Staphylococcus aureus ultrastructure and the action of bactericidal antibiotics

Abstract

The bacterial envelope of Staphylococcus aureus consists of the plasma membrane and a multi- layered peptidoglycan cell wall with the exoplasm between them. This structure is essential for growth, division, protection and turgor maintenance. The cell wall is also of clinical importance as its synthesis is the target of various antibiotics. The aim of this study was to investigate the maintenance of S. aureus viability, focusing on the roles of the exoplasm, cell wall dynamics and the action of bactericidal antibiotics. I optimised the electron microscopy techniques for a thorough analysis of the bacterial envelope with focus on the exoplasmic space, and present results that establish the basis for further research into the maintenance of this compartment and its importance for cell viability. My analysis of the morphological changes observed during the action of bactericidal antibiotics was performed as part of a collaborative interdisciplinary investigation into cell wall peptidoglycan homeostasis. I tested a simple predictive model for bacterial life and death based on cell wall homeostasis, and present evidence which stresses the essentiality of peptidoglycan synthesis and hydrolysis for growth; as interruption of either leads to cell death while loss of both leads to stasis. The bactericidal mechanisms that lead to cell death, due to β-lactams and vancomycin, involve a complex process of multiple highly-regulated hydrolases with redundancy in function. The major peptidoglycan hydrolase SagB is a growth-associated hydrolase and is implicated in an increase in size and number of cell wall perforating holes that lead to antibiotic- induced lysis. β-lactam antibiotics also have an additional mode of cell killing that culminates in plasmolysis. This β-lactam-induced death derives from the continued action of various hydrolases involved in cell separation, such as Atl, provoking large cell wall perforating holes and the early septal scission of incomplete septa that lead to plasmolysis and lysis. My project has set the bactericidal activity of antibiotics within a simple, overarching model for cell viability, growth and division. This sets a framework for the development of new control regimes for important pathogens