Making a point: the role of DivIVA in streptococcal polar anatomy.

Like the cells of other bacteria that are not rod shaped, Streptococcus pneumoniae cells manage to avoid potentially distressing changes in their surface-to-volume ratio as they grow. The work reported by Fadda et al. in this issue (9) on the streptococcal DivIVA protein suggests new ideas on how this protein may contribute to transformation of the midcell into two pointed poles that result in duplication of the cell volume with no net increase in cell diameter, a mechanism that prevents changes in the surface-to-volume ratio during growth and division. Some nearly spherical bacteria, like Staphylococcus aureus, have circumvented the issue of surface-to-volume ratio maintenance by placing a sagittal septum at midcell while maintaining a nearly constant diameter. Once finished, the S. aureus septum splits in the middle, and two polar, hemispherical caps are gradually reshaped from the resultant almost flat moieties (33). In contrast, the ovoid S. pneumoniae cells grow and divide simultaneously by recreating two pointed polar caps at midcell. Although a septum is formed, it does not have continuity in the sagittal plane until the very end of division. In some respects the S. pneumoniae septation process resembles constriction, like that observed in Escherichia coli dividing cells, except that the resultant polar surface contributes a larger portion of the cell surface in the case of the streptococcal cell than in the case of the rod-shaped E. coli cell.