Polysaccharases for microbial exopolysaccharides

The T7-related Pseudomonas putida phage φ15 displays virion-associated biofilm degradation properties

Formation of a protected biofilm environment is recognized as one of the major causes of the increasing antibiotic resistance development and emphasizes the need to develop alternative antibacterial strategies, like phage therapy. This study investigates the in vitro degradation of single-species Pseudomonas putida biofilms, PpG1 and RD5PR2, by the novel phage ϕ15, a ‘T7-like virus’ with a virion-associated exopolysaccharide (EPS) depolymerase. Phage ϕ15 forms plaques surrounded by growing opaque halo zones, indicative for EPS degradation, on seven out of 53 P. putida strains. The absence of haloes on infection resistant strains suggests that the EPS probably act as a primary bacterial receptor for phage infection. Independent of bacterial strain or biofilm age, a time and dose dependent response of ϕ15-mediated biofilm degradation was observed with generally a maximum biofilm degradation 8 h after addition of the higher phage doses (10⁴ and 10⁶ pfu) and resistance development after 24 h. Biofilm age, an in vivo very variable parameter, reduced markedly phage-mediated degradation of PpG1 biofilms, while degradation of RD5PR2 biofilms and ϕ15 amplification were unaffected. Killing of the planktonic culture occurred in parallel with but was always more pronounced than biofilm degradation, accentuating the need for evaluating phages for therapeutic purposes in biofilm conditions. EPS degrading activity of recombinantly expressed viral tail spike was confirmed by capsule staining. These data suggests that the addition of high initial titers of specifically selected phages with a proper EPS depolymerase are crucial criteria in the development of phage therapy.

Threshold concentrations of biomass and iron for pressure drop increase in spiral-wound membrane elements

In a model feed channel for spiral-wound membranes the quantitative relationship of biomass and iron accumulation with pressure drop development was assessed. Biofouling was stimulated by the use of tap water enriched with acetate at a range of concentrations (1-1000 μgCl(-1)). Autopsies were performed to quantify biomass concentrations in the fouled feed channel at a range of Normalized Pressure Drop increase values (NPD(i)). Active biomass was determined with adenosinetriphosphate (ATP) and the concentration of bacterial cells with Total Direct Cell count (TDC). Carbohydrates (CH) were measured to include accumulated extracellular polymeric substances (EPS). The paired ATP and CH concentrations in the biofilm samples were significantly (p<0.001; R(2)=0.62) correlated and both parameters were also significantly correlated with NPD(i) (p<0.001). TDC was not correlated with the pressure drop in this study. The threshold concentration for an NPD(i) of 100% was 3.7 ng ATP cm(-2) and for CH 8.1 μg CH cm(-2). Both parameters are recommended for diagnostic membrane autopsy studies. Iron concentrations of 100-400 mg m(-2) accumulated in the biofilm by adsorption were not correlated with the observed NPD(i), thus indicating a minor role of Fe particulates at these concentrations in fouling of spiral-wound membrane.

Bacteriophage resistance mechanisms

Phages are now acknowledged as the most abundant microorganisms on the planet and are also possibly the most diversified. This diversity is mostly driven by their dynamic adaptation when facing selective pressure such as phage resistance mechanisms, which are widespread in bacterial hosts. When infecting bacterial cells, phages face a range of antiviral mechanisms, and they have evolved multiple tactics to avoid, circumvent or subvert these mechanisms in order to thrive in most environments. In this Review, we highlight the most important antiviral mechanisms of bacteria as well as the counter-attacks used by phages to evade these systems.

Characterization of a new acid stable exo-beta-1,3-glucanase of Rhizoctonia solani and its action on microbial polysaccharides

A new acid stable exo-beta-1,3-glucanase of Rhizoctonia solani purified from a commercial source 'Kitarase-M', by a combination of ammonium sulfate precipitation, ion-exchange and gel filtration methods, had specific activity of 0.26 U/mg protein, Km and Vmax values of 0.78 mg/ml and 0.27 mM/min/mg protein, respectively. It had molecular weight of 62 kDa with optimum activity at 40 degrees C temperature and pH 5.0, with high stability at pH of 3-7. Unique amino acid sequence was found at N-terminal end. The substrate specificity studies confirmed that it is an exo-beta-1,3-glucanase. It could hydrolyze curdlan powder to release glucose.

Extracellular polysaccharides produced by cooling water tower biofilm bacteria and their possible degradation

The extracellular polymers (EPS) of biofilm bacteria that can cause heat and mass transfer problems in cooling water towers in the petrochemical industry were investigated. In addition, these microorganisms were screened for their ability to grow and degrade their own EPS and the EPS of other species. Twelve bacteria producing the most EPS were isolated from cooling water towers and characterized biochemically by classic and commercial systems. These were species of Pseudomonas, Burkholderia, Aeromonas, Pasteurella, Pantoea, Alcaligenes and Sphingomonas. EPS of these species were obtained by propan-2-ol precipitation and centrifugation from bacterial cultures in media enriched with glucose, sucrose or galactose. EPS yields were of 1.68-4.95 g l(-1). These EPS materials were characterized for total sugar and protein contents. Their total sugar content ranged from 24 to 56% (g sugar g(-1) EPS), and their total protein content ranged from 10 to 28% (g protein g(-1) EPS). The monosaccharide compositions of EPS were determined by HPLC. Generally, these compositions were enriched in galactose and glucose, with lesser amounts of mannose, rhamnose, fructose and arabinose. All bacteria were investigated in terms of EPS degradation. Eight of the bacteria were able to utilize EPS from Burkholderia cepacia, seven of the bacteria were able to utilize EPS from Pseudomonas sp. and Sphingomonas paucimobilis. The greatest viscosity reduction of B. cepacia was obtained with Pseudomonas sp. The results show that the bacteria in this study are able to degrade EPS from biofilms in cooling towers.

Diatom-derived carbohydrates as factors affecting bacterial community composition in estuarine sediments

Microphytobenthic biofilms in estuaries, dominated by epipelic diatoms, are sites of high primary productivity. These diatoms exude large quantities of extracellular polymeric substances (EPS) comprising polysaccharides and glycoproteins, providing a substantial pool of organic carbon available to heterotrophs within the sediment. In this study, sediment slurry microcosms were enriched with either colloidal carbohydrates or colloidal EPS (cEPS) or left unamended. Over 10 days, the fate of these carbohydrates and changes in beta-glucosidase activity were monitored. Terminal restriction fragment length polymorphism (T-RFLP), DNA sequencing, and quantitative PCR (Q-PCR) analysis of 16S rRNA sequences were used to determine whether sediment bacterial communities exhibited compositional shifts in response to the different available carbon sources. Initial heterotrophic activity led to reductions in carbohydrate concentrations in all three microcosms from day 0 to day 2, with some increases in beta-glucosidase activity. During this period, treatment-specific shifts in bacterial community composition were not observed. However, by days 4 and 10, the bacterial community in the cEPS-enriched sediment diverged from those in colloid-enriched and unamended sediments, with Q-PCR analysis showing elevated bacterial numbers in the cEPS-enriched sediment at day 4. Community shifts were attributed to changes in cEPS concentrations and increased beta-glucosidase activity. T-RFLP and sequencing analyses suggested that this shift was not due to a total community response but rather to large increases in the relative abundance of members of the gamma-proteobacteria, particularly Acinetobacter-related bacteria. These experiments suggest that taxon- and substrate-specific responses within the bacterial community are involved in the degradation of diatom-derived extracellular carbohydrates.

Comparison of dyes for easy detection of extracellular cellulases in fungi

To evaluate which dye is effective in a plate assay for detecting extracellular cellulase activity produced by fungi, four chromogenic dyes including remazol brilliant blue, phenol red, congo red, and tryphan blue, were compared using chromagenic media. For the comparison, 19 fungal species belonging to three phyla, ascomycota, basidiomycota, and zygomycota were inoculated onto yeast nitrogen-based media containing different carbon substrates such as cellulose (carboxylmethyl and avicel types) and cellobiose labeled with each of the four dyes. Overall, the formation of clear zone on agar media resulting from the degradation of the substrates by the enzymes secreted from the test fungi was most apparent with media containing congo red. The detection frequency of cellulase activity was also most high on congo red-supplemented media. The results of this study showed that congo red is better dye than other three dyes in a plate assay for fungal enzyme detection.

Production of ?-glucan and related glucan-hydrolases by Botryosphaeria rhodina

AIMS

Characterization of beta-glucan production from Botryosphaeria rhodina DABAC-P82 by detecting simultaneously glucan-hydrolytic enzymes and their localization, culture medium rheology and oxygen transfer.

METHODS AND RESULTS

Mycelium growth, beta-glucan production, substrate consumption and glucan-hydrolytic enzymes were monitored both in shaken flasks and in a 3-l stirred-tank bioreactor. Glucan production (19.7 and 15.2 g l(-1), in flask and bioreactor, respectively) was accompanied by extra-cellular and cell-bound beta-glucanase and beta-glucosidase activities. In the bioreactor scale, in the time interval of 0-78 h the apparent viscosity of the culture broth exhibited a general increase; thereafter, it began to reduce, probably because of the above glucan-hydrolytic activities. Moreover, the culture media collected after 45 h behaved as solid-like materials at shear rates smaller than 0.001 s(-1), as pseudo-plastic liquids in the middle shear rate range and as Newtonian ones at shear rates greater than 1000 s(-1).

CONCLUSION

The greatest beta-glucan accumulation in the bioreactor was found to be associated with nitrogen and dissolved oxygen concentrations smaller than 0.15 g l(-1) and 25%, respectively, and with the peak points of the glucan-degrading enzymes.

SIGNIFICANCE AND IMPACT OF THE STUDY

A careful analysis of the critical factors (such as, culture broth rheology, oxygen mass transfer and glucan-hydrolytic enzymes) limiting the beta-glucan production by B. rhodina is a prerequisite to maximize beta-glucan yield and production, as well as to define the process flow sheet capable of maximizing biopolymer recovery, solvent re-utilization and glucose consumption.

Biodegradability of extracellular polymeric substances produced by aerobic granules

This study investigated the biodegradability of extracellular polymeric substances (EPS) produced by aerobic granules. Aerobic granules were precultivated with synthetic wastewater in a lab-scale sequencing batch reactor. EPS were extracted from aerobic granules and were then fed as the sole carbon source to their own producers. Results showed that about 50% of EPS produced by aerobic granules could be utilized by their producers under aerobic starvation condition. The average biodegradation rate of the granule EPS in terms of chemical oxygen demand was five times slower than that of acetate, but 50 times faster than that of nonbiodegradable EPS produced by aerobic granules. The nonbiodegradable EPS was mainly found on the outer shell of aerobic granule. EPS produced by aerobic granules basically comprised two major components, i.e., biodegradable and nonbiodegradable EPS. The biodegradable EPS could serve as a useful energy source to sustain the growth of aerobic granules under starvation. This study provides experimental evidence that part of the EPS produced by aerobic granules would be biodegradable, but only nonbiodegradable EPS would play a crucial role in maintaining the structural integrity of aerobic granule.

Paenibacillus soli sp. nov., a xylanolytic bacterium isolated from soil

Two novel polysaccharide-degrading bacteria (strains DCY03T and DCY04) were isolated from a soil sample of a ginseng field in the Republic of Korea and were identified as representing members of the genus Paenibacillus on the basis of phenotypic characteristics and phylogenetic inference based on 16S rRNA gene sequences. Cells of the two isolates were Gram-positive, spore-forming, non-motile, straight rods. Based on DNA–DNA relatedness data, the strains were considered to belong to the same species. The DNA G+C content ranged from 56.6 to 57.0 mol%. The predominant cellular fatty acid was anteiso-C15 : 0 (63.8–62.8 %). Levels of 16S rRNA gene sequence similarity between the two novel isolates and the type strains of recognized Paenibacillus species were 91.4–96.5 %. Strains DCY03T and DCY04 could clearly be distinguished from phylogenetically closely related Paenibacillus species on the basis of DNA–DNA relatedness data and phenotypic characteristics. Therefore, on the basis of these data, the two isolates are considered to represent a novel species of the genus Paenibacillus, for which the name Paenibacillus soli sp. nov. is proposed. The type strain is DCY03T (=KCTC 13010T=LMG 23604T).

Biosynthesis of a thermostable gellan lyase by newly isolated and characterized strain of Geobacillus stearothermophilus 98

The thermophilic strain able to degrade gellan was isolated from Bulgarian hot spring. According to its morphological and biochemical properties and by partial sequencing of its 16S rDNA, it was classified as Geobacillus stearothermophilus. It grew in a synthetic medium with gellan as the only carbon source with a specific growth rate of 0.69 h(-1) and generation time of 60 min. The strain produced thermostable gellan lyase extracellularly during exponential phase. Its synthesis was inducible; the enzyme was not registered in culture liquid without gellan. The enzyme activity was increased tenfold in conditions of continuous cultivation compared to data from batch fermentations and enzyme productivity was almost sixfold higher. The enzyme showed optimal activity at 75 degrees C in a very large pH area 4-8.5. This enzyme is the first reported thermostable gellan lyase, its residual activity was 100% after 24 h incubation at 60 degrees C and its half-life was 60 min at 70 degrees C.

Cellulomonas terrae sp. nov., a cellulolytic and xylanolytic bacterium isolated from soil

A bacterial strain (DB5(T)), with polysaccharide-degrading activities, was isolated from garden soil in Daejeon, Republic of Korea. The cells were Gram-positive, aerobic or facultatively anaerobic, non-motile straight rods. Phylogenetic analysis based on 16S rRNA gene sequences showed that this strain belongs to the genus Cellulomonas and that it is most closely related to Cellulomonas xylanilytica LMG 21723(T) and Cellulomonas humilata ATCC 25174(T) (98.0 and 97.9% similarity, respectively). Chemotaxonomic data also supported the classification of strain DB5(T) in the genus Cellulomonas, i.e. L-ornithine as the cell-wall diamino acid, anteiso-C(15:0) and iso-C(15:0) as the major fatty acids, MK-9(H(4)) as the predominant menaquinone and the presence of diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylglycerol and phosphatidylinositol mannosides in the polar lipid profile. The results of DNA-DNA hybridization in combination with chemotaxonomic and physiological data demonstrated that strain DB5(T) (=KCTC 19081(T)=NBRC 100819(T)) should be classified as the type strain of a novel species within the genus Cellulomonas, for which the name Cellulomonas terrae sp. nov. is proposed.

Structural characterisation of a highly branched exopolysaccharide produced by Lactobacillus delbrueckii subsp. bulgaricus NCFB2074

Lactobacillus delbrueckii subsp. bulgaricus NCFB2074 when grown in skimmed milk secretes a highly branched exopolysaccharide. The exopolysaccharide has a heptasaccharide repeat unit and is composed of glucose and galactose in the molar ratio 3:4. Using chemical techniques and 1D and 2D NMR spectroscopy the polysaccharide has been shown to possess the following repeat unit structure: [carbohydrate structure: see text].

The interaction of phage and biofilms

Biofilms present complex assemblies of micro-organisms attached to surfaces. they are dynamic structures in which various metabolic activities and interactions between the component cells occur. When phage come in contact with biofilms, further interactions occur dependent on the susceptibility of the biofilm bacteria to phage and to the availability of receptor sites. If the phage also possess polysaccharide-degrading enzymes, or if considerable cell lysis is effected by the phage, the integrity of the biofilm may rapidly be destroyed. Alternatively, coexistence between phage and host bacteria within the biofilm may develop. Although phage have been proposed as a means of destroying or controlling biofilms, the technology for this has not yet been successfully developed.

Development of a plate technique for screening of polysaccharide-degrading microorganisms by using a mixture of insoluble chromogenic substrates

A plate assay based on the visible solubilization of small substrate particles and the formation of haloes on Petri dishes, containing a mixture of different dye-labelled polysaccharides as substrates, provides a specific, reliable and rapid simultaneous detection of corresponding polysaccharide-degrading microorganisms. It has potential for increasing the efficacy of screening of microorganisms, utilizing different polysaccharides, in large numbers of natural samples. Diversely colored insoluble forms of amylose, xylan and hydroxyethyl-cellulose (HE-cellulose) were prepared as chromogenic substrates by using the cross-linking reagent 1,4-butanediol diglycidyl ether and the dyes Brilliant Red 3B-A, Cibacron Blue 3GA and Reactive Orange 14. Using the method, the bacteria with amylase or xylanase or cellulase or a combination of these activities were screened from soil and sludge samples, selected and identified according to 16S rDNA sequencing.

Molecular organization of exopolysaccharide (EPS) encoding genes on the lactococcal bacteriophage adsorption blocking plasmid, pCI658

The lactococcal plasmid pCI658 (58 kb) isolated from Lactococcus lactis ssp. cremoris HO2 encodes the production of a hydrophilic exopolysaccharide (EPS) which consists primarily of galactose and glucuronic acid and which interferes with adsorption of phages ø712 and øc2 to cell surface receptors. Examination of the nucleotide sequence of a 21.8-kb region of the plasmid revealed a large genetic cluster consisting of at least 23 putative EPS biosynthetic determinants in addition to the presence of insertion sequences at the 5(') and 3(') ends. According to homology searches, the genes were organized in specific regions involved in regulation, synthesis and export of the EPS. The predicted products of individual genes exhibited significant homology to exopolysaccharide, capsular polysaccharide (CPS), and lipopolysaccharide (LPS) gene products from a variety of Gram positive and Gram negative bacteria. Evidence of a gene encoding UDP-glucose dehydrogenase is also presented and this is the first description of such a gene in Lactococcus.

Effect of medium composition and temperature and pH changes on exopolysaccharide yields and stability during Streptococcus thermophilus LY03 fermentations

To increase the exopolysaccharide (EPS) yields from Streptococcus thermophilus LY03 and to unravel the nature of the EPS degradation process, fermentation experiments were carried out with this strain in a customized MRS medium, using different additional carbohydrates or amino acids possibly related to growth and EPS production. No significant increase of the EPS yields or activities of the enzymes alpha-phosphoglucomutase, UDP-glucose pyrophosphorylase and UDP-galactose 4-epimerase that are correlated with EPS production, or of the activity of dTDP-glucose pyrophosphorylase involved in the rhamnose synthetic branch of EPS biosynthesis, was observed. The EPS monomer composition remained unchanged for all experiments. Fermentations with a sudden temperature increase or lowered pH were carried out as well to try to avoid EPS degradation upon prolonged fermentation. It was demonstrated that EPS degradation took place enzymatically. Incubations of purified high-molecular-mass EPS with cell-free culture supernatant or cell extracts showed its degradation by enzymes with an endo-activity. This glycohydrolytic activity probably encompasses several enzymes having a molecular mass lower than 50,000 and 10,000 Da, and seems to be rather stable at high temperature and low pH. These results contribute to a better understanding of the physiological and chemical factors influencing EPS production and degradation.

Evidence for protection of nitrogenase from O(2) by colony structure in the aerobic diazotroph Gluconacetobacter diazotrophicus

Gluconacetobacter diazotrophicus is an endophytic diazotroph of sugarcane which exhibits nitrogenase activity when growing in colonies on solid media. Nitrogenase activity of G. diazotrophicus colonies can adapt to changes in atmospheric partial pressure of oxygen (pO(2)). This paper investigates whether colony structure and the position of G. diazotrophicus cells in the colonies are components of the bacterium's ability to maintain nitrogenase activity at a variety of atmospheric pO(2) values. Colonies of G. diazotrophicus were grown on solid medium at atmospheric pO(2) of 2 and 20 kPa. Imaging of live, intact colonies by confocal laser scanning microscopy and of fixed, sectioned colonies by light microscopy revealed that at 2 kPa O(2) the uppermost bacteria in the colony were very near the upper surface of the colony, while the uppermost bacteria of colonies cultured at 20 kPa O(2) were positioned deeper in the mucilaginous matrix of the colony. Disruption of colony structure by physical manipulation or due to 'slumping' associated with colony development resulted in significant declines in nitrogenase activity. These results support the hypothesis that G. diazotrophicus utilizes the path-length of colony mucilage between the atmosphere and the bacteria to achieve a flux of O(2) that maintains aerobic respiration while not inhibiting nitrogenase activity.

Purification and properties of a glucuronan lyase from Sinorhizobium meliloti M5N1CS (NCIMB 40472)

A glucuronan lyase extracted from Sinorhizobium meliloti strain M5N1CS was purified to homogeneity by anion-exchange chromatography. The purified enzyme corresponds to a monomer with a molecular mass of 20 kDa and a pI of 4.9. A specific activity was found only for polyglucuronates leading to the production of 4,5-unsaturated oligoglucuronates. The enzyme activity was optimal at pH 6.5 and 50 degrees C. Zn(2+), Cu(2+), and Hg(2+) (1 mM) inhibited the enzyme activity. No homology of the enzyme N-terminal amino acid sequence was found with any of the previously published protein sequences. This enzyme purified from S. meliloti strain M5N1CS corresponding to a new lyase was classified as an endopolyglucuronate lyase.

The catalytic activities of the bifunctional Azotobacter vinelandii mannuronan C-5-epimerase and alginate lyase AlgE7 probably originate from the same active site in the enzyme

The Azotobacter vinelandii genome encodes a family of seven secreted Ca(2+)-dependent epimerases (AlgE1--7) catalyzing the polymer level epimerization of beta-D-mannuronic acid (M) to alpha-L-guluronic acid (G) in the commercially important polysaccharide alginate. AlgE1--7 are composed of two types of protein modules, A and R, and the A-modules have previously been found to be sufficient for epimerization. AlgE7 is both an epimerase and an alginase, and here we show that the lyase activity is Ca(2+)-dependent and also responds similarly to the epimerases in the presence of other divalent cations. The AlgE7 lyase degraded M-rich alginates and a relatively G-rich alginate from the brown algae Macrocystis pyrifera most effectively, producing oligomers of 4 (mannuronan) to 7 units. The sequences cleaved were mainly G/MM and/or G/GM. Since G-moieties dominated at the reducing ends even when mannuronan was used as substrate, the AlgE7 epimerase probably stimulates the lyase pathway, indicating a complex interplay between the two activities. A truncated form of AlgE1 (AlgE1-1) was converted to a combined epimerase and lyase by replacing the 5'-798 base pairs in the algE1-1 gene with the corresponding A-module-encoding DNA sequence from algE7. Furthermore, substitution of an aspartic acid residue at position 152 with glycine in AlgE7A eliminated almost all of both the lyase and epimerase activities. Epimerization and lyase activity are believed to be mechanistically related, and the results reported here strongly support this hypothesis by suggesting that the same enzymatic site can catalyze both reactions.

Structural characterisation of the exopolysaccharide produced by Streptococcus thermophilus EU20

Streptococcus thermophilus EU20 when grown on skimmed milk secretes a high-molecular-weight exopolysaccharide that is composed of glucose, galactose and rhamnose in a molar ratio of 2:3:2. Using chemical techniques and 1D and 2D-NMR spectroscopy (1H and 13C) the polysaccharide has been shown to possess a heptasaccharide repeating unit having the following structure: [chemical structure: see text]. Treatment of the polysaccharide with mild acid (0.5 M TFA, 100 degrees C for 1 h) liberates two oligosaccharides; the components correspond to the repeating unit and a hexasaccharide equivalent to the repeating unit minus the terminal alpha-L-Rhap.

Purification and characterisation of a beta-galactosidase from Aspergillus aculeatus with activity towards (modified) exopolysaccharides from Lactococcus lactis subsp. cremoris B39 and B891

Beta-galactosidase from Aspergillus aculeatus was purified from a commercial source for its hydrolytic activity towards (modified) exopolysaccharides (EPSs) produced by Lactococcus lactis subsp. cremoris B39 and B891. The enzyme had a molecular mass of approximately 120 kDa, a pI between 5.3 and 5.7 and was optimally active at pH 5.4 and 55-60 degrees C. Based on the N-terminal amino acid sequence, the enzyme probably belongs to family 35 of the glycosyl hydrolases. The catalytic mechanism was shown to be retaining and transglycosylation products were demonstrated using lactose as a substrate. The beta-galactosidase was also characterised using its activity towards two EPSs having lactosyl side chains attached to different backbone structures. The enzyme degraded O-deacetylated EPS B891 faster than EPS B39. Furthermore, the presence of acetyl groups in EPS B891 slowed down the hydrolysing rate, but the enzyme was still able to release all terminally linked galactose.

Structural characterisation and enzymic modification of the exopolysaccharide produced by Lactococcus lactis subsp. cremoris B891

Lactococcus lactis subsp. cremoris B891 grown on whey permeate produced an exopolysaccharide containing D-Gal and D-Glc in a molar ratio of 2:3. The polysaccharide was partially O-acetylated. By means of HF solvolysis, O-deacetylation, enzymic modification, sugar linkage analysis and ID/2D NMR studies the exopolysaccharide was shown to be composed of repeating units with the following structure: [structure: see text].