Bgs4 is responsible for the SS formation and necessary for the integrity of the PS base needed for its anchorage into the cell wall. (A) TEM details of septum maturation. WT complete septa are straight and increase in thickness by a second round of SS synthesis (left). Bgs4 absence causes the total absence of SS. The complete septa are twisted and the middle region is extremely thin as a result of the defect in PS completion. During maturation, the thin central region is repaired by addition of new PS (right). Cells were grown as in Fig. 2 A. (B) Details of PS anchorage or retraction during septum maturation. WT PS starts in the cell wall (Cw) surface. During septum growth and maturation the PS is progressively anchored into the cell wall (top). In Bgs4 absence, the defective PS progressively retracts from the cell wall, leaving a space filled by MD (bottom). Bars: (A) 1 µm; (B) 0.5 µm.