Leuconostoc Strains Research Articles

Antimicrobial Efficacy of Leuconostoc spp. Isolated from Indian Meat against Escherichia coli and Listeria monocytogenes in Spinach Leaves.

Five Leuconostoc strains (CM17, CM19, PM30, PM32, and PM36) previously isolated from Indian meat showed promising antimicrobial activity against food pathogens in screening assay. This study evaluates the efficacy of these isolates against Escherichia coli Microbial Type Culture Collection and Gene Bank (MTCC) 443 and Listeria monocytogenes (MTCC 657) in spinach leaves. Challenge studies were conducted by inoculating E. coli and L. monocytogenes at 6 to 7 Log10 CFU/g of the leaves respectively and treating them with cell free supernatant (CFS) of 48 h cultures of the isolates. The samples were stored at 4°C and analyzed over a period of 5 d. The study was conducted in triplicates and statistical analysis was carried out using one-way Anova. The counts of the pathogens did not increase over the 5 d period in the control samples, without any treatment. Whereas in the case of CFS treatments, significant reduction (p<0.05) was observed in both E. coli and L. monocytogenes from 1 to 5 d with all the 5 strains as compared to the control. The counts of Listeria dropped by 0.5 to 1 log by 5 d, with PM 36 showing the highest reduction (1 log). In the case of E. coli, 1.1 to 1.5 log reduction was observed by 5 d, with again PM 36 showing the highest reduction (1.5). The overall results indicate that the isolates (specifically PM36) not only showed efficacy in in vitro studies but are also proved to be effective in food matrix making them potential clean label antimicrobial alternatives for food application.

The incorporation of sourdough in sugar-reduced biscuits: a promising strategy to improve techno-functional and sensory properties

The demand for sugar-reduced, low-sugar, or even sugar-free food products is increasing. Sweet bakery products are the second main source of added sugar after sugary drinks. The reduction of sugar causes a loss of product quality, since sugar contributes to structure and flavour. The most common sugar replacers on the market are polyols, which act as sweet bulking agents. However, alternatives, which can be considered as ‘clean-label’ are in high demand. Sourdough technology was applied using the bi-functional lactic acid bacteria strain Leuconostoc citreum TR116. This strain is able to produce mannitol, which contributes to sweetness, and/or exopolysaccharides to compensate structure loss in sugar-reduced products. Besides the full-sugar biscuit (C1), biscuits reduced in sugar by 75% by wheat starch (C2) or by commercially available mannitol (C3) were considered as controls. Wheat starch as a sugar replacer created a lower biscuit firmness (− 10.7 N), while mannitol increased the hardness significantly (+ 12.9 N). Both sugar replacers caused less biscuit spreading and a poorer sensory profiles. The addition of sourdough (5% or 10%) in a sugar-reduced biscuit by wheat starch (C2) improved the viscoelastic properties, dough stickiness and dough hardness, as well as biscuit firmness. Furthermore, it contributed to colour (lowered the ΔE value) and increased sweetness and flavour intensity (+ 140%; + 139%). The predicted GI of C2-biscuits (73.5) were lower than C3-biscuits (80.8). Sourdough did not influence the release of reducing sugars during digestion. In conclusion, 10% sourdough incorporation represents a useful tool to overcome quality loss caused by the reduction of sugar by improving texture, taste and flavour.

Antibacterial and antibiofilm activity of the bacteriocin-producing strain Leuconostoc mesenteroides CHBY46 isolated from Algerian dromedary milk against Pseudomonas aeruginosa and Staphylococcus aureus biofilms

In order to control biofilm formation of pathogenic and spoilage bacteria in foods, some species of Leuconostoc are very important in food industries, as they increase the shelf life of foods during preservation. In this study the strain CHBY46 a bacteriocin-producing strain belonging to Leuconostoc ge-nus isolated from dromedary milk in the south of Algeria was characterized by 16S rRNA gene sequencing and MALDITOF MS mass spectrometry, tested for its antibacterial and antibiofilm activities against Pseudomonas aeruginosa and Staphylococcus aureus. The produced bacteriocin was partially puri-fied with sulfate ammonium precipitation and RP-HPLC. The strain CHBY46 was classified as Leuc. mesenteroides after molecular identification. Among the bacteria tested the pathogens Staph. aureus ATCC 29213 and Ps. aeru-ginosa ATCC 27653 were sensitive to this bacteriocin with 480 AU/ml. Antibi-ofilm activity was investigated by crystal violet assay. The bacteriocin of Leuc. mesenteroides CHBY 46 exhibited significant biofilm inhibition ; 35.58% with Ps. aeruginosa, and 42.11% with Staph. aureus. Tricine-SDS-PAGE analysis of the partially purified bacteriocin indicated a low molecular weight of approximately 3.5 kDa. Therfore, we conclude that bacteriocins from Leuco-nostoc have the potential as a therapeutic strategy against pathogen’s bio-films, which contribute, to bacterial pathogenicity and resistance toward antibiotics or being used in foods as adjunsts to contribute food safety.

Glycosylation of caffeic acid and structural analogues catalyzed by novel glucansucrases from Leuconostoc and Weissella species

Twelve Leuconostoc and seven Weissella strains with extracellular glucansucrase activity were obtained from an analysis of 41 lactic acid bacteria. Culture supernatants of all glucansucrase positive strains catalyzed the glycosylation of caffeic acid with sucrose as donor substrate. Eighteen enzymes produced one major peak, which was identified as caffeic acid-4′-O-α-D-monoglucoside by LC-MS and NMR spectroscopy. Only W. beninensis DSM 22752 formed significant amounts of the corresponding 3´-O-α-D-monoglucoside. The Weissella strain and five Leuconostoc strains with high glycosylation activity were selected for further studies. All glucansucrases catalyzed the glycosylation of the catechol protocatechuic acid, a side-chain truncated analogue of caffeic acid. The Leuconostoc enzymes displayed a preference for the 4′-O-α-D isomer, while the DSM 22752 glucansucrase also produced the protocatechuic acid-3′-O-α-D-monoglucoside. Lower activities with non-catecholic caffeic acid derivatives and no activity with mono-methylated caffeic acid were observed with all glucansucrases. Time-course analyses confirmed that glucansucrase from L. citreum DSM 5577 was the most efficient biocatalyst for catechol glucosylation with yields of up to 74% caffeic acid glucosides after 24 h. The enzyme displayed a high regio-preference for the 4′-O-α-d-isomer and formed less than 10% oligoglucosides. Gel electrophoretic analysis and activity staining of the PEG-enriched enzyme showed a single protein band with a molecular mass of 171 kDa. The DSM 5577 glucansucrase was tolerant against the co-solvents dimethyl sulfoxide and ethanol. Kinetic analysis revealed a KM of 27.6 mM for caffeic acid and 31 mM for sucrose with kcat values of 131 s−1 and 438 s−1.

Contribution of autochthonous lactic acid bacteria to the typical flavour of raw goat milk cheeses

In this study, a collection of 298 wild strains of lactic acid bacteria (LAB) isolated from raw goat milk cheeses, belonging to 8 genera and 24 species, was investigated for their ability to generate volatile compounds. Sensory evaluation showed that goat milk cultures of Leuconostoc and Lactococcus strains reached the highest scores for dairy odour attributes. Fifty six LAB strains with the highest sensory evaluation scores (one per each ten, or fraction, of those belonging to the same species) were selected for gas chromatography–mass spectrometry (GC–MS) analysis. A total of 34 volatile compounds (five carboxylic acids, eleven alcohols, six aldehydes, six ketones, one ester, and five miscellaneous compounds) were detected in lactic curds made with each one of 56 selected LAB strains. The number of volatile compounds in individual curds varied from 15 to 33 while the total abundance of volatile compounds in individual curds ranged from 1.98- to 17.50-fold the total abundance in uninoculated curd. Major differences in volatile generation were related to amino acid catabolism.

Effect of Bacteriophages on Viability and Growth of Co-cultivated Weissella and Leuconostoc in Kimchi Fermentation.

This study aimed to understand the survival and growth patterns of bacteriophage-sensitive Weissella and Leuconostoc strains involved in kimchi fermentation. Dongchimi kimchi was prepared, and Weissella and Leuconostoc were co-cultivated in the dongchimi broth. Weissella cibaria KCTC 3807 growth was accompanied by rapid lysis with an increase in the bacteriophage quantity. Leuconostoc citreum KCCM 12030 followed the same pattern. The bacteriophage-insensitive strains W. cibaria KCTC 3499 and Leuconostoc mesenteroides KCCM 11325 survived longer under low pH as their growth was not accompanied by bacteriophages. The bacteriophage lysate of W. cibaria KCTC 3807 accelerated and promoted the growth of Leuconostoc. Overall, our results show that bacteriophages might affect the viability and population dynamics of lactic acid bacteria during kimchi fermentation.

Use of a Selected Leuconostoc Citreum Strain as a Starter for Making a "Yeast-Free" Bread.

The aim of this study was the characterization and selection of bacterial strains suitable for the production of a “yeast-free” bread. The strains Leuconostoc citreum C2.27 and Weissella confusa C5.7 were selected for their leavening and acidification capabilities and individually used as starters in bread-making tests. Liquid type-II sourdoughs, singly inoculated with the two selected strains, were characterized and employed for bread-making, through the set-up of a biotechnological protocol without the use of baker’s yeast as a leavening agent. Aiming to verify the ability of the selected strains to dominate the fermentation process, bacteria and yeasts were isolated from liquid sourdoughs and doughs, genetically characterized and identified. Both the selected strains were suitable for the production of bread, even if L. citreum C2.27 showed the highest leavening capacity and was able to dominate the dough microbiota. The effects of different salt concentrations on the selected strain performances were also investigated. The applicability of the developed protocol, adapted for the production of the typical Apulian bread, “puccia”, and the suitability of the strain L. citreum C2.27 were confirmed at pilot scale in an industrial bakery. The puccia bread, which was produced with the liquid sourdough fermented with L. citreum C2.27, without baker’s yeast and salt, was similar in appearance to the conventional product containing baker’s yeast and was judged positively by a sensory analysis.

Extrusion of Dissolved Oxygen by Exopolysaccharide From Leuconostoc mesenteroides and Its Implications in Relief of the Oxygen Stress.

Strains of Leuconostoc are generally facultatively anaerobic and exposure to oxygen might be detrimental; therefore, strategies to combat the oxygen stress are essential for these bacteria to survive and flourish in the oxygenic atmosphere. Despite the extensive applications in industry, the fundamental issues concerning the aerobic life of Leuconostocs remain to be addressed. In this study, we have demonstrated that Leuconostoc mesenteroides CGMCC10064 cultivated in sucrose medium would acquire a growth advantage over that in glucose medium under oxygenic conditions, as reflected by more viable cells and less accumulation of reactive oxygen species. Further analysis showed that the growth advantage was dependent on exopolysaccharide (EPS) synthesized by a secreted glucansucrase. Determination of the dissolved oxygen in the culture suggested that the growth improvement was mediated by extrusion of dissolved oxygen from the aqueous circumstances. Growth experiments performed with the purified EPS showed that supplementation of 5 g/L EPS in the medium could improve the aerobic growth of L. mesenteroides by ∼10-fold. Moreover, the purified EPS was also effective in promoting the aerobic growth of oxygen-sensitive Lactobacillus and Bifidobacterium. These results demonstrate that EPS of L. mesenteroides plays a critical role in relief of the oxygen stress, and suggest the potential of the EPS in manufacture as well as preservation of oxygen-sensitive probiotics.

Glucansucrase catalyzed synthesis and functional characterization of nordihydroguaiaretic acid glucosides

Nordihydroguaiaretic acid (NDGA) is the major lignan of the creosote bush Larrea tridentata known for its antioxidative and pharmacological properties. Here we present the identification of glucansucrases for NDGA glucosylation and the physicochemical and biological characterization of the glucosides. Extracellular glucansucrase of L. pseudomesenteroides DSM 20193 was selected from 19 glucansucrase positive Leuconostoc and Weissella strains. Kinetic analysis of the PEG-fractionated enzyme revealed a KM of 6.6 mM and a kcat of 2.6 s−1 for NDGA. Full-factorial design methodology was used to optimize conversion resulting in 95.5% total NDGA glucosides. In total 7 glucosides were detected by LC–MS ranging from mono- to triglucoside. The 4-O-α-D-monoglucoside and the symmetrical 4,4’-O-α-D-diglucoside were the major products in all biotransformations. Water solubility and half-life stability at 45 °C increased significantly in the order diglucoside > monoglucoside > aglycon. Analysis of cellular antioxidative capacity exhibited a time-dependent activity increase pointing towards glucoside hydrolysis. Accordingly, NDGA-glucosides impaired metastasis of triple negative breast cancer cells to the same degree as the aglycon with 35% reduction of cell migration by the mono- and 34% reduction by the diglucoside after 20 h.

Bacteria Isolated from Antarctic Permafrost are Efficient Antibiotic Producers

Nontraditional sources were shown to be promising for obtaining bacterial isolates characterized by efficient production of antibiotics, including those affecting the strains with multidrug resistance. From Antarctic permafrost, 32 bacterial strains were isolated, and their ability to synthesize antimicrobal compounds efficient against 12 test strains, including the ones with multidrug resistance, was studied in submerged cultures. Out of 13 strains producing antibiotic compounds with different spectra of action, seven were chosen for further study. Their species identification was carried out by microbiological techniques and by analysis of their 16S rRNA gene sequences. Six strains of spore-forming bacteria were identified as Bacillus species (B. licheniformis, B. mojavensis, B. safensis, and B. subtilis), while one strain was identified as Gordonia terrae. Antimicrobial activity of two B. mojavensis strains (INA 01149 and INA 01151), two B. safensis strains (INA 01153 and INA 01154), and B. licheniformis strain INA 01155 against the antibiotic-resistant tester strain Leuconostoc mesenteroides VKPM B-4177 (VR) was revealed. Antibiotic activity of two B. safensis strains (INA 01153 and INA 01154) against tester bacteria, including Pseudomonas aeruginosa ATCC 27853 and Staphylococcus aureus INA 00761 with multidrug resistance has not been reported previously. Antibiotic producers isolated from Antarctic permafrost samples are of interest to medicine due to the global issue of increasing antibiotic resistance of pathogenic bacteria.

Optimization and some characteristics of bacteriocin produced by Enterococcus sp. CM9 collected from Mauritanian Camel milk

The use of bacteriocin as a bioconservation is very promising in the agri-food industry to substitute the chemical preservatives. The production of bacteriocin by lactic acid bacteria (LAB) depending on various conditions, the purpose of the present work was to assess the effect of some ingredients of medium on the bacteriocin production. Twenty-four strains of LAB of the genera Leuconostoc, Lactobacillus, and Enterococcus, isolated from camel milk, were tested for antibacterial activity and bacteriocin production. Among these strains, three isolates showed bacteriocinogenic activity, two Enterococcus strains (bacteriocin CM9, bacteriocin CM18), and one Leuconostoc strain (bacteriocin CHBK310). The bacteriocin produced by Enterococcus sp CM9 was subject of optimisation study. The modified MRS with nitrogen sources in media containing tryptone, meat extract and yeast extract and with potassium sources in media containing 2% of K2HPO4 allowed higher activity with inhibition zone of 10 mm and this production remained stable at incubation temperatures from 25 to 40°C with an inhibition zone which reached 12 mm at 25°C. The diffusion of this bacteriocin in the neutrality condition, allowed a better evaluation of its inhibitory effect. The activity of the bacteriocin was not affected, neither by acidic or basic pH nor by heat treatments, its activity remaining stable even after 2 weeks at -20 and 4°C. The bacteriocin CM9 belonged to class II, had a molecular weight of 7.6 kDa. These results indicated that the growth and diffusion conditions have a role in the bacteriocin production and its activity.

Selection of Leuconostoc strains isolated from artisanal Serrano Catarinense cheese for use as adjuncts in cheese manufacture.

Serrano Catarinense cheese is a raw bovine milk cheese produced in the region of Santa Catarina, Brazil. Twelve representative strains of Leuconostoc isolated from 20 samples of this artisanal cheese were selected and submitted for evaluation of the acidifying, proteolytic, autolytic, aminopeptidase and lipolytic activities, NaCl and acid resistance, production of dextran and biogenic amines and antimicrobial activity. The aim was to genetically and technologically characterize the Leuconostoc strains in order to use them in mixed starter cultures for cheese manufacture. Leuconostoc mesenteroides subsp. mesenteroides was the species that accounted for the largest proportion of isolates of Leuconostoc genus. Two leuconostoc isolates stood out in the acidifying activity, with reduction in pH of 1.12 and 1.04 units. The isolates showed low proteolytic and autolytic activity. Most of the isolates were dextran producers, presented good resistance to the salt and pH conditions of the cheese and showed antimicrobial activity against cheese pathogen bacteria, and none of them produced biogenic amines. These results allowed the selection of five strains (UEL 04, UEL 12, UEL 18, UEL 21 and UEL 28) as good candidates for use as adjunct cultures for cheese manufacture. © 2018 Society of Chemical Industry.

Characterization of highly branched dextran produced by Leuconostoc citreum B-2 from pineapple fermented product

A strain Leuconostoc citreum B-2 was isolated from homemade fermentation product of pineapple and its polysaccharide yield was 28.3g/L after cultivating the strain in Man-Rogosa-Sharpe (MRS) medium with 75g/L sucrose. The exopolysaccharide (EPS) was characterized by gas chromatography (GC), Fourier-transform infrared (FT-IR) spectra, high-performance size-exclusion chromatography (HPSEC), nuclear magnetic resonance (NMR) spectroscopy and scanning electron microscope (SEM) analysis in present study. The monosaccharide composition of EPS was glucose and molecular weight was 3.77×106Da. FT-IR and NMR spectra revealed that the B-2 EPS was composed of 75% α-(1→6) linked d-glucopyranose units existing in the main chain with 19% α-(1→3) branching and only a few α-(1→2) branching. The SEM of the dried EPS appeared irregular sheets with glittering surface and compact structure. Water solubility index and water holding capacity of B-2 EPS were 80% and 450%, respectively. All the mentioned characteristics suggested that the EPS has a potential application in the food, cosmetic and pharmaceuticals industry.

Response of Leuconostoc strains against technological stress factors: Growth performance and volatile profiles

The ability of twelve strains belonging to three Leuconostoc species (Leuconostoc mesenteroides, Leuconostoc lactis and Leuconostoc pseudomesenteroides) to grow under diverse sub-lethal technological stress conditions (cold, acidic, alkaline and osmotic) was evaluated in MRS broth. Two strains, Leuconostoc lactis Ln N6 and Leuconostoc mesenteroides Ln MB7, were selected based on their growth under sub-lethal conditions, and volatile profiles in RSM (reconstituted skim milk) at optimal and under stress conditions were analyzed. Growth rates under sub-lethal conditions were strain- and not species-dependent. Volatilomes obtained from the two strains studied were rather diverse. Particularly, Ln N6 (Ln. lactis) produced more ethanol and acetic acid than Ln MB7 (Ln. mesenteroides) and higher amounts and diversity of the rest of volatile compounds as well, at all times of incubation. For the two strains studied, most of stress conditions applied diminished the amounts of ethanol and acetic acid produced and the diversity and levels of the rest of volatile compounds. These results were consequence of the different capacity of the strains to grow under each stress condition tested.

Optimization, chain conformation and characterization of exopolysaccharide isolated from Leuconostoc mesenteroides DRP105

Response surface methodology (RSM) was used to optimize the fermentation condition of exopolysaccharide (EPS) producing strain Leuconostoc mesenteroides DRP105. Result showed that the optimum condition was sucrose 86.83g/L, tryptone 15.47g/L, initial pH7.18 and maximum yield was 53.79±0.78g/L in 36h fermentation. Chain conformation was characterized by Congo red test, β-elimination and circular dichroism (CD), which indicated that the EPS was O-linkage and exhibited random coil structure in aqueous solution. CD results concluded hydrogen-bond interaction and chirality might be connected with concentration. Purified EPS has a higher degradation temperature of 279.42°C, suggesting high thermal stability of the EPS. The absolute value of zeta potential and particle size were enhanced with increasing concentration. Crude EPS and its purified fraction were found to have moderate DPPH, hydroxyl, superoxide anion radicals scavenging activities and reducing power. This study provided a high yield EPS with unique characteristics for industrial applications.

ANTIMICROBIAL ACTIVITY OF LEUCONOSTOC LACTIS STRAIN BT17, ISOLATED FROM A SPONTANEOUSLY FERMENTED CEREAL BEVERAGE (BOZA)

The antimicrobial activity of Leuconostoc lactis BT17, isolated from a spontaneously fermented cereal beverage (boza) against some test microorganisms (Escherichia coli ATCC 25922, Salmonella sp. and Klebsiella pneumoniae) was determined by co-culturing of the strain Leuconostoc lactis BT17 with each of the test microorganisms under static conditions. It was found that the strain Leuconostoc lactis BT17 inhibited the growth of the test microorganisms and no viable cells of Escherichia coli, Salmonella sp. and Klebsiella pneumoniae were detected at the 48-th hour of the co-culturing. The reduction of the number of viable cells in the mixed populations was a result of the produced lactic acid by Leuconostoc lactis BT17 in the culture medium, which modify the conditions unlikely to growth of the test microorganisms.

Rheology and bioactivity of high molecular weight dextrans synthesised by lactic acid bacteria

Dextrans synthesised by three Leuconostoc mesenteroides strains, isolated from mammalian milks, were studied and compared with dextrans produced by Lc. mesenteroides and Lactobacillus sakei strains isolated from meat products. Size exclusion chromatography coupled with multiangle laser light scattering detection analysis demonstrated that the dextrans have molecular masses between 1.74×108Da and 4.41×108Da. Rheological analysis of aqueous solutions of the polymer revealed that all had a pseudoplastic behaviour under shear conditions and a random, and flexible, coil structure. The dextrans showed at shear zero a difference in viscosity, which increased as the concentration increased. Also, the purified dextrans were able to immunomodulate in vitro human macrophages, partially counteracting the inflammatory effect of Escherichia coli O111:B4 lipopolysaccharide. During prolonged incubation on a solid medium containing sucrose, dextran-producing bacteria showed two distinct phenotypes not related to the genus or species to which they belonged. Colonies of Lc. mesenteroides CM9 from milk and Lb. sakei MN1 from meat formed stable and compact mucoid colonies, whereas the colonies of the other three Leuconostoc strains became diffuse after 72h. This differential behaviour was also observed in the ability of the corresponding strains to bind to Caco-2 cells. Strains forming compact mucoid colonies showed a high level of adhesion when grown in the presence of glucose, which decreased in the presence of sucrose (the condition required for dextran synthesis). However no influence of the carbon source was detected for the adhesion ability of the other Lc. mesenteroides strains, which showed variable levels of binding to the enterocytes.

Leuconostoc strains isolated from dairy products: Response against food stress conditions

A systematic study about the intrinsic resistance of 29 strains (26 autochthonous and 3 commercial ones), belonging to Leuconostoc genus, against diverse stress factors (thermal, acidic, alkaline, osmotic and oxidative) commonly present at industrial or conservation processes were evaluated. Exhaustive result processing was made by applying one-way ANOVA, Student's test (t), multivariate analysis by Principal Component Analysis (PCA) and Matrix Hierarchical Cluster Analysis. In addition, heat adaptation on 4 strains carefully selected based on previous data analysis was assayed. The strains revealed wide diversity of resistance to stress factors and, in general, a clear relationship between resistance and Leuconostoc species was established. In this sense, the highest resistance was shown by Leuconostoc lactis followed by Leuconostoc mesenteroides strains, while Leuconostoc pseudomesenteroides and Leuconostoc citreum strains revealed the lowest resistance to the stress factors applied. Heat adaptation improved thermal cell survival and resulted in a cross–resistance against the acidic factor. However, all adapted cells showed diminished their oxidative resistance. According to our knowledge, this is the first study regarding response of Leuconostoc strains against technological stress factors and could establish the basis for the selection of “more robust” strains and propose the possibility of improving their performance during industrial processes.

Functional analysis of truncated and site-directed mutagenesis dextransucrases to produce different type dextrans.

Dextrans with distinct molecular size and structure are increasingly being used in the food and pharmaceutical industries. Dextran is produced by dextransucrase (DSR, EC2.4.5.1), which is produced by Leuconostoc mesenteroides. DSR belongs to glycosyl hydrolase family (GH70) and synthesizes branched α-glucan (dextran) with both 5% α(1-3) and 95% α(1-6) glycosidic linkages. The DSR gene dex-YG (Genebank, Accession No. DQ345760) was cloned from the wild strain Leuconostoc mesenteroides 0326. This study generated a series of C-terminally truncated variants of dextransucrase and substituting the amino-acid residues in the active site of DSR. With shorter length of DSR, its polysaccharide-synthesizing capability was impaired heavily, whereas oligosaccharide (acting as prebiotics)-synthesizing capability increased significantly, efficiently producing special sizes of dextran. All truncated mutant enzymes were active. Results demonstrated that the catalytic domain dextransucrase was likely in 800 aa or less. Based on the three-dimensional structure model of dextransucrase built through homology modeling methods, the DSR and its mutants with the acceptor substrate of maltose and donor substrate of sucrose were studied by molecular-docking method. Substituting these amino-acid residues significantly affected enzyme activities. Compared with the wild-type dextran, mutant enzymes catalyzed the synthesis of a-glucan with 1-9% α(1-3) and 90-98% α(1-6) branching linkages. Some mutants introduced a small amount of α(1-4) linkages and α(1-2) linkages. This strategy can be effectively used for the rational protein design of dextransucrase.

English

The various properties of lactic acid bacteria have made them good auxiliary in the manufacturing process in agro food industries and farms. They are widely used as probiotics which can be defined as living microorganisms that have beneficial effects on human health probiotics could replace antibiotic growth promoters in livestock without creating new threats such as that observed with antibiotics. Before their use as probiotics lactic acid bacteria require a perfect knowledge in view to their biochemical and genetic characteristics because it is difficult to differentiate morphologically some Leuconostoc and Lactobacillus strains using morphological characteristics. This study was undertaken in order to evaluate the probiotics potential of Leuconostoc strains isolated from traditional fermented cassava. The results showed that 5 strains of Leuconostoc have antibacterial activity against Staphylococcus aureus (MetiR), Klebsiella pneumoniae (BLSE), Escherichia coli, Salmonella typhimurium and Pseudomonas aeruginosa. The molecular identification of species using the conserved region of the 16S rRNA helped distinguish the species Leuconostoc mesenteroides. All these results showed that the studied Leuconostoc strains could be used as potential probiotics for the biopreservation of various foods. Key words: Leuconostoc, antibacterial activity, polymerase chain reaction (PCR), sequencing, probiotics.