Cell wall architecture and the role of wall teichoic acid in Staphylococcus aureus

The bacterium Staphylococcus aureus only synthesises peptidoglycan during cell division at the septum using a complex protein biosynthetic apparatus called the divisome. It divides sequentially in three orthogonal planes, using heritable features within the peptidoglycan architecture to maintain this process over generations. The ‘rib’ features that form this ‘memory’ are remnants of a large belt of peptidoglycan called the ‘piecrust’ that is formed at the initiation of septation and before the septal plate. After division, the ribs remain as orthogonal features, which are bisected by further ‘piecrust’ features from ensuing division cycles. This results in a characteristic pattern of different age peptidoglycan sectors, delineated by ribs of a different architecture.
As well as maintaining cellular viability and shape the peptidoglycan layer also acts as a scaffold for many other polymers, including wall teichoic acid (WTA). WTA is known to direct and modulate cell wall hydrolase activity. There has been recent debate as to its subcellular localisation. In this study, using the bacterial two-hybrid assay, four putative WTA biosynthesis enzymes were found to interact with numerous members of the divisome. Microscopy techniques localised WTA across the entire cell surface except on the piecrust and rib features. It was hypothesised that WTA blocks the rest of the peptidoglycan thereby directing the localisation of hydrolases and other proteins.
The localisation of peptidoglycan hydrolases was studied. Those found to localise to the rib and piecrust features (Atl(glucosaminidase), Atl(amidase), SagB and ScaH) showed a distinct pattern which was completely disrupted in a strain missing WTA. Conversely those (SceD) not associated with rib/piecrust showed no difference to localisation with loss of WTA. The processed forms of Atl (glucosaminidase and amidase) demonstrated different binding properties during the cell cycle and a model to illustrate the cell cycle dependent binding is proposed.