SmdA is a novel cell morphology determinant in Staphylococcus aureus

Cell division and cell wall synthesis in staphylococci need to be precisely coordinated and controlled to allow the cell to multiply while maintaining their nearly spherical shape. The mechanisms ensuring correct placement of the division plane and synthesis of new cell wall have been studied intensively, however, hitherto unknown factors and proteins are likely to play key roles in this complex interplay. We here identified and investigated a protein with major influence on cell morphology in Staphylococcus aureus . The protein, named SmdA (for staphylococcal morphology determinant A), is a membrane-protein with septum-enriched localization. By CRISPRi knockdown and overexpression combined with different microscopy techniques, we demonstrate that proper levels of SmdA is necessary for cell division, including septum formation and cell splitting. We also identified conserved residues in SmdA that are critical for its functionality. Pulldown- and bacterial two-hybrid interaction experiments showed that SmdA interacts with several known cell division- and cell wall synthesis proteins, including penicillin binding proteins (PBPs) and EzrA. Notably, SmdA also affects susceptibility to cell wall targeting antibiotics, particularly in methicillin-resistant S. aureus (MRSA). Together, our results show that S. aureus is dependent on balanced amounts of membrane-attached SmdA in order to carry out proper cell division. Importance Staphylococcus aureus is an important human and animal pathogen. Antibiotic resistance is a major problem in treatment of staphylococcal infections, and cell division and cell wall synthesis factors have previously been shown to modulate susceptibility to antibiotics in this species. In the current work we investigated the function of an essential protein named SmdA, which was identified based on its septal localization and knockdown phenotype resulting in defective cellular morphologies. We demonstrate that this protein is critical for normal cell division in S. aureus . Depletion of SmdA sensitize resistant staphylococci to β-lactam antibiotics. This work thus reveals a new staphylococcal cell division factor and a potential future target for narrow spectrum antimicrobials or compounds to resensitize antibiotic resistant staphylococcal strains.