Nucleotide sequence of the cell wall proteinase gene of Streptococcus cremoris Wg2

Identification of proteolytic bacteria from Yunnan fermented foods and their use to reduce the allergenicity of β-lactoglobulin

Beta-lactoglobulin (β-LG) is considered to be the major allergenic protein in milk. Lactic acid bacteria (LAB) possess a protein hydrolysis system that holds great promise for hydrolyzing β-LG and reducing its allergenicity. Therefore, this study aimed to screen LAB with β-LG hydrolysis activity from Yunnan traditional fermented foods. The results showed that Pediococcus pentosaceus C1001, Pediococcus acidilactici E1601-1, and Lactobacillus paracasei E1601-2, could effectively hydrolyze β-LG and further reduce its sensitization (more than 40%). All 3 lactic acid bacteria hydrolyzed β-LG allergenic fragments V41-K60 and L149-I162. Moreover, they encode a variety of genes related to proteolysis, such as aminopeptidase pepC and pepN, proline peptidase pepIP and endopeptidase pepO, and L. paracasei E1601-2 contains extracellular protease coding gene prtP. And they encode a variety of genes associated with hydrolyzed proteins. The 3 strains screened in this study can be used to develop hypoallergenic dairy products.

Lactococcus cell envelope proteases enable lactococcal growth in minimal growth media supplemented with high molecular weight proteins of plant and animal origin

Lactic acid bacteria (LAB) have evolved into fastidious microorganisms that require amino acids from environmental sources. Some LAB have cell envelope proteases (CEPs) that drive the proteolysis of high molecular weight proteins like casein in milk. CEP activity is typically studied using casein as the predominant substrate, even though CEPs can hydrolyze other protein sources. Plant protein hydrolysis by LAB has rarely been connected to the activity of specific CEPs. This study aims to show the activity of individual CEPs using LAB growth in a minimal growth medium supplemented with high molecular weight casein or potato proteins. Using Lactococcus cremoris MG1363 as isogenic background to express CEPs, we demonstrate that CEP activity is directly related to growth in the protein-supplemented minimal growth media. Proteolysis is analyzed based on the amino acid release, allowing a comparison of CEP activities and analysis of amino acid utilization by L. cremoris MG1363. This approach provides a basis to analyze CEP activity on plant-based protein substrates as casein alternatives and to compare activity of CEP homologs.

Characterization of Cell-Envelope Proteinases from Two Lacticaseibacillus casei Strains Isolated from Parmigiano Reggiano Cheese

In the present work, two cell-envelope proteinases (CEPs) from Lacticaseibacillus casei strains PRA205 and 2006 were characterized at both the biochemical and genetic levels. The genomes of both L. casei strains included two putative CEPs genes prtP2 and prtR1, but only prtR1 was transcribed. The extracted PrtR1 proteinases were serine proteinases with optimal activity at 40 °C and pH 7.5, and were activated by Ca²⁺ ions. Interestingly, PrtR1 from L. casei PRA205 exhibited high residual activity at pH 4 and at 5 °C, suggesting its possible exploitation for fermented food production. The caseinolytic activity against αS1- and β-casein indicated that both PrtR1s belonged to the PI/PIII type. These PrtR1s cleaved β-casein peptide bonds preferentially when amino acid M or N was present at the P1 subsite and amino acids A and D were at the P1' subsite. Several bioactive peptides were found to be released from PrtR1 after αs1- and β-casein hydrolysis.

Modeled Structure of the Cell Envelope Proteinase of Lactococcus lactis

The cell envelope proteinase (CEP) of Lactococcus lactis is a large extracellular protease covalently linked to the peptidoglycan of the cell wall. Strains of L. lactis are typically auxotrophic for several amino acids and in order to grow to high cell densities in milk they need an extracellular protease. The structure of the entire CEP enzyme is difficult to determine experimentally due to the large size and due to the attachment to the cell surface. We here describe the use of a combination of structure prediction tools to create a structural model for the entire CEP enzyme of Lactococcus lactis. The model has implications for how the bacterium interacts with casein micelles during growth in milk, and it has implications regarding the energetics of the proteolytic system. Our model for the CEP indicates that the catalytic triad is activated through a structural change caused by interaction with the substrate. The CEP of L. lactis might become a useful model for the mode of action for enzymes belonging to the large class of S8 proteinases with a PA (protease associated) domain and a downstream fibronectin like domain.

Soluble Lactobacillus delbrueckii subsp. bulgaricus 92059 PrtB proteinase derivatives for production of bioactive peptide hydrolysates from casein

The proteinase-encoding prtB gene of Lactobacillus (Lb.) delbrueckii (d.) subsp. bulgaricus 92059 was cloned and sequenced. Two soluble, secreted, C-terminally His-tagged derivatives were constructed and expressed in Lactococcus lactis by means of the NICE® Expression System. In both obtained derivatives PrtBb and PrtB2, the C-terminal, cell wall-binding domain was deleted. In addition, in derivative PrtB2, the C-terminal part of the B domain was deleted and the signal sequence was replaced by a lactococcal export signal. The affinity-purified derivatives were both proteolytically active. Peptide hydrolysates produced from casein with each of the derivatives showed identical peptide composition, as determined by liquid chromatography-mass spectrometry. Comparison of the peptides generated to those generated with living Lb. d. subsp. bulgaricus 92059 cells (Kliche et al. Appl Microbiol Biotechnol 101:7621-7633, 2017) showed that β-casein was the casein fraction most susceptible to hydrolysis and that some significant differences were observed between the products obtained by either the derivatives or living Lb. d. subsp. bulgaricus 92059 cells. When tested for biological activity, the hydrolysate obtained with PrtBb showed 50% inhibition of angiotensin-converting enzyme at a concentration of 0.5 mg/ml and immunomodulation/anti-inflammation in an in vitro assay of TNF-α induced NFκB activation at concentrations of 5 and 2.5 mg/ml, respectively. The enzymatically obtained hydrolysate did not show any pro-inflammatory or cytotoxic activity.

Development and evaluation of an efficient heterologous gene knock-in reporter system in Lactococcus lactis

Background

Lactococcus lactis is a food grade probiotics and widely used to express heterologous proteins. Generally, target genes are knocked into the L. lactis genome through double-crossover recombination to express heterologous proteins stably. However, creating marker-less heterologous genes knocked-in clones is laborious. In this study, an efficient heterologous gene knock-in reporter system was developed in L. lactis NZ9000.

Results

Our knock-in reporter system consists of a temperature-sensitive plasmid pJW and a recombinant L. lactis strain named NZB. The pJW contains homologous arms, and was constructed to knock-in heterologous genes at a fixed locus of NZ9000 genome. lacZ (β-galactosidase) gene was knocked into the chromosome of NZ9000 as a counter-selective marker through the plasmid pJW to generate NZB. The engineered NZB strain formed blue colonies on X-Gal plate. The desired double-crossover mutants formed white colonies distinctive from the predominantly blue colonies (parental and plasmid-integrated clones) when the embedded lacZ was replaced with the target heterologous genes carried by pJW in NZB.

Conclusions

By using the system, the heterologous gene knocked-in clones are screened by colony phenotype change rather than by checking colonies individually. Our new knock-in reporter system provides an efficient method to create heterologous genes knocked-in clones.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-017-0770-1) contains supplementary material, which is available to authorized users.

Screening for proteolytically active lactic acid bacteria and bioactivity of peptide hydrolysates obtained with selected strains

In a screening for proteolytically active lactic acid bacteria, three strains, Lactobacillus delbrueckii ssp. lactis 92202, Lactobacillus helveticus 92201, and Lactobacillus delbrueckii ssp. bulgaricus 92059, showed the highest activities following growth in milk. All three strains degraded α- and β-casein, but did not hydrolyse κ-casein. HPLC analysis of skim milk fermentation revealed increasing amounts of peptides after 5 and 10 h with Lb. d. ssp. bulgaricus 92059. Hydrolysates obtained with Lb. d. ssp. lactis 92202 and Lb. d. ssp. bulgaricus 92059 revealed the highest angiotensin-converting enzyme-inhibitory effect. The effect was dose dependent. Almost no effect (<10%) was seen for Lb. helveticus 92201. For Lb. d. ssp. bulgaricus 92059, maximal inhibition of approx. 65% was reached after 25 h of fermentation. In an in vitro assay measuring potential immunomodulation, hydrolysates of the three strains yielded anti-inflammatory activities in the presence of TNF-α. However, the effects were more pronounced at lower hydrolysate concentrations. In the absence of TNF-α, slight pro-inflammatory effects were observed. The hydrolysate of Lb. d. ssp. bulgaricus 92059, when purified by means of solid-phase extraction, exhibited pro-inflammatory activity. Sour whey containing Lb. d. ssp. bulgaricus 92059 cells showed pro-inflammatory activity while cell-free sour whey was clearly anti-inflammatory. In the purified hydrolysate, 20 different α- and β-casein (CN)-derived peptides could be identified by LC-MS. Most peptides originated from the central and C-terminal regions of β-casein. Peptide length was between 9 (β-CN(f 59-67)) and 22 amino acids (β-CN(f 117-138)).

Genetic, enzymatic and metabolite profiling of the Lactobacillus casei group reveals strain biodiversity and potential applications for flavour diversification

AIMS

The Lactobacillus casei group represents a widely explored group of lactic acid bacteria, characterized by a high level of biodiversity. In this study, the genetic and phenotypic diversity of a collection of more than 300 isolates of the Lact. casei group and their potential to produce volatile metabolites important for flavour development in dairy products, was examined.

METHODS AND RESULTS

Following confirmation of species by 16S rRNA PCR, the diversity of the isolates was determined by pulsed-field gel electrophoresis. The activities of enzymes involved in the proteolytic cascade were assessed and significant differences among the strains were observed. Ten strains were chosen based on the results of their enzymes activities and they were analysed for their ability to produce volatiles in media with increased concentrations of a representative aromatic, branched chain and sulphur amino acid. Volatiles were assessed using gas chromatography coupled with mass spectrometry. Strain-dependent differences in the range and type of volatiles produced were evident.

CONCLUSIONS

Strains of the Lact. casei group are characterized by genetic and metabolic diversity which supports variability in volatile production.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provides a screening approach for the knowledge-based selection of strains potentially enabling flavour diversification in fermented dairy products.

Diversity in proteinase specificity of thermophilic lactobacilli as revealed by hydrolysis of dairy and vegetable proteins

Ability of industrially relevant species of thermophilic lactobacilli strains to hydrolyze proteins from animal (caseins and β-lactoglobulin) and vegetable (soybean and wheat) sources, as well as influence of peptide content of growth medium on cell envelope-associated proteinase (CEP) activity, was evaluated. Lactobacillus delbrueckii subsp. lactis (CRL 581 and 654), L. delbrueckii subsp. bulgaricus (CRL 454 and 656), Lactobacillus acidophilus (CRL 636 and 1063), and Lactobacillus helveticus (CRL 1062 and 1177) were grown in a chemically defined medium supplemented or not with 1 % Casitone. All strains hydrolyzed mainly β-casein, while degradation of αs-caseins was strain dependent. Contrariwise, κ-Casein was poorly degraded by the studied lactobacilli. β-Lactoglobulin was mainly hydrolyzed by CRL 656, CRL 636, and CRL 1062 strains. The L. delbrueckii subsp. lactis strains, L. delbrueckii subsp. bulgaricus CRL 656, and L. helveticus CRL 1177 degraded gliadins in high extent, while the L. acidophilus and L. helveticus strains highly hydrolyzed soy proteins. Proteinase production was inhibited by Casitone, the most affected being the L. delbrueckii subsp. lactis species. This study highlights the importance of proteolytic diversity of lactobacilli for rational strain selection when formulating hydrolyzed dairy or vegetable food products.

Casein hydrolysis by Bifidobacterium longum KACC91563 and antioxidant activities of peptides derived therefrom

Milk protein is a well-known precursor protein for the generation of bioactive peptides using lactic acid bacteria. This study investigated the antioxidant activity of bovine casein hydrolysate after fermentation with Bifidobacterium longum KACC91563 using the 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) assay and total phenolic content (TPC). The antioxidant activities of the 24-h and 48-h hydrolysates were higher than that of the 4-h hydrolysate (2,045.5 and 1,629.3 μM gallic acid equivalents, respectively, vs. 40.3 μM) in the ABTS assay. In contrast, TPC values showed activities of 43.2 and 52.4 μM gallic acid equivalents for the 4-h and 24-h hydrolysates, respectively. Three fractions (≥10 kDa, ≥3 but <10 kDa, and <3 kDa) were separated from the 24-h hydrolysate by ultrafiltration. Among these fractions, the <3 kDa fraction exhibited the highest antioxidant activity (936.7 μM) compared with the other fractions (42.1 and 34.2 μM for >10 kDa and 3-10 kDa fractions, respectively). Through liquid chromatography-electrospray ionization-tandem mass spectrometry analysis, 2 peptides, VLSLSQSKVLPVPQK and VLSLSQSKVLPVPQKAVPYPQRDMPIQA, containing the fragment VLPVPQ that has antioxidant properties, were identified in the <3kDa fraction after 24h of hydrolysis. The present study demonstrates the possibility of antioxidant peptide production from bovine casein using Bifidobacterium longum.

Bacterial display and screening of posttranslationally thioether-stabilized peptides

A major hurdle in the application of therapeutic peptides is their rapid degradation by peptidases. Thioether bridges effectively protect therapeutic peptides against breakdown, thereby strongly increasing bioavailability, enabling oral and pulmonary delivery and potentially significantly optimizing the receptor interaction of selected variants. To efficiently select optimal variants, a library of DNA-coupled thioether-bridged peptides is highly desirable. Here, we present a unique cell surface display system of thioether-bridged peptides and successfully demonstrate highly selective screening. Peptides are posttranslationally modified by thioether bridge-installing enzymes in Lactococcus lactis, followed by export and sortase-mediated covalent coupling to the lactococcal cell wall. This allows the combinatorial optimization and selection of medically and economically highly important therapeutic peptides with strongly enhanced therapeutic potential.

Simultaneous presence of PrtH and PrtH2 proteinases in Lactobacillus helveticus Strains improves breakdown of the pure alphas1-casein

Lactobacillus helveticus can possess one or two cell envelope proteinases (CEPs), called PrtH2 and PrtH. The aim of this work was to explore the diversity of 15 strains of L. helveticus, isolated from various origins, in terms of their proteolytic activities and specificities on pure caseins or on milk casein micelles. CEP activity differed 14-fold when the strains were assayed on a synthetic substrate, but no significant differences were detected between strains possessing one or two CEPs. No correlation was observed between the proteolytic activities of the strains and their rates of acidification in milk. The kinetics of hydrolysis of purified α(s1)- and β-casein by L. helveticus whole cells was monitored using Tris-Tricine sodium dodecyl sulfate (SDS) electrophoresis, and for four strains, the peptides released were identified using mass spectrometry. While rapid hydrolysis of pure β-casein was observed for all strains, the hydrolysis kinetics of α(s1)-casein was the only criterion capable of distinguishing between the strains based on the number of CEPs. Fifty-four to 74 peptides were identified for each strain. When only PrtH2 was present, 22 to 30% of the peptides originated from α(s1)-casein. The percentage increased to 41 to 49% for strains in which both CEPs were expressed. The peptide size ranged from 6 to 33 amino acids, revealing a broad range of cleavage specificities, involving all classes of amino acids (Leu, Val, Ala, Ile, Glu, Gln, Lys, Arg, Met, and Pro). Regions resistant to proteolysis were identified in both caseins. When strains were grown in milk, a drastic reduction in the number of peptides was observed, reflecting changes in accessibility and/or peptide assimilation during growth.

Functionality of sortase A in Lactococcus lactis

Lactococcus lactis IL1403 harbors a putative sortase A (SrtA) and 11 putative sortase substrates that carry the canonical LPXTG signature of such substrates. We report here on the functionality of SrtA to anchor five LPXTG substrates to the cell wall, thus suggesting that SrtA is the housekeeping sortase in L. lactis IL1403.

The presence of prtP proteinase gene in natural isolate Lactobacillus plantarum BGSJ3-18

AIMS

The study of proteolytic activity and examination of proteinase gene region organization in proteolytically active Lactobacillus plantarum strains from different natural sources.

METHODS AND RESULTS

A set of 37 lactobacilli was distinguished by using multiplex PCR assay. Results showed that 34 strains were Lact. plantarum and three of them were Lact. paraplantarum. The examination of proteolytic activity revealed that 28 Lact. plantarum and two Lact. paraplantarum hydrolyse beta-casein. Further analyses of all proteolytically active Lact. plantarum with primers specific for different types of CEPs demonstrated that strain BGSJ3-18 has prtP catalytic domain as well as prtP-prtM intergenic region showing more than 95% sequence identity with the same regions present in Lact. paracasei, Lact. casei and L. lactis. No presence of prtB, prtH or prtR proteinase genes was detected in any of tested Lact. plantarum strains.

CONCLUSIONS

One out of 28 analysed Lact. plantarum strains harbours the prtP-like gene. The other proteolytically active Lact. plantarum probably possesses a different type of extracellular proteinase(s).

SIGNIFICANCE AND IMPACT OF THE STUDY

It is the first report about the presence of the prtP-like gene in Lact. plantarum, which illustrates the mobility of this gene and its presence in different species.

Variation in specificity of the PrtP extracellular proteinases in Lactococcus lactis and Lactobacillus paracasei subsp. paracasei

Comparison of cell-wall-bound extracellular proteinases (CEPs) from Lactobacillus paracasei (LBP) ssp. paracasei natural isolates BGHN14, BGAR75 and BGAR76 with Lactococcus lactis (LCL) ssp. cremoris Wg2, in their action on alpha(S1)-, beta- and kappa-casein was done. The CEPs of LBP strains were able to degrade alpha(S1)- and beta-caseins and their caseinolytic specificity depended on the type of buffer used. These CEPs, compared with LCL Wg2, differ in four amino acid residues in small segments predicted to be involved in substrate binding. The most striking features of this comparison are the presence of Ala instead of Ser(329) and the presence of Thr instead of Asn(256) and Ala(299), in the subtilisin-like region of the CEP in LBP natural isolates. Additional conservative amino acid substitution Leu to Ile(364) was found.

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.

The Listeria monocytogenes virulence factor InlJ is specifically expressed in vivo and behaves as an adhesin

The food-borne pathogen Listeria monocytogenes is adapted to a diversity of environments, such as soil, food, body fluids, and the cytosol of eukaryotic cells. The transition between saprophytic and pathogenic life is mediated through complex regulatory pathways that modulate the expression of virulence factors. Here we examined the expression of inlJ, a recently identified gene encoding a protein of the LPXTG-internalin family and involved in pathogenesis. We show that inlJ expression is controlled neither by the major listerial regulator of virulence genes, PrfA, nor by AxyR, a putative AraC regulator encoded by a gene adjacent to inlJ and divergently transcribed. The InlJ protein is not produced by bacteria grown in vitro in brain heart infusion medium or replicating in the cytosol of tissue-cultured cells. In contrast, it is efficiently produced and localized at the surface of bacteria present in the liver and blood of infected animals. Strikingly, the expression of inlJ by a heterologous promoter in L. monocytogenes or L. innocua promotes bacterial adherence to human cells in vitro. Taken together, these results strongly suggest that InlJ acts as a novel L. monocytogenes sortase-anchored adhesin specifically expressed during infection in vivo.

DNA sequence analysis of a cryptic plasmid pL2 from Lactococcus lactis subsp. lactis

A cryptic plasmid pL2 was isolated from Lactococcus lactis subsp. lactis and its complete nucleotide sequence was determined (5,299 bp, GenBank accession No. DQ917780). Its replication mode was identified as a theta-type belonging to the pAM beta 1 family. Analysis of the nucleotide sequence revealed that pL2 contained a transfer origin, a replication origin, and five putative open reading frames (ORF1-5). ORF1 (386 amino acids) was homologous to replication protein RepB. The other open reading frames were cryptic. A small counter-transcript RNA (ctRNA) coding region was also observed.

Mucosal immunisation of mice with malaria protein on lactic acid bacterial cell walls

Mice were orally and nasally immunised with a malaria parasite protein, MSA2, expressed in Lactococcus lactis, covalently attached to the peptidoglycan (MSA2cP), or non-covalently rebound to L. lactis cell walls, live Lactobacillus reuteri and Lb. salivarius (MSA2cA). Although there was marked variation within mice of the same inbred strain, the data suggest that the characteristics of serum IgG antibodies formed against MSA2 are influenced by the mouse strain and mode of MSA2 presentation. MSA2-specific IgA antibodies in sera and faecal pellets, and IFNgamma-secreting spleen cells were detectable in some immunised animals. MSA2cA on lactobacilli, although not effective in eliciting serum IgG antibodies, showed a potential for eliciting IgA antibodies in the gut. The presence of MSA2 in L. lactis tended to divert the antibody response from lactococcal antigens. The data suggest that anti-MSA2 and anti-lactococcal antibody responses are not necessarily associated in the mice. The findings are related to the use of lactic acid bacteria expressing heterologous proteins for therapy and vaccination.

Proteolytic systems of lactic acid bacteria

Lactic acid bacteria (LAB) have a very long history of use in the manufacturing processes of fermented foods and a great deal of effort was made to investigate and manipulate the role of LAB in these processes. Today, the diverse group of LAB includes species that are among the best-studied microorganisms and proteolysis is one of the particular physiological traits of LAB of which detailed knowledge was obtained. The proteolytic system involved in casein utilization provides cells with essential amino acids during growth in milk and is also of industrial importance due to its contribution to the development of the organoleptic properties of fermented milk products. For the most extensively studied LAB, Lactococcus lactis, a model for casein proteolysis, transport, peptidolysis, and regulation thereof is now established. In addition to nutrient processing, cellular proteolysis plays a critical role in polypeptide quality control and in many regulatory circuits by keeping basal levels of regulatory proteins low and removing them when they are no longer needed. As part of the industrial processes, LAB are challenged by various stress conditions that are likely to affect metabolic activities, including proteolysis. While environmental stress responses of LAB have received increasing interest in recent years, our current knowledge on stress-related proteolysis in LAB is almost exclusively based on studies on L. lactis. This review provides the current status in the research of proteolytic systems of LAB with industrial relevance.

Conversion of Lactococcus lactis cell envelope proteinase specificity by partial allele exchange

AIMS

To determine whether conversion of lactocepin substrate binding regions by gene replacement can alter lactocepin specificity in Lactococcus lactis starter bacteria without affecting other important strain properties.

METHODS AND RESULTS

We utilized two-step gene replacement to convert substrate-binding determinants in the L. lactis prtP genes encoding group h (bitter) lactocepin in two industrial strains into the corresponding group b (nonbitter) variant. Analysis of lactocepin activity toward alpha(s1)-casein (f 1-23) by reversed-phase high-pressure liquid chromatography demonstrated enzyme specificity among isogenic derivatives had been altered in a manner that was consistent with predicted amino acid substitutions in substrate binding regions. Milk acidification properties of some mutants were not statistically different (P > 0.05) from wild-type parent strains, and strain propensity for autolysis was also not significantly (P > 0.05) changed.

CONCLUSIONS

Conversion of lactocepin substrate binding regions by allele exchange can effectively alter lactocepin specificity in industrial strains of L. lactis without significantly affecting other important strain properties.

SIGNIFICANCE AND IMPACT OF THE STUDY

Methodology outlined in this study can be used to alter lactocepin specificity in commercial starter cultures with a propensity for bitter flavour defect, and prtP derivatives developed by this approach should be suitable for commercial application.

Immunogenicity of a malaria parasite antigen displayed by Lactococcus lactis in oral immunisations

A putative protective protein from Plasmodium falciparum merozoites, MSA2, was expressed in two different ways on the cell surface of the Gram-positive food-grade bacterium, Lactococcus lactis. The first display format exploits an LPXTG-type anchoring motif of the lactococcal proteinase PrtP to covalently anchor MSA2 to the genetically modified producer cells. In a second display format, MSA2 was fused to the peptidoglycan-binding domain (Protein Anchor) of the lactococcal cell wall hydrolase AcmA and was non-covalently rebound to the surface of non-genetically modified, non-living high-binder L. lactis cells, termed Gram-positive enhancer matrix (GEM) particles. The L. lactis recombinants carrying covalently bound MSA2 were used to immunise rabbits through nasal and oral routes. The highest levels of IgG antibodies reacting with near-native MSA2 on merozoites was elicited by oral administration. Intestinal antibodies to MSA2 were produced only after oral immunisation. MSA2-specific T(h)-cell activation could be demonstrated. Based on these results, the immunogenicity in oral immunisations of MSA2, bound non-covalently to non-genetically modified L. lactis GEM particles, was compared with MSA2 that was bound covalently to genetically modified L. lactis. These two forms elicited similar titres of serum antibodies. The results illustrate the potential of using non-genetically modified L. lactis as a safe vaccine delivery vehicle to elicit systemic antibodies, thereby avoiding the dissemination of recombinant DNA into the environment.

Amino acid profiles of lactic acid bacteria, isolated from kefir grains and kefir starter made from them

The characteristics of cell growth, lactic acid production, amino acid release and consumption by single-strain cultures of lactic acid bacteria (isolated from kefir grains), and by a multiple-strain kefir starter prepared from them, were studied. The change in the levels of free amino acids was followed throughout the kefir process: single-strain kefir bacteria and the kefir starter (Lactococcus lactis C15-1%+Lactobacillus helveticus MP12-3%+(Streptococcus thermophilus T15+Lactobacillus bulgaricus HP1 = 1:1)-3%) were cultivated in pasteurized (92 degrees C for 20 min) cow's milk (3% fat content) at 28 degrees C for 5 h (the kefir starter reached pH 4.7) and subsequently grown at 20 degrees C for 16 h; storage was at 4 degrees C for 168 h. The strain L. helveticus MP12 was unrivaled with respect to free amino acid production (53.38 mg (100 g)(-1)) and cell growth (17.8 x 10(8) CFU ml(-1)); however, it manifested the lowest acidification activity. L. bulgaricus HP1 released approximately 3.7 times less amino acids, nearly 5 times lower cell growth, and produced about 1.2 times more lactic acid. S. thermophilus T15 demonstrated dramatically complex amino acid necessities for growth and metabolism. With L. lactis C15, the highest levels of growth and lactic acid synthesis were recorded (18.3 x 10(8) CFU ml(-1) and 7.8 g l(-1) lactic acid at the 21st hour), and as for free amino acid production, it approximated L. bulgaricus HP1 (17.03 mg (100 g)(-1) maximum concentration). In the L. lactis C15 culture, the amino acids were used more actively throughout the first exponential growth phase (by the 10th hour) than during the second growth phase. The unique properties of the L. helveticus MP12 strain to produce amino acids were employed to create a symbiotic bioconsortium kefir culture, which, under conditions of kefir formation, enhanced lactic acid production and shortened the time required to reach pH 4.7; intensified cell growth activity, resulting in a respective 90- and 60-fold increase in the concentration of lactobacilli and cocci in the mixed culture compared to individual cultures; and accumulated free amino acids in the final kefir with higher total concentrations (56.88 mg (100 g)(-1)) and an individual concentration of essential amino acids (1.5 times) greater than that of yogurt.

Variable bacteriocin production in the commercial starter Lactococcus lactis DPC4275 is linked to the formation of the cointegrate plasmid pMRC02

Lactococcus lactis DPC4275 is a bacteriocin-producing transconjugant of the industrial starter strain DPC4268. Strain DPC4275 was generated through conjugal transfer by mating DPC4268 with L. lactis MG1363 containing the 60-kb plasmid pMRC01, which encodes the genetic determinants for the lantibiotic lacticin 3147 and for a phage resistance mechanism of the abortive infection type. The many significant applications of this strain prompted a genetic analysis of its apparently unstable bacteriocin-producing phenotype. Increased levels of lacticin 3147 produced by DPC4275 were associated with the appearance of an 80-kb plasmid, designated pMRC02, which was derived from DNA originating from pMRC01 (60 kb) and a resident DPC4268 proteinase plasmid, pMT60 (60 kb). Indeed, pMRC02 was shown to be derived from the insertion of a 17-kb fragment of pMRC01, encompassing the lacticin 3147 operon, into pMT60. The presence of pMRC02 at a high copy number was found to correlate with increased levels of lacticin 3147 in DPC4275 compared to the wild-type containing pMRC01. Subsequent transfer of pMRC02 into the plasmid-free strain MG1363 by electroporation allowed a direct phenotypic comparison with pMRC01, also studied in the MG1363 background. Plasmid pMRC02 displayed phage resistance similar to that by pMRC01, although it was less potent, as demonstrated by a larger plaque size for phage c2 infection of MG1363(pMRC02). While this locus is flanked by IS946 elements, the sequencing of pMT60-pMRC01 junction sites established that this event was unlikely to be insertion sequence mediated and most probably occurred by homologous recombination followed by deletion of most of pMRC01. This was not a random occurrence, as nine other transconjugants investigated were found to have the same junction sites. Such derivatives of commercial strains producing increased levels of bacteriocin could be exploited as protection cultures for food applications.

Determination of the domain of the Lactobacillus delbrueckii subsp. bulgaricus cell surface proteinase PrtB involved in attachment to the cell wall after heterologous expression of the prtB gene in Lactococcus lactis

Belonging to the subtilase family, the cell surface proteinase (CSP) PrtB of Lactobacillus delbrueckii subsp. bulgaricus differs from other CSPs synthesized by lactic acid bacteria. Expression of the prtB gene under its own promoter was shown to complement the proteinase-deficient strain MG1363 (PrtP(-) PrtM(-)) of Lactococcus lactis subsp. cremoris. Surprisingly, the maturation process of PrtB, unlike that of lactococcal CSP PrtPs, does not require a specific PrtM-like chaperone. The carboxy end of PrtB was previously shown to be different from the consensus anchoring region of other CSPs and exhibits an imperfect duplication of 59 amino acids with a high lysine content. By using a deletion strategy, the removal of the last 99 amino acids, including the degenerated anchoring signal (LPKKT), was found to be sufficient to release a part of the truncated PrtB into the culture medium and led to an increase in PrtB activity. This truncated PrtB is still active and enables L. lactis MG1363 to grow in milk supplemented with glucose. By contrast, deletion of the last 806 amino acids of PrtB led to the secretion of an inactive proteinase. Thus, the utmost carboxy end of PrtB is involved in attachment to the bacterial cell wall. Proteinase PrtB constitutes a powerful tool for cell surface display of heterologous proteins like antigens.

Surface display of the receptor-binding region of the Lactobacillus brevis S-layer protein in Lactococcus lactis provides nonadhesive lactococci with the ability to adhere to intestinal epithelial cells

Lactobacillus brevis is a promising lactic acid bacterium for use as a probiotic dietary adjunct and a vaccine vector. The N-terminal region of the S-layer protein (SlpA) of L. brevis ATCC 8287 was recently shown to mediate adhesion to various human cell lines in vitro. In this study, a surface display cassette was constructed on the basis of this SlpA receptor-binding domain, a proteinase spacer, and an autolysin anchor. The cassette was expressed under control of the nisA promoter in Lactococcus lactis NZ9000. Western blot assay of lactococcal cell wall extracts with anti-SlpA antibodies confirmed that the SlpA adhesion domain of the fusion protein was expressed and located within the cell wall layer. Whole-cell enzyme-linked immunosorbent assay and immunofluorescence microscopy verified that the SlpA adhesion-mediating region was accessible on the lactococcal cell surface. In vitro adhesion assays with the human intestinal epithelial cell line Intestine 407 indicated that the recombinant lactococcal cells had gained an ability to adhere to Intestine 407 cells significantly greater than that of wild-type L. lactis NZ9000. Serum inhibition assay further confirmed that adhesion of recombinant lactococci to Intestine 407 cells was indeed mediated by the N terminus-encoding part of the slpA gene. The ability of the receptor-binding region of SlpA to adhere to fibronectin was also confirmed with this lactococcal surface display system. These results show that, with the aid of the receptor-binding region of the L. brevis SlpA protein, the ability to adhere to gut epithelial cells can indeed be transferred to another, nonadhesive, lactic acid bacterium.

Lactococcus lactis DPC5598, a plasmid-free derivative of a commercial starter, provides a valuable alternative host for culture improvement studies

AIMS

To generate a plasmid-free derivative of an extensively used industrial starter strain Lactococcus lactis DPC4268, which could be used as a backbone strain for starter improvement programmes.

METHODS AND RESULTS

DPC4268 containing four large plasmids was subjected to high temperature plasmid curing resulting in derivatives, each with a different plasmid complement of one, two or three different plasmids in addition to a plasmid-free derivative. Industrially relevant phenotypes were assigned to each plasmid on the basis of detailed phenotypic and genetic analyses and these were (a) proteinase activity (Prt, 60 kb) (b) lactose fermentation (Lac, 55 kb) (c) bacteriophage adsorption inhibition (Ads, 44 kb) and (d) type I restriction/modification (R/M, 40 kb). The plasmid-free variant of DPC4268 was shown to be transformable at frequencies comparable to the common laboratory strain L. lactis MG1614. Furthermore its genome was demonstrated to be significantly different from the laboratory strains L. lactis MG1614 and the recently sequenced L. lactis IL1403 genomes by pulsed-field gel electrophoresis.

CONCLUSIONS

This study produced an easily transformable plasmid-free derivative which was genomically different from both MG1614 and IL1403. In addition, important plasmid-borne industrial traits, including two phage-resistance mechanisms, were identified in DPC4268.

SIGNIFICANCE AND IMPACT OF THE STUDY

L. DPC4268 is a vitally important commercial strain used in the manufacture of Cheddar cheese. The generation of a plasmid-free derivative may provide an important backbone strain as a basis for future strain improvement purposes.

Contribution of Lactococcus lactis cell envelope proteinase specificity to peptide accumulation and bitterness in reduced-fat Cheddar cheese

Bitterness is a flavor defect in Cheddar cheese that limits consumer acceptance, and specificity of the Lactococcus lactis extracellular proteinase (lactocepin) is widely believed to be a key factor in the development of bitter cheese. To better define the contribution of this enzyme to bitterness, we investigated peptide accumulation and bitterness in 50% reduced-fat Cheddar cheese manufactured with single isogenic strains of Lactococcus lactis as the only starter. Four isogens were developed for the study; one was lactocepin negative, and the others produced a lactocepin with group a, e, or h specificity. Analysis of cheese aqueous extracts by reversed-phase high-pressure liquid chromatography confirmed that accumulation of alpha(S1)-casein (f 1-23)-derived peptides f 1-9, f 1-13, f 1-16, and f 1-17 in cheese was directly influenced by lactocepin specificity. Trained sensory panelists demonstrated that Cheddar cheese made with isogenic starters that produced group a, e, or h lactocepin was significantly more bitter than cheese made with a proteinase-negative isogen and that propensity for bitterness was highest in cells that produced group h lactocepin. These results confirm the role of starter proteinase in bitterness and suggest that the propensity of some industrial strains for production of the bitter flavor defect in cheese could be altered by proteinase gene exchange or gene replacement.

Nucleotide sequence and characterization of the cell envelope proteinase plasmid in Lactococcus lactis subsp. cremoris HP

AIMS

The major cell envelope proteinase (lactocepin; EC 3.4.21.96) produced by Lactococcus lactis cheese starter bacteria is required for starter growth and acid production in milk. The aim of this study was to characterize a lactocepin plasmid from a L. lactis subsp. cremoris cheese starter strain.

METHODS AND RESULTS

A restriction map of the lactocepin plasmid pHP003 from strain HP was constructed, fragments were cloned in Escherichia coli vectors, and the complete DNA sequence (13,433 bp) was determined. Among 120 industrial L. lactis starter strains screened, five contained the same specificity-type lactocepin as pHP003. The lactocepin gene in these strains was invariably linked with a partially-deleted abiB gene.

CONCLUSION

The lactocepin specificity type of strain HP, conferred by a known configuration of key residues, is relatively uncommon. The gene is invariably linked with a partially deleted abiB gene on each lactocepin plasmid.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first complete sequence reported for a lactocepin plasmid, and provides the basis for better understanding, or manipulation, of lactocepin production.

Transcriptional pattern of genes coding for the proteolytic system of Lactococcus lactis and evidence for coordinated regulation of key enzymes by peptide supply

The transcription of 16 genes encoding 12 peptidases (pepC, pepN, pepX, pepP, pepA, pepF2, pepDA1, pepDA2, pepQ, pepT, pepM, and pepO1), P(I) and P(III) proteinases (prtP1 and prtP3), and three transport systems (dtpT, dtpP, and opp-pepO1) of Lactococcus lactis MG1363 was analyzed in response to different environmental factors. Promoter fusions with luciferase reporter genes and/or mRNA analysis were used to study the effects of sugar sources, growth at 37 degrees C, and peptide supply on the transcription of these genes. Only transcription of the pepP gene is modulated by the source of sugar. The presence of potential catabolite-responsive element (CRE) boxes in its promoter region suggests that expression of this gene is directly controlled by catabolic repression. Elevated temperature had no significant effect on the level of transcription of these genes. prtP1, prtP3, pepC, pepN, pepX, and the opp-pepO1 operon are the most highly expressed genes in chemically defined medium, and their expression is repressed 5- to 150-fold by addition of peptide sources such as Casitone in the medium. Moreover, the transcription of prtP1, prtP3, pepC, pepN, and the opp-pepO1 operon is repressed two- to eight-fold by the dipeptides leucylproline and prolylleucine. The transcription of pepDA2 might also be repressed by the peptide sources, but this effect is not observed on the regulation of dtpT, pepP, pepA, pepF2, pepDA1, pepQ, pepT, pepM, and the dtpP operon. The significance of these results with respect to the functions of different components of the proteolytic system in L. lactis are discussed.

The autoproteolysis of Lactococcus lactis lactocepin III affects its specificity towards beta-casein

The effect of autoproteolysis of Lactococcus lactis lactocepin III on its specificity towards beta-casein was investigated. beta-Casein degradation was performed by using either an autolysin-defective derivative of L. lactis MG1363 carrying the proteinase genes of L. lactis SK11, which was unable to transport oligopeptides, or autoproteolyzed enzyme purified from L. lactis SK11. Comparison of the peptide pools by high-performance liquid chromatography analysis revealed significant differences. To analyze these differences in more detail, the peptides released by the cell-anchored proteinase were identified by on-line coupling of liquid chromatography to mass spectrometry. More than 100 oligopeptides were released from beta-casein by the cell-anchored proteinase. Analysis of the cleavage sites indicated that the specificity of peptide bond cleavage by the cell-anchored proteinase differed significantly from that of the autoproteolyzed enzyme.

Streptococcus thermophilus cell wall-anchored proteinase: release, purification, and biochemical and genetic characterization

Streptococcus thermophilus CNRZ 385 expresses a cell envelope proteinase (PrtS), which is characterized in the present work, both at the biochemical and genetic levels. Since PrtS is resistant to most classical methods of extraction from the cell envelopes, we developed a three-step process based on loosening of the cell wall by cultivation of the cells in the presence of glycine (20 mM), mechanical disruption (with alumina powder), and enzymatic treatment (lysozyme). The pure enzyme is a serine proteinase highly activated by Ca(2+) ions. Its activity was optimal at 37 degrees C and pH 7.5 with acetyl-Ala-Ala-Pro-Phe-paranitroanilide as substrate. The study of the hydrolysis of the chromogenic and casein substrates indicated that PrtS presented an intermediate specificity between the most divergent types of cell envelope proteinases from lactococci, known as the PI and PIII types. This result was confirmed by the sequence determination of the regions involved in substrate specificity, which were a mix between those of PI and PIII types, and also had unique residues. Sequence analysis of the PrtS encoding gene revealed that PrtS is a member of the subtilase family. It is a multidomain protein which is maturated and tightly anchored to the cell wall via a mechanism involving an LPXTG motif. PrtS bears similarities to cell envelope proteinases from pyogenic streptococci (C5a peptidase and cell surface proteinase) and lactic acid bacteria (PrtP, PrtH, and PrtB). The highest homologies were found with streptococcal proteinases which lack, as PrtS, one domain (the B domain) present in cell envelope proteinases from all other lactic acid bacteria.

Deletion of various carboxy-terminal domains of Lactococcus lactis SK11 proteinase: effects on activity, specificity, and stability of the truncated enzyme

The Lactococcus lactis SK11 cell envelope proteinase is an extracellular, multidomain protein of nearly 2,000 residues consisting of an N-terminal serine protease domain, followed by various other domains of largely unknown function. Using a strategy of deletion mutagenesis, we have analyzed the function of several C-terminal domains of the SK11 proteinase which are absent in cell envelope proteinases of other lactic acid bacteria. The various deletion mutants were functionally expressed in L. lactis and analyzed for enzyme stability, activity, (auto)processing, and specificity toward several substrates. C-terminal deletions of first the cell envelope W (wall) and AN (anchor) domains and then the H (helix) domain leads to fully active, secreted proteinases of unaltered specificity. Gradually increasing the C-terminal deletion into the so-called B domain leads to increasing instability and autoproteolysis and progressively less proteolytic activity. However, the mutant with the largest deletion (838 residues) from the C terminus and lacking the entire B domain still retains proteolytic activity. All truncated enzymes show unaltered proteolytic specificity toward various substrates. This suggests that the main role played by these domains is providing stability or protection from autoproteolysis (B domain), spacing away from the cell (H domain), and anchoring to the cell envelope (W and AN domains). In addition, this study allowed us to more precisely map the main C-terminal autoprocessing site of the SK11 proteinase and the epitope for binding of group IV monoclonal antibodies.

Genetic characterization of a cell envelope-associated proteinase from Lactobacillus helveticus CNRZ32

A cell envelope-associated proteinase gene (prtH) was identified in Lactobacillus helveticus CNRZ32. The prtH gene encodes a protein of 1,849 amino acids and with a predicted molecular mass of 204 kDa. The deduced amino acid sequence of the prtH product has significant identity (45%) to that of the lactococcal PrtP proteinases. Southern blot analysis indicates that prtH is not broadly distributed within L. helveticus. A prtH deletion mutant of CNRZ32 was constructed to evaluate the physiological role of PrtH. PrtH is not required for rapid growth or fast acid production in milk by CNRZ32. Cell surface proteinase activity and specificity were determined by hydrolysis of alpha(s1)-casein fragment 1-23 by whole cells. A comparison of CNRZ32 and its prtH deletion mutant indicates that CNRZ32 has at least two cell surface proteinases that differ in substrate specificity.

Surface proteins of gram-positive bacteria and mechanisms of their targeting to the cell wall envelope

The cell wall envelope of gram-positive bacteria is a macromolecular, exoskeletal organelle that is assembled and turned over at designated sites. The cell wall also functions as a surface organelle that allows gram-positive pathogens to interact with their environment, in particular the tissues of the infected host. All of these functions require that surface proteins and enzymes be properly targeted to the cell wall envelope. Two basic mechanisms, cell wall sorting and targeting, have been identified. Cell well sorting is the covalent attachment of surface proteins to the peptidoglycan via a C-terminal sorting signal that contains a consensus LPXTG sequence. More than 100 proteins that possess cell wall-sorting signals, including the M proteins of Streptococcus pyogenes, protein A of Staphylococcus aureus, and several internalins of Listeria monocytogenes, have been identified. Cell wall targeting involves the noncovalent attachment of proteins to the cell surface via specialized binding domains. Several of these wall-binding domains appear to interact with secondary wall polymers that are associated with the peptidoglycan, for example teichoic acids and polysaccharides. Proteins that are targeted to the cell surface include muralytic enzymes such as autolysins, lysostaphin, and phage lytic enzymes. Other examples for targeted proteins are the surface S-layer proteins of bacilli and clostridia, as well as virulence factors required for the pathogenesis of L. monocytogenes (internalin B) and Streptococcus pneumoniae (PspA) infections. In this review we describe the mechanisms for both sorting and targeting of proteins to the envelope of gram-positive bacteria and review the functions of known surface proteins.

Electron microscopy studies of cell-wall-anchored cellulose (Avicel)-binding protein (AbpS) from Streptomyces reticuli

Streptomyces reticuli produces a 35-kDa cellulose (Avicel)-binding protein (AbpS) which interacts strongly with crystalline cellulose but not with soluble types of cellulose. Antibodies that were highly specific for the NH2-terminal part of AbpS were isolated by using truncated AbpS proteins that differed in the length of the NH2 terminus. Using these antibodies for immunolabelling and investigations in which fluorescence, transmission electron, or immunofield scanning electron microscopy was used showed that the NH2 terminus of AbpS protrudes from the murein layer of S. reticuli. Additionally, inspection of ultrathin sections of the cell wall, as well as biochemical experiments performed with isolated murein, revealed that AbpS is tightly and very likely covalently linked to the polyglucane layer. As AbpS has also been found to be associated with protoplasts, we predicted that a COOH-terminal stretch consisting of 17 hydrophobic amino acids anchors the protein to the membrane. Different amounts of AbpS homologues of several Streptomyces strains were synthesized.

Molecular and biotechnological aspects of microbial proteases

Proteases represent the class of enzymes which occupy a pivotal position with respect to their physiological roles as well as their commercial applications. They perform both degradative and synthetic functions. Since they are physiologically necessary for living organisms, proteases occur ubiquitously in a wide diversity of sources such as plants, animals, and microorganisms. Microbes are an attractive source of proteases owing to the limited space required for their cultivation and their ready susceptibility to genetic manipulation. Proteases are divided into exo- and endopeptidases based on their action at or away from the termini, respectively. They are also classified as serine proteases, aspartic proteases, cysteine proteases, and metalloproteases depending on the nature of the functional group at the active site. Proteases play a critical role in many physiological and pathophysiological processes. Based on their classification, four different types of catalytic mechanisms are operative. Proteases find extensive applications in the food and dairy industries. Alkaline proteases hold a great potential for application in the detergent and leather industries due to the increasing trend to develop environmentally friendly technologies. There is a renaissance of interest in using proteolytic enzymes as targets for developing therapeutic agents. Protease genes from several bacteria, fungi, and viruses have been cloned and sequenced with the prime aims of (i) overproduction of the enzyme by gene amplification, (ii) delineation of the role of the enzyme in pathogenecity, and (iii) alteration in enzyme properties to suit its commercial application. Protein engineering techniques have been exploited to obtain proteases which show unique specificity and/or enhanced stability at high temperature or pH or in the presence of detergents and to understand the structure-function relationships of the enzyme. Protein sequences of acidic, alkaline, and neutral proteases from diverse origins have been analyzed with the aim of studying their evolutionary relationships. Despite the extensive research on several aspects of proteases, there is a paucity of knowledge about the roles that govern the diverse specificity of these enzymes. Deciphering these secrets would enable us to exploit proteases for their applications in biotechnology.

A deficiency in aspartate biosynthesis in Lactococcus lactis subsp. lactis C2 causes slow milk coagulation

A mutant of fast milk-coagulating (Fmc+) Lactococcus lactis subsp. lactis C2, designated L. lactis KB4, was identified. Although possessing the known components essential for utilizing casein as a nitrogen source, which include functional proteinase (PrtP) activity and oligopeptide, di- and tripeptide, and amino acid transport systems, KB4 exhibited a slow milk coagulation (Fmc-) phenotype. When the amino acid requirements of L. lactis C2 were compared with those of KB4 by use of a chemically defined medium, it was found that KB4 was unable to grow in the absence of aspartic acid. This aspartic acid requirement could also be met by aspartate-containing peptides. The addition of aspartic acid to milk restored the Fmc+ phenotype of KB4. KB4 was found to be defective in pyruvate carboxylase and thus was deficient in the ability to form oxaloacetate and hence aspartic acid from pyruvate and carbon dioxide. The results suggest that when lactococci are propagated in milk, aspartate derived from casein is unable to meet fully the nutritional demands of the lactococci, and they become dependent upon aspartate biosynthesis.