Transformation of Streptococcus lactis Protoplasts by Plasmid DNA

Evolution of Food Fermentation Processes and the Use of Multi-Omics in Deciphering the Roles of the Microbiota

Food fermentation has been practised since ancient times to improve sensory properties and food preservation. This review discusses the process of fermentation, which has undergone remarkable improvement over the years, from relying on natural microbes and spontaneous fermentation to back-slopping and the use of starter cultures. Modern biotechnological approaches, including genome editing using CRISPR/Cas9, have been investigated and hold promise for improving the fermentation process. The invention of next-generation sequencing techniques and the rise of meta-omics tools have advanced our knowledge on the characterisation of microbiomes involved in food fermentation and their functional roles. The contribution and potential advantages of meta-omics technologies in understanding the process of fermentation and examples of recent studies utilising multi-omics approaches for studying food-fermentation microbiomes are reviewed. Recent technological advances in studying food fermentation have provided insights into the ancient wisdom in the practice of food fermentation, such as the choice of substrates and fermentation conditions leading to desirable properties. This review aims to stimulate research on the process of fermentation and the associated microbiomes to produce fermented food efficiently and sustainably. Prospects and the usefulness of recent advances in molecular tools and integrated multi-omics approaches are highlighted.

The Evolution of gene regulation research in Lactococcus lactis

Lactococcus lactis is a major microbe. This lactic acid bacterium (LAB) is used worldwide in the production of safe, healthy, tasteful and nutritious milk fermentation products. Its huge industrial importance has led to an explosion of research on the organism, particularly since the early 1970s. The upsurge in the research on L. lactis coincided not accidentally with the advent of recombinant DNA technology in these years. The development of methods to take out and re-introduce DNA in L. lactis, to clone genes and to mutate the chromosome in a targeted way, to control (over)expression of proteins and, ultimately, the availability of the nucleotide sequence of its genome and the use of that information in transcriptomics and proteomics research have enabled to peek deep into the functioning of the organism. Among many other things, this has provided an unprecedented view of the major gene regulatory pathways involved in nitrogen and carbon metabolism and their overlap, and has led to the blossoming of the field of L. lactis systems biology. All of these advances have made L. lactis the paradigm of the LAB. This review will deal with the exciting path along which the research on the genetics of and gene regulation in L. lactis has trodden.

An Amazing Journey

This article describes my early life and the chance events leading to my becoming a microbiologist and then my embarking on a career developing the plasmid biology and genetics of lactococci used in milk fermentations.

Molecular characterization and structural instability of the industrially important composite metabolic plasmid pLP712

The widely used plasmid-free Lactococcus lactis strain MG1363 was derived from the industrial dairy starter strain NCDO712. This strain carries a 55.39 kb plasmid encoding genes for lactose catabolism and a serine proteinase involved in casein degradation. We report the DNA sequencing and annotation of pLP712, which revealed additional metabolic genes, including peptidase F, d-lactate dehydrogenase and α-keto acid dehydrogenase (E3 complex). Comparison of pLP712 with other large lactococcal lactose and/or proteinase plasmids from L. lactis subsp. cremoris SK11 (pSK11L, pSK11P) and the plant strain L. lactis NCDO1867 (pGdh442) revealed their close relationship. The plasmid appears to have evolved through a series of genetic events as a composite of pGdh442, pSK11L and pSK11P. We describe in detail a scenario by which the metabolic genes relevant to the growth of its host in a milk environment have been unified on one replicon, reflecting the evolution of L. lactis as it changed its biological niche from plants to dairy environments. The extensive structural instability of pLP712 allows easy isolation of derivative plasmids lacking genes for casein degradation and/or lactose catabolism. Plasmid pLP712 is transferable by transduction and conjugation, and both of these processes result in significant molecular rearrangements. We report the detailed molecular analysis of insertion sequence element-mediated genetic rearrangements within pLP712 and several different mechanisms, including homologous recombination and adjacent deletion. Analysis of the integration of the lactose operon into the chromosome highlights the fluidity of the MG1363 integration hotspot and the potential for frequent movement of genes between plasmids and chromosomes in Lactococcus.

Effect of Proteolytic Enzymes on Transfection and Transformation of Streptococcus lactis Protoplasts

With both chymotrypsin and mutanolysin used to form protoplasts, consistent transformation frequencies of 10 to 10 transformants and transfectants per mug of DNA were achieved. The procedure was used to transform protoplasts of Streptococcus cremoris CS224 at low frequency (5 transformants per mug of DNA).

In Vivo Cloning of lac Genes in Streptococcus lactis ML3

The isolation and characterization of a Streptococcus lactis ML3 strain which possessed a recombinant lactose plasmid is described. The recombination events generating this plasmid occurred in vivo in a recombination-deficient strain and appeared to be mediated by transposition events. Restriction mapping revealed that the recombinant plasmid, pDA0307, contained a region of the lactose plasmid, pSK08, linked to another resident plasmid, pSK07. Copy number determinations indicated that the lac genes were present at approximately 20 copies per cell in pDA0307, whereas the lac genes are normally present at approximately 10 copies per cell in pSK08. The strain containing pDA0307 displayed a 21 to 54% increase in the expression of the Lac enzyme phospho-beta-d-galactosidase. However, the strain containing pDA0307 both grew and produced lactic acid in milk at rates identical to that of a strain containing pSK08. This result suggests that lac gene dosage of plasmid-linked lac genes was not limiting the rate at which these derivatives of S. lactis ML3 fermented milk.

Controlled release of protein from viable Lactococcus lactis cells

Overexpression of the lactococcal CsiA protein affects the cell wall integrity of growing cells and leads to leakage of intracellular material. This property was optimized and exploited for the targeted release of biologically active compounds into the extracellular environment, thereby providing a new delivery system for bacterial proteins and peptides. The effects of different levels of CsiA expression on the leakage of endogenous lactate dehydrogenase and nucleic acids were measured and related to the impact of CsiA expression on Lactococcus lactis cell viability and growth. A leakage phenotype was obtained from cells expressing both recombinant and nonrecombinant forms of CsiA. As proof of principle, we demonstrated that CsiA promotes the efficient release of the heterologous Listeria bacteriophage endolysin LM4 in its active form. Under optimized conditions, native and heterologous active-molecule release is possible without affecting cell viability. The ability of CsiA to release intracellular material by controlled lysis without the requirement for an external lytic agent provides a technology for the control of both the extent of lysis and its timing. Taken together, these results demonstrate the potential of this novel approach for applications including product recovery in industrial fermentations, food processing, and medical therapy.

Controlled expression of CluA in Lactococcus lactis and its role in conjugation

CluA is a 136 kDa surface-bound protein encoded by the chromosomally located sex factor of Lactococcus lactis MG1363 and is associated with cell aggregation linked to high-frequency transfer of the sex factor. To further investigate the involvement of CluA in these phenomena, the cluA gene was cloned on a plasmid, downstream from the lactococcal nisA promoter. In a sex-factor-negative MG1363 derivative, nisin-controlled CluA expression resulted in aggregation, despite the absence of the other genes of the sex factor. Therefore, CluA is the only sex factor component responsible for aggregation. The direct involvement of CluA in the establishment of cell-to-cell contact for aggregate formation was observed by electron microscopy using immunogold-labelled CluA antibodies. Inactivation of cluA in an MG1363 background led to a dramatic decrease in sex factor conjugation frequency compared to the parental strain. Increasing levels of CluA expressed in trans in the cluA-inactivated donor strain facilitated a gradual restoration of conjugation frequency, reaching that of the parental strain. In conclusion, CluA is essential for efficient sex factor transfer in conjugation of L. lactis.

Engineering Lactococcus lactis for production of mannitol: high yields from food-grade strains deficient in lactate dehydrogenase and the mannitol transport system

Mannitol is a sugar polyol claimed to have health-promoting properties. A mannitol-producing strain of Lactococcus lactis was obtained by disruption of two genes of the phosphoenolpyruvate (PEP)-mannitol phosphotransferase system (PTS(Mtl)). Genes mtlA and mtlF were independently deleted by double-crossover recombination in strain L. lactis FI9630 (a food-grade lactate dehydrogenase-deficient strain derived from MG1363), yielding two mutant (Delta ldh Delta mtlA and Delta ldh Delta mtlF) strains. The new strains, FI10091 and FI10089, respectively, do not possess any selection marker and are suitable for use in the food industry. The metabolism of glucose in nongrowing cell suspensions of the mutant strains was characterized by in vivo (13)C-nuclear magnetic resonance. The intermediate metabolite, mannitol-1-phosphate, accumulated intracellularly to high levels (up to 76 mM). Mannitol was a major end product, one-third of glucose being converted to this hexitol. The double mutants, in contrast to the parent strain, were unable to utilize mannitol even after glucose depletion, showing that mannitol was taken up exclusively by PEP-PTS(Mtl). Disruption of this system completely blocked mannitol transport in L. lactis, as intended. In addition to mannitol, approximately equimolar amounts of ethanol, 2,3-butanediol, and lactate were produced. A mixed-acid fermentation (formate, ethanol, and acetate) was also observed during growth under controlled conditions of pH and temperature, but mannitol production was low. The reasons for the alteration in the pattern of end products under nongrowing and growing conditions are discussed, and strategies to improve mannitol production during growth are proposed.

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.

Commercial bacterial starter cultures for fermented foods of the future

Starter cultures for fermented foods are today developed mainly by design rather than by screening. The design principles are based on knowledge of bacterial metabolism and physiology as well as on the interaction with the food product. In the genomics era, we will obtain a wealth of data making design on a rational basis even simpler. The design tools available are food grade tools for genetic, metabolic and protein engineering and an increased use of laboratory automation and high throughput screening methods. The large body of new data will influence the future patterns of regulation. It is currently difficult to predict in what direction the future regulatory requirements will influence innovation in the food industry. It can either become a promoting force for the practical use of biotechnology to make better and safer products, or it can be limiting the use of starter cultures to a few strains with official approval. Successful cultures based on modern technology is expected to be launched in the areas of: probiotics, bioprotection, general improvement of yield and performance for the existing culture market and probably the introduction of cultures for fermenting other food products. A scientific basis for dramatic innovations that could transform the culture industry is currently being established.

Insertion sequence analysis of protoplast fused strains of Lactococcus lactis ssp. cremoris

The use of insertion sequence probes for the analysis of fusants obtained following protoplast fusion is described. Hybridization of both total and plasmid DNA from parent and fusant strains with probes to IS904 and ISS1 showed that of the four protoplast fusions examined, three appeared to involve a rearrangement of genetic material while in the fourth the fusant appeared similar to one of the parental strains. This method of analysis provides more information about the changes induced by protoplast fusion than that obtained by monitoring the acquisition or loss of individual characteristics.

Improved cloning vectors and transformation procedure for Lactococcus lactis

Four shuttle vectors (pMIG 1, 2, 2H and 3) have been constructed based on the broad host-range plasmid pCK1. All the pMIG vectors possess a multiple cloning site containing 12 or more unique restriction enzyme sites, and are stably maintained at either high or low copy number in Lactococcus lactis and in Escherichia coli. By cloning the E. coli pUC replicon into one of these vectors a plasmid was constructed which can replicate to high copy number in recA strains of E. coli. The broad host-range of the pCK1 replicon may enable these cloning vectors to be used in a number of Gram-positive bacteria. One of these vectors was used to optimize an electroporation procedure for transformation of a commonly used plasmid-cured strain MG1363 of L. lactis which routinely yielded 1 x 10(7) to 5 x 10(7) transformants micrograms-1 supercoiled DNA using stored, snap-frozen cells. This transformation efficiency was obtained by growing the cells in medium containing the cell wall weakening agent glycine, to an upper limit of 2.5% w/v. Although growth of L. lactis strain MG1363 was inhibited by the use of 0.5 mol l-1 sucrose as an osmotic stabilizer, the presence of sucrose in the electroporation buffer was critical for high transformation efficiency. Other variables which were tested for their effect on the efficiency of transformation were cell concentration, DNA concentration, pulse time and field strength. These results provide a model procedure which can be followed to optimize conditions for the genetic transformation of various strains of L. lactis.

Gene expression in Lactococcus lactis

Lactic acid bacteria are of major economic importance, as they occupy a key position in the manufacture of fermented foods. A considerable body of research is currently being devoted to the development of lactic acid bacterial strains with improved characteristics, that may be used to make fermentations pass of more efficiently, or to make new applications possible. Therefore, and because the lactococci are designated 'GRAS' organisms ('generally recognized as safe') which may be used for safe production of foreign proteins, detailed knowledge of homologous and heterologous gene expression in these organisms is desired. An overview is given of our current knowledge concerning gene expression in Lactococcus lactis. A general picture of gene expression signals in L. lactis emerges that shows considerable similarity to those observed in Escherichia coli and Bacillus subtilis. This feature allowed the expression of a number of L. lactis-derived genes in the latter bacterial species. Several studies have indicated, however, that in spite of the similarities, the expression signals from E. coli, B. subtilis and L. lactis are not equally efficient in these three organisms.

In vivo genetic exchange of a functional domain from a type II A methylase between lactococcal plasmid pTR2030 and a virulent bacteriophage

The conjugative plasmid pTR2030 confers bacteriophage resistance to lactococci by two independent mechanisms, an abortive infection mechanism (Hsp+) and a restriction and modification system (R+/M+). pTR2030 transconjugants of lactococcal strains are used in the dairy industry to prolong the usefulness of mesophilic starter cultures. One bacteriophage which has emerged against a pTR2030 transconjugant is not susceptible to either of the two defense systems encoded by the plasmid. Phage nck202.50 (phi 50) is completely resistant to restriction by pTR2030. A region of homology between pTR2030 and phi 50 was subcloned, physically mapped, and sequenced. A region of 1,273 bp was identical in both plasmid and phage, suggesting that the fragment had recently been transferred between the two genomes. Sequence analysis confirmed that the transferred region encoded greater than 55% of the amino domain of the structural gene for a type II methylase designated LlaI. The LlaI gene is 1,869 bp in length and shows organizational similarities to the type II A methylase FokI. In addition to the amino domain, upstream sequences, possibly containing the expression signals, were present on the phage genome. The phage phi 50 fragment containing the methylase amino domain, designated LlaPI, when cloned onto the shuttle vector pSA3 was capable of modifying another phage genome in trans. This is the first report of the genetic exchange between a bacterium and a phage which confers a selective advantage on the phage. Definition of the LlaI system on pTR2030 provides the first evidence that type II systems contribute to restriction and modification phenotypes during host-dependent replication of phages in lactococci.

Analysis of the lacZ sequences from two Streptococcus thermophilus strains: comparison with the Escherichia coli and Lactobacillus bulgaricus beta-galactosidase sequences

The lacZ gene from Streptococcus thermophilus A054, a commercial yogurt strain, was cloned on a 7.2 kb PstI fragment in Escherichia coli and compared with the previously cloned lacZ gene from S. thermophilus ATCC 19258. Using the dideoxy chain termination method, the DNA sequences of both lacZ structural genes were determined and found to be 3071 bp in length. When the two sequences were more closely analysed, 21 nucleotide differences were detected, of which only nine resulted in amino acid changes in the proteins, the remainder occurring in wobble positions of the respective codons. Only three bases separated the termination codon for the lacS gene from the initiation codon for lacZ, suggesting that the lactose utilization genes are organized as an operon. The amino acid sequence of the beta-galactosidase, derived from the DNA sequence, corresponds to a protein with a molecular mass of 116860 Da. Comparison of the S. thermophilus amino acid sequences with those from Lactobacillus bulgaricus, E. coli and Klebsiella pneumoniae showed 48, 35 and 32.5% identity respectively. Although little sequence homology was observed at the DNA level, many regions conserved in the amino acid sequence were identified when the beta-galactosidase proteins from S. thermophilus, E. coli and L. bulgaricus were compared.

Cloning, expression, and sequence determination of a bacteriophage fragment encoding bacteriophage resistance in Lactococcus lactis

A number of host-encoded phage resistance mechanisms have been described in lactococci. However, the phage genome has not been exploited as a source of additional resistance determinants. A 4.5-kb BamHI-HindIII fragment of phage nck202.50 (phi 50) was subcloned in streptococcus-Escherichia coli shuttle plasmid pSA3 and introduced into Lactococcus lactis NCK203 and MG1363 by protoplast transformation. This cloned phage fragment directed a bacteriophage resistance phenotype designated Per (phage-encoded resistance). Both phi 50 and a distantly related phage, nck202.48 (phi 48), formed small plaques on strain NCK213 at a slightly reduced efficiency of plaquing on the Per+ host. The per locus was further reduced to a 1.4-kb fragment through in vitro deletion analysis. The 1.4-kb fragment was sequenced, and the Per phenotype was found to be associated with a ca. 500-bp region rich in direct and inverted repeats. We present evidence that the Per region contains a phage origin of replication which, in trans, may interfere with phage replication by titration of DNA polymerase or other essential replication factors. It was demonstrated that the Per+ phenotype is not a result of reduced adsorption or action of a restriction and modification system. Per+ activity was not detected against six independent phages which were previously shown to be sensitive to the Hsp+ mechanism. The mutually exclusive resistance mechanisms could be combined to confer resistance to both types of phages (Hsp resistant and Per resistant) in a single host. This is the first description in lactococci of a phage resistance phenotype, other than superinfection immunity, originating from a lactococcal phage genome.

Genetics of the proteolytic system of lactic acid bacteria

The proteolytic system of lactic acid bacteria is of eminent importance for the rapid growth of these organisms in protein-rich media. The combined action of proteinases and peptidases provides the cell with small peptides and essential amino acids. The amino acids and peptides thus liberated have to be translocated across the cytoplasmic membrane. To that purpose, the cell contains specific transport proteins. The internalized peptides are further degraded to amino acids by intracellular peptidases. The world-wide economic importance of the lactic acid bacteria and their proteolytic system has led to an intensive research effort in this area and a considerable amount of biochemical data has been collected during the last two decades. Since the development of systems to genetically manipulate lactic acid bacteria, data on the genetics of enzymes and processes involved in proteolysis are rapidly being generated. In this review an overview of the latest genetic data on the proteolytic system of lactic acid bacteria will be presented. As most of the work in this field has been done with lactococci, the emphasis will, inevitably, be on this group of organisms. Where possible, links will be made with other species of lactic acid bacteria.

Localization, cloning, and expression of genetic determinants for bacteriophage resistance (Hsp) from the conjugative plasmid pTR2030

Genetic determinants for a bacteriophage resistance mechanism (Hsp+) encoded by plasmid pTR2030 (46.2 kilobases [kb]) were localized by mapping an 11.5-kb deletion that accompanied the transition of Lactococcus lactis LMA12-4 transconjugants (M. E. Sanders, P. J. Leonard, W. D. Sing, and T. R. Klaenhammer, Appl. Environ. Microbiol. 52:1001-1007, 1986) from phage resistance to phage sensitivity. The deleted 34.7-kb replicon (pTR2023, Hsp-) retained its conjugative ability, demonstrating that the phage resistance and conjugal transfer determinants were genetically distinct. The Hsp region of pTT2030, which was contained within a 13.6-kb BglII fragment, was cloned into the BamHI site of bacteriophage lambda EMBL3, and Hsp was subcloned into the Escherichia coli-Streptococcus shuttle vector pSA3. The recombinant plasmids pTK6 and pTK9 were recovered in E. coli HB101 and contained a 13.6-kb insert in opposite orientations. L. Lactis MG1363 transformants carrying pTK6 or pTK9 exhibited a significant reduction in plaque size, in addition to a slight reduction in the efficiency of plaquing for both prolate and small isometric phages. Phenotypic reactions observed for the recombinant plasmids suggest that pTR2030-encoded Hsp acts similarly against both prolate and small isometric phages. Tn5 mutagenesis was used to define the region essential for the expression of the Hsp+ phenotype. Any of four insertions within a 3-kb region resulted in the loss of phage resistance, whereas a further 26 insertions outside this locus had no effect on Hsp expression. In vitro deletion analysis confirmed that the 3-kb region contained all the information necessary for the observed resistance.

Genetic transformation of intact Lactococcus lactis subsp. lactis by high-voltage electroporation

To apply recombinant DNA techniques for genetic manipulation of the industrially important lactococci, an efficient and reliable high-frequency transformation system must be available. High-voltage electric pulses have been demonstrated to enhance uptake of DNA into protoplasts and intact cells of numerous gram-negative and gram-positive microorganisms. The objective of this study was to develop a system for electroporating intact cells of Lactococcus lactis subsp. lactis LM0230 (previously designated Streptococcus lactis LM0230) with a commercially available electroporation unit (BTX Transfector 100; BTX, Inc., San Diego, Calif.). Parameters which influenced the efficiency of transformation included growth phase and final concentration of cells, ionic strength of the suspending medium, concentration of plasmid DNA, and the amplitude and duration of the pulse. Washed suspensions of intact cells suspended in deionized distilled water were subjected to one high-voltage electric pulse varying in voltage (300 to 900 V corresponding to field strengths of 5 to 17 kV/cm) and duration (100 microseconds to 1 s). Transformation efficiencies of 10(3) transformants per microgram of DNA were obtained when dense suspensions (final concentration, 5 x 10(10) CFU/ml) of stationary-phase cells were subjected to one pulse with a peak voltage of 900 V (field strength, 17 kV/cm) and a pulse duration of 5 ms in the presence of plasmid DNA. Dilution of porated cells in broth medium followed by an expression period of 2 h at 30 degrees C was beneficial in enhancing transformation efficiencies. Plasmids ranging in size from 9.8 to 30.0 kilobase pairs could be transformed by this procedure.

Isolation and characterization of a temperate bacteriophage from the ruminal anaerobe Selenomonas ruminantium

A temperate bacteriophage was obtained from an isolate of the ruminal anaerobe Selenomonas ruminantium. Clear plaques that became turbid on further incubation occurred on a lawn of host bacteria. Cells picked from a turbid plaque produced healthy liquid cultures, but these often lysed on storage. Mid-log-phase liquid cultures incubated with the bacteriophage lysed and released infectious particles with a titer of up to 3 X 10(7) PFU/ml. A laboratory strain of S. ruminantium, HD-4, was also sensitive to this bacteriophage, which had an icosohedral head (diameter, 50 nm) and a flexible tail (length, 140 nm). The bacteriophage contained 30 kilobases of linear, double-stranded DNA, and a detailed restriction map was constructed. The lysogenic nature of infection was demonstrated by hybridization of bacteriophage DNA to specific restriction fragments of infected host genomic DNA and by identification of a bacteriophage genomic domain which may participate in integration of the bacteriophage DNA. Infection of S. ruminantium in vitro was demonstrated by two different methods of cell transformation with purified bacteriophage DNA.

Conjugative mobilization as an alternative vector delivery system for lactic streptococci

Due to the current variability in applying polyethylene glycol-mediated protoplast transformation to lactic streptococci, a study was undertaken to assess the feasibility of conjugative mobilization as an alternative method for vector delivery. By using the broad-host-range conjugative plasmid pVA797, the partially homologous cloning vector pVA838 was successfully introduced into various strains of Streptococcus lactis, Streptococcus cremoris, Streptococcus lactis subsp. diacetylactis, Streptococcus thermophilus, and Streptococcus faecalis. Frequencies ranged from 10(-2) to 10(-6) transconjugants per recipient. Both pVA797 and pVA838 were acquired intact, without alteration in functionality. Also, the shuttle vector pSA3, which shares partial homology with pVA797, was mobilized via conjugation. The use of S. lactis LM2301 as the intermediate donor allowed the use of physiologic and metabolic characteristics for recipient differentiation. The construction of a vector containing a "DNA cassette" conferring mobilization and the resolution, segregation, and stability of the cointegrates, pVA797, pVA838, and pSA3, are also reported.

A method for genetic transformation of nonprotoplasted Streptococcus lactis

Plasmid transformation of whole cells of Streptococcus lactis LM0230 was demonstrated. The procedure required polyethylene glycol and incubation in hypertonic media, but did not require enzymatic cell wall digestion. Conditions were optimized, yielding 5 X 10(5) transformants per micrograms of pSA3 DNA. Variables tested for effect on transformation efficiency included molecular weight, concentration, and pH of polyethylene glycol; cell density; plating media; DNA concentration; heat shock; and incubation of cells in hypertonic buffer. DNAs transformed included pSA3, pVA856, pTV1, and c2 phi. Transformation from DNA-DNA ligation mixes, with DNA not purified through density gradients, and with previously frozen cells was also achieved. The method described here for transformation of nonprotoplasted cells of LM0230 is unique, and to date has not been applied successfully to other lactic acid bacteria.

Restriction enzyme analysis of lactose and bacteriocin plasmids from Streptococcus lactis subsp. diacetylactis WM4 and cloning of BclI fragments coding for bacteriocin production

The 131.1-kilobase (kb) bacteriocin production (Bac) plasmid pNP2 and the 63.6-kb lactose metabolism (Lac) plasmid pCS26, from Streptococcus lactis subsp. diacetylactis WM4, as well as pWN8, a 116.7-kb recombinant plasmid from a Lac+ transconjugant, were analyzed with restriction enzymes to determine the origin of pWN8. Plasmid pWN8 conferred a Lac+ Bac- phenotype, contained DNA derived from pCS26 and pNP2, and, like pNP2, exhibited self-transmissibility (Tra+). In cloning attempts, Bac+ transformant S. lactis KSH1 was isolated. The recombinant plasmid, pKSH1, contained three BclI fragments from pNP2. Bac- transformants which individually contained each of the three fragments were also identified. Comparison of restriction maps of pKSH1 and pNP2 revealed an 18.4-kb region common to both plasmids, involving two of the three BclI fragments. S. lactis KSH1 also exhibited greater inhibitory activity against the indicator strain S. diacetylactis 18-16 than did a strain containing the 131.1-kb Bac plasmid.

Protoplast fusion of Lactobacillus fermentum

Tetracycline-resistant (Tetr) erythromycin-resistant (Eryr) fusants of Lactobacillus fermentum 604 carrying a 10-megadalton Tetr plasmid and L. fermentum 605 carrying a 38-megadalton Eryr plasmid were obtained by means of polyethylene glycol-induced protoplast fusion.

High-efficiency transformation of Streptococcus lactis protoplasts by plasmid DNA

Streptococcus lactis IL1403 protoplasts were transformed by plasmid pIL204 (5.5 kilobases), which conferred erythromycin resistance with an average efficiency of 5 X 10(6) transformants per microgram of supercoiled DNA. The procedure used and transformation efficiencies obtained were close to those described for Bacillus subtilis (G. Chang and S. N. Cohen, Mol. Gen. Genet. 168:111-115, 1979).

Partial characterization of the genetic basis for sucrose metabolism and nisin production in Streptococcus lactis

We attempted to identify the genetic loci for sucrose-fermenting ability (Suc+), nisin-producing ability (Nip+), and nisin resistance (Nisr) in certain strains of Streptococcus lactis. To obtain genetic evidence linking the Suc+ Nip+ Nisr phenotype to a distinct plasmid, both conjugal transfer and transformation were attempted. A conjugation procedure modified to protect the recipients against the inhibitory action of nisin allowed the conjugal transfer of the Suc+ Nip+ Nisr marker from three Suc+ Nip+ Nisr donors to various recipients. The frequency of transfer ranged from 1.7 x 10(-4) to 5.6 x 10(-8) per input donor, depending on the mating pair. However, no additional plasmid DNA was apparent in these transconjugants. Transformation of S. lactis LM0230 to the Suc+ Nip+ Nisr phenotype by using the plasmid pool of S. lactis ATCC 11454 was not achieved, even though other plasmids present in the pool were successfully transferred. However, two results imply the involvement of plasmid DNA in coding for the Suc+ Nip+ Nisr phenotype. The Suc+ Nip+ Nisr marker was capable of conjugal transfer to a recipient deficient in host-mediated homologous recombination (Rec-), and the Suc+ Nip+ Nisr marker exhibited bilateral plasmid incompatibility with a number of lactose plasmids found in S. lactis. Although our results indicate that the Suc+ Nip+ Nisr phenotype is plasmid encoded, no physical evidence linking this phenotype to a distinct plasmid was obtained.

Effect of Ca2+ ions on plasmid transformation of Streptococcus lactis protoplasts

The effects of Mg2+ and Ca2+ ions on the efficiency of the plasmid transformation of lysozyme-treated Streptococcus lactis protoplasts were compared. A 33-megadalton plasmid, pLP712, coding for lactose fermentation and a 6.5-megadalton plasmid, pGB301, coding for erythromycin and chloramphenicol resistance were used as model plasmids, and S. lactis MG1614 was the recipient. Replacing Mg2+ with Ca2+ in the transformation buffer was found to increase transformant frequency more than 10-fold with both plasmids.

Construction of cloning, promoter-screening, and terminator-screening shuttle vectors for Bacillus subtilis and Streptococcus lactis

Shuttle vectors have been constructed which are suitable both for the selection of regulatory sequences and for gene cloning in Bacillus subtilis and Streptococcus lactis. The promoter screening vectors contain a promoterless chloramphenicol acetyltransferase gene; the insertion of suitable DNA fragments upstream of the gene restored the enzyme activity. With a related set of vectors, transcription termination signals can be selected.

Cloning and expression of a Streptococcus cremoris proteinase in Bacillus subtilis and Streptococcus lactis

Previously, curing experiments suggested that plasmid pWV05 (17.5 megadaltons [Md]) of Streptococcus cremoris Wg2 specifies proteolytic activity. A restriction enzyme map of pWV05 was constructed, the entire plasmid was subcloned in Escherichia coli with plasmids pBR329 and pACYC184. A 4.3-Md HindIII fragment could not be cloned in an uninterrupted way in E. coli but could be cloned in two parts. Both fragments showed homology with the 9-Md proteinase plasmid of S. cremoris HP. The 4.3-Md HindIII fragment was successfully cloned in Bacillus subtilis on plasmid pGKV2 (3.1 Md). Crossed immunoelectrophoresis of extracts of B. subtilis carrying the recombinant plasmid (pGKV500; 7.4 Md) showed that the fragment specifies two proteins of the proteolytic system of S. cremoris Wg2. PGKV500 was introduced in a proteinase-deficient Streptococcus lactis strain via protoplast transformation. Both proteins were also present in cell-free extracts of S. lactis(pGKV500). In S. lactis, pGKV500 enables the cells to grow normally in milk with rapid acid production, indicating that the 4.3-Md HindIII fragment of plasmid pWV05 specifies the proteolytic activity of S. cremoris Wg2.

Transformation and fusion of Streptococcus faecalis protoplasts

Nonconjugative plasmids were transferred by protoplast fusion among Streptococcus faecalis strains and from Streptococcus sanguis to S. faecalis. S. faecalis protoplasts were also transformed with several different plasmids, including the Tn917 delivery vehicle pTV1. Transformation was reproducible, but low in frequency (10(-6) transformants per viable protoplast). A new shuttle vector (pAM610), able to replicate in Escherichia coli and S. faecalis, was constructed and transformed into S. faecalis protoplasts. pAM610 was mobilized by the conjugative plasmid pAM beta 1 in matings among S. faecalis strains and from S. sanguis to S. faecalis. Chimeric derivatives of pAM610 were also transformed into S. faecalis.