Demonstration of the role of cell wall homeostasis in Staphylococcus aureus growth and the action of bactericidal antibiotics

Bacterial cell wall peptidoglycan is essential, maintaining both cellular integrity and morphology, in the face of internal turgor pressure. Peptidoglycan synthesis is important as it is targeted by cell wall antibiotics including methicillin and vancomycin. Here we have used the major human pathogen Staphylococcus aureus, to elucidate both cell wall dynamic processes essential for growth (life), and the bactericidal effects of cell wall antibiotics (death), based on the principle of coordinated peptidoglycan synthesis and hydrolysis. Death of S. aureus due to depletion of the essential, two-component, positive regulatory system for peptidoglycan hydrolase activity (WalKR) is prevented by addition of otherwise bactericidal cell wall antibiotics, resulting in stasis. In contrast, cell wall antibiotics kill via the activity of peptidoglycan hydrolases in the absence of concomitant synthesis. Both methicillin and vancomycin treatment lead to the appearance of perforating holes throughout the cell wall due to peptidoglycan hydrolases. Methicillin alone, also results in plasmolysis and mis-shapen septa with the involvement of the major peptidoglycan hydrolase Atl, a process that is inhibited by vancomycin. The bactericidal effect of vancomycin involves the peptidoglycan hydrolase SagB. In the presence of cell wall antibiotics, inhibition of peptidoglycan hydrolase activity using the novel inhibitor complestatin, results in reduced killing, while conversely deregulation of hydrolase activity via loss of wall teichoic acids increases the death rate. For S. aureus, independent regulation of cell wall synthesis and hydrolysis can lead to cell growth, death, or stasis, with implications for the development of new control regimes for this important pathogen.