Transformation ofLactobacillus caseiby electroporation

Recent advances in genetic tools for engineering probiotic lactic acid bacteria

Synthetic biology has grown exponentially in the last few years, with a variety of biological applications. One of the emerging applications of synthetic biology is to exploit the link between microorganisms, biologics, and human health. To exploit this link, it is critical to select effective synthetic biology tools for use in appropriate microorganisms that would address unmet needs in human health through the development of new game-changing applications and by complementing existing technological capabilities. Lactic acid bacteria (LAB) are considered appropriate chassis organisms that can be genetically engineered for therapeutic and industrial applications. Here, we have reviewed comprehensively various synthetic biology techniques for engineering probiotic LAB strains, such as clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 mediated genome editing, homologous recombination, and recombineering. In addition, we also discussed heterologous protein expression systems used in engineering probiotic LAB. By combining computational biology with genetic engineering, there is a lot of potential to develop next-generation synthetic LAB with capabilities to address bottlenecks in industrial scale-up and complex biologics production. Recently, we started working on Lactochassis project where we aim to develop next generation synthetic LAB for biomedical application.

Contribution of Lactobacilli on Intestinal Mucosal Barrier and Diseases: Perspectives and Challenges of Lactobacillus casei

The intestine barrier, the front line of normal body defense, relies on its structural integrity, microbial composition and barrier immunity. The intestinal mucosal surface is continuously exposed to a complex and dynamic community of microorganisms. Although it occupies a relatively small proportion of the intestinal microbiota, Lactobacilli has been discovered to have a significant impact on the intestine tract in previous studies. It is undeniable that some Lactobacillus strains present probiotic properties through maintaining the micro-ecological balance via different mechanisms, such as mucosal barrier function and barrier immunity, to prevent infection and even to solve some neurology issues by microbiota-gut-brain/liver/lung axis communication. Notably, not only living cells but also Lactobacillus derivatives (postbiotics: soluble secreted products and para-probiotics: cell structural components) may exert antipathogenic effects and beneficial functions for the gut mucosal barrier. However, substantial research on specific effects, safety and action mechanisms in vivo should be done. In clinical application of humans and animals, there are still doubts about the precise evaluation of Lactobacilli's safety, therapeutic effect, dosage and other aspects. Therefore, we provide an overview of central issues on the impacts of Lactobacillus casei (L. casei) and their products on the intestinal mucosal barrier and some diseases and highlight the urgent need for further studies.

Conjugative DNA Transfer From E. coli to Transformation-Resistant Lactobacilli

Lactic acid bacteria (LAB) belonging to the genus classically known as Lactobacillus, recently split into 25 different genera, include many relevant species for the food industry. The well-known properties of lactobacilli as probiotics make them an attractive model also for vaccines and therapeutic proteins delivery in humans. However, scarce tools are available to accomplish genetic modification of these organisms, and most are only suitable for laboratory strains. Here, we test bacterial conjugation as a new tool to introduce genetic modifications into many biotechnologically relevant laboratory and wild type lactobacilli. Using mobilizable shuttle plasmids from a donor Escherichia coli carrying either RP4 or R388 conjugative systems, we were able to get transconjugants to all tested Lactocaseibacillus casei strains, including many natural isolates, and to several other genera, including Lentilactobacillus parabuchneri, for which no transformation protocol has been reported. Transconjugants were confirmed by the presence of the oriT and 16S rRNA gene sequencing. Serendipitously, we also found transconjugants into researcher-contaminant Staphylococcus epidermidis. Conjugative DNA transfer from E. coli to S. aureus was previously described, but at very low frequencies. We have purified this recipient strain and used it in standard conjugation assays, confirming that both R388 and RP4 conjugative systems mediate mobilization of plasmids into S. epidermidis. This protocol could be assayed to introduce DNA into other Gram-positive microorganisms which are resistant to transformation.

Immobilization of Cd Using Mixed Enterobacter and Comamonas Bacterial Reagents in Pot Experiments with Brassica rapa L

Enterobacter sp. A11 and Comamonas sp. A23 were isolated and identified. Coculturing these two strains with Cd(II) led to the production of biofilm, H₂S, and succinic acid (SA), and Cd(II) was adsorbed by cells and formed CdS precipitates. After centrifugation, 97% Cd(II) was removed from the coculture. Proteomic and metabolomic analyses of the cocultured bacteria revealed that H₂S and SA production pathways, metal transportation, and TCA cycle were active under Cd(II) stress. In vitro addition of SA enhanced the production of H₂S and biofilm formation and Cd(II) adsorption. Two-season greenhouse pot experiments with Brassica rapa L. were performed with and without the coculture bacteria. Compared with the control, the average Cd amounts of the two-season pot experiments of the aboveground plants were decreased by 71.3%, 62.8%, and 38.6%, and the nonbioavailable and immobilized Cd in the soils were increased by 211.8%, 213.4%, and 116.7%, for low-, medium-, and high- Cd-spiked soils, respectively. The two strains survived well in soil during plant growth using plate counting, quantitative real-time PCR, and metagenomics analysis. Our results indicate that the combination of Enterobacter and Comamonas strains with the production of H₂S and biofilm are important effectors for the highly efficient immobilization of Cd.

Multiple pulse electroporation of lactic acid bacteria Lactococcus lactis and Lactobacillus casei

Genetic manipulation of lactic acid bacteria is often difficult due to the inability to transform them with high efficiency. Multi-pulse electroporation offers a simple approach to increase transformation efficiencies. Using cells grown with 1% glycine and pretreated with lithium acetate and dithiothreitol, multi-pulse electroporation (five pulses of 12.5 kV cm^-1) of Lactococcus lactis JB704 cells resulted in a transformation efficiency of up to 1.2 × 10⁶ colony forming units (CFU) μg^-1 pGK13, an 8-fold increase in the transformation efficiency compared to single pulse electroporation. Other cell growth and pretreatment conditions with JB704 resulted in lower transformation efficiencies but had 4-fold to 27-fold higher transformation efficiencies with the five pulse electroporations. With similarly grown and pretreated Lactobacillus casei 32G cells, multi-pulse electroporation (five pulses of 7.5 kV cm^-1) resulted in a mean transformation efficiency of 7.3 × 10³ CFU μg^-1 pTRKH2, a 4-fold increase in the transformation efficiency compared to single pulse electroporation.

Secretion of M2e:HBc fusion protein by Lactobacillus casei using Cwh signal peptide

The ability to serve as a delivery vehicle for various interesting biomolecules makes lactic acid bacteria (LAB) very useful in several applications. In the medical field, recombinant LAB expressing pathogenic antigens at different cellular locations have been used to elicit both mucosal and systemic immune responses. Expression–secretion vectors (ESVs) with a signal peptide (SP) are pivotal for protein expression and secretion. In this study, the genome sequence of Lactobacillus casei ATCC334 was explored for new SPs using bioinformatics tools. Three new SPs of the proteins Cwh, SurA and SP6565 were identified and used to construct an ESV based on our Escherichia coli–L. casei shuttle vector, pRCEID-LC13.9. Functional testing of these constructs with the green fluorescence protein (GFP) gene showed that they could secrete the GFP. The construct with CwhSP showed the highest GFP secretion. Consequently, CwhSP was selected to develop an ESV construct carrying a synthetic gene encoding the extracellular domain of the matrix 2 protein fused with the hepatitis B core antigen (M2e:HBc). This ESV was shown to efficiently express and secrete the M2e:HBc fusion protein. The identified SPs and the developed ESVs can be exploited for expression and secretion of homologous and heterologous proteins in L. casei.

The novel Cwh signal peptide selected from the Lactobacillus casei genome by using a bioinformatics approach was successfully used in the expression–secretion vector for heterologous protein secretion.

Efficient transformation of Staphylococcus aureus using multi-pulse electroporation

A new multi-pulse electroporation system was evaluated to transform Staphylococcus aureus. Compared to the conventional electroporation system, it yielded high transformation efficiency to obtain more than 3.9×10⁵S. aureus RN4220 transformed cells/1μg plasmid DNA using a single electroporation by manipulating the poring pulse and transfer pulse.

Room temperature electrocompetent bacterial cells improve DNA transformation and recombineering efficiency

Bacterial competent cells are essential for cloning, construction of DNA libraries, and mutagenesis in every molecular biology laboratory. Among various transformation methods, electroporation is found to own the best transformation efficiency. Previous electroporation methods are based on washing and electroporating the bacterial cells in ice-cold condition that make them fragile and prone to death. Here we present simple temperature shift based methods that improve DNA transformation and recombineering efficiency in E. coli and several other gram-negative bacteria thereby economizing time and cost. Increased transformation efficiency of large DNA molecules is a significant advantage that might facilitate the cloning of large fragments from genomic DNA preparations and metagenomics samples.

Abiotic Gene Transfer: Rare or Rampant?

Phylogenetic studies reveal that horizontal gene transfer (HGT) plays a prominent role in evolution and genetic variability of life. Five biotic mechanisms of HGT among prokaryotic organisms have been extensively characterized: conjugation, competence, transduction, gene transfer agent particles, and transitory fusion with recombination, but it is not known whether they can account for all natural HGT. It is even less clear how HGT could have occurred before any of these mechanisms had developed. Here, we consider contemporary conditions and experiments on microorganisms to estimate possible roles of abiotic HGT-currently and throughout evolution. Candidate mechanisms include freeze-and-thaw, microbeads-agitation, and electroporation-based transformation, and we posit that these laboratory techniques have analogues in nature acting as mechanisms of abiotic HGT: freeze-and-thaw cycles in polar waters, agitation by sand at foreshores and riverbeds, and lightning-triggered electroporation in near-surface aqueous habitats. We derive conservative order-of-magnitude estimates for rates of microorganisms subjected to freeze-and-thaw cycles, sand agitation, and lightning-triggered electroporation, at 10²⁴, 10¹⁹, and 10¹⁷ per year, respectively. Considering the yield of viable transformants, which is by far the highest in electroporation, we argue this may still favor lightning-triggered transformation over the other two mechanisms. Electroporation-based gene transfer also appears to be the most general of these abiotic candidates, and perhaps even of all known HGT mechanisms. Future studies should provide improved estimates of gene transfer rates and cell viability, currently and in the past, but to assess the importance of abiotic HGT in nature will likely require substantial progress-also in knowledge of biotic HGT.

Development of an Escherichia coli-Lactobacillus casei shuttle vector for heterologous protein expression in Lactobacillus casei

There is an increasing interest to develop various lactic acid bacteria (LAB) species as mucosal delivery vehicles, for which the development of a variety of cloning and expression systems for these bacteria is of primary importance. This study reports the complete nucleotide sequence of the cryptic plasmid pRCEID7.6 derived from the chicken probiotic LAB strain Lactobacillus casei TISTR1341. Sequence analysis and comparison showed that pRCEID7.6 is composed of nine putative open reading frames. The replicon origin of pRCEID7.6 consisted of untranslated origin of replication and translated replication protein B sequences. This region was used to construct Escherichia coli/L. casei shuttle vectors carrying erythromycin and chloramphenicol resistance genes as selective markers. Segregation and structural stability of the vectors in L. casei was sufficient for most genetic applications. The feasibility of this vector for heterologous protein expression in L. casei was determined by cloning in pRCEID-LC7.6, the gene encoding the nucleocapsid protein (NP), from the influenza A virus under the control of the homologous promoter from the lactate dehydrogenase gene. L. casei carrying this recombinant plasmid was shown to successfully express the NP protein. Therefore, this shuttle vector can be used for further study in the development of mucosal delivery vehicles.

Inhibition of pathogenic and spoilage bacteria by a novel biofilm-forming Lactobacillus isolate: a potential host for the expression of heterologous proteins

BACKGROUND

Bacterial biofilms are a preferred mode of growth for many types of microorganisms in their natural environments. The ability of pathogens to integrate within a biofilm is pivotal to their survival. The possibility of biofilm formation in Lactobacillus communities is also important in various industrial and medical settings. Lactobacilli can eliminate the colonization of different pathogenic microorganisms. Alternatively, new opportunities are now arising with the rapidly expanding potential of lactic acid bacteria biofilms as bio-control agents against food-borne pathogens.

RESULTS

A new isolate Lactobacillus plantarum PA21 could form a strong biofilm in pure culture and in combination with several pathogenic and food-spoilage bacteria such as Salmonella enterica, Bacillus cereus, Pseudomonas fluorescens, and Aeromonas hydrophila. Exposure to Lb. plantarum PA21 significantly reduced the number of P. fluorescens, A. hydrophila and B. cereus cells in the biofilm over 2-, 4- and 6-day time periods. However, despite the reduction in S. enterica cells, this pathogen showed greater resistance in the presence of PA21 developed biofilm, either in the planktonic or biofilm phase. Lb. plantarum PA21 was also found to be able to constitutively express GFP when transformed with the expression vector pMG36e which harbors the gfp gene as a reporter demonstrating that the newly isolated strain can be used as host for genetic engineering.

CONCLUSION

In this study, we evaluate the ability of a new Lactobacillus isolate to form strong biofilm, which would provide the inhibitory effect against several spoilage and pathogenic bacteria. This new isolate has the potential to serve as a safe and effective cell factory for recombinant proteins.

High efficiency electrotransformation of Lactobacillus casei

We investigated whether protocols allowing high efficiency electrotransformation of other lactic acid bacteria were applicable to five strains of Lactobacillus casei (12A, 32G, A2-362, ATCC 334 and BL23). Addition of 1% glycine or 0.9 M NaCl during cell growth, limitation of the growth of the cell cultures to OD600 0.6-0.8, pre-electroporation treatment of cells with water or with a lithium acetate (100 mM)/dithiothreitol (10 mM) solution and optimization of electroporation conditions all improved transformation efficiencies. However, the five strains varied in their responses to these treatments. Transformation efficiencies of 10(6) colony forming units μg(-1) pTRKH2 DNA and higher were obtained with three strains which is sufficient for construction of chromosomal gene knock-outs and gene replacements.

Lightning-triggered electroporation and electrofusion as possible contributors to natural horizontal gene transfer

Phylogenetic studies show that horizontal gene transfer (HGT) is a significant contributor to genetic variability of prokaryotes, and was perhaps even more abundant during the early evolution. Hitherto, research of natural HGT has mainly focused on three mechanisms of DNA transfer: conjugation, natural competence, and viral transduction. This paper discusses the feasibility of a fourth such mechanism--cell electroporation and/or electrofusion triggered by atmospheric electrostatic discharges (lightnings). A description of electroporation as a phenomenon is followed by a review of experimental evidence that electroporation of prokaryotes in aqueous environments can result in release of non-denatured DNA, as well as uptake of DNA from the surroundings and transformation. Similarly, a description of electrofusion is followed by a review of experiments showing that prokaryotes devoid of cell wall can electrofuse into hybrids expressing the genes of their both precursors. Under sufficiently fine-tuned conditions, electroporation and electrofusion are efficient tools for artificial transformation and hybridization, respectively, but the quantitative analysis developed here shows that conditions for electroporation-based DNA release, DNA uptake and transformation, as well as for electrofusion are also present in many natural aqueous environments exposed to lightnings. Electroporation is thus a plausible contributor to natural HGT among prokaryotes, and could have been particularly important during the early evolution, when the other mechanisms might have been scarcer or nonexistent. In modern prokaryotes, natural absence of the cell wall is rare, but it is reasonable to assume that the wall has formed during a certain stage of evolution, and at least prior to this, electrofusion could also have contributed to natural HGT. The concluding section outlines several guidelines for assessment of the feasibility of lightning-triggered HGT.

Sequencing and analysis of three plasmids from Lactobacillus casei TISTR1341 and development of plasmid-derived Escherichia coli-L. casei shuttle vectors

Pyrosequencing followed by conventional PCR and sequencing was used to determine the complete nucleotide sequence of three plasmids (pRCEID2.9, pRCEID3.2, and pRCEID13.9) from the Lactobacillus casei strain TISTR1341. The plasmid sequences were found to be almost identical, respectively, to those of pLA106, pLA105, and pLA103 from Lactobacillus acidophilus strain TK8912, suggesting that these strains may be related. Sequence analysis and comparison indicated that pRCEID2.9 replicates by a rolling circle (RC) mechanism, while pRCEID3.2 and pRCEID13.9 probably follow a theta-type mode of replication. Replicons of pRCEID2.9 and pRCEID13.9 were used to develop Escherichia coli/L. casei compatible shuttle vectors, which were stably maintained in different genetic backgrounds. Real-time quantitative PCR analysis showed copy numbers of around 4 and 15, respectively, for the pRCEID13.9- and pRCEID2.9-derived shuttle vectors per chromosome equivalent. The functionality of vector pRCEID-LC13.9 was proved by cloning and expressing in L. casei of a green fluorescent protein gene variant from Aequorea victoria under the control of the promoter from a homologous lactate dehydrogenase gene. The new vectors might complement those currently in use for the exploitation of L. casei as a cellular factory and in other biotechnological applications.

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.

Plasmid uptake by bacteria: a comparison of methods and efficiencies

The ability to introduce individual molecules of plasmid DNA into cells by transformation has been of central importance to the recent rapid advancement of plasmid biology and to the development of DNA cloning methods. Molecular genetic manipulation of bacteria requires the development of plasmid-mediated transformation systems that include (1) chemical transformation, (2) electro-transformation, (3) biolistic transformation, and (4) sonic transformation, leading to the introduction of exogenous plasmid DNA into bacterial cells. In this review, the manipulation properties and transformation efficiencies of these techniques are described. In addition to these methods, a conceptually novel transformation technique, namely the hydrogel exposure method, was developed. The hydrogel exposure method, based on the Yoshida effect, provides a significant advance over chemical means for transforming many strains of Escherichia coli and a variety of other bacterial species. The new term "tribos transformation" has been proposed for this novel technique. We also determined that, compared to conventional methods, the hydrogel exposure method is a novel and convenient method by which to transform bacteria.

Suppressive effect on activation of macrophages by Lactobacillus casei strain Shirota genes determining the synthesis of cell wall-associated polysaccharides

Although many Lactobacillus strains used as probiotics are believed to modulate host immune responses, the molecular natures of the components of such probiotic microorganisms directly involved in immune modulation process are largely unknown. We aimed to assess the function of polysaccharide moiety of the cell wall of Lactobacillus casei strain Shirota as a possible immune modulator which regulates cytokine production by macrophages. A gene survey of the genome sequence of L. casei Shirota hunted down a unique cluster of 10 genes, most of whose predicted amino acid sequences had similarities to various extents to known proteins involved in biosynthesis of extracellular or capsular polysaccharides from other lactic acid bacteria. Gene knockout mutants of eight genes from this cluster resulted in the loss of reactivity to L. casei Shirota-specific monoclonal antibody and extreme reduction of high-molecular-mass polysaccharides in the cell wall fraction, indicating that at least these genes are involved in biosynthesis of high-molecular-mass cell wall polysaccharides. By adding heat-killed mutant cells to mouse macrophage cell lines or to mouse spleen cells, the production of tumor necrosis factor alpha, interleukin-12 (IL-12), IL-10, and IL-6 was more stimulated than by wild-type cells. In addition, these mutants additively enhanced lipopolysaccharide-induced IL-6 production by RAW 264.7 mouse macrophage-like cells, while wild-type cells significantly suppressed the IL-6 production of RAW 264.7. Collectively, these results indicate that this cluster of genes of L. casei Shirota, which have been named cps1A, cps1B, cps1C, cps1D, cps1E, cps1F, cps1G, and cps1J, determine the synthesis of the high-molecular-mass polysaccharide moiety of the L. casei Shirota cell wall and that this polysaccharide moiety is the relevant immune modulator which may function to reduce excessive immune reactions during the activation of macrophages by L. casei Shirota.

Casitone-dependent transcriptional regulation of the prtP and prtM genes in the natural isolate Lactobacillus paracasei subsp. paracasei

The prtP-prtM intergenic region of Lactobacillus paracasei subsp. paracasei BGHN 14 was cloned and sequenced. The nucleotide sequence of the prtP-prtM intergenic region in BGHN 14, containing divergently orientated P(prtP) and P(prtP) promoters, was shorter by 35 bp in comparison with that in lactococci. The nucleotide sequence involved in casitone-dependent transcriptional regulation of the lactococcal prt genes was not found in the BGHN14. The activity of P(prtM) in L. lactis NZ9000 was very low and insignificantly changed in the presence of casitone, whereas P(prtP) was completely inactive. When L. casei ATCC393(T) was used as host, both P(prtP) and P(prtM) were active and strongly regulated by casitone. The results strongly indicate that the mechanisms of the casitone-dependent regulation of the prt genes in BGHN14 and lactococci are different.

Comparative sequence analysis of plasmids from Lactobacillus delbrueckii and construction of a shuttle cloning vector

While plasmids are very commonly associated with the majority of the lactic acid bacteria, they are only very rarely associated with Lactobacillus delbrueckii, with only four characterized to date. In this study, the complete sequence of a native plasmid, pDOJ1, from a strain of Lactobacillus delbrueckii subsp. bulgaricus was determined. It consisted of a circular DNA molecule of 6,220 bp with a G+C content of 44.6% and a characteristic ori and encoded six open reading frames (ORFs), of which functions could be predicted for three-a mobilization (Mob) protein, a transposase, and a fused primase-helicase replication protein. Comparative analysis of pDOJ1 and the other available L. delbrueckii plasmids (pLBB1, pJBL2, pN42, and pLL1212) revealed a very similar organization and amino acid identities between 85 and 98% for the putative proteins of all six predicted ORFs from pDOJ1, reflecting a common origin for L. delbrueckii plasmids. Analysis of the fused primase-helicase replication gene found a similar fused organization only in the theta replicating group B plasmids from Streptococcus thermophilus. This observation and the ability of the replicon to function in S. thermophilus support the idea that the origin of plasmids in L. delbrueckii was likely from S. thermophilus. This may reflect the close association of these two species in dairy fermentations, particularly yogurt production. As no vector based on plasmid replicons from L. delbrueckii has previously been constructed, an Escherichia coli-L. delbrueckii shuttle cloning vector, pDOJ4, was constructed from pDOJ1, the p15A ori, the chloramphenicol resistance gene of pCI372, and the lacZ polylinker from pUC18. This cloning vector was successfully introduced into E. coli, L. delbrueckii subsp. bulgaricus, S. thermophilus, and Lactococcus lactis. This shuttle cloning vector provides a new tool for molecular analysis of Lactobacillus delbrueckii and other lactic acid bacteria.

Cloning and sequencing of plasmid pLC494 isolated from human intestinal Lactobacillus casei: construction of an Escherichia coli-Lactobacillus shuttle vector

The complete nucleotide sequence of plasmid pLC494 isolated from Lactobacillus casei L-49 was determined. Plasmid pLC494 is an 8846-bp long circular molecule with a G+C content of 41.5%. Two putative open reading frames, ORF4 (282 amino acids) and ORF5 (169 amino acids), were identified as replication proteins A and B that revealed 100 and 99% similarity, respectively, with the replication proteins of plasmid pLA103 from Lactobacillus acidophilus TK8912. Upstream of ORF4 were the four repeat regions (three perfect 22-bp repeats and one imperfect motif), a putative ribosome binding site, a -10 region, and a -35 region. The shuttle vector pJLE4942 (5318 bp) was constructed using repA from pLC494, a multiple cloning site, ColE1 ori, the ori of gram-negative bacteria from vector pUC19, and the chloramphenicol resistance gene from pJIR418 as a selection marker. Transformation of several lactic acid bacteria with the vector pJLE4942 indicated that this vector might be useful as a genetic tool for the intestinal lactobacilli.

Optimization of technical conditions for the transformation of Lactobacillus acidophilus strains by electroporation

AIMS

To optimize the conditions for electroporating foreign plasmid DNA into Lactobacillus acidophilus ATCC 43121.

METHODS AND RESULTS

The conditions of electroporation were optimized to improve the transformation efficiency. Plasmid pNZ123 containing multicloning site and chloramphenicol resistance was employed to construct a cloning vector. The optimum electroporation conditions for the maximum transformation efficiency were a pulse strength of 12.5 kV cm(-1), a pulse number of 10, a pulse interval of 500 ms, and pNZ123 plasmid DNA concentration of 25 ng microl(-1). Under the optimum conditions the transformation efficiency of L. acidophilus ATCC 43121 was 1.84 +/- 0.13 x 10(4) (+/- standard error of measurements) CFU per mug of plasmid DNA. Other strains of L. acidophilus showed transformation efficiencies ranging from 1.38 +/- 0.02 x 10(4) to 9.32 +/- 0.54 x 10(4) under these conditions. A green fluorescent protein (GFP) was successfully expressed and detected by fluorescence microscopy when the pKU::slpA-GFP, pNZ123 containing GFP gene, was transformed in L. acidophilus ATCC 43121 under the optimum conditions.

CONCLUSIONS

The results suggest that electrical parameters, antibiotic concentration, and host specificity play important roles to determine transformation efficiency of lactobacilli. The optimum conditions for the transformation of L. acidophilus ATCC 43121 may be applied to improve transformation efficiency of other lactobacilli.

SIGNIFICANCE AND IMPACT OF THE STUDY

The optimized conditions for electrotransformation may provide a mean to improve the introduction of foreign DNA into L. acidophilus to be used as a vehicle for a heterologous protein expression.

Sequence analysis of the plasmid genome of the probiotic strain Lactobacillus paracasei NFBC338 which includes the plasmids pCD01 and pCD02

Lactobacillus paracasei NFBC338 is a probiotic strain that was isolated from the human gastrointestinal tract (GIT) and contains a plasmid genome of 80kb. Using a shotgun sequencing approach, two of the plasmids, pCD01 (19,882bp) and pCD02 (8554bp) have been completely sequenced, and four contiguous sequences (Contigs) have been assembled. Bioinformatic analysis of pCD01 revealed that it contains 23 putative open reading frames (ORFs) and that it contains regions characterised by potential replication functions and multidrug resistance (MDR). In contrast, the content of pCD02 is mainly cryptic, although, it does contain two insertion sequence (IS) elements. Indeed, up to 17% of the entire plasmid genome encodes putative transposable elements. In addition, there are a number of interesting ORFs distributed over the four Contigs that show significant homology to genes such as those involved in adherence and biotin metabolism, which may prove beneficial to Lb. paracasei NFBC338 under certain environmental conditions. This study provides a novel insight into the rich plasmid complement of this probiotic Lactobacillus strain, which may potentially be exploited as the basis for development of improved genetic tools for probiotic lactobacilli.

Modified electroporation protocol for Lactobacilli isolated from the chicken crop facilitates transformation and the use of a genetic tool

Isolates of Lactobacillus spp. from a collection of potentially probiotic strains isolated from the crops of broiler chickens were found to be non-electrotransformable using published techniques. One strain of Lactobacillus salivarius was shown to develop electrocompetence when an overnight culture was incubated in fresh medium. The effect was enhanced if glycine was incorporated into the fresh growth medium. When these modifications were applied to a number of other crop isolates of Lactobacillus spp., electrocompetence could be detected in approximately half the strains tested. Two temperature sensitive plasmid vectors that had been used for the genetic modification of other lactic acid bacteria were introduced into a crop strain of Lb. salivarius. Both showed temperature sensitivity at 42 degrees C and above but were relatively stable at 37 degrees C. The genetic tool harbouring an IS element allowed the delivery of the plasmid to multiple independent sites in the host chromosome. Harnessing such genetic tools will facilitate the future genetic analysis of the host bacterium.

Identification and genetic characterization of a novel proteinase, PrtR, from the human isolate Lactobacillus rhamnosus BGT10

A novel proteinase, PrtR, produced by the human vaginal isolate Lactobacillus rhamnosus strain BGT10 was identified and genetically characterized. The prtR gene and flanking regions were cloned and sequenced. The deduced amino acid sequence of PrtR shares characteristics that are common for other cell envelope proteinases (CEPs) characterized to date, but in contrast to the other cell surface subtilisin-like serine proteinases, it has a smaller and somewhat different B domain and lacks the helix domain, and the anchor domain has a rare sorting signal sequence. Furthermore, PrtR lacks the insert domain, which otherwise is situated inside the catalytic serine protease domain of all CEPs, and has a different cell wall spacer (W) domain similar to that of the cell surface antigen I and II polypeptides expressed by oral and vaginal streptococci. Moreover, the PrtR W domain exhibits significant sequence homology to the consensus sequence that has been shown to be the hallmark of human intestinal mucin protein. According to its alpha(S1)- and beta-casein cleavage efficacy, PrtR is an efficient proteinase at pH 6.5 and is distributed throughout all L. rhamnosus strains tested. Proteinase extracts of the BGT10 strain obtained with Ca(2+)-free buffer at pH 6.5 were proteolytically active. The prtR promoter-like sequence was determined, and the minimal promoter region was defined by use of prtR-gusA operon fusions. The prtR expression is Casitone dependent, emphasizing that nitrogen depletion elevates its transcription. This is in correlation with the catalytic activity of the PrtR proteinase.

Optimization of the green fluorescent protein (GFP) expression from a lactose-inducible promoter in Lactobacillus casei

An expression vector for Lactobacillus casei has been constructed containing the inducible lac promoter and the gene encoding ultraviolet visible green fluorescent protein (GFP(UV)) as reporter. Different conditions to grow L. casei were assayed and fluorescence as well as total protein synthesized were quantified. The maintenance of neutral pH had the greatest incidence on GFP(UV) expression, followed by aeration and a temperature of 30 degrees C. Environmental factors favoring GFP(UV) accumulation did not exactly correlate with those enhancing fluorescence. Therefore, oxygenation, by stirring the culture, had the greatest influence on the proportion of fluorescent protein, which is in accordance with the structural requirements of this protein. The highest yield obtained was 1.3 microg of GFP per mg of total protein, from which 55% was fluorescent.

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.

Electrotransformation of Lactobacillus delbrueckii subsp. bulgaricus and L. delbrueckii subsp. lactis with various plasmids

We describe, for the first time, a detailed electroporation procedure for Lactobacillus delbrueckii. Three L. delbrueckii strains were successfully transformed. Under optimal conditions, the transformation efficiency was 10(4) transformants per microg of DNA. Using this procedure, we identified several plasmids able to replicate in L. delbrueckii and integrated an integrative vector based on phage integrative elements into the L. delbrueckii subsp. bulgaricus chromosome. These vectors provide a good basis for developing molecular tools for L. delbrueckii and open the field of genetic studies in L. delbrueckii.

Construction of compatible wide-host-range shuttle vectors for lactic acid bacteria and Escherichia coli

A new collection of shuttle cloning vectors has been constructed that can be used in a broad host range, because they carry replication origins which are functional in Escherichia coli (p15A, pWV01, ColE1), Lactococcus lactis, lactobacilli, and Bacillus subtilis (pAMbeta1, pWV01). These plasmids contain the lacZ-T1T2 cassette from pJDC9, which allows the X-gal selection and cloning of DNA fragments that could cause plasmid instability in E. coli. In addition, they have been proved to be structurally and segregationally stable in Lactobacillus casei, in which their copy number has been determined by real-time quantitative PCR. Furthermore, the antibiotic resistance markers (beta-lactamase, chloramphenicol acetyl transferase, and erythromycin transacetylase) and the theta and rolling circle replicating origins have been combined to obtain this set of compatible plasmids (pIA family) that can be cotransformed, both in lactic acid bacteria and in E. coli.

Transport of D-xylose in Lactobacillus pentosus, Lactobacillus casei, and Lactobacillus plantarum: evidence for a mechanism of facilitated diffusion via the phosphoenolpyruvate:mannose phosphotransferase system

We have identified and characterized the D-xylose transport system of Lactobacillus pentosus. Uptake of D-xylose was not driven by the proton motive force generated by malolactic fermentation and required D-xylose metabolism. The kinetics of D-xylose transport were indicative of a low-affinity facilitated-diffusion system with an apparent K(m) of 8.5 mM and a V(max) of 23 nmol min(-1) mg of dry weight(-1). In two mutants of L. pentosus defective in the phosphoenolpyruvate:mannose phosphotransferase system, growth on D-xylose was absent due to the lack of D-xylose transport. However, transport of the pentose was not totally abolished in a third mutant, which could be complemented after expression of the L. curvatus manB gene encoding the cytoplasmic EIIB(Man) component of the EII(Man) complex. The EII(Man) complex is also involved in D-xylose transport in L. casei ATCC 393 and L. plantarum 80. These two species could transport and metabolize D-xylose after transformation with plasmids which expressed the D-xylose-catabolizing genes of L. pentosus, xylAB. L. casei and L. plantarum mutants resistant to 2-deoxy-D-glucose were defective in EII(Man) activity and were unable to transport D-xylose when transformed with plasmids containing the xylAB genes. Finally, transport of D-xylose was found to be the rate-limiting step in the growth of L. pentosus and of L. plantarum and L. casei ATCC 393 containing plasmids coding for the D-xylose-catabolic enzymes, since the doubling time of these bacteria on D-xylose was proportional to the level of EII(Man) activity.

A rapid and safe plasmid isolation method for efficient engineering of recombinant lactobacilli expressing immunogenic or tolerogenic epitopes for oral administration

Recombinant lactobacilli are being developed which can be used as expression and delivery vectors of heterologous antigens in oral vaccination and other therapeutic applications. Because most Lactobacillus strains do not accept ligation mixtures, sufficiently pure plasmid DNA needs to be isolated from Lactobacillus casei to transform other Lactobacillus strains. The isolation of plasmid DNA from Gram-positive lactobacilli is complicated by the resilience of the peptidoglycan layer. Here a rapid, safe and efficient method is described that combines enzymatic breakdown of the cell wall and purification of the plasmid by commercially available DNA-binding columns. For the lysis-resistant L. casei strain, this method yields high levels of pure plasmid DNA that can be used for common molecular techniques, such as digestion and transformation, with high efficiency.

Genetics of lactobacilli in food fermentations

Lactobacilli play a substantial role in food biotechnology and influence our quality of life by their fermentative and probiotic properties. Despite their obvious importance in fermentation ecology and biotechnology only recent years have brought some insight into the genetics of lactobacilli. These genetic investigations allow the elucidation of traits determinative for competitiveness and ecology and thus product safety and quality. They have concentrated only on a small selection of lactobacilli whereas others are hardly touched or remained recalcitrant to genetic analysis and manipulation. The knowledge gained on the biochemistry, physiology, ecology and especially genetics is a prerequisite for the deliberate application and improved handling of lactobacilli in traditional and novel applications. In this review, the achievements in the genetics of lactobacilli are described including detection systems, genetic elements, host vector systems, gene cloning and expression and risk assessment of genetically engineered lactobacilli.

Electroporation-mediated transformation of Arcanobacterium (Actinomyces) pyogenes

Plasmids derived from pNG2 or RSF1010 were introduced into strains of Arcanobacterium (Actinomyces) pyogenes by electroporation. Electroporation conditions were varied systematically to give a maximum electroporation frequency of 3.7 x 10(5) CFU/microgram DNA at 1.5 kV/cm and 246 omega, resulting in a time constant of approximately 10 ms. The A. pyogenes transformants expressed plasmid-encoded resistance to chloramphenicol, erythromycin, kanamycin, and streptomycin. The source of incoming DNA affected the growth rate of transformants, but not the electroporation efficiency. This is the first report of genetic transformation of the veterinary pathogen A. pyogenes.

Plasmids in Lactobacillus

This review describes Lactobacillus plasmids on distribution, structure, function, vector construction, vector stability, application, and prospective. About 38% of species of the genus Lactobacillus were found to contain plasmids with different sizes (from 1.2 to 150 kb) and varied numbers (1 or more). Some Lactobacillus plasmids with small sizes were highly similar to those of single strand plasmids from other Gram-positive bacteria. The extensive sequence homologies of plus origins, replication initiation proteins, minus origins, cointegration sites, and the presence of single strand intermediates supported the fact that these small Lactobacillus plasmids replicate with a rolling-circle replication mechanism. Some Lactobacillus plasmid replicons were of broad host range that could function in other Gram-positive bacteria, and even in Escherichia coli, while replicons of other Gram-positive bacteria also function in Lactobacillus. Although most Lactobacillus plasmids are cryptic, some plasmid-encoded functions have been discovered and applied to vector construction and Lactobacillus identification, detection, and modification.

Regulation of expression of the Lactobacillus pentosus xylAB operon

The xylose cluster of Lactobacillus pentosus consists of five genes, two of which, xylAB, form an operon and code for the enzymes involved in the catabolism of xylose, while a third encodes a regulatory protein, XylR. By introduction of a multicopy plasmid carrying the xyl operator and by disruption of the chromosomal xylR gene, it was shown that L. pentosus xylR encodes a repressor. Constitutive expression of xylAB in the xylR mutant is repressed by glucose, indicating that glucose repression does not require XylR. The xylR mutant displayed a prolonged lag phase compared to wild-type bacteria when bacteria were shifted from glucose to xylose medium. Differences in the growth rate in xylose medium at different stages of growth are not correlated with differences in levels of xylAB transcription in L. pentosus wild-type or xylR mutant bacteria but are positively correlated in Lactobacillus casei with a plasmid containing xylAB. Glucose repression was further investigated with a ccpA mutant. An 875-bp internal fragment of the ccpA gene of L. pentosus was isolated by PCR and used to construct a ccpA knockout mutant. Transcription analysis of L. pentosus xylA showed that CcpA is involved in glucose repression. CcpA was also shown to be involved in glucose repression of the alpha-amylase promoter of Lactobacillus amylovorus by demonstrating that glucose repression of the chloramphenicol acetyltransferase gene under control of the alpha-amylase promoter is strongly reduced in the L. pentosus ccpA mutant strain.

Plasmid integration in a wide range of bacteria mediated by the integrase of Lactobacillus delbrueckii bacteriophage mv4

Bacteriophage mv4 is a temperate phage infecting Lactobacillus delbrueckii subsp. bulgaricus. During lysogenization, the phage integrates its genome into the host chromosome at the 3' end of a tRNA(Ser) gene through a site-specific recombination process (L. Dupont et al., J. Bacteriol., 177:586-595, 1995). A nonreplicative vector (pMC1) based on the mv4 integrative elements (attP site and integrase-coding int gene) is able to integrate into the chromosome of a wide range of bacterial hosts, including Lactobacillus plantarum, Lactobacillus casei (two strains), Lactococcus lactis subsp. cremoris, Enterococcus faecalis, and Streptococcus pneumoniae. Integrative recombination of pMC1 into the chromosomes of all of these species is dependent on the int gene product and occurs specifically at the pMC1 attP site. The isolation and sequencing of pMC1 integration sites from these bacteria showed that in lactobacilli, pMC1 integrated into the conserved tRNA(Ser) gene. In the other bacterial species where this tRNA gene is less or not conserved; secondary integration sites either in potential protein-coding regions or in intergenic DNA were used. A consensus sequence was deduced from the analysis of the different integration sites. The comparison of these sequences demonstrated the flexibility of the integrase for the bacterial integration site and suggested the importance of the trinucleotide CCT at the 5' end of the core in the strand exchange reaction.