FIG. 9.
Competition experiments. (A) Competition between a minority strain, MSf7-2 (Strr, stationary-phase-adapted flat strain), identified by plating with streptomycin (■), and a majority strain, MS1-1 (Rifr, exponential-phase-adapted, normal-colony-morphology strain), plated with (•) and without (○) rifampin. Five microliters of a 3-day-stationary-phase culture of MSf7-2 was mixed with 5 ml of a 3-day-stationary-phase culture of MS1-1. After 30 days, the number of rifampin-resistant MS1-1 cells dropped to ≤1 × 104. (B) Reverse competition between the MSf7-2 majority strain (■, plated with streptomycin) and the MS1-1 minority strain (•, plated with rifampin). (C) Shown are an MS1-1 control (no MSf7-2 added), plated with (•) and without (○) rifampin; an MSf7-2 control (no MS1-1 added), plated with (■) and without (□) streptomycin; and an MS1 control to the strain used in panel D (no MSf7-2 added), plated without antibiotics (▵). (D) Competition between a minority strain, MSf7-2 (■), and a majority strain, MS1 (▵). All competitions were performed three times with similar results. Plating errors were within 10% of the viable counts. Note that although the proportion of the majority cells to minority cells inoculated was always 1,000:1 (vol/vol), there appeared to be 10,000-fold more MS1-1 cells than MSf7-2 cells in Fig. 8A, compared with only 500-fold more MSf7-2 cells in Fig. 8B. This was due to the fact that flat cultures clumped more, which reduced the viable counts of flat strains approximately 10-fold compared with counts for the wild type under similar conditions.