Osmoregulated Periplasmic Glucan Synthesis Is Required for Erwinia chrysanthemi Pathogenicity

Abstract

Erwinia chrysanthemi is a phytopathogenic enterobacterium causing soft rot disease in a wide range of plants. Osmoregulated periplasmic glucans (OPGs) are intrinsic components of the gram-negative bacterial envelope. We cloned the opgGH operon of E. chrysanthemi, encoding proteins involved in the glucose backbone synthesis of OPGs, by complementation of the homologous locus mdoGH of Escherichia coli. OpgG and OpgH show a high level of similarity with MdoG and MdoH, respectively, and mutations in the opgG or opgH gene abolish OPG synthesis. The opg mutants exhibit a pleiotropic phenotype, including overproduction of exopolysaccharides, reduced motility, bile salt hypersensitivity, reduced protease, cellulase, and pectate lyase production, and complete loss of virulence. Coinoculation experiments support the conclusion that OPGs present in the periplasmic space of the bacteria are necessary for growth in the plant host.Osmoregulated periplasmic glucans (OPGs) are a family of oligosaccharides found in the periplasmic space of gram-negative bacteria. Their two common features are the presence of glucose as the sole constituent sugar and their increased level in media of low osmolarity (5).Members of the family Enterobacteriaceae and related bacteria synthesize a family of linear and branched OPGs that are variously substituted. The linear backbone is constituted by glucose units joined by ␤,1-2 linkages, and the branches are made of one glucose unit linked to the main chain by a ␤,1-6 linkage. In Escherichia coli, the backbone, containing 7 to 13 glucose units, is substituted with phosphoglycerol, phosphoethanolamine, and succinyl residues (19). In Erwinia chrysanthemi, the backbone contains 5 to 12 glucose units substituted with succinyl and acetyl residues (9), and in Pseudomonas syringae, the backbone, consisting of 6 to 13 glucose units, is not substituted (38). In E. coli, the OPG backbone is synthesized by the products of the mdoGH operon located in the vicinity of pyrC, a gene involved in the biosynthesis of uracil (6). In this bacterium, the defect in OPG synthesis does not confer an easily selectable phenotype in laboratory conditions. Thus, the mdoGH locus was cloned using the linked selectable genetic marker pyrC (23).Many factors are involved in the virulence of pathogenic bacteria, and OPGs appear to be among them. In P. syringae pv. syringae, the causal agent of brown spot disease of the common bean (Phaseolus vulgaris), the hrpM mutant, obtained after transposon mutagenesis, was isolated because it failed to incite disease on the host plant and to cause the hypersensitive reaction on a non-host plant such as tobacco (29). The hrpM mutant does not synthesize OPGs, and the hrpM locus complements the OPG synthesis defect of the mdoH200::Tn10 mutant of E. coli. The amino acid sequences of HrpM and MdoH are 75.5% identical and 87.5% similar (25). More recently, a transposon insertion in a gene similar to hrpM/mdoH was isolated because it severely reduces the virulence in Pseudomonas aeruginosa PA14, an opp...

Inactivation of pecS restores the virulence of mutants devoid of osmoregulated periplasmic glucans in the phytopathogenic bacterium Dickeya dadantii

“…In the opgG single mutant strain, pectinase activity remained inducible by PGA, given that activity was three fold higher after growth in medium containing PGA, but pectinase activities were decreased by a factor of two as compared with the wild-type strain (Fig. 3), as previously described (Page et al, 2001). For the pecS single mutant strain, pectinase activity remained inducible by PGA as activity was three times higher after growth in medium containing PGA, but pectinase activity was increased by a factor of two as compared with the wildtype strain (Fig.…”

“…Increased synthesis of exopolysaccharides is responsible for increased resistance to ROS in the opgG strain It was recently shown that in the symbiotic bacterium Sinorhizobium meliloti, exopolysaccharides had a protective effect against ROS (Lehman & Long, 2013) and exopolysaccharides are much more abundant around bacteria in opg strains as compared with the wild-type strain (Page et al, 2001). Second, in E. coli, it was shown that closing of the tricarboxylic acid (TCA) cycle generated ROS synthesis within bacteria, thus inducing the ROS stress response (Kohanski et al, 2008), and in a previous proteomic analysis (Bouchart et al, 2007), it was shown that in an opgG strain, increasing energy level was required as compared with a wild-type strain and resulted in a closed TCA cycle.…”

“…In Enterobacteria and related bacteria, both products of the opgGH operon are required for synthesis of the glucose backbone of OPGs. opgG or opgH mutant strains are completely devoid of OPGs and exhibit a reduced or non-virulent phenotype, indicating that these glucans belong to the common virulence factors of many zoopathogens and phytopathogens (Loubens et al, 1993;Mahajan-Miklos et al, 1999;Page et al, 2001).…”

Osmoregulated periplasmic glucans of Salmonella enterica serovar Typhimurium are required for optimal virulence in mice

“…have emerged from the present study. For example, opgGH mutants of Erwinia chrysanthemi showed increased capsular polysaccharide synthesis (giving rise to mucoid colonies) as well as hypersensitivity towards bile salts (Cogez et al, 2001;Page et al, 2001). No change in colony morphology or change in sensitivity towards bile salts was observed for strain SG111 (Fig S1a).…”

“…OPGs have also been demonstrated to play a significant role in establishing successful pathogenic or symbiotic associations with plant hosts (Arellano-Reynoso et al, 2005;Bhagwat & Keister, 1995;Page et al, 2001). Among animal pathogens, the role of OPGs has been examined in Brucella abortus (ArellanoReynoso et al, 2005), where synthesis of glucans is not osmoregulated (Briones et al, 1997).…”