Date of Award
8-6-2020
Degree Name
Doctor of Philosophy (PhD)
Degree Type
Dissertation
Abstract
Single-celled organisms must adapt their physiology to persist and propagate across a wide range of environmental conditions. The growth and division of bacterial cells depends on continuous synthesis of an essential extracellular barrier: the peptidoglycan cell wall, a polysaccharide matrix that counteracts turgor pressure and confers cell shape. This dissertation identifies mechanisms that ensure robust peptidoglycan integrity and metabolism in Escherichia coli across growth environments and examines how these adaptations impact cell growth, division, morphogenesis, and antimicrobial resistance. The majority of enzymes involved in synthetizing and remodeling the peptidoglycan cell wall appear functionally redundant. In this dissertation (Chapters 2 and 3), I report that a subset of these enzymes (~ 25%) are not redundant and are instead specialized for growth and division in diverse pH environments. In Chapter 2, I demonstrate two semi-redundant cell wall synthases, PBP1a and PBP1b, are specialized for growth opposing pH ranges: PBP1a promotes cell wall integrity in alkaline medium, whereas PBP1b promotes cell wall integrity in acidic medium. Differential utilization of PBP1a and PBP1b across pH environments impacts intrinsic β-lactam resistance. In Chapter 5, I present evidence that PBP1b protects cells from β-lactam antibiotics by localizing to, and possibly repairing, lesions in the peptidoglycan matrix. In Chapter 3, I expand the repertoire of pH specialist cell wall enzymes to include peptidoglycan amidase AmiB, which I discover is preferentially active in acidic medium. E. coli mutants encoding AmiB as the sole amidase form long chains of unseparated cells in neutral pH medium but separate normally in acidic medium. Computational analysis indicates acidic pH may alleviate autoinhibition of the enzyme’s active site. Altogether, my findings in Chapter 2 and 3 suggest unique sets of cell wall enzymes are active in different environments, and the collective activity of these enzymes permits E. coli growth and division across a wide range of ecological niches. Moreover, my work demonstrates the active repertoire of cell wall enzymes may influence emergent properties of the cell, including its intrinsic antibiotic resistance. Work in Chapter 4 examines the impact of environmental pH on the E. coli cell division machinery, an essential specialized cell wall synthesis complex that controls cell size. Previous studies indicate nutrient availability influences the assembly of the cytokinetic ring, but it remained unclear whether non-metabolic signals are also perceived by the division machinery. Here, I demonstrate that in acidic medium the division activator FtsN hyperaccumulates at midcell, stimulating cytokinesis at a reduced cell volume. In alkaline medium, FtsN accumulation is reduced, delaying cytokinesis until a larger cell size is achieved. My findings in Chapter 4 reveal the composition and activity of the cell division apparatus varies based on the cell’s growth environment, and these modifications impact homeostatic cell size. As a whole, this dissertation provides substantial evidence that the peptidoglycan cell wall remains robust across environmental conditions due to plasticity within its synthesis machinery. At the same time, this work also suggests small differences within the construction of the cell wall are sufficient to alter fundamental properties of the cell, including cell morphology and antimicrobial resistance.
Language
English (en)
Chair and Committee
Petra Levin
Recommended Citation
Mueller, Elizabeth A., "Plasticity ensures robust cell wall metabolism in Escherichia coli" (2020). Arts & Sciences Electronic Theses and Dissertations. 3250.
https://openscholarship.wustl.edu/art_sci_etds/3250