth counts and RPF broth counts (P optimal concentration of rifampicin required to clear stationary-phase bacteria, we examined the effect of incremental drug concentrations on both log- (7 days) and stationary-phase (60 days) cultures; see Figure ?Figure22. In log-phase cultures, apparent bacterial clearance was achieved at/before 20 days with rifampicin concentrations of >2 mg/L (Figure ?Figure2A2A). However, to achieve the same antimicrobial effect in stationary-phase cultures, significantly higher drug concentrations (>32 mg/L) were required (Figure ?Figure2B2B). FIGURE 2 Determination of rifampicin activity against log-phase and stationary-phase M. tuberculosis using time kill curve analysis. Rifampicin at the concentration from 16 to 0 mg/L were incubated with a 7 days log-phase culture (A) and rifampicin at the concentration ... To delineate the level of residual viable M.