Identification and characterization of a dextranase gene of Streptococcus criceti

Biochemical properties of a Flavobacterium johnsoniae dextranase and its biotechnological potential for Streptococcus mutans biofilm degradation

Cariogenic biofilms have a matrix rich in exopolysaccharides (EPS), mutans and dextrans, that contribute to caries development. Although several physical and chemical treatments can be employed to remove oral biofilms, those are only partly efficient and use of biofilm-degrading enzymes represents an exciting opportunity to improve the performance of oral hygiene products. In the present study, a member of a glycosyl hydrolase family 66 from Flavobacterium johnsoniae (FjGH66) was heterologously expressed and biochemically characterized. The recombinant FjGH66 showed a hydrolytic activity against an early EPS-containing S. mutans biofilm, and, when associated with a α-(1,3)-glucosyl hydrolase (mutanase) from GH87 family, displayed outstanding performance, removing more than 80% of the plate-adhered biofilm. The mixture containing FjGH66 and Prevotella melaninogenica GH87 α-1,3-mutanase was added to a commercial mouthwash liquid to synergistically remove the biofilm. Dental floss and polyethylene disks coated with biofilm-degrading enzymes also degraded plate-adhered biofilm with a high efficiency. The results presented in this study might be valuable for future development of novel oral hygiene products.

Leuconostoc mesenteroides and Liquorilactobacillus mali strains, isolated from Algerian food products, are producers of the postbiotic compounds dextran, oligosaccharides and mannitol

Six lactic acid bacteria (LAB) isolated from Algerian sheep's milk, traditional butter, date palm sap and barley, which produce dextran, mannitol, oligosaccharides and vitamin B2 have been characterized. They were identified as Leuconostoc mesenteroides (A4X, Z36P, B12 and O9) and Liquorilactobacillus mali (BR201 and FR123). Their exopolysaccharides synthesized from sucrose by dextransucrase (Dsr) were characterized as dextrans with (1,6)-D-glucopyranose units in the main backbone and branched at positions O-4, O-2 and/or O-3, with D-glucopyranose units in the side chain. A4X was the best dextran producer (4.5 g/L), while the other strains synthesized 2.1-2.7 g/L. Zymograms revealed that L. mali strains have a single Dsr with a molecular weight (Mw) of ~ 145 kDa, while the Lc. mesenteroides possess one or two enzymes with 170-211 kDa Mw. As far as we know, this is the first detection of L. mali Dsr. Analysis of metabolic fluxes from sucrose revealed that the six LAB produced mannitol (~ 12 g/L). The co-addition of maltose-sucrose resulted in the production of panose (up to 37.53 mM), an oligosaccharide known for its prebiotic effect. A4X, Z36P and B12 showed dextranase hydrolytic enzymatic activity and were able to produce another trisaccharide, maltotriose, which is the first instance of a dextranase activity encoded by Lc. mesenteroides strains. Furthermore, B12 and O9 grew in the absence of riboflavin (vitamin B2) and synthesized this vitamin, in a defined medium at the level of ~ 220 μg/L. Therefore, these LAB, especially Lc. mesenteroides B12, are good candidates for the development of new fermented food biofortified with functional compounds.

Glycoside hydrolases active on microbial exopolysaccharide α-glucans: structures and function

Glucose is the most abundant monosaccharide in nature and is an important energy source for living organisms. Glucose exists primarily as oligomers or polymers and organisms break it down and consume it. Starch is an important plant-derived α-glucan in the human diet. The enzymes that degrade this α-glucan have been well studied as they are ubiquitous throughout nature. Some bacteria and fungi produce α-glucans with different glucosidic linkages compared with that of starch, and their structures are quite complex and not fully understood. Compared with enzymes that degrade the α-(1→4) and α-(1→6) linkages in starch, biochemical and structural studies of the enzymes that catabolize α-glucans from these microorganisms are limited. This review focuses on glycoside hydrolases that act on microbial exopolysaccharide α-glucans containing α-(1→6), α-(1→3), and α-(1→2) linkages. Recently acquired information regarding microbial genomes has contributed to the discovery of enzymes with new substrate specificities compared with that of previously studied enzymes. The discovery of new microbial α-glucan-hydrolyzing enzymes suggests previously unknown carbohydrate utilization pathways and reveals strategies for microorganisms to obtain energy from external sources. In addition, structural analysis of α-glucan degrading enzymes has revealed their substrate recognition mechanisms and expanded their potential use as tools for understanding complex carbohydrate structures. In this review, the author summarizes the recent progress in the structural biology of microbial α-glucan degrading enzymes, touching on previous studies of microbial α-glucan degrading enzymes.

The role of dextran production in the metabolic context of Leuconostoc and Weissella Tunisian strains

High molecular weight dextrans improve the rheological properties of fermented products and have immunomodulatory and antiviral activity. We report on 5.84 × 10⁷-2.61 × 10⁸ Da dextrans produced by Leuconostoc lactis AV1n, Weissella cibaria AV2ou and Weissella confusa V30 and FS54 strains. Dextransucrases catalyze dextran synthesis by sucrose hydrolysis concomitant with fructose generation. The four bacteria have dextransucrases with molecular weight of about 160 kDa detected by zymograms. Each bacterium showed different interplay of dextran production and metabolic fluxes. All bacteria produced lactate, and AV2ou apart, synthesized mannitol from fructose. FS54 hydrolyzed dextran blue and the concentration of dextran produced by this bacterium decreased during the stationary phase. The AV1n binding to Caco-2 cells and polystyrene plates was higher under conditions for dextran synthesis. Thus, this is the first instance of a Weissella dextranase, associated with a dextransucrase ability, and of a positive influence of dextran on adhesion and aggregation properties of a bacterium.

Identification and characterization of an autolysin gene, atlA, from Streptococcus criceti

AtlA of Streptococcus mutans is a major autolysin and belongs to glycoside hydrolase family 25 with cellosyl of Streptomyces coelicolor. The autolysin gene (atlA) encoding AtlA was identified from S. criceti. AtlA of S. criceti comprises the signal sequence in the N-terminus, the putative cell-wall-binding domain in the middle, and the catalytic domain in the C-terminus. Homology modeling analysis of the catalytic domain of AtlA showed the resemblance of the spatial arrangement of five amino acids around the predicted catalytic cavity to that of cellosyl. Recombinant AtlA and its four point mutants, D655A, D747A, W831A, and D849A, were evaluated on zymogram of S. criceti cells. Lytic activity was destroyed in the mutants D655A and D747A and diminished in the mutants W831A and D849A. These results suggest that Asp655 and Asp747 residues are critical for lytic activity and Trp831 and Asp849 residues are also associated with enzymatic activity.

Identification and characterization of an autolysin gene, atlh, from Streptococcus downei

An autolysin gene, atlh, was identified and sequenced from Streptococcus downei MFe28 using degenerate polymerase chain reaction (PCR) and the gene-walking method. Atlh protein encoded by atlh is composed of 879 amino acids, with a molecular weight of 95,902.26. Atlh possesses four 15-amino-acid residue repeats in the putative cell-wall-binding domain and has a catalytic domain in the C-terminus. The deduced amino acid sequence of atlh showed homology to S. mutans autolysin AtlA (68.4% similarity). Inactivation of atlh resulted in elongated chain formation compared to the parent strain. Recombinant proteins Atlh and its derivatives were constructed and analyzed by zymography. Zymographic analysis revealed that the Asp-771 residue of Atlh was essential for lytic activity and that lytic activity was not diminished by the deletion of repetitive regions in the putative cell-wall-binding domain of Atlh. Biofilm assay showed that the wild-type strain formed glucose- and sucrose-dependent biofilms, the atlh mutant diminished this ability. These results suggest that Atlh is associated with cell separation and biofilm formation.