Abstract
The cell wall of Bacillus subtilis is a rigid structure on the outside of the cell that forms the first barrier between the bacterium and the environment, and at the same time maintains cell shape and withstands the pressure generated by the cell's turgor. In this review, the chemical composition of peptidoglycan, teichoic and teichuronic acids, the polymers that comprise the cell wall, and the biosynthetic pathways involved in their synthesis will be discussed, as well as the architecture of the cell wall. B. subtilis has been the first bacterium for which the role of an actin-like cytoskeleton in cell shape determination and peptidoglycan synthesis was identified and for which the entire set of peptidoglycan synthesizing enzymes has been localised. The role of the cytoskeleton in shape generation and maintenance will be discussed and results from other model organisms will be compared to what is known for B. subtilis. Finally, outstanding questions in the field of cell wall synthesis will be discussed.
Highlights
The cell wall is a critical structural component of each bacterial cell, except for those few bacteria that lack a cell wall (Mollicutes). It determines bacterial cell shape and bears the stress generated by the intracellular pressure, called turgor
After assembly of the Z-ring, all other cell division proteins, including the division specific PBP2b localise to the division site and synthesis of the septum follows. In both B. subtilis and E. coli, the rate of PG synthesis during cell division is notably increased compared to synthesis during cell growth along the lateral wall (Cooper and Hsieh, 1988; Daniel and Errington, 2003; de Pedro et al, 1997; Kuru et al, 2012; Woldringh et al, 1987)
A recent pull down in B. subtilis with a LipidII analogue containing a photocrosslinker identified penicillin-binding proteins (PBPs) 1, 3, 4, 5 and X bound to LipidII next to several other known cell wall synthesis and division proteins (EzrA, LytE, MurG, FtsX among others) (Sarkar et al, 2016)
Summary
The cell wall is a critical structural component of each bacterial cell, except for those few bacteria that lack a cell wall (Mollicutes). The integrity of the cell wall is of critical importance to cell viability. In both Grampositive and Gram-negative bacteria, the scaffold of the cell wall consists of the cross-linked polymer peptidoglycan (PG). In Gram-negative bacteria the cell wall lies in the periplasmic space, between the inner and the outer membrane of the cell, and consists of only 1 to 3 layers of PG. Gram-positive bacteria, like Bacillus subtilis, lack an outer membrane and so the cell wall constitutes the contact area with the external milieu (Figure 1). The Gram-positive cell wall contains 10 to 30 layers of PG, as well as covalently linked teichoic and teichuronic acid polymers
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