Abstract
The cell wall (CW) of bacteria is an intricate arrangement of macromolecules, at least constituted of peptidoglycan (PG) but also of (lipo)teichoic acids, various polysaccharides, polyglutamate and/or proteins. During bacterial growth and division, there is a constant balance between CW degradation and biosynthesis. The CW is remodeled by bacterial hydrolases, whose activities are carefully regulated to maintain cell integrity or lead to bacterial death. Each cell wall hydrolase (CWH) has a specific role regarding the PG: (i) cell wall amidase (CWA) cleaves the amide bond between N-acetylmuramic acid and L-alanine residue at the N-terminal of the stem peptide, (ii) cell wall glycosidase (CWG) catalyses the hydrolysis of the glycosidic linkages, whereas (iii) cell wall peptidase (CWP) cleaves amide bonds between amino acids within the PG chain. After an exhaustive overview of all known conserved catalytic domains responsible for CWA, CWG, and CWP activities, this review stresses that the CWHs frequently display a modular architecture combining multiple and/or different catalytic domains, including some lytic transglycosylases as well as CW binding domains. From there, direct physiological and collateral roles of CWHs in bacterial cells are further discussed.
Highlights
The first bacterial cell wall hydrolase (CWH) was discovered in 1921 by the Scottish bacteriologist Sir Alexander Fleming, who is well known for his 1928 discovery of the antibiotic penicillin (Fleming, 1929)
Biophysical investigations with state-of-the-art approaches have been performed on a handful of model bacteria, namely B. subtilis, S. aureus, or S. pneumoniae for CW-monoderm bacteria but quite restricted to E. coli for LPS-diderm bacteria (Vollmer et al, 2008a; Turner et al, 2010; Beaussart et al, 2014)
Beyond the simplistic dichotomy of Gram-positive vs. Gram-negative bacteria, there is a need to encompass the full biodiversity of the bacterial kingdom considering the divergent molecular compositions and structural arrangements of the CW
Summary
The first bacterial cell wall hydrolase (CWH) was discovered in 1921 by the Scottish bacteriologist Sir Alexander Fleming, who is well known for his 1928 discovery of the antibiotic penicillin (Fleming, 1929). A so-called autolysin can induce cell lysis (and not just CW remodeling) when its expression is not tightly controlled (Typas et al, 2012) For these reasons, in this manuscript, we will favor the term CWHs over lysins, and describe them with respect to their catalytic activity into CW amidases, CW glycosidases and/or CW peptidases rather than endolysins, exolysins and autolysins. The proportion of cross-linking is significantly higher in myco-diderm bacteria compared to LPSdiderm bacteria as observed for Mycobacterium species vs E. coli (Alderwick et al, 2015) In both CW-monoderm and LPS-diderm bacteria, modifications to the basic PG structure such as N-glycosylation, O-acetylation and/or N-deacetylation occur frequently and many of them are species-specific (Markiewicz and Popowska, 2011). Modifications to the basic PG structure occur at several levels, namely in the disaccharide backbone, the bridge regions, and the peptide stem (Humann and Lenz, 2009)
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