FIG. 1.

Growth (A) and penicillin-induced killing (B) of the parent S. gordonii strain (squares) and its tolerant mutant Tol1 (circles) monitored by optical density (OD), viable counts, and nonquantitative PCR of 16S rRNA genes. (A) During growth, both optical densities and viable counts increased in parallel, while nonquantitative rRNA gene amplification remained stable. After addition of 500× MIC of penicillin G (B), the viable counts decreased sharply in the parent strain while remaining stable in the Tol1 mutant. In contrast, optical densities and rRNA gene densitometries remained steady for both organisms. The experiments were performed in triplicate on three independent occasions. Interexperimental variation was ≤20%.