Heterologous expression of the Lactococcus lactis bacteriocin, nisin, in a dairy Enterococcus strain

Nisin variants from Streptococcus and Staphylococcus successfully express in NZ9800

AIMS

Increases in antimicrobial resistance have meant that the antimicrobial potential of lantibiotics is now being investigated irrespective of the nature of the producing organism. The aim of this study was to investigate whether natural nisin variants produced by non-Generally Recognized as Safe (GRAS) strains, such as nisin H, nisin J and nisin P, could be expressed in a well-characterized GRAS host.

METHODS AND RESULTS

This study involved cloning the nisin A promoter and leader sequence fused to nisin H, nisin J or nisin P structural gene sequences originally produced by Streptococcus hyointestinalis DPC 6484, Staphylococcus capitis APC 2923 and Streptococcus agalactiae DPC 7040, respectively. This resulted in their expression in Lactococcus lactis NZ9800, a genetically modified strain that does not produce nisin A.

CONCLUSIONS

Induction of the nisin controlled gene expression system demonstrates that these three nisin variants could be acted on by nisin A machinery provided by the host strain.

SIGNIFICANCE AND IMPACT OF THE STUDY

Describes the first successful heterologous production of three natural nisin variants by a GRAS strain, and demonstrates how such systems could be harnessed not only for lantibiotic production but also in the expansion of their structural diversity and development for use as future biotherapeutics.

Efficient Production of Nisin A from Low-Value Dairy Side Streams Using a Nonengineered Dairy Lactococcus lactis Strain with Low Lactate Dehydrogenase Activity

Nisin is commonly used as a biopreservative in foods. For industrial production, nisin-producing Lactococcus lactis strains are usually grown to high cell densities to achieve the highest possible nisin titer. However, accumulation of lactic acid eventually halts production, even in pH-controlled fermentations. Here, we describe a nisin-producing L. lactis strain Ge001, which was obtained after transferring the nisin gene cluster from L. lactis ATCC 11454, by conjugation, into the natural mutant L. lactis RD1M5, with low lactate dehydrogenase activity. The ability of Ge001 to produce nisin was tested using dairy waste as the fermentation substrate. To accommodate redox cofactor regeneration, respiration conditions were used, and to alleviate oxidative stress and to reduce adsorption of nisin onto the producing cells, we found it to be beneficial to add 1 mM Mn²⁺ and 100 mM Ca²⁺, respectively. A high titer of 12 084 IU/mL nisin could be reached, which is comparable to the highest titers reported using expensive, rich media. Summing up, we here present a 100% natural, robust, and sustainable approach for producing food-grade nisin and acetoin from readily available dairy waste.

A Single Plasmid of Nisin-Controlled Bovine and Human Lactoferrin Expressing Elevated Antibacterial Activity of Lactoferrin-Resistant Probiotic Strains

Lactoferrin (LF) is a multifunctional protein found in mammals, and it shows broad-spectrum antimicrobial activity. To improve the functional properties of specific probiotics in order to provide both the beneficial characteristics of lactic acid bacteria and the biological activity of LF, cDNAs of bovine LF (BLF), human LF (HLF), or porcine LF (PLF) were cloned into a nisin-inducible plasmid. These were then transformed into the selected eight probiotics, which are LF-resistant hosts. Expression of recombinant LFs (rLFs) was analyzed via SDS-PAGE and Western blot analysis. Although the selected host strains may not contain the nisRK genes (NisK, the sensor kinase; NisR, the regulator protein), the components of autoregulation, a low level of LFs expression can be successfully induced by using nisin within bacterial cells in a time-dependent manner in three engineered clones, including Lactobacillus delbrueckii/HLF, L. delbrueckii/BLF, and L. gasseri/BLF. Lactobacillus delbrueckii and Lactobacillus gasseri originate from yogurt and human milk, respectively, and both strains are functional probiotic strains. Therefore, we further compared the antibacterial activities of disrupted recombinant probiotic clones, conventional strains (host control), and vector control ones by using agar diffusion and broth inhibition analysis, and the expression of rLFs in the above three clones considerately improved their antibacterial efficacies against four important food-borne pathogens, namely, Escherichia coli, Staphylococcus aureus, Enterococcus faecalis, and Salmonellaenterica. In conclusion, this study provides a simple strategy for the production of functional LFs (BLF and HLF) in both functional and LF-resistant hosts for applications in the field.

Antimicrobial Activity of Lactococcus lactis subsp. lactis Isolated from a Stranded Cuvier's Beaked Whale (Ziphius cavirostris) against Gram-Positive and -Negative Bacteria

In the present study, we isolated and characterized Lactococcus lactis (L. lactis) subsp. lactis from a female Cuvier’s beaked whale (Ziphius cavirostris) stranded in Shizuoka, Japan. Only five isolates (CBW1-5), grown on Lactobacilli de Man Rogosa Sharpe (MRS) agar plates prepared using 50% artificial seawater, were positive in L. lactis species-specific primer PCR. Their 16S rRNA sequences were highly similar to those of L. lactis subsp. lactis JCM 5805^T. The Gram reaction, motility, gas production from glucose, catalase production, and growth conditions were consistent with those of the type strain. Additionally, carbohydrate utilization of the strains was consistent with previously reported marine organism-derived strains. The pH-neutralized cell-free culture supernatant of strain CBW2 inhibited the growth of Bacillus subtilis subsp. subtilis ATCC 6051 and Vibrio alginolyticus ATCC 17749, whereas protease treatment eliminated or diminished its inhibitory activity. The strain possesses a precursor of the nisin structural gene (nisA), which showed 100% homology with nisin Z, and nisin biosynthesis-related genes (nisB, nisC, nisT, nisP, nisF, nisI, and nisRK), suggesting that the strain produces a nisin-like substance. This study provides fundamental information on whale-derived L. lactis subsp. lactis which may be useful for reducing the carriage of B. subtilis subsp. subtilis and V. alginolyticus.

Nisin Production by Enterococcus hirae DF105Mi Isolated from Brazilian Goat Milk

The purpose of this study was to select the promising biopreservation bacteriocin producer strain from goat milk and characterize the expressed bacteriocin, related to its physiological and biochemical properties and specificity of operon encoding production and expression of antimicrobial peptide. Brazilian goat milk was used as the source for the selection of bacteriocin-producing lactic acid bacteria. One strain (DF105Mi) stood out for its strong activity against several Listeria monocytogenes strains. Selected strain was identified based on the biochemical and physiological characteristics and 16s rRNA analysis. The bacteriocin production and inhibitory spectrum of strain DF105Mi were studied, together with the evaluation of the effect of temperature, pH, and chemicals on bacteriocin stability and production, activity, and adsorption to target cells as well as to the cell surface of bacteriocin producers. Physiological and bio-molecular analyses based on targeting of different genes, parts of nisin operon were performed in order to investigate the hypothesis that the studied strain can produce and express nisin. Based on biochemical, physiological, and 16s rRNA analysis, the strain DF105Mi was classified as Enterococcus hirae. The selected strain produces a bacteriocin which is stable in a wide range of pH (2.0-12.0), temperature (up to 120 °C), presence of selected chemicals and presents adsorption affinity to different test organisms, process influenced by environmental conditions. Higher bacteriocin production by Ent. hirae DF105Mi was recorded during stationary growth phase, but only when the strain was cultured at 37 °C. The strain's genetic analysis indicated presence of the genes coding for the production of the bacteriocin nisin. This result was confirmed by cross-checking the sensitivity of the produced strain to commercial nisin A. The strong anti-Listeria activity, bacteriocin adsorption, and stability of produced bacteriocin indicate that Ent. hirae DF105Mi presents a differentiated potential application for biopreservation of fermented dairy products.

Inhibitory Effect of Substances Produced by Native Lactococcus lactis Strains of Tropical Fruits towards Food Pathogens

The use of peptides produced by lactic acid bacteria (LAB) as antimicrobial agents in food emerged from the increasing need of replacing chemicals with natural substances to ensure their safety and quality. A total of 30 LAB belonging to the genus Lactococcus sp. (10) and Enterococcus sp. (20) were isolated from native fruits of Ecuador subtropical rainforest. Among Lactococcus species, the isolates assigned Gt28, Gt29, and Ella8, identified as Lactococcus lactis subsp. lactis with 99% identity, showing highly inhibitory potential against four food pathogens were further characterized. The treatment of cell-free supernatant with proteolytic enzymes indicated the protein nature of released components, which displayed a broad antimicrobial activity against Gram-positive and -negative bacteria. Polymerase chain reaction analysis indicated the presence of lacticin 3147 gene in all isolates, lactococcin M gene in Gt28 and Gt29 but not in Ella8 and lactococcin A gene in Gt28 only. The antimicrobial activity was not linked to the presence of structural nisin gene as no amplification product was obtained. Treatment of Salmonella enterica ATCC 51741 and Escherichia coli ATCC 25922 at both vegetative and exponential phase of growth with the cell-free supernatant of Gt28 resulted in complete inactivation upon 3 h suggesting its bactericidal mode of action. An increment on inhibitory activity occurred when partial purified bacteriocin Gt28 was combined with ethylenediaminetetraacetic acid rather than bacteriocin only, indicating that the cells were sensitized in vitro by the chelator agent acting synergistically to induce the killing of pathogenic cells.

Processing and Structure of the Lantibiotic Peptide Nso From the Human Gut Bacterium Blautia obeum A2-162 analysed by Mass Spectrometry

A previously reported gene cluster encoding four nisin-like peptides, three with the same sequence (NsoA1-3) and the unique NsoA4, produced antimicrobial activity in the presence of trypsin after heterologous expression in Lactococcus lactis. Protein extracts were separated by SDS gel electrophoresis or immunoprecipitation using an antibody to the NsoA2 leader. Tryptic peptides observed by LC-MS/MS covered the complete sequence of preNsoA1-3 and part of the leader sequence of preNsoA4 and confirmed the expression and the predicted sequences of the preNsoA peptides. Further, the data revealed that the preNsoA1-3 peptides were partly modified with dehydrations and formation of lanthionine rings. A certain amount of fully modified preNsoA1-3 was observed. Details of modifications of the core peptide and the C-terminal tryptic peptide TATCGCHITGK covering rings D and E indicated that 22% of these preNsoA1-3 peptides were completely modified. A lower amount of ring formation is estimated for rings A-C. Intact masses of immunoprecipitation-derived peptides determined by LC-MS accurately matched the expected preNsoA precursor peptides. The most abundant peptides detected were preNsoA2-3-8H₂O followed by preNsoA1-8H₂O and other states of dehydration. The results confirm incomplete processing of preNsoA peptides in the heterologous system, with the formation of a certain amount of fully modified peptides.

Discovery of a novel lantibiotic nisin O from Blautia obeum A2-162, isolated from the human gastrointestinal tract

A novel lanC-like sequence was identified from the dominant human gut bacterium Blautia obeum strain A2-162. This sequence was extended to reveal a putative lantibiotic operon with biosynthetic and transport genes, two sets of regulatory genes, immunity genes, three identical copies of a nisin-like lanA gene with an unusual leader peptide, and a fourth putative lanA gene. Comparison with other nisin clusters showed that the closest relationship was to nisin U. B. obeum A2-162 demonstrated antimicrobial activity against Clostridium perfringens when grown on solid medium in the presence of trypsin. Fusions of predicted nsoA structural sequences with the nisin A leader were expressed in Lactococcus lactis containing the nisin A operon without nisA. Expression of the nisA leader sequence fused to the predicted structural nsoA1 produced a growth defect in L. lactis that was dependent upon the presence of biosynthetic genes, but failed to produce antimicrobial activity. Insertion of the nso cluster into L. lactis MG1614 gave an increased immunity to nisin A, but this was not replicated by the expression of nsoI. Nisin A induction of L. lactis containing the nso cluster and nisRK genes allowed detection of the NsoA1 pre-peptide by Western hybridization. When this heterologous producer was grown with nisin induction on solid medium, antimicrobial activity was demonstrated in the presence of trypsin against C. perfringens, Clostridium difficile and L. lactis. This research adds to evidence that lantibiotic production may be an important trait of gut bacteria and could lead to the development of novel treatments for intestinal diseases.

Mechanistic Understanding of Lanthipeptide Biosynthetic Enzymes

Lanthipeptides are ribosomally synthesized and post-translationally modified peptides (RiPPs) that display a wide variety of biological activities, from antimicrobial to antiallodynic. Lanthipeptides that display antimicrobial activity are called lantibiotics. The post-translational modification reactions of lanthipeptides include dehydration of Ser and Thr residues to dehydroalanine and dehydrobutyrine, a transformation that is carried out in three unique ways in different classes of lanthipeptides. In a cyclization process, Cys residues then attack the dehydrated residues to generate the lanthionine and methyllanthionine thioether cross-linked amino acids from which lanthipeptides derive their name. The resulting polycyclic peptides have constrained conformations that confer their biological activities. After installation of the characteristic thioether cross-links, tailoring enzymes introduce additional post-translational modifications that are unique to each lanthipeptide and that fine-tune their activities and/or stability. This review focuses on studies published over the past decade that have provided much insight into the mechanisms of the enzymes that carry out the post-translational modifications.

Investigation of genes involved in nisin production in Enterococcus spp. strains isolated from raw goat milk

Different strains of Lactococcus lactis are capable of producing the bacteriocin nisin. However, genetic transfer mechanisms allow the natural occurrence of genes involved in nisin production in members of other bacterial genera, such as Enterococcus spp. In a previous study, nisA was identified in eight enterococci capable of producing antimicrobial substances. The aim of this study was to verify the presence of genes involved in nisin production in Enterococcus spp. strains, as well as nisin expression. The nisA genes from eight Enterococcus spp. strains were sequenced and the translated amino acid sequences were compared to nisin amino-acid sequences previously described in databases. Although containing nisin structural and maturation related genes, the enterococci strains tested in the present study did not present the immunity related genes (nisFEG and nisI). The translated sequences of nisA showed some point mutations, identical to those presented by Lactococcus strains isolated from goat milk. All enterococci were inhibited by nisin, indicating the absence of immunity and thus that nisin cannot be expressed. This study demonstrated for the first time the natural occurrence of nisin structural genes in Enterococcus strains and highlights the importance of providing evidence of a link between the presence of bacteriocin genes and their expression.

Combined effect of a mixture of tetracycline, acid, and detergent, and nisin against Enterococcus faecalis and Actinomyces viscosus biofilms

OBJECTIVES

To evaluate the combined effect of a mixture of tetracycline, acid, and detergent (MTAD) and Nisin against Enterococcus faecalis (E. faecalis) and Actinomyces viscosus (A. viscosus) biofilms.

METHODS

This study was conducted between June and December 2013 in collaboration with Dental Caries Research Chair, College of Dentistry, King Saud University, Riyadh, Saudi Arabia. Single-species biofilms (n=9/species/observation period) were generated on membrane filter discs and subjected to 5, 10, or 15 minute incubation with MTADN (MTAD with 3% Nisin), 5.25% sodium hypochlorite (NaOCl), or normal saline. The colony forming units were counted using the Dark field colony counter.

RESULTS

A 100% bactericidal effect of 5.25% NaOCl was noted during the 3 observation periods; a significant reduction (p=0.000) in mean survival rates of E. faecalis (77.3+13.6) and A. viscosus (39.6+12.6) was noted after 5 minutes exposure to MTADN compared with normal saline (78000000+5291503) declining to almost no growth after 10 and 15 minutes. The survival rates of the E. faecalis and A. viscosus biofilm were no different after treatment with MTADN and 5.25% NaOCl at the 3 observation periods (p=1.000).

CONCLUSION

A combination of MTAD and Nisin was as effective as NaOCl against E. faecalis and A. viscosus biofilms.

An agmatine-inducible system for the expression of recombinant proteins in Enterococcus faecalis

Background

Scientific interest in Enterococcus faecalis has increased greatly over recent decades. Some strains are involved in food fermentation and offer health benefits, whereas others are vancomycin-resistant and cause infections that are difficult to treat. The limited availability of vectors able to express cloned genes efficiently in E. faecalis has hindered biotechnological studies on the bacterium’s regulatory and pathogenicity-related genes. The agmatine deiminase (AGDI) pathway of E. faecalis, involved in the conversion of agmatine into putrescine, is driven by a response inducer gene aguR.

Results

This study describes that the exposure to the induction factor (agmatine) results in the transcription of genes under the control of the aguB promoter, including the aguBDAC operon. A novel E. faecalis expression vector, named pAGEnt, combining the aguR inducer gene and the aguB promoter followed by a cloning site and a stop codon was constructed. pAGEnt was designed for the overexpression and purification of a protein fused to a 10-amino-acid His-tag at the C-terminus. The use of GFP as a reporter of gene expression in E. faecalis revealed that under induction with 60 mM agmatine, fluorescence reached 40 arbitrary units compared to 0 in uninduced cells.

Conclusion

pAGEnt vector can be used for the overexpression of recombinant proteins under the induction of agmatine in E. faecalis, with a close correlation between agmatine concentration and fluorescence when GFP was used as reporter.

Control of tyramine and histamine accumulation by lactic acid bacteria using bacteriocin forming lactococci

The aim of this study was to evaluate the competitive effects of three bacteriocin producing strains of Lactococcus lactis subsp. lactis against two aminobiogenic lactic acid bacteria, i.e. the tyramine producing strain Enterococcus faecalis EF37 and the histamine producing strain Streptococcus thermophilus PRI60, inoculated at different initial concentrations (from 2 to 6 log cfu/ml). The results showed that the three L. lactis subsp. lactis strains were able to produce bacteriocins: in particular, L. lactis subsp. lactis VR84 and EG46 produced, respectively, nisin Z and lacticin 481, while for the strains CG27 the bacteriocin has not been yet identified, even if its peptidic nature has been demonstrated. The co-culture of E. faecalis EF37 in combination with lactococci significantly reduced the growth potential of this aminobiogenic strain, both in terms of growth rate and maximum cell concentration, depending on the initial inoculum level of E. faecalis. Tyramine accumulation was strongly reduced when E. faecalis EF37 was inoculated at 2 log cfu/ml and, to a lesser extent, at 3 log cfu/ml, as a result of a lower cell load of the aminobiogenic strain. All the lactococci were more efficient in inhibiting streptococci in comparison with E. faecalis EF37; in particular, L. lactis subsp. lactis VR84 induced the death of S. thermophilus PRI60 and allowed the detection of histamine traces only at higher streptococci inoculum levels (5-6 log cfu/ml). The other two lactococcal strains did not show a lethal action against S. thermophilus PRI60, but were able to reduce its growth extent and histamine accumulation, even if L. lactis subsp. lactis EG46 was less effective when the initial streptococci concentration was 5 and 6 log cfu/ml. This preliminary study has clarified some aspects regarding the ratio between bacteriocinogenic strains and aminobiogenic strains with respect to the possibility to accumulate BA and has also showed that different bacteriocins can have different effects on BA production on the same strain. This knowledge is essentially aimed to use bacteriocinogenic lactococci as a predictable strategy against aminobiogenic bacteria present in cheese or other fermented foods.

Cloning and optimization of a nisin biosynthesis pathway for bacteriocin harvest

Nisin is an important antimicrobial peptide that has enormous applications in biotechnology. Despite many encouraging efforts, its overproduction has been a long-standing challenge due to the complexity of the underlying pathway and the difficulty in genetic modification of lactic acid bacteria. Here, we cloned an entire nisin biosynthesis pathway from a nisin-producing strain (Lactococcus lactis K29) into a plasmid and transplanted the plasmid into a nisin deficient strain Lactococcus lactis MG1363, resulting in successful heterologous expression of bioactive recombinant nisin. To increase nisin harvest, we also overexpressed nisA, a gene responsible for nisin precursor production, with a set of constitutive promoters. To further optimize nisin yield, we minimized the metabolic cost of the engineered strains by integrating nisA overexpression cassettes and the recombinant pathway into a single circuit. With our rational construction and optimization, our engineered optimized strain is able to produce bioactive nisin with a yield of 1098 IU/mL, which is more than six times higher than that of the original strain.

Antagonistic lactic acid bacteria isolated from goat milk and identification of a novel nisin variant Lactococcus lactis

Background

The raw goat milk microbiota is considered a good source of novel bacteriocinogenic lactic acid bacteria (LAB) strains that can be exploited as an alternative for use as biopreservatives in foods. The constant demand for such alternative tools justifies studies that investigate the antimicrobial potential of such strains.

Results

The obtained data identified a predominance of Lactococcus and Enterococcus strains in raw goat milk microbiota with antimicrobial activity against Listeria monocytogenes ATCC 7644. Enzymatic assays confirmed the bacteriocinogenic nature of the antimicrobial substances produced by the isolated strains, and PCR reactions detected a variety of bacteriocin-related genes in their genomes. Rep-PCR identified broad genetic variability among the Enterococcus isolates, and close relations between the Lactococcus strains. The sequencing of PCR products from nis-positive Lactococcus allowed the identification of a predicted nisin variant not previously described and possessing a wide inhibitory spectrum.

Conclusions

Raw goat milk was confirmed as a good source of novel bacteriocinogenic LAB strains, having identified Lactococcus isolates possessing variations in their genomes that suggest the production of a nisin variant not yet described and with potential for use as biopreservatives in food due to its broad spectrum of action.

Cloning and heterologous expression of plnE, -F, -J and -K genes derived from soil metagenome and purification of active plantaricin peptides

Plantaricin gene-specific primers were used to obtain plnE, -F, -J and -K structural gene amplicons from soil metagenome. These amplicons were cloned and expressed in pET32a (+) vector in Escherichia coli BL21 (DE3). PlnE, -F, -J and -K peptides were expressed as His-tagged-fusion proteins and were separated by Ni(2+) -chelating affinity chromatography. The peptides were released from the fusion by enterokinase cleavage and separated from the carrier thioredoxin. The cleaved peptides were further analysed for antimicrobial activity and found to be active against Listeria innocua NRRL B33314, Micrococcus luteus MTCC 106 and lactic acid bacteria, such as Enterococcus casseliflavus NRRL B3502, Lactococcus lactis lactis NRRL 1821, Lactobacillus curvatus NRRL B4562 and Lactobacillus plantarum NRRL B4496. E. coli has been successfully exploited as a host for heterologous expression with a significant yield of fused and cleaved peptides in the range of 8-12 and 1-1.5 mg/l of the culture, respectively. Heterologous expression, therefore, can be used to overcome the constraints of low yield often reported from a native strain.

Identification of a nisI promoter within the nisABCTIP operon that may enable establishment of nisin immunity prior to induction of the operon via signal transduction

Certain strains of Lactococcus lactis produce the broad-spectrum bacteriocin nisin, which belongs to the lantibiotic class of antimicrobial peptides. The genes encoding nisin are organized in three contiguous operons: nisABTCIP, encoding production and immunity (nisI); nisRK, encoding regulation; and nisFEG, also involved in immunity. Transcription of nisABTCIP and nisFEG requires autoinduction by external nisin via signal transducing by NisRK. This organization poses the intriguing question of how sufficient immunity (NisI) can be expressed when the nisin cluster enters a new cell, before it encounters external nisin. In this study, Northern analysis in both Lactococcus and Enterococcus backgrounds revealed that nisI mRNA was present under conditions when no nisA transcription was occurring, suggesting an internal promoter within the operon. The nisA transcript was significantly more stable than nisI, further substantiating this. Reverse transcriptase PCR analysis revealed that the transcription initiated just upstream from nisI. Fusing this region to a lacZ gene in a promoter probe vector demonstrated that a promoter was present. The transcription start site (TSS) of the nisI promoter was mapped at bp 123 upstream of the nisI translation start codon. Ordered 5' deletions revealed that transcription activation depended on sequences located up to bp -234 from the TSS. The presence of poly(A) tracts and computerized predictions for this region suggested that a high degree of curvature may be required for transcription initiation. The existence of this nisI promoter is likely an evolutionary adaptation of the nisin gene cluster to enable its successful establishment in other cells following horizontal transfer.

In situ activity of a bacteriocin-producing Lactococcus lactis strain. Influence on the interactions between lactic acid bacteria during sourdough fermentation

AIMS

To biochemically characterize the bacteriocin produced by Lactococcus lactis ssp. lactis M30 and demonstrate its effect on lactic acid bacteria (LAB) during sourdough propagation.

METHODS AND RESULTS

A two-peptide bacteriocin produced by L. lactis ssp. lactis M30 was purified by ion exchange, hydrophobic interaction and reversed phase chromatography. Mass spectrometry of the two peptides and sequence analysis of the ltnA2 gene showed that the bacteriocin was almost identical to lacticin 3147. During a 20-day period of sourdough propagation the stability of L. lactis M30 was demonstrated, with concomitant inhibition of the indicator strain Lactobacillus plantarum 20, as well as the non-interference with the growth of the starter strain Lact. sanfranciscensis CB1.

CONCLUSIONS

In situ active bacteriocins influence the microbial consortium of sourdough LAB and can "support" the dominance of insensitive strains during sourdough fermentation.

SIGNIFICANCE AND IMPACT OF THE STUDY

The in situ bacteriocinogenic activity of selected lactococci enables the persistence of insensitive Lact. sanfranciscensis strains, useful to confer good characteristics to the dough, at a higher cell concentration with respect to other LAB of the same ecosystem.