Fig. 1: Dap action on B. subtilis strains (WT, ΔmreB and ΔugtP strains) and on POPG vesicles by EM and OM.

a In the absence of Dap, the ΔmreB and ΔugtP mutant strains display abnormal surfaces visible by SEM, whereas the OM does not display such disparities. b Exposed for 15 min to sub-MIC Dap, SEM displays in the WT and ΔmreB strains, but not in the ΔugtP strain, the creasing of the bacterial cell wall surface (yellow arrows). In all the strains, we observe pronounced deformations at the cell poles (green arrow) and submicron protuberances emerging, seemly of lipidic nature given its spherical shape (white arrow and insets). In contrast, the OM displays bulbous micron-sized swellings, primarily coming out of the bacterial poles in the two mutants, but not in the WT cells. Thus, the cell wall delimits the Dap-induced morphological alterations. c Exposed for 15 min to over-MIC, the OM also displays bulbous micron-sized swellings in the WT B. subtilis. All the rest of deformations observed at sub-MIC are found also at over-MIC in all the strains. d–f TEM of POPG vesicles exposed to Dap: In the absence of Dap, the POPG vesicles display spherical shape; at sub-MIC, a number of tubulations emerge from, and interconnect, the vesicles (black arrows); at over-MIC, the tubulations are omnipresent interconnecting the vesicles and the surface of the vesicles becomes creased. g Detail of the tubulations that appear in the ΔmreB cells at over-MIC visualised by SEM. Their size, shape, and interconnective character, is comparable to those observed in POPG vesicles shown in e and f. These tubules are only noticed with ΔmreB mutant; thus, they are not created by the SEM sample preparation.