Temporal distribution of Singaporean ST239 and ST22 populations. (A
B,) Maximum clade credibility tree of the ST239 (A) and ST22 (B) populations based on BEAST analysis. Tips of the tree are constrained by isolation dates; the time scale is shown below the tree. The three colored boxes in the ST239 tree represent the three main ST239 clades present during the study period of 2000 to 2010, clades A, B and C. Isolates that were identified as potential hyper-mutators (Figure 3) were not included in the BEAST analysis. The mutation rate for the ST22 population was calculated as 1.4 × 10-6 mutations per year (95% HPD, 1.27 × 10-6 to 1.65 × 10-6), which was similar to the previously published rate for ST22 . The estimated mutation rate for ST239 of 2.2 × 10-6 mutations per year (95% HPD, 1.96 × 10-6 to 2.51 × 10-6) was slower than the previous estimate of Harris et al. , but probably reflects the more sophisticated analysis method used in this study and also a more extensive data set. The colored balls on the tips of the tree indicate the origins of the isolate (see legend to figure). For an illustration of the posterior support of the trees see Additional file 3.