Dissecting cell division in the human pathogen Staphylococcus aureus

Bacterial cell division is a fundamental process mediated by a large collection of proteins, collectively called the divisome. The divisome is responsible for the appropriate synthesis of new cell wall peptidoglycan to produce the septum, allowing the production of two new daughter cells. Although divisome components have been identified, their precise roles are not well understood. Many cell division proteins interact and coordination of information from the cytoplasm, through the membrane, to the peptidoglycan biosynthesis machinery is required. FtsZ is a key cytoplasmic component, which initiates septum formation resulting in new peptidoglycan synthesis via penicillin binding proteins (PBPs). In Staphylococcus aureus, cell division components, including EzrA, have been identified by a bacterial two hybrid analysis. EzrA, a membrane-associated protein, interacts with both cytoplasmic proteins and those with periplasmic domains. It is therefore proposed to act as an interface between FtsZ in the cytoplasm and PBPs in the membrane forming a scaffold for other cell division components. In this study, a combination of protein labelling and super-resolution microscopy approaches have been used to study the architecture of the cell division process in S. aureus. Conventional wide field fluorescence microscopy shows EzrA and FtsZ as uniform rings at the division site. Super resolution fluorescence microscopy of a strain, in which the only copy of EzrA was tagged with a fluorophore, revealed that EzrA does not form a homogenous ring but it is rather a collection of ‘patches’ distributed at the division site. FtsZ precedes EzrA and also forms heterogeneous structures around the midcell. Unsuccessful attempts were made to investigate the localisation of the essential PBP2 at molecular level using fluorescent proteins or fluorescent derivatives of β-lactams. A model of septum formation in S. aureus, in which a decreasing concentration gradient of cell division components wraps around the septum and forms a framework for peptidoglycan synthesis, has been developed.