Osmoregulated periplasmic glucans in Proteobacteria

Abstract

Large amounts of osmoregulated periplasmic glucans (OPGs) are found in the periplasmic space of Proteobacteria. Four families of OPGs are described on the basis of structural features of the polyglucose backbone. Depending on the species considered, OPGs can be modified to various extent by a variety of substituents. Genes governing the backbone synthesis are identified in a limited number of species. They belong to three unrelated families. OPG synthesis is subject to osmoregulation and feedback control. Osmoregulation can occur at the level of gene expression and/or at the level of enzyme activity. Mutants defective in OPG synthesis have a highly pleiotropic phenotype, indicative of an overall alteration of their envelope properties. Mutants of this kind were obtained as attenuated or avirulent derivatives of plant or animals pathogen. Thus, OPGs appear to be important intrinsic components of the Gram‐negative bacterial envelope, which can be essential in extreme conditions found in nature, and especially when bacteria must interact with an eukaryotic host.

Bacterial cyclic β‐(1,2)‐glucans sequester iron to protect against iron‐induced toxicity

“…Knockout of ndvB compromised the production of the extracellular cyclic b-(1,2)-glucan, but had no effect on the production of xanthan (Figs 2C and EV3A). As glucan is normally synthesized under low-osmotic conditions and accumulates in the periplasmic space of Gram-negative bacteria [17,19], we isolated glucan from the periplasmic fraction by sizeexclusion chromatography followed by analysis by HPLC and MALDI-MS. Furthermore, the growth defect exhibited by glucan-deficient ndvB mutant in medium containing DETAPAC, BPS, and DP could be rescued by addition of FeSO 4 but not by MnCl 2 or ZnSO 4 , which further corroborated the iron specificity of the growth defect (Figs 1A-F and EV1E-G; Appendix Table S1).…”

“…Although glucan is localized primarily in the periplasm, several bacteria also produce secreted glucan [17,18]. We have found that extracellular supplementation of cyclic b-(1,2)-glucan could rescue the growth deficiency exhibited by the glucan-deficient Xcc mutants under iron-restricted conditions.…”

“…Although glucan is primarily localized in the periplasm, several bacteria also produce secreted glucan [19]. Earlier studies suggested diverse, albeit poorly understood biological roles of secreted and periplasmic glucans, particularly in host-bacteria interactions, osmoadaptation, and cell signaling [17,19,20]. However, despite glucan being present in many Gram-negative bacteria, their physiological role has not been established.…”

Functional Roles of (1,3)-β-glucans and Related Polysaccharides

“…Low molecular weight periplasmic glucans that have a linear or cyclic backbone structure are intrinsic components of some Gram-negative bacterial cell envelopes (Breedveld and Miller, 1994;Kennedy, 1996;Bohin, 2000). Typical amongst the -Proteobacteria is the production of structurally diverse cyclic glucans.…”

“…Periplasmic glucans are considered to be important intrinsic components of the Gram-negative cell envelope since mutants defective in their synthesis display a variety of pleiotropic phenotypes suggestive of significant structural alteration of the cell envelope (Bohin, 2000;Breedveld and Miller, 1994;Kennedy, 1996). The glucans occur in the periplasm usually only during growth of the bacteria in a medium of very low osmolality (100 milliosmoles per kilogram H 2 O) and can accumulate to a high concentration (10-100 mM), representing 5-10% of the cellular dry weight (Bohin, 2000;Talaga et al, 2002).…”