Piptedoglycan Dynamics in Bacillus subtilis

Abstract

Gram-positive bacterial cell morphology, growth, and division rely on a delicate balance of peptidoglycan synthesis and controlled degradation to maintain the structural integrity of the cell wall. The process of growth involves synthesising new wall material underneath the existing cell wall on the surface of the cell membrane. This growth requires the cell wall degrading enzymes to distinguish between the newly generated wall and the old wall, selectively degrading the outer layers of the wall to facilitate cell enlargement. As the B. subtilis genome encodes 42 genes that are potentially involved in peptidoglycan degradation, systematic deletion of the known autolytic enzymes was used combined with phenotypic analysis for both cell morphological changes and the ability to become motile. From this work, it was found that only 1 of 2 specific autolytic enzymes is functionally required for growth (CwlO and LytE). Although these two autolytic enzymes exhibit a significant degree of functional redundancy, they are required for slightly different aspects of cell morphology. Only CwlO, in concert with CwlQ or CwlS, was also found to be required for the efficient insertion of flagella through the cell wall. Further analysis showed that CwlO activity with respect to cell growth required the activity of a peptidoglycan carboxypeptidase (DacA), but this was not required for flagellar insertion, suggesting that CwlO has two distinct modes of action. In summary, the results of the work presented in this thesis show that the majority of the predicted cell wall degrading enzymes are dispensable, and only 2 enzymes, CwlO and LytE, have critical roles in maintaining normal cell morphology. Interestingly, this study also reveals that the two key autolytic enzymes seem to have distinct modes of action and potentially differ in their substrate specificity. In this respect, a model for cell growth is presented that tries to amalgamate the results of this work with previously published ideas to explain how cell growth is coordinated with respect to peptidoglycan synthesis and degradation without compromising cell integrity and maintaining cell morphology. This model potentially outlines the basic mechanism of cell wall metabolism in Gram-positive rod-shaped bacteria. It also seems that aspects of the mechanism are also conserved in other bacterial species with a different cell morphology as well as in Gram-negative bacteria. These results clearly indicate that the autolytic enzymes and their regulatory mechanisms are potentially interesting novel targets for the development of small molecule antibacterial compounds.