Compensatory Evolution of pbp Mutations Restores the Fitness Cost Imposed by β-Lactam Resistance in Streptococcus pneumoniae
Figure 3
Electron microscopy analysis of cell morphology of pbp mutants.
Microphotographs of the wild-type strain (Cp1015) revealed the typical morphology of S. pneumoniae, with a correct septum placement, division and symmetry of daughter cells. In the pbp2b28 mutant, two subpopulations were identified, namely rod-like and coccoid cells. The rod-like shaped cells showed multiple septa with incorrect formation and placement. The coccoid-shape cells exhibited multiple septa and intracellular accumulation of the cell wall, with the atypical septum localization resulting in irregular cell divisions and asymmetrical daughter cells (see arrows). The pbp2b28 pbp2x28 mutant partially restored the coccoid morphology, but conserved some cell alterations similar to those of the single pbp2b28 mutant. The triple pbp mutant showed a similar morphology to wt cells. Additional microphotographs are shown in Figs. S12, S13, S14 and S15 in Supporting Information S1 (bar scale, 1.6 µm).