The eight clusters of the transcriptomic profiling of S. meliloti 1021 following a shift to acidic pH. The diagrams of clusters A to H show the mean M value (y-axis) obtained by the Sm6kOligo microarray analyses for each time point (x-axis) after pH shift. The standard deviations are represented by the vertical lines crossing each point of the graph. The dotted line divides the cellular response into two parts. The location of the dotted line was chosen according to the observation that most of the cellular response happened in the first 20 minutes.