FIG. 2.

Growth rate curves for plasmid-containing cells. The plasmid-containing cells were grown overnight with chloramphenicol to select for retention of the plasmid and then transferred to Luria-Bertani broth without chloramphenicol at 30°C. The cultures were periodically diluted to continuously maintain them in logarithmic growth. The optical density at 600 nanometers of the cells was recorded at the times shown. The values were corrected for the periodic dilutions. A duplicate experiment gave virtually identical results. Diamonds, no plasmid; open triangles, pALA136 (vector); squares, pALA2529 (mvp⁺). Panel A, determinations for strain BR825; panel B, determinations for strain CC5220, which expresses mvp genes from the chromosome and is immune to killing by the plasmid mvp system. Virtually identical growth curves were obtained when viable counts were assayed (see Fig. 3; data not shown).