Molecular surveillance of the subtle septum: discovering a new mode of peptidoglycan synthesis in streptococci.

The process of septation requires precise temporal and spatial organization of penicillin binding proteins (PBPs) and associated proteins for the deposition of new cell wall material. In most bacteria, the filamentous protein FtsZ organises PBPs into assemblies at the midcell which then constrict inwards as peptidoglycan is synthesised, eventually closing the septa. Tsui et al. (2014), through the use of fluorescent d-amino acids and high resolution microscopy, report that PBP2x of Streptococcus pneumoniae is directed to a discrete location within the septal aperture during the later stages of cell division. Once at this new site, PBP2x catalyses the de novo synthesis of peptidoglycan, which is imaged by the authors as a central 'spot', distinct from material made by other PBPs at the outer ring. This discovery, which represents a novel mode of cell wall assembly, was made in a directed capsular knockout of strain D39, thereby avoiding potential mechanistic complications in commonly used laboratory strain R6. These findings prompt not only a partial rethink of septum formation in S. pneumoniae, but consideration of the modes of PBP localization and the subtleties that can influence phenotypic study.