Spore Peptidoglycan

Abstract

Bacterial endospores possess multiple integument layers, one of which is the cortex peptidoglycan wall. The cortex is essential for the maintenance of spore core dehydration and dormancy and contains structural modifications that differentiate it from vegetative cell peptidoglycan and determine its fate during spore germination. Following the engulfment stage of sporulation, the cortex is synthesized within the intermembrane space surrounding the forespore. Proteins responsible for cortex synthesis are produced in both the forespore and mother cell compartments. While some of these proteins also contribute to vegetative cell wall synthesis, others are sporulation specific. In order for the bacterial endospore to germinate and resume metabolism, the cortex peptidoglycan must first be degraded through the action of germination-specific lytic enzymes. These enzymes are present, yet inactive, in the dormant spore and recognize the muramic-δ-lactam modification present in the cortex. Germination-specific lytic enzymes across Bacillaceae and Clostridiaceae share this specificity determinant, which ensures that the spore cortex is hydrolyzed while the vegetative cell wall remains unharmed. Bacillus species tend to possess two redundant enzymes, SleB and CwlJ, capable of sufficient cortex degradation, while the clostridia have only one, SleC. Additional enzymes are often present that cannot initiate the cortex degradation process, but which can increase the rate of release of small fragments into the medium. Between the two families, the enzymes also differ in the enzymatic activities they possess and the mechanisms acting to restrict their activation until germination has been initiated.

The Bacillus subtilis germinant receptor GerA triggers premature germination in response to morphological defects during sporulation

“…The presence of phase‐dark spores in the mutants analyzed above and the reduction in this class of spores in the cells lacking a functional GerA receptor raised the possibility that morphological defects in spore development inappropriately trigger the GerA receptor leading to premature germination and the more severe phenotypes observed. Two effectors in the germination pathway that act downstream of the germinant receptors that are critical for the transition from phase‐bright to phase‐dark spores are the spore cortex lytic enzymes CwlJ and SleB (reviewed in Popham and Bernhards, ). Both proteins are synthesized during sporulation and packaged in the dormant spore.…”

“…Germination is thought to begin by the release of monovalent ions from the core, followed by the release of Ca 2+ ‐DPA through a putative channel complex in the inner spore membrane. These steps are followed by the degradation of the spore cortex by two partially redundant spore cortex lytic enzymes that specifically target the muramic delta lactam, leaving the germ cell wall intact (Popham and Bernhards, ). Ca 2+ ‐DPA release and cortex degradation allow an influx of water and the transition from a desiccated phase‐bright spore to a swollen phase‐dark one.…”

Building peptidoglycan inside eukaryotic cells: A view from symbiotic and pathogenic bacteria

“…A second level of uncertainty refers to regulators, acting at the transcriptional or post‐transcriptional level, that control expression and activity of PG enzymes and for which we are still at a very early stage of knowledge. Some exceptions include: (a) the essential transmembrane protein kinases bearing PASTA (penicillin‐binding protein and serine/threonine kinase associated) domains that control PG synthesis and cell wall homeostasis in low G + C Gram‐positive bacteria (Dubrac, Bisicchia, Devine, & Msadek, ; Jones & Dyson, ); (b) the sigma‐type regulators that regulate expression of PG enzymes driving the synthesis of an unique PG during spore formation and germination in the mother cell, the forespore and the mature spore (Popham & Bernhards, ); and, (c) proteins that associate in complexes with synthases and hydrolases regulating their activity by protein‐protein interactions (Egan, Cleverley, Peters, Lewis, & Vollmer, ; Pazos, Peters, & Vollmer, ). Intriguingly, most PG enzymes controlled by these regulators are periplasmic synthases and hydrolases, reinforcing the idea of macromolecular PG as a sensor device that integrates external signals upon exposure to stress.…”

“…This is especially relevant for enzymes predicted to act in identical bonds of the PG sacculus -the repeatedly discussed redundancy-or for those enzymes with multiple paralogs of yet unknown function. The global analysis performed 'at the protein level' will be, in my opinion, insightful to understand why these apparent multiple copies exist and if compensatory effects formation and germination in the mother cell, the forespore and the mature spore (Popham & Bernhards, 2015); and, (c) proteins that associate in complexes with synthases and hydrolases regulating their activity by protein-protein interactions (Egan, Cleverley, Peters, Lewis, & Vollmer, 2017;Pazos, Peters, & Vollmer, 2017). Intriguingly, most PG enzymes controlled by these regulators are periplasmic synthases and hydrolases, reinforcing the idea of macromolecular PG as a sensor device that integrates external signals upon exposure to stress.…”

Clostridium perfringens Sporulation and Sporulation-Associated Toxin Production

“…Beneath the coat is the outer membrane, which does not provide protection to dormant spores but is essential for spore formation and is presumably lost by shearing forces after short periods. The spore peptidoglycan cortex underlines the outer membrane, with a structure similar to that of peptidoglycan in a growing cell wall (16). The cortex plays an essential role in spore core dehydration and therefore directly contributes to spore resistance to environmental stress and chemicals.…”