Cell wall metabolism in Bacillus subtilis

Abstract

Cell wall is a unique and essential component of bacterial cell. It defines cell shape and protects cell from bursting through its own internal osmotic pressure. It also represents a significant drain on the cells resources, particularly in Gram positives, where the wall accounts for more than 20 % of the dry weight of the cell, and approximately 50 % of ‘‘old’’ cell wall is degraded and new material made to permit cell growth. After the discovery of penicillin, there has been active study of bacterial cell wall structure and metabolism, as it represents the major target for antibacterial compounds. The biosynthetic pathways for cell wall precursors has been well investigated in bacteria generally, but the coordination of cell wall metabolic processes and the fate of turnover cell wall materials have only been well characterised in Gram-negative bacteria (e.g Escherichia coli). In Gram-positive bacteria, it has generally been accepted that the old wall is released from the surface and lost to the environment during growth, with apparent recycling of this material during stationary phase for Bacillus subtilis. It is also known that the Gram-positive wall is subject to significant post-synthetic processing, involving the linkage of wall teichoic acids and the cleavage of molecules from the structure, e.g. D-alanine, although the function of these is unclear. Understanding the importance of these processes has relevance for both the pathogenicity and biotechnological use of bacteria, as well as for understanding bacterial cell biology. As it is known that the peptidoglycan fragments (e.g muropeptides) induce the innate immune response in higher organisms and so act as a signal for infection, particularly for Gram-positive bacteria. Thus, understanding how they are generated and recycled by the bacteria may offer potential insights into novel therapeutics, also the accumulation of cell wall muropeptides should be avoided in biotechnological products. In this thesis, the D-alanine metabolism was manipulated to understand the mechanistic details of cell wall metabolism and D-alanine recycling in B. subtilis, using genetic, biochemical, bioinformatics and fluorescent microscopy approaches. Through these analyses, a D-alanine transporter (DatA, formerly YtnA) was identified by genetic screening. The roles of DatA and the carboxypeptidases, LdcB and DacA, in recycling of cell wall derived D-alanine have experimentally been confirmed. We also found that D-alanine aminotransferase (Dat) can act to synthesis D-alanine under certain conditions. From the data obtained a model for peptidoglycan assembly (coordinated synthesis and turnover) during growth of B. subtilis has been developed to take into account the various aspects of cell wall metabolism.