Abstract
The peptidoglycan (PG) is a macromolecular component of the bacterial cell wall that maintains the shape and integrity of the cell. The PG of Caulobacter crescentus, unlike that of many other Gram-negative bacteria, has repeatedly been shown to contain significant amounts of glycine. This compositional peculiarity has been deemed an intrinsic characteristic of this species. By performing a comprehensive qualitative and quantitative analysis of the C. crescentus PG by high-performance liquid chromatography (HPLC) and mass spectrometry (MS), we show here that glycine incorporation into the C. crescentus PG depends on the presence of exogenous glycine in the growth medium. High levels of glycine were detected at the fifth position of the peptide side chains of PG isolated from C. crescentus cells grown in the complex laboratory medium PYE or in defined medium (M2G) supplemented with casamino acids or glycine alone. In contrast, glycine incorporation was undetectable when cells were grown in M2G medium lacking glycine. Remarkably, glycine incorporation into C. crescentus peptidoglycan occurred even in the presence of low millimolar to sub-millimolar concentrations of free glycine. High glycine content in the PG had no obvious effects on growth rates, mode of PG incorporation or cell morphology. Hence, the C. crescentus PG is able to retain its physiological functions in cell growth and morphogenesis despite significant alterations in its composition, in what we deem to be unprecedented plasticity.
Highlights
The peptidoglycan (PG) sacculus, which is a mesh-like macromolecule situated on the exterior side of the cytoplasmic membrane, is an essential cellular component of a large variety of bacterial species
It has been shown that C. crescentus PG samples contain high levels of pentapeptide side chains and that roughly half of them terminated with a glycine residue [21]
It has been assumed since that the PG of C. crescentus is characterized by a high Penta(Gly) peptide content, an idea supported by later PG analyses [22,31,32]
Summary
The peptidoglycan (PG) sacculus, which is a mesh-like macromolecule situated on the exterior side of the cytoplasmic membrane, is an essential cellular component of a large variety of bacterial species. Cellular growth and morphogenesis are tightly connected to the regulated processes of PG synthesis, remodeling and degradation [1,2,4,5,6]. Genetic or chemical inactivation of PG synthesizing or modifying enzymes (or of regulatory factors of these enzymes) can result in growth inhibition and cell lysis. The innate immune system has evolved to use PG features as triggers of pathogen recognition pathways and immune cell activation [7], reflecting the nearubiquitous presence of the peptidoglycan throughout the bacterial kingdom
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