Effect of mutation accumulation on pneumococcal competence.

<p>(A, B) Activity of an <i>ssbB</i>’-<i>luc</i> competence reporter in (A) 12 lineages of strain OAP5 (▵<i>mutX</i>) and (B) 12 lineages of strain OAP6 (wild-type <i>mutX</i>) before (black squares) and after (red triangles) 15 growth cycles of serial transfer. Symbols represent means ± SEM for 12 independent lineages. Because assays were started by inoculation of individual colonies into each sample well rather than using standardized starter cultures, growth times of samples were variable. Results are therefore displayed by plotting luciferase activity against optical density of the culture. (C) Distribution of 48 SNPs identified during resequencing of 11 lineages in the &▵;<i>mutX</i> background and ordered by location in the R6 reference genome. (D) Frequency of competence development among 11 sequenced &▵;<i>mutX</i> lineages as compared to the number of nonsynonymous SNPs identified in coding regions of each genome. Competence frequency was determined as the proportion of replicate samples for a given isolate that displayed luciferase activity above a baseline level during the assay. This baseline was defined by the set of samples that did not develop a burst of luminescence at any point but rather showed consistently low activity levels (approximately 1000 RLU/OD₆₂₀ at the start of exponential phase) that slowly declined further during growth.