Integrative food-grade expression system based on the lactose regulon of Lactobacillus casei

Sugar Utilization-Associated Food-Grade Selection Markers in Lactic Acid Bacteria and Yeast

This comprehensive review explores the development of food-grade selection markers in lactic acid bacteria and yeast; some of their strains are precisely defined as safe microorganisms and are crucial in the food industry. Lactic acid bacteria, known for their ability to ferment carbohydrates into lactic acid, provide essential nutrients and contribute to immune responses. With its strong fermentation capabilities and rich nutritional profile, yeast finds use in various food products. Genetic engineering in these microorganisms has grown rapidly, enabling the expression of enzymes and secondary products for food production. However, the focus is on ensuring safety, necessitating food-grade selection markers. Traditional antibiotic and heavy metal resistance selection markers pose environmental and health risks, prompting the search for safer alternatives. Complementary selection markers, such as sugar utilization markers, offer a promising solution. These markers use carbohydrates as carbon sources for growth and are associated with the natural metabolism of lactic acid bacteria and yeast. This review discusses the use of specific sugars, such as lactose, melibiose, sucrose, D-xylose, glucosamine, and N-acetylglucosamine, as selection markers, highlighting their advantages and limitations. In summary, this review underscores the importance of food-grade selection markers in genetic engineering and offers insights into their applications, benefits, and challenges, providing valuable information for researchers in the field of food microbiology and biotechnology.

One-step construction of a food-grade expression system based on the URA3 gene in Kluyveromyces lactis

Proteins from food-grade expression systems can be used in food products and medical applications. Herein, we describe a one-step method of constructing an expression vector in Kluyveromyces lactis by combining a URA3-deficient strain and a plasmid vector with no drug-resistant selection. Adjacent DNA elements of the vector were assembled in a targeted manner through a reaction with a special recombinase to form a plasmid vector using a one-step reaction. The unnecessary fragments containing the pUC origin and the ampicillin resistance gene were removed, and the vector was isolated and purified before transformation. A single transformation of the vector can produce a URA3-deficient strain. PCR assay, sequencing, and western blot analysis all indicated that the method of vector construction and target protein expression (mCherry and human serum albumin) were successful. This method may potentially be applied to any species containing the URA3 gene; this system has the potential to become a safe and powerful tool for promoting protein expression in food-safe species.

Engineering integrative vectors based on phage site-specific recombination mechanism for Lactococcus lactis

Background

Site-specific integration system allows foreign DNA to be integrated into the specific site of the host genome, enabling stable expression of heterologous protein. In this study, integrative vectors for secretion and surface display of proteins were constructed based on a lactococcal phage TP901–1 integrating system.

Results

The constructed integration system comprises of a lactococcal promoter (P_nisA or P₁₇₀), phage attachment site (attP) from bacteriophage TP901–1, a signal peptide (USP45 or SPK1) for translocation of the target protein, and a PrtP₃₄₄ anchor domain in the case of the integrative vectors for surface display. There were eight successfully constructed integrative vectors with each having a different combination of promoter and signal peptide; pS1, pS2, pS3 and pS4 for secretion, and pSD1, pSD2, pSD3 and pSD4 for surface display of desired protein. The integration of the vectors into the host genome was assisted by a helper vector harbouring the integrase gene. A nuclease gene was used as a reporter and was successfully integrated into the L. lactis genome and Nuc was secreted or displayed as expected. The signal peptide SPK1 was observed to be superior to USP45-LEISSTCDA fusion in the secretion of Nuc. As for the surface display integrative vector, all systems developed were comparable with the exception of the combination of P₁₇₀ promoter with USP45 signal peptide which gave very low signals in whole cell ELISA.

Conclusion

The engineered synthetic integrative vectors have the potential to be used for secretion or surface display of heterologous protein production in lactococcal expression system for research or industrial purposes, especially in live vaccine delivery.

Advanced Strategies for Food-Grade Protein Production: A New E. coli/Lactic Acid Bacteria Shuttle Vector for Improved Cloning and Food-Grade Expression

Food-grade production of recombinant proteins in Gram-positive bacteria, especially in LAB (i.e., Lactococcus, Lactobacillus, and Streptococcus), is of great interest in the areas of recombinant enzyme production, industrial food fermentation, gene and metabolic engineering, as well as antigen delivery for oral vaccination. Food-grade expression relies on hosts generally considered as safe organisms and on clone selection not dependent on antibiotic markers, which limit the overall DNA manipulation workflow, as it can be carried out only in the expression host and not in E. coli. Moreover, many commercial expression vectors lack useful elements for protein purification. We constructed a “shuttle” vector containing a removable selective marker, which allows feasible cloning steps in E. coli and subsequent protein expression in LAB. In fact, the cassette can be easily excised from the selected recombinant plasmid, and the resulting marker-free vector transformed into the final LAB host. Further useful elements, as improved MCS, 6xHis-Tag, and thrombin cleavage site sequences were introduced. The resulting vector allows easy cloning in E. coli, can be quickly converted in a food-grade expression vector and harbors additional elements for improved recombinant protein purification. Overall, such features make the new vector an improved tool for food-grade expression.

Effect of Respiratory Growth on the Metabolite Production and Stress Robustness of Lactobacillus casei N87 Cultivated in Cheese Whey Permeate Medium

Cheese whey permeate (WP) is a low-cost feedstock used for the production of biomass and metabolites from several lactic acid bacteria (LAB) strains. In this study, Lactobacillus casei N87 was cultivated in an optimized WP medium (WPM) to evaluate the effect of anaerobic and respiratory conditions on the growth performances (kinetics, biomass yield), consumption of sugars (lactose, galactose, glucose) and citrate, metabolite production [organic acids, volatile organic compounds (VOCs)] and stress survival (oxidative, heat, freezing, freeze-drying). The transcription of genes involved in the main pathways for pyruvate conversion was quantified through Real Time-PCR to elucidate the metabolic shifts due to respiratory state. Cultivation in WPM induced a diauxic growth in both anaerobic and respiratory conditions, and L. casei N87 effectively consumed the lactose and galactose present in WPM. Genomic information suggested that membrane PTS system and tagatose-6-P pathway mediated the metabolism of lactose and galactose in L. casei N87. Respiration did not affect specific growth rate and biomass production, but significantly altered the pyruvate conversion pathways, reducing lactate accumulation and promoting the formation of acetate, acetoin and diacetyl to ensure the redox balance. Ethanol was not produced under either cultivation. Pyruvate oxidase (pox), acetate kinase (ack), α-acetolactate decarboxylase (ald), acetolactate synthase (als) and oxaloacetate decarboxylase (oad) genes were up-regulated under respiration, while L-lactate dehydrogenase (ldh), pyruvate formate lyase (pfl), pyruvate carboxylase (pyc), and phosphate acetyltransferase (pta) were down regulated by oxygen. Transcription analysis was consistent with metabolite production, confirming that POX-ACK and ALS-ALD were the alternative pathways activated under aerobic cultivation. Respiratory growth affected the production of volatile compounds useful for the development of aroma profile in several fermented foods, and promoted the survival of L. casei N87 to oxidative stresses and long-term storage. This study confirmed that the respiration-based technology coupled with cultivation on low-cost medium may be effectively exploited to produce competitive and functional starter and/or adjunct cultures. Our results, additionally, provided further information on the activation and regulation of metabolic pathways in homofermentative LAB grown under respiratory promoting conditions.

A Single-Plasmid Genome Editing System for Metabolic Engineering of Lactobacillus casei

Genome engineering of Lactobacillus casei, an important industrial microorganism for dairy fermented product, currently relies on inefficient and time-consuming double crossover events. In this study, we developed an easy-to-use genome engineering strategy for metabolic engineering of L. casei for acetoin production. Plasmid pMSP456-Cre, that contains prophage recombinase operon LCABL_13040-50-60 driven by the nisin-controlled inducible expression (NICE) system and the site-specific recombinase gene cre under the control of the promoter of the lactose operon from L. casei, was constructed. Using this plasmid, integration of a hicD3 gene linear donor cassette (up-lox66-cat-lox71-down) was catalyzed by the LCABL_13040-50-60 recombinase and the cat gene was excised by the Cre/lox system with an efficiency of 60%. To demonstrate this system for sequential and iterative knocking out genes in L. casei, another three genes (pflB, ldh and pdhC) related to acetoin production were deleted with the efficiencies of 60, 40, and 60%, respectively. The yielding quadruple mutant could produce a ∼18-fold higher amount of acetoin than the wild-type and converted 59.8% of glucose to acetoin in aerobic. Therefore, these results proved this simple genome engineering strategy have potential in metabolic engineering of L. casei for production of high value-added metabolites.

Vector Development Timeline for Mucosal Vaccination and Treatment of Disease Using Lactococcus lactis and Design Approaches of Next Generation Food Grade Plasmids

Lactococcus lactis has been used historically in fermentation and food preservation processes as it is considered safe for human consumption (GRAS—Generally Recognized As Safe). Nowadays, in addition to its wide use in the food industry, L. lactis has been used as a bioreactor for the production of molecules of medical interest, as well as vectors for DNA delivery. These applications are possible due to the development of promising genetic tools over the past few decades, such as gene expression, protein targeting systems, and vaccine plasmids. Thus, this review presents some of these genetic tools and their evolution, which allow us to envision new biotechnological and therapeutic uses of L. lactis. Constitutive and inductive expression systems will be discussed, many of which have been used successfully for heterologous production of different proteins, tested on animal models. In addition, advances in the construction of new plasmids to be used as potential DNA vaccines, delivered by this microorganism, will also be viewed. Finally, we will focus on the scene of gene expression systems known as “food-grade systems” based on inducing compounds and safe selection markers, which eliminate the need for the use of compounds harmful to humans or animal health and potential future prospects for their applications.

Construction and characterization of thymidine auxotrophic (ΔthyA) recombinant Lactobacillus casei expressing bovine lactoferricin

BACKGROUND

Lactobacillus casei (L. casei) is well known for its probiotic property in human and animals. Lactoferricin (Lfcin) polypeptide can effectively modulate host immune responses and have antimicrobial activity in vivo and in vitro. In order to develop a food-grade L. casei system constitutively expressing bovine Lfcin, this study constructed a thymidine auxotrophy (ΔthyA) recombinant L. casei.

RESULTS

Based on the thymidylate synthase gene (thyA) insert site, LFEC(Lfcin expression cassette)was inserted into L. casei genome through homologous recombination, successfully expressed and could be stably inherited. The recombinant L. casei, ΔthyA L. casei-LFEC, is sensitive to chloramphenicol and limited when cultured without thymine. Meanwhile, ΔthyA L. casei-LFEC has both good antibacterial activity against Escherichia coli and Staphylococcus aureus and antiviral activity against porcine epidemic diarrhea virus (PEDV).

CONCLUSIONS

We successfully constructed a recombinant L. casei strain expressing Lfcin, ΔthyA L. casei-LFEC, which could only survive in the presence of thymine, and had excellent antimicrobial and antiviral activity with good genetic stability and sensitivity. This research provides a cost-effective alternative to the antibiotics with additional biological functions and wider applicability prospect. Using ΔthyA as the selectable mark instead of antibiotic to construct genetic engineering L.casei provides a safe and effective approach of feed additives in livestock raising.

A review of food-grade vectors in lactic acid bacteria: from the laboratory to their application

Lactic acid bacteria (LAB) have a long history of use in fermented foods and as probiotics. Genetic manipulation of these microorganisms has great potential for new applications in food safety, as well as in the development of improved food products and in health. While genetic engineering of LAB could have a major positive impact on the food and pharmaceutical industries, progress could be prevented by legal issues related to the controversy surrounding this technology. The safe use of genetically modified LAB requires the development of food-grade cloning systems containing only the DNA from homologous hosts or generally considered as safe organisms, and not dependent antibiotic markers. The rationale for the development of cloning vectors derived from cryptic LAB plasmids is the need for new genetic engineering tools, therefore a vision from cryptic plasmids to applications in food-grade vectors for LAB plasmids is shown in this review. Replicative and integrative vectors for the construction of food-grade vectors, and the relationship between resistance mechanism and expression systems, will be treated in depth in this paper. Finally, we will discuss the limited use of these vectors, and the problems arising from their use.

Reporter systems for in vivo tracking of lactic acid bacteria in animal model studies

Bioluminescence (BLI) and fluorescence imaging (FI) allow for non-invasive detection of viable microorganisms from within living tissue and are thus ideally suited for in vivo probiotic studies. Highly sensitive optical imaging techniques detect signals from the excitation of fluorescent proteins, or luciferase-catalyzed oxidation reactions. The excellent relation between microbial numbers and photon emission allow for quantification of tagged bacteria in vivo with extreme accuracy. More information is gained over a shorter period compared to traditional pre-clinical animal studies. The review summarizes the latest advances in in vivo bioluminescence and fluorescence imaging and points out the advantages and limitations of different techniques. The practical application of BLI and FI in the tracking of lactic acid bacteria in animal models is addressed.

Reduction of D-lactate content in sauerkraut using starter cultures of recombinant Leuconostoc mesenteroides expressing the ldhL gene

The D-form of lactate, which causes metabolic stress upon excessive dietary intake, is mainly produced by Leuconostoc sp., the predominant species in sauerkraut. To shift the metabolic flux of d-lactate from pyruvate to l-lactate, we expressed the l-lactate dehydrogenase (ldhL) gene in Leuconostoc mesenteroides ATCC 8293. The ldhL gene from Lactobacillus plantarum was introduced into L. mesenteroides using the shuttle vectors pLeuCM and pLeuCM42. To elevate the expression level of ldhL in L. mesenteroides, the nucleotides for pyruvate kinase promoter were fused to ldhL and cloned into above vectors to construct pLC18pkL and pLC42pkL. As results, introduction of pLC42pkL in L. mesenteroides significantly improved both l-LDH activity and l-lactate productivity during fermentation, decreasing the d-/l-lactate ratio. When used as a starter culture for sauerkraut fermentation, recombinant L. mesenteroides harboring pLC42pkL increased l-lactate concentration and decreased d-lactate concentration compared to the wild type strain. We newly developed a recombinant L. mesenteroides which has high l-lactate dehydrogenase activity and applied this strain to minimize the harmful effect of d-lactate during the sauerkraut fermentation. To the best of our knowledge, we demonstrate for the first time the effective use of recombinant Leuconostoc sp. for quality improvement of fermented foods.

Implications of new research and technologies for malolactic fermentation in wine

The initial conversion of grape must to wine is an alcoholic fermentation (AF) largely carried out by one or more strains of yeast, typically Saccharomyces cerevisiae. After the AF, a secondary or malolactic fermentation (MLF) which is carried out by lactic acid bacteria (LAB) is often undertaken. The MLF involves the bioconversion of malic acid to lactic acid and carbon dioxide. The ability to metabolise L-malic acid is strain specific, and both individual Oenococcus oeni strains and other LAB strains vary in their ability to efficiently carry out MLF. Aside from impacts on acidity, LAB can also metabolise other precursors present in wine during fermentation and, therefore, alter the chemical composition of the wine resulting in an increased complexity of wine aroma and flavour. Recent research has focused on three main areas: enzymatic changes during MLF, safety of the final product and mechanisms of stress resistance. This review summarises the latest research and technological advances in the rapidly evolving study of MLF and investigates the directions that future research may take.

Directed chromosomal integration and expression of the reporter gene gusA3 in Lactobacillus acidophilus NCFM

Lactobacillus acidophilus NCFM is a probiotic microbe that survives passage through the human gastrointestinal tract and interacts with the host epithelium and mucosal immune cells. The potential for L. acidophilus to express antigens at mucosal surfaces has been investigated with various antigens and plasmid expression vectors. Plasmid instability and antibiotic selection complicate the possibility of testing these constructs in human clinical trials. Integrating antigen encoding genes into the chromosome for expression is expected to eliminate selection requirements and provide genetic stability. In this work, a reporter gene encoding a β-glucuronidase (GusA3) was integrated into four intergenic chromosomal locations. The integrants were tested for genetic stability and GusA3 activity. Two locations were selected for insertion downstream of constitutively highly expressed genes, one downstream of slpA (LBA0169), encoding a highly expressed surface-layer protein, and one downstream of phosphopyruvate hydratase (LBA0889), a highly expressed gene with homologs in other lactic acid bacteria. An inducible location was selected downstream of lacZ (LBA1462), encoding a β-galactosidase. A fourth location was selected in a low-expression region. The expression of gusA3 was evaluated from each location by measuring GusA3 activity on 4-methyl-umbelliferyl-β-d-glucuronide (MUG). GusA3 activity from both highly expressed loci was more than three logs higher than the gusA3-negative parent, L. acidophilus NCK1909. GusA3 activity from the lacZ locus was one log higher in cells grown in lactose than in glucose. The differences in expression levels between integration locations highlights the importance of rational targeting with gene cassettes intended for chromosomal expression.

A food-grade system for inducible gene expression in Lactobacillus plantarum using an alanine racemase-encoding selection marker

Food-grade gene expression systems for lactic acid bacteria are useful for applications in the food industry. We describe a new food-grade host/vector system for Lactobacillus plantarum based on pSIP expression vectors and the use of the homologous alanine racemase gene (alr) as selection marker. A new series of expression vectors were constructed by exchanging the erythromycin resistance gene (erm) in pSIP vectors by the L. plantarum WCFS1 alr gene. The vectors were applied for the overexpression of β-galactosidase genes from L. reuteri L103 and L. plantarum WCFS1 in an alr deletion mutant of L. plantarum WCFS1. The expression levels obtained in this way, i.e. without the use of antibiotics, were comparable to the levels obtained with the conventional system based on selection for erythromycin resistance. The new system is suitable for the production of ingredients and additives for the food industry.

Cloning Vectors Based on Cryptic Plasmids Isolated from Lactic Acid Bacteria:Their Characteristics and Potential Applications in Biotechnology

Lactic acid bacteria (LAB) are Gram positive bacteria, widely distributed in nature, and industrially important as they are used in a variety of industrial food fermentations. The use of genetic engineering techniques is an effective means of enhancing the industrial applicability of LAB. However, when using genetic engineering technology, safety becomes an essential factor for the application of improved LAB to the food industry. Cloning and expression systems should be derived preferably from LAB cryptic plasmids that generally encode genes for which functions can be proposed, but no phenotypes can be observed. However, some plasmid-encoded functions have been discovered in cryptic plasmids originating from Lactobacillus, Streptococcus thermophilus, and Pediococcus spp. and can be used as selective marker systems in vector construction. This article presents information concerning LAB cryptic plasmids, and their structures, functions, and applications. A total of 134 cryptic plasmids collated are discussed.

Characterization of endogenous plasmids from Lactobacillus salivarius UCC118

The genome of Lactobacillus salivarius UCC118 comprises a 1.83-Mb chromosome, a 242-kb megaplasmid (pMP118), and two smaller plasmids of 20 kb (pSF118-20) and 44 kb (pSF118-44). Annotation and bioinformatic analyses suggest that both of the smaller plasmids replicate by a theta replication mechanism. Furthermore, it appears that they are transmissible, although neither possesses a complete set of conjugation genes. Plasmid pSF118-20 encodes a toxin-antitoxin system composed of pemI and pemK homologs, and this plasmid could be cured when PemI was produced in trans. The minimal replicon of pSF118-20 was determined by deletion analysis. Shuttle vector derivatives of pSF118-20 were generated that included the replication region (pLS203) and the replication region plus mobilization genes (pLS208). The plasmid pLS203 was stably maintained without selection in Lactobacillus plantarum, Lactobacillus fermentum, and the pSF118-20-cured derivative strain of L. salivarius UCC118 (strain LS201). Cloning in pLS203 of genes encoding luciferase and green fluorescent protein, and expression from a constitutive L. salivarius promoter, demonstrated the utility of this vector for the expression of heterologous genes in Lactobacillus. This study thus expands the knowledge base and vector repertoire of probiotic lactobacilli.

Expression of citrate permease gene of plasmid pCM1 isolated from Lactococcus lactis subsp. lactis biovar diacetylactis NIAI N-7 in Lactobacillus casei L-49-4

Recombinant vector pJLECit (8,232 bp) was constructed using citrate permease gene contained in the 3,919-bp fragment of plasmid pCM1 (8,280 bp) isolated from Lactococcus lactis subsp. lactis biovar diacetylactis NIAI N-7, repA and ori from pLU1, and pMB1 ori and the erythromycin resistance gene from pJIR418. Lactobacillus casei L-49-4 (plasmid-free mutant of strain L-49) harboring the constructed pJLECit converted citrate into diacetyl/acetoin. Citrate uptake rate of resting cells was the highest at pH 5.5 and 10 mM citrate concentration. Diacetyl formation activity by the cell-free extracts of Lb. casei L-49-4 (pJLECit) grown in de Man-Rogosa-Sharpe (MRS) broth was higher than that of cells grown in MRS broth without citrate. On the other hand, diacetyl reductase activity of cells grown in MRS broth was lower than that of cells grown in MRS broth without citrate.

Sorbitol synthesis by an engineered Lactobacillus casei strain expressing a sorbitol-6-phosphate dehydrogenase gene within the lactose operon

Sorbitol is claimed to have important health-promoting effects and Lactobacillus casei is a lactic acid bacterium relevant as probiotic and used as a cheese starter culture. A sorbitol-producing L. casei strain might therefore be of considerable interest in the food industry. A recombinant strain of L. casei was constructed by the integration of a d-sorbitol-6-phosphate dehydrogenase-encoding gene (gutF) in the chromosomal lactose operon (strain BL232). gutF expression in this strain followed the same regulation as that of the lac genes, that is, it was repressed by glucose and induced by lactose. (13)C-nuclear magnetic resonance analysis of supernatants of BL232 resting cells demonstrated that, when pre-grown on lactose, cells were able to synthesize sorbitol from glucose. Inactivation of the l-lactate dehydrogenase gene in BL232 led to an increase in sorbitol production, suggesting that the engineered route provides an alternative pathway for NAD(+) regeneration.

Use of an α-Galactosidase Gene as a Food-Grade Selection Marker for Streptococcus thermophilus

The alpha-galactosidase gene (aga) of Lactococcus raffinolactis ATCC 43920 was previously shown to be an efficient food-grade selection marker in Lactococcus lactis and Pediococcus acidilactici but not in Streptococcus thermophilus. In this study, we demonstrated that the alpha-galactosidase of L. raffinolactis is thermolabile and inoperative at 42 degrees C, the optimal growth temperature of S. thermophilus. An in vitro assay indicated that the activity of this alpha-galactosidase at 42 degrees C was only 3% of that at 30 degrees C, whereas the enzyme retained 23% of its activity at 37 degrees C. Transformation of Strep. thermophilus RD733 with the shuttle-vector pNZ123 bearing the aga gene of L. raffinolactis (pRAF301) generated transformants that were stable and able to grow on melibiose and raffinose at 37 degrees C or below. The transformed cells possessed 6-fold more alpha-galactosidase activity after growth on melibiose than cells grown on lactose. Slot-blot analyses of aga mRNA indicated that repression by lactose occurred at the transcriptional level. The presence of pRAF301 did not interfere with the lactic acid production when the transformed cells of Strep. thermophilus were grown at the optimal temperature in milk. Using the recombinant plasmid pRAF301, which carries a chloramphenicol resistance gene in addition to aga, we showed that both markers were equally efficient at differentiating transformed from nontransformed cells. The aga gene of L. raffinolactis can be used as a highly efficient selection marker in Strep. thermophilus.

Analysis of promoter sequences from Lactobacillus helveticus CNRZ32 and their activity in other lactic acid bacteria

AIMS

To clone and analyse seven putative promoter fragments (pepC, pepN, pepX, pepO, pepE, pepO2, hsp17) from Lactobacillus helveticus CNRZ32 for their expression in Lact. helveticus CNRZ32, Lact. casei ATCC334 and Lactococcus lactis MG1363.

METHODS AND RESULTS

Promoter fragments were fused to the promoter-less beta-glucuronidase (gusA) gene on pNZ272(RBS-) (ATG-). The resulting constructs were evaluated for their ability to drive the expression of active GusA with 0.5 mmol l(-1) 5-bromo-4-chloro-3-indolyl-beta-D-glucuronide. All promoters except P(pepN)::gusA were active in the examined strains. Northern hybridization was performed to examine the promoter strength. Sequence analysis of these promoters identified well conserved putative ribosomal binding and putative -10 hexamers sites.

CONCLUSIONS

Seven promoter fragments from Lact. helveticus CNRZ32 were recognized in the lactic acid bacteria, Lact. casei ATCC334 and L. lactis MG1363, as well as in Escherichia coli. P(pepN)::gusA could not be maintained in the strains examined because of toxicity associated with heterologous protein over-expression driven by P(pepN).

SIGNIFICANCE AND IMPACT OF THE STUDY

This study revealed that desirable levels of heterologous food-grade protein production in GRAS organisms can be obtained with the application of natural promoter fragments from closely related organisms.

General and specialized vectors derived from pBM02, a new rolling circle replicating plasmid of Lactococcus lactis

This paper reports the construction of several general cloning vectors and a specialized depurative vector based on a new lactococcal plasmid that replicates by the rolling circle mechanism [pBM02; Plasmid 49 (2003) 118]. Most vectors are shuttle vectors for Escherichia coli-Lactococcus lactis and carry replicons of both ColE1 and pBM02 plasmids (ColE1 is used even though the pBM02 replicon is fully active in both Gram-positive and Gram-negative organisms). Segregational and structural studies indicated that the new vectors were stable enough for the majority of applications. Further, since the basic replicon is compatible with plasmid derivatives of pWV01 and pSH71, they can be maintained in the same cell with members of the two largest vector series for L. lactis and other lactic acid bacteria, the pGK, and the pNZ series.

Expression of Streptococcus pneumoniae antigens, PsaA (pneumococcal surface antigen A) and PspA (pneumococcal surface protein A) by Lactobacillus casei

A number of recent research works in lactic acid bacteria aim towards the design of new strains that could be used as live vectors for the delivery of antigens for oral vaccination, or other therapeutic molecules. In this work, an inducible expression system based on the Lactobacillus casei lactose operon promoter was used to express three important surface antigens of Streptococcus pneumoniae in this lactic acid bacterium: a virulence-related pneumococcal surface antigen (PsaA) and two variants of the virulence factor PspA (pneumococcal surface protein A). Expression of the three proteins was induced upon growth on lactose and strongly repressed by glucose. These proteins were produced intracellularly. Also, secretion to the growth medium was achieved by means of a fusion to the secreting and processing signals from the L. casei surface proteinase. Interestingly, while secreted PspA proteins were found in the culture supernatants, PsaA remained trapped in the cell wall. Expression of pneumococcal antigens in a food-grade organism opens an alternative for mucosal vaccination against this important pathogen.

Application of the shsp gene, encoding a small heat shock protein, as a food-grade selection marker for lactic acid bacteria

Plasmid pSt04 of Streptococcus thermophilus contains a gene encoding a protein with homology to small heat shock proteins (A. Geis, H. A. M. El Demerdash, and K. J. Heller, Plasmid 50:53-69, 2003). Strains cured from the shsp plasmids showed significantly reduced heat and acid resistance and a lower maximal growth temperature. Transformation of the cloned shsp gene into S. thermophilus St11 lacking a plasmid encoding shsp resulted in increased resistance to incubation at 60 degrees C or pH 3.5 and in the ability to grow at 52 degrees C. A food-grade cloning system for S. thermophilus, based on the plasmid-encoded shsp gene as a selection marker, was developed. This approach allowed selection after transfer of native and recombinant shsp plasmids into different S. thermophilus and Lactococcus lactis strains. Using a recombinant plasmid carrying an erythromycin resistance (Em(r)) gene in addition to shsp, we demonstrated that both markers are equally efficient in selecting for plasmid-bearing cells. The average transformation rates in S. thermophilus (when we were selecting for heat resistance) were determined to be 2.4 x 10(4) and 1.0 x 10(4) CFU/0.5 micro g of DNA, with standard deviations of 0.54 x 10(4) and 0.32 x 10(4), for shsp and Em(r) selection, respectively. When we selected for pH resistance, the average transformation rates were determined to be 2.25 x 10(4) and 3.8 x 10(3) CFU/0.5 micro g of DNA, with standard deviations of 0.63 x 10(4) and 3.48 x 10(3), for shsp and Em(r) selection, respectively. The applicability of shsp as a selection marker was further demonstrated by constructing S. thermophilus plasmid pHRM1 carrying the shsp gene as a selection marker and the restriction-modification genes of another S. thermophilus plasmid as a functional trait.

New integration vector using a cellulase gene as a screening marker for Lactobacillus

The new integration vector for Lactobacillus, pJC4, was developed using the extracellular endoglucanase A gene (celA) of Clostridium thermocellum as a screening marker. pJC4 was transformed into four Lactobacillus species, Lb. johnsonii, Lb. gasseri, Lb. bulgaricus, and Lb. plantarum. In each species, the pJC4 integrants were easily and accurately detected by the appearance of a clear halo on a cellulase screening plate without any false transformants. Polymerase chain reaction and Southern hybridization indicated that all transformants with clear halos contained pJC4 in their chromosomal DNAs. The celA gene could be a useful screening marker for other lactic acid bacteria.

Significant differences between Lactobacillus casei subsp. casei ATCC 393T and a commonly used plasmid-cured derivative revealed by a polyphasic study

Many studies on Lactobacillus casei subsp. casei (L. casei) have been carried out using strain ATCC 393 (pLZ15-). Four strains of L. casei ATCC 393T and three of ATCC 393 (pLZ15-) were compared using phenotypic methods and many of the available genotyping techniques. These tests showed that strains of ATCC 393T obtained from independent public type-culture collections were significantly different from the plasmid-free (pLZ15-) strains of ATCC 393T. These findings were confirmed by sequencing the first 580 nt (domain I) of the 16S and 23S rDNAs of the strains. Complete sequencing of the 16S rDNA of one representative strain from each group revealed that strain ATCC 393T from culture collections was 99% similar to Lactobacillus zeae ATCC 15820T and that the strain so far considered as L. casei ATCC 393 (pLZ15-) was, in turn, 100% similar to L. casei ATCC 334 and Lactobacillus paracasei subsp. paracasei ATCC 4022. All data obtained in this work indicate that the ancestral strain of ATCC 393 (pLZ15-) might never have been the strain that is now available from culture collections.

Genetically modified lactic acid bacteria: applications to food or health and risk assessment

Lactic acid bacteria have a long history of use in fermented food products. Progress in gene technology allows their modification by introducing new genes or by modifying their metabolic functions. These modifications may lead to improvements in food technology (bacteria better fitted to technological processes, leading to improved organoleptic properties em leader ), or to new applications including bacteria producing therapeutic molecules that could be delivered by mouth. Examples in these two fields will be discussed, at the same time evaluating their potential benefit to society and the possible risks associated with their use. Risk assessment and expected benefits will determine the future use of modified bacteria in the domains of food technology and health.

Inducible promoter-repressor system from the Lactobacillus casei phage phiFSW

With the aim to extend the presently available inducible gene expression systems for lactobacilli, we have isolated a thermoinducible promoter-repressor cassette from the temperate Lactobacillus casei phage phiFSW-TI in Escherichia coli. The phiFSW-TI promoter fragment was abutted to the plasmid-borne promoterless beta-glucuronidase (gusA) reporter gene and shown to direct its transcription in L. casei. In addition, the functionality of the promoter-repressor system was verified in the L. casei phiFSW-TI lysogen by showing that the gusA reporter gene, controlled by the isolated phiFSW-TI promoter, was repressed at 28 degrees C and expressed at 42 degrees C. Moreover, a homology search revealed that the C terminus of the isolated phiFSW repressor shows a high similarity to the small mutS-related domain of the MutS2 protein family that is unprecedented for phage-encoded repressor proteins.

Construction of a xylanase-producing strain of Brevibacterium lactofermentum by stable integration of an engineered xysA gene from Streptomyces halstedii JM8

A xylanolytic strain of Brevibacterium lactofermentum containing the Streptomyces halstedii His-tagged xysA gene was generated. The new strain contains DNA derived from S. halstedii, expresses xylanolytic activity, and was obtained by an integrative process mediated by a conjugative plasmid targeted to a dispensable chromosomal region located downstream from the essential cell division gene ftsZ. The His-tagged Xys1 enzyme was constitutively expressed under the control of the kan promoter from Tn5 and was easily purified by use of Ni-nitrilotriacetic acid-agarose. The new strain is stable for more than 200 generations, lacks any known antibiotic resistance gene, and does not need any selective pressure to maintain the integrated gene. This strategy can be used to integrate any gene into the B. lactofermentum chromosome and to maintain it stably without the use of antibiotics for selection.