Secretory expression of a heterologous nattokinase in <Emphasis Type="Italic">Lactococcus lactis</Emphasis>

Nattokinase has been reported as an oral health product for the prevention of atherosclerosis. We developed a novel strategy to express a nattokinase from Bacillus subtilis in a live delivery vehicle, Lactococcus lactis. Promoter P _nisZ and signal peptide SP_Usp were used for inducible and secretory expression of nattokinase in L. lactis. Western blotting analysis demonstrated that nattokinase was successfully expressed, and about 94% of the enzyme was secreted to the culture. The recombinant nattokinase showed potent fibrinolytic activity, equivalent to 41.7 urokinase units per milliliter culture. Expression and delivery of such a fibrinolytic enzyme in the food-grade vehicle L. lactis would facilitate the widespread application of nattokinase in the control and prevention of thrombosis diseases.     

Enhancement of Nisin Production by <Emphasis Type="Italic">Lactococcus lactis</Emphasis> subsp. <Emphasis Type="Italic">lactis</Emphasis>

Lactococcus lactis subsp lactis BSA (L. lactis BSA) was isolated from a commercial fermented product (BSA Food Ingredients, Montreal, Canada) containing mixed bacteria that are used as starter for food fermentation. In order to increase the bacteriocin production by L. lactis BSA, different fermentation conditions were conducted. They included different volumetric combinations of two culture media (the Man, Rogosa and Sharpe (MRS) broth and skim milk), agitation level (0 and 100 rpm) and concentration of commercial nisin (0, 0.15, and 0.30 µg/ml) added into culture media as stimulant agent for nisin production. During fermentation, samples were collected and used for antibacterial evaluation against Lactobacillus sakei using agar diffusion assay. Results showed that medium containing 50 % MRS broth and 50 % skim milk gave better antibacterial activity as compared to other medium formulations. Agitation (100 rpm) did not improve nisin production by L. lactis BSA. Adding 0.15 µg/ml of nisin into the medium-containing 50 % MRS broth and 50 % skim milk caused the highest nisin activity of 18,820 AU/ml as compared to other medium formulations. This activity was 4 and ~3 times higher than medium containing 100 % MRS broth without added nisin (~4700 AU/ml) and 100 % MRS broth with 0.15 µg/ml of added nisin (~6650 AU/ml), respectively.     

Characterization of <Emphasis Type="Italic">Lactococcus lactis</Emphasis> subsp. <Emphasis Type="Italic">lactis</Emphasis> isolated from surface waters

A group of nine presumptive enterococci was isolated on enterococcal selective media Slanetz-Bartley agar and/or kanamycin-esculin-azide agar during a screening of Enterococcus spp. in surface waters. All strains formed a homogeneous cluster separated from all enterococcal species using rep-PCR fingerprinting with the (GTG)(5) primer but they matched fingerprints revealed by Lactococcus lactis subsp. lactis representatives. Further identification using extensive biotyping and automated ribotyping with EcoRI (RiboPrinter(R) microbial characterization system) confirmed all strains as L. lactis subsp. lactis in full correspondence with the (GTG)(5)-PCR. We demonstrated that L. lactis subsp. lactis strains occur in different surface waters and can be confused with enterococci due to their positive growth on selective enterococcal media as well as positive results in tests commonly used for identification of the genus Enterococcus (esculin hydrolysis, acetoin and pyrrolidonyl arylamidase production, growth at 10 degrees C and in 6.5% NaCl). The (GTG)(5)-PCR fingerprinting was revealed as a reliable and fast method for the identification of L. lactis subsp lactis while automated ribotyping with EcoRI proved to be a good tool for intrasubspecies typing purposes.     

Cytoplasmic expression of a thermostable invertase from <Emphasis Type="Italic">Thermotoga maritima</Emphasis> in <Emphasis Type="Italic">Lactococcus lactis</Emphasis>

Objectives

To evaluate the secretory and cytoplasmic expression of a thermostable Thermogata maritima invertase in Lactococcus lactis.

Results

The thermostable invertase from T. maritima was cloned with and without the USP45 secretory peptide into the pNZ8148 vector for nisin-inducible expression in L. lactis. The introduction of an USP45 secretion peptide at the N-terminal of the enzyme led to a loss of protein solubility. Computational homology modeling and hydrophobicity studies indicated that the USP45 peptide exposes a stretch of hydrophobic amino acids on the protein surface resulting in lower solubility. Removal of the USP45 secretion peptide allowed a soluble and functional invertase to be expressed intracellularly in L. lactis. Immobilized metal affinity chromatography purification of the cell lysate with nickel-NTA gave a single protein band on SDS-PAGE, while E. coli-expressed invertase consistently co-purified with an additional band. The yields of the purified invertase from E. coli and L. lactis were 14.1 and 6.3 mg/l respectively.

Conclusions

Invertase can be expressed in L. lactis and purified in a functional form. L. lactis is a suitable host for the production of food-grade invertase for use in the food and biotechnology industries.

     

The <Emphasis Type="Italic">Lactococcus lactis</Emphasis> FabF fatty acid synthetic enzyme can functionally replace both the FabB and FabF proteins of <Emphasis Type="Italic">Escherichia coli</Emphasis> and the FabH protein of <Emphasis Type="Italic">Lactococcus lactis</Emphasis>

The genome of Lactococcus lactis encodes a single long chain 3-ketoacyl-acyl carrier protein synthase. This is in contrast to its close relative, Enterococcus faecalis, and to Escherichia coli, both of which have two such enzymes. In E. faecalis and E. coli, one of the two long chain synthases (FabO and FabB, respectively) has a role in unsaturated fatty acid synthesis that cannot be satisfied by FabF, the other long chain synthase. Since L. lactis has only a single long chain 3-ketoacyl-acyl carrier protein synthase (annotated as FabF), it seemed likely that this enzyme must function both in unsaturated fatty acid synthesis and in elongation of short chain acyl carrier protein substrates to the C18 fatty acids found in the cellular phospholipids. We report that this is the case. Expression of L. lactis FabF can functionally replace both FabB and FabF in E. coli, although it does not restore thermal regulation of phospholipid fatty acid composition to E. coli fabF mutant strains. The lack of thermal regulation was predictable because wild-type L. lactis was found not to show any significant change in fatty acid composition with growth temperature. We also report that overproduction of L. lactis FabF allows growth of an L. lactis mutant strain that lacks the FabH short chain 3-ketoacyl-acyl carrier protein synthase. The strain tested was a derivative (called the ∆fabH bypass strain) of the original fabH deletion strain that had acquired the ability to grow when supplemented with octanoate. Upon introduction of a FabF overexpression plasmid into this strain, growth proceeded normally in the absence of fatty acid supplementation. Moreover, this strain had a normal rate of fatty acid synthesis and a normal fatty acid composition. Both the ∆fabH bypass strain that overproduced FabF and the wild type strain incorporated much less exogenous octanoate into long chain phospholipid fatty acids than did the ∆fabH bypass strain. Incorporation of octanoate and decanoate labeled with deuterium showed that these acids were incorporated intact as the distal methyl and methylene groups of the long chain fatty acids.     

Cloning and Characterization of a Novel <Emphasis Type="Italic">tuf</Emphasis> Promoter from <Emphasis Type="Italic">Lactococcus lactis</Emphasis> subsp. <Emphasis Type="Italic">lactis</Emphasis> IL1403

Genetic engineering of lactic acid bacteria (LAB) requires a reliable gene expression system. Especially, a stable promoter is an important genetic element to induce gene expression in such a system. We report on a novel tuf promoter (Ptuf) of Lactococcus lactis subsp. lactis IL1403 that was screened and selected through analysis of previously published microarray data. Ptuf activity was examined and compared with three other known lactococcal promoters (PdnaJ, PpfkA, and Pusp45) using different bacteria as expression hosts. Each promoter was, respectively, fused to the promoterless and modified bmpB gene as a reporter, and we estimated promoter activity through BmpB expression. All promoters were active in IL1403, and Ptuf activity was strongest among them. The activity of each promoter differed by host bacteria (Lactobacillus plantarum Lb25, Lactobacillus reuteri ATCC23272, and Escherichia coli Top10F’). Ptuf had the highest activity in IL1403 when growth reached late log phase. The activity of each promoter correlated with the expression of each cognate gene in the microarray data (R ² = 0.7186, P = 0.06968). This study revealed that novel food-grade promoters such as IL1403 Ptuf can be selected from microarray data for food-grade microorganisms and Ptuf can be used to develop a reliable gene expression system in L. lactis.     

Investigation of the antibiotic complex produced by <Emphasis Type="Italic">Lactococcus lactis</Emphasis> subsp. <Emphasis Type="Italic">lactis</Emphasis> 194, Variant K

Phase variation in the culture of the environmental strain Lactococcus lactis subsp. lactis 194 resulted in the formation of two types of colonies differing by 15% in antibiotic activity. The active variant 194-K produced an antibiotic complex with a broad spectrum of antibacterial and antifungal activity. Five components (194-A, B, C, D, and E) were isolated from the complex by solid-phase extraction and thin-layer chromatography. Components 194-A and 194-B were hydrophobic neutral compounds soluble in organic solvents. Component 194-A possessed fungicidal activity, whereas component 194-B exhibited only bactericidal activity. Physicochemical studies of the isolated components 194-A and 194-B revealed that they had no analogs in the Berdy database of biologically active substances (BNPD) and appeared to be novel antibiotics. Component 194-C was a hydrophilic polar compound inhibiting growth of gram-positive and gramnegative bacteria. Component 194-D belonged to peptide antibiotics; it inhibited growth of only gram-positive bacteria and was similar to nisin A in biological properties but differed in electrophoretic mobility and molecular mass.     

Identification and Characteristics of Nisin Z-Producing <Emphasis Type="Italic">Lactococcus lactis</Emphasis> subsp. <Emphasis Type="Italic">lactis</Emphasis> Isolated from Kimchi

We isolated bacteriocin-producing Lactococcus lactis subsp. lactis from Kimchi. The bacteriocin inhibited strains of Clostridium perfringens, C. difficile, Listeria monocytogenes, vancomycin-resistant Enterococcus, and one out of four methicillin-resistant Staphylococcus aureus strains, as well as some closely related lactic acid bacteria. In tricine-SDS-PAGE, the bacteriocin migrated with an apparent molecular weight of about 4 kDa to the same location as nisin A and crude nisin Z. The gene encoding this bacteriocin was found to be identical to that of nisin Z with direct PCR sequence methods. The inhibitory activity was stable against heat and pH, but it was lost at 100°C for 1 h and at 121°C for 15 min. The bacteriocin was inactivated by proteolytic enzymes, but was not affected by lysozyme, lipase, catalase, or β-glucosidase. There were some differences in characteristics from those of nisins described previously. Received: 21 June 2002 / Accepted: 22 July 2002     

Modeling <Emphasis Type="Italic">Lactococcus lactis</Emphasis> using a genome-scale flux model

Background  

Genome-scale flux models are useful tools to represent and analyze microbial metabolism. In this work we reconstructed the metabolic network of the lactic acid bacteria Lactococcus lactis and developed a genome-scale flux model able to simulate and analyze network capabilities and whole-cell function under aerobic and anaerobic continuous cultures. Flux balance analysis (FBA) and minimization of metabolic adjustment (MOMA) were used as modeling frameworks.     

Hyaluronic acid production by recombinant <Emphasis Type="Italic">Lactococcus lactis</Emphasis>

A gene encoding a new xylanase, named xynZG, was cloned by the genome-walking PCR method from the nematophagous fungus Plectosphaerella cucumerina. The genomic DNA sequence of xynZG contains a 780 bp open reading frame separated by two introns with the sizes of 50 and 46 bp. To our knowledge, this would be the first functional gene cloned from P. cucumerina. The 684 bp cDNA was cloned into vector pHBM905B and transformed into Pichia pastoris GS115 to select xylanase-secreting transformants on RBB-xylan containing plate. The optimal secreting time was 3 days at 25°C and enzymatic activities in the culture supernatants reached the maximum level of 362 U ml⁻¹. The molecular mass of the enzyme was estimated to be 19 kDa on SDS-PAGE. The optimal pH and temperature of the purified enzyme is 6 and 40°C, respectively. The purified enzyme is stable at room temperature for at least 10 h. The K _m and V _max values for birchwood xylan are 2.06 mg ml⁻¹ and 0.49 mmol min⁻¹mg⁻¹, respectively. The inhibitory effects of various mental ions were investigated. It is interesting to note that Cu²⁺ ion, which strongly inhibits most other xylanases studied, reduces enzyme activity by only 40%. Furthermore, enzyme activity is unaffected by EDTA even at a concentration of 5 mM.     

Enhanced expression of heterologous inulinase in <Emphasis Type="Italic">Kluyveromyces lactis</Emphasis> by disruption of <Emphasis Type="Italic">hap1</Emphasis> gene

Inulinase gene (Kcinu) derived from Kluyveromyces cicerisporus was expressed extracellularly in Kluyveromyces lactis using an episomal vector directed by Kcinu promoter. The influence of hap1 gene disruption on the expression of inulinase was studied. Inulinase activity in the supernatant of the recombinant Klhap1Δ strain was 391 U ml⁻¹ after cultured 120 h, which was 2.2-fold that of the wild type host. The relative inulinase mRNA level of the Klhap1Δ strain was 11.3-fold that of the wild type strain, and the expression plasmid was more stable in the mutant host. Based on these results, the disruption of hap1 facilitated the high and stable expression of inulinase controlled by Kcinu promoter in K. lactis.     

Purification and partial characterization of bacteriocin produced by <Emphasis Type="Italic">Lactococcus lactis</Emphasis> ssp. <Emphasis Type="Italic">lactis</Emphasis> LL171

Lactic acid bacteria (LAB) are possessing ability to synthesize antimicrobial compounds (like bacteriocin) during their growth. In this regard, novel bacteriocin compound secreting capability of LAB isolated from Tulum Cheese in Turkey was demonstrated. The synthesized bacteriocin was purified by ammonium sulphate precipitation, dialysis and gel filtration. The molecular weight (≈3.4 kDa) of obtained bacteriocin was confirmed by SDS-PAGE, which revealed single peptide band. Molecular identification of LAB strain isolated from Tulum Cheese was conducted using 16S rDNA gene sequencing as Lactococcus lactis ssp. lactis LL171. The amino acid sequences (KKIDTRTGKTMEKTEKKIELSLKNMKTAT) of the bacteriocin from Lactococcus lactis ssp. lactis LL171 was found unique and novel than reported bacteriocins. Further, the bacteriocin was possessed the thermostable property and active at wide range of pH values from 1 to 11. Thus, bacteriocin reported in this study has the potential applications property as food preservative agent.     

A propeptide toolbox for secretion optimization of <Emphasis Type="Italic">Flavobacterium meningosepticum</Emphasis> endopeptidase in <Emphasis Type="Italic">Lactococcus lactis</Emphasis>

Background

Lactic acid bacteria are a family of “generally regarded as safe” organisms traditionally used for food fermentation. In recent years, they have started to emerge as potential chassis for heterologous protein production. And more recently, due to their beneficial properties in the gut, they have been examined as potential candidates for mucosal delivery vectors, especially for acid-sensitive enzymes. One such application would be the delivery of gluten-digesting endopeptidases for the treatment of celiac disease. To facilitate these applications, an efficient recombinant protein expression toolbox is required, especially for recombinant protein secretion. While current tools for enhancing protein secretion consist mainly of signal peptides, secretion propeptides have also been observed to play a crucial role for protein secretion and improved yields.

Results

To expand the propeptide library for secretion optimization, we have mined and characterized three naturally occurring propeptides from the sequenced genomes of 109 Lactococcus species. These newly-mined propeptides were introduced after the N-terminal USP45 secretion signal to characterize and compare their effects on the secretion of Escherichia coli thioredoxin (TRX) and Flavobacterium meningosepticum prolyl endopeptidase (Fm PEP) in Lactococcus lactis NZ9000. All three propeptides, along with the positive control LEISSTCDA, improved volumetric secretion yields by 1.4–2.3-folds. However, enhancement of secretion yield is dependent on protein of interest. For TRX, the optimal combination of USP45 signal peptide and LEISSTCDA produced a 2.3-fold increase in secretion yields. Whilst for Fm PEP, propeptide 1 with USP45 signal peptide improved volumetric secretion yields by 2.2-fold compared to a 1.4-fold increase by LEISSTCDA. Similar trends in Fm PEP activity and protein yield also demonstrated minimal effect of the negative charged propeptides on PEP activity and thus folding.

Conclusions

Overall, we have characterized three new propeptides for use in L. lactis secretion optimization. From success of these propeptides for improvement of secretion yields, we anticipate this collection to be valuable to heterologous protein secretion optimisation in lactic acid bacteria. We have also demonstrated for the first time, secretion of Fm PEP in L. lactis for potential use as a therapy agent in celiac disease.

     

Improvement of nisin production in pH feed-back controlled,fed-batch culture by <Emphasis Type="Italic">Lactococcus lactis</Emphasis>subsp<Emphasis Type="Italic">. lactis</Emphasis>

Nisin production by Lactococcus lactis subsp. lactisin fed-batch culture was doubled by using a pH feed-back controlled method. Sucrose concentration was controlled at 10 g l^–1 giving 5010 IU nisin ml^–1 compared to 2660 IU nisin ml^–1 in batch culture.     

High efficiency electrotransformation of <Emphasis Type="Italic">Lactococcus lactis</Emphasis> spp. <Emphasis Type="Italic">lactis</Emphasis>cells pretreated with lithium acetate and dithiothreitol

Background  

A goal for the food industry has always been to improve strains of Lactococcus lactis and stabilize beneficial traits. Genetic engineering is used extensively for manipulating this lactic acid bacterium, while electropolation is the most widely used technique for introducing foreign DNA into cells. The efficiency of electrotransformation depends on the level of electropermealization and pretreatment with chemicals which alter cell wall permeability, resulting in improved transformation efficiencies is rather common practice in bacteria as in yeasts and fungi. In the present study, treatment with lithium acetate (LiAc) and dithiothreitol (DTT) in various combinations was applied to L. lactis spp. lactis cells of the early-log phase prior to electroporation with plasmid pTRKH3 (a 7.8 kb shuttle vector, suitable for cloning into L. lactis). Two strains of L. lactis spp. lactis were used, L. lactis spp. lactis LM0230 and ATCC 11454. To the best of our knowledge these agents have never been used before with L. lactis or other bacteria.     

Production of a thermophilic maltooligosyl-trehalose synthase in <Emphasis Type="Italic">Lactococcus lactis</Emphasis>

The thermoacidophilic archaeon Sulfolobus solfataricus MT4 encodes a maltooligosyltrehalose synthase (MTS), that catalyzes an intramolecular transglycosylation process converting the glycosidic linkages at the reducing end of dextrins from alpha-1,4 into alpha-1,1. In this research the gene encoding MTS was cloned and expressed in Lactococcus lactis NZ9000 using the so-called NICE system. Growth conditions of the recombinant strain were optimized in flask experiments in relation to enzyme production. Batch experiments in 2 L-fermenters were performed on the best identified semidefined medium and 256 U L(-1) of recombinant MTS were produced. Purified recombinant MTS shows its optimal activity at 70 degrees C and pH 5.5, prefers maltoheptaose and maltohexaose as substrates, and demonstrates minimal side hydrolytic activity.     

Growth Kinetics of <Emphasis Type="Italic">Lactococcus lactis</Emphasis> ssp. <Emphasis Type="Italic">diacetylactis</Emphasis> Harboring Different Plasmid Content

The effect of plasmid content on growth of Lactococcus lactis ssp. diacetylactis harboring different plasmids and on plasmid stability was studied. Strain DRC-2C is a plasmid Lac⁺- and Prt⁺-free strain. Strain DRC-2 utilizes lactose as carbohydrate and has proteinase activity. The plasmid-free strain DRC-2C exhibited none of these features. Plasmid-encoded properties were clearly identified. Results showed that plasmid content decreased bacterial growth in terms of the specific growth rate determined. Slightly lower specific growth rate and lactic acid production were observed in the strain of higher plasmid content owing to the plasmid presence, causing metabolic burden to the host cell. The plasmid profile results showed that the number of bands in the two strains before and after fermentation were the same. This indicated that the plasmids were stably maintained and unchanged during the fermentation. Received: 27 July 2002 / Accepted: 27 August 2002