Role of the biofilm master regulator CsgD in cross-regulation between biofilm formation and flagellar synthesis

CsgD, the master regulator of biofilm formation, activates the synthesis of curli fimbriae and extracellular polysaccharides in Escherichia coli. To obtain insights into its regulatory role, we have identified a total of 20 novel regulation target genes on the E. coli genome by using chromatin immunoprecipitation (ChIP)-on-chip analysis with a high-density DNA microarray. By DNase I footprinting, the consensus CsgD-binding sequence predicted from a total of 18 target sites was found to include AAAAGNG(N(2))AAAWW. After a promoter-lacZ fusion assay, the CsgD targets were classified into two groups: group I genes, such as fliE and yhbT, are repressed by CsgD, while group II genes, including yccT and adrA, are activated by CsgD. The fliE and fliEFGH operons for flagellum formation are directly repressed by CsgD, while CsgD activates the adrA gene, which encodes an enzyme for synthesis of cyclic di-GMP, a bacterial second messenger, which in turn inhibits flagellum production and rotation. Taking these findings together, we propose that the cell motility for planktonic growth is repressed by CsgD, thereby promoting the switch to biofilm formation.     

The Disulfide Bonding System Suppresses CsgD-Independent Cellulose Production in Escherichia coli

The bacterial extracellular matrix encases cells and protects them from host-related and environmental insults. The Escherichia coli master biofilm regulator CsgD is required for the production of the matrix components curli and cellulose. CsgD activates the diguanylate cyclase AdrA, which in turn stimulates cellulose production through cyclic di-GMP (c-di-GMP). Here, we identified and characterized a CsgD- and AdrA-independent cellulose production pathway that was maximally active when cultures were grown under reducing conditions or when the disulfide bonding system (DSB) was compromised. The CsgD-independent cellulose activation pathway was dependent on a second diguanylate cyclase, called YfiN. c-di-GMP production by YfiN was repressed by the periplasmic protein YfiR, and deletion of yfiR promoted CsgD-independent cellulose production. Conversely, when YfiR was overexpressed, cellulose production was decreased. Finally, we found that YfiR was oxidized by DsbA and that intraprotein YfiR disulfide bonds stabilized YfiR in the periplasm. Altogether, we showed that reducing conditions and mutations in the DSB system caused hyperactivation of YfiN and subsequent CsgD-independent cellulose production.     

Cyclic di-GMP signalling controls virulence properties of Salmonella enterica serovar Typhimurium at the mucosal lining

Cyclic di-GMP (c-di-GMP), a novel secondary signalling molecule present in most bacteria, controls transition between motility and sessility. In Salmonella enterica serovar Typhimurium ( S. typhimurium ) high c-di-GMP concentrations favour the expression of a biofilm state through expression of the master regulator CsgD. In this work, we investigate the effect of c-di-GMP signalling on virulence phenotypes of S. typhimurium. After saturation of the cell with c-di-GMP by overexpression of a di-guanylate cyclase, we studied invasion and induction of a pro-inflammatory cytokine in epithelial cells, basic phenotypes that are major determinants of S. typhimurium virulence. Elevated c-di-GMP had a profound effect on invasion into and IL-8 production by the gastrointestinal epithelial cell line HT-29. Invasion was mainly inhibited through CsgD and the extracellular matrix component cellulose, while inhibition of the pro-inflammatory response occurred through CsgD, which inhibited the secretion of monomeric flagellin. Our results suggest that transition between biofilm formation and virulence in S. typhimurium at the epithelial cell lining is mediated by c-di-GMP signalling through CsgD and cellulose expression.     

Genetic determinants of biofilm development of opaque and translucent Vibrio parahaemolyticus

Vibrio parahaemolyticus isolates display variation in colony morphology, alternating between opaque (OP) and translucent (TR) cell types. Phase variation is the consequence of genetic alterations in the locus encoding the quorum sensing output regulator OpaR. Here, we show that both cell types form stable, but distinguishable biofilms that differ with respect to attachment and detachment profiles to polystyrene, pellicle formation and stability at the air/medium interface, and submerged biofilm architecture and dispersion at a solid/liquid interface. The pellicle, which is a cohesive mat of cells, was exploited to identify mutants having altered or defective biofilm formation. Transposon insertion mutants were obtained with defects in genes affecting multiple cell surface characteristics, including extracellular polysaccharide, mannose-sensitive haemagglutinin type 4 pili and polar (but not lateral) flagella. Other insertions disrupted genes coding for potential secreted proteins or transporters of secreted proteins, specifically haemolysin co-regulated protein and an RTX toxin-like membrane fusion transporter, as well as potential modifiers of cell surface molecules (nagAC operon). The pellicle screen also identified mutants with lesions in regulatory genes encoding H-NS, a CsgD-like repressor and an AraC-like protein. This work initiates the characterization of V. parahaemolyticus biofilm formation in the OP and TR cell types and identifies a diverse repertoire of cell surface elements that participate in determining multicellular architecture.     

牛磺胆酸作用下副溶血弧菌全基因组比较转录谱的表达分析

目的利用DNA芯片技术研究副溶血弧菌对牛磺胆酸刺激反应的全局性基因转录变化概况,找出其中的表达调控变化规律,为副溶血弧菌基因转录调控网络的构建提供实验和理论依据。方法副溶血弧菌分别在正常和添加了50mmol／L牛磺胆酸的培养基中孵育至对数中期,收集菌体,提取RNA,利用全基因组DNA芯片分析比较两者基因转录变化。并应用聚类分析比较其中的变化规律。结果比较转录谱分析证实一共有255个基因的转录表达发生显著性变化,和对照组相比,上调的基因明显占主导优势。而在这些变化的基因中,关于蛋白合成和硫代谢以及谷氨酸合成相关的基因均呈现明显的转录上调变化。结论我们利用DNA芯片技术描绘出了副溶血弧菌在添加牛磺胆酸后全部基因转录水平变化的概图,并发现了蛋白合成,硫代谢和谷氨酸合成相关的基因的变化规律,这给我们下一步的转录调控网络研究提供了良好的靶标。     

Quorum sensing and silencing in Vibrio parahaemolyticus

The quorum regulatory cascade is poorly characterized in Vibrio parahaemolyticus, in part because swarming and virulence factors--the hallmarks of the organism--are repressed by this scheme of gene control, and quorum sensing seems to be silenced in many isolates. In these studies, we examine a swarming-proficient, virulent strain and identify an altered-function allele of the quorum regulator luxO that is demonstrated to produce a constitutively active mimic of LuxO～P. We find that LuxO* affects the expression of three small regulatory RNAs (Qrrs) and the activity of a translational fusion in opaR, the output regulator. Tests for epistasis showed that luxO* is dominant over luxO and that opaR is dominant over luxO. Thus, information flow through the central elements of the V. parahaemolyticus quorum pathway is proven for the first time. Quorum-sensing output was explored using microarray profiling: the OpaR regulon encompasses ～5.2% of the genome. OpaR represses the surface-sensing and type III secretion system 1 (T3SS1) regulons. One novel discovery is that OpaR strongly and oppositely regulates two type VI secretion systems (T6SS). New functional consequences of OpaR control were demonstrated: OpaR increases the cellular cyclic di-GMP (c-di-GMP) level, positively controls chitin-induced DNA competency, and profoundly blocks cytotoxicity toward host cells. In expanding the previously known quorum effects beyond the induction of the capsule and the repression of swarming to elucidate the global scope of genes in the OpaR regulon, this study yields many clues to distinguishing traits of this Vibrio species; it underscores the profoundly divergent survival strategies of the quorum On/Off phase variants.     

Role of MetR and PurR in the activation of glyA by CsgD in Escherichia coli K-12

The csgD gene of Escherichia coli is required for the expression of curli fibres, surface fibres that are important for biofilm formation and infection. Previously, we demonstrated that expression of CsgD from a multicopy plasmid increased expression of the glyA gene, which codes for serine hydroxymethyltransferase. We show here that this activation requires the participation of both known regulatory proteins, MetR and PurR. The adjacent divergently transcribed gene hmp was weakly induced by CsgD, but its induction did not require MetR or PurR. The effect of CsgD on the expression of several pur and met genes was also tested.