Growth of S. meliloti 1021 before and after a shift to low pH. An S. meliloti 1021 preculture has been grown in VMM buffered at pH 7.0 until it reached an o.D.580 of 0.8 (dotted line with triangles). Afterwards the pre-culture has been separated into even parts, centrifuged and re-suspended in VMM at pH 5.75 and VMM at pH 7.0, respectively. The growth of the pH 5.75 culture is given by lines with crosses and the growth of the pH 7.0 culture is given by lines with plus-symbols. The arrows in the diagram indicate the time points where cell culture probes were taken for transcriptional profiling. Remarks indicate the time in minutes passed after the splitting of the S. meliloti preculture.