Controlled overproduction of proteins by lactic acid bacteria

Development of Zn2+-controlled expression system for lactic acid bacteria and its application in engineered probiotics

Lactococcus lactis and Streptococcus thermophilus are considered as ideal chassis of engineered probiotics, while food-grade genetic tools are limited in those strains. Here, a Zn2+-controlled gene expression (ZICE) system was identified in the genome of S. thermophilus CGMCC7.179, including a transcriptional regulator sczAst and a promoter region of cation transporter czcD (PczcDst). Specific binding of the SczAst to the palindromic sequences in PczcDst was demonstrated by EMSA analysis, suggesting the regulation role of SczAst on PczcDst. To evaluate their possibility to control gene expression in vivo, the sczAst-PczcDst was employed to drive the expression of green fluorescence protein (GFP) gene in L. lactis NZ9000 and S. thermophilus CGMCC7.179, respectively. Both of the transformants could express GFP under Zn2+ induction, while no fluorescence without Zn2+ addition. For optimal conditions, Zn2+ was used at a final concentration of 0.8 mM in L. lactis and 0.16 mM in S. thermophilus at OD600 close to 0.4, and omitting yeast extract powder in the medium unexpectedly improved GFP expression level by 2.2-fold. With the help of the ZICE system, engineered L. lactis and S. thermophilus strains were constructed to secret cytokine interleukin-10 (IL-10) with immunogenicity, and the IL-10 content in the supernatant of the engineered L. lactis was 59.37 % of that under the nisin controlled expression system. This study provided a tightly controlled expression system by the food-grade inducer Zn2+, having potential in development of engineered probiotics.

Lipidated variants of the antimicrobial peptide nisin produced via incorporation of methionine analogs for click chemistry show improved bioactivity

The increase in antibiotic resistance calls for accelerated molecular engineering strategies to diversify natural products for drug discovery. The incorporation of non-canonical amino acids (ncAAs) is an elegant strategy for this purpose, offering a diverse pool of building blocks to introduce desired properties into antimicrobial lanthipeptides. We here report an expression system using Lactococcus lactis as a host for non-canonical amino acid incorporation with high efficiency and yield. We show that incorporating the more hydrophobic analog ethionine (instead of methionine) into nisin improves its bioactivity against several Gram-positive strains we tested. New-to-nature variants were further created by click chemistry. By azidohomoalanine (Aha) incorporation and subsequent click chemistry, we obtained lipidated variants at different positions in nisin or in truncated nisin variants. Some of them show improved bioactivity and specificity against several pathogenic bacterial strains. These results highlight the ability of this methodology for lanthipeptide multi-site lipidation, to create new-to-nature antimicrobial products with diverse features, and extend the toolbox for (lanthi)peptide drug improvement and discovery.

Development of high cell density Limosilactobacillus reuteri KUB-AC5 for cell factory using oxidative stress reduction approach

BACKGROUND

Expression systems for lactic acid bacteria have been developed for metabolic engineering applications as well as for food-grade recombinant protein production. But the industrial applications of lactic acid bacteria as cell factories have been limited due to low biomass formation resulted in low efficiency of biomanufacturing process. Limosilactobacillus reuteri KUB-AC5 is a safe probiotic lactic acid bacterium that has been proven as a gut health enhancer, which could be developed as a mucosal delivery vehicle for vaccines or therapeutic proteins, or as expression host for cell factory applications. Similar to many lactic acid bacteria, its oxygen sensitivity is a key factor that limits cell growth and causes low biomass production. The aim of this study is to overcome the oxidative stress in L. reuteri KUB-AC5. Several genes involved in oxidative and anti-oxidative stress were investigated, and strain improvement for higher cell densities despite oxidative stress was performed using genetic engineering.

RESULTS

An in-silico study showed that L. reuteri KUB-AC5 genome possesses an incomplete respiratory chain lacking four menaquinone biosynthesis genes as well as a complete biosynthesis pathway for the production of the precursor. The presence of an oxygen consuming enzyme, NADH oxidase (Nox), leads to high ROS formation in aerobic cultivation, resulting in strong growth reduction to approximately 25% compared to anaerobic cultivation. Recombinant strains expressing the ROS scavenging enzymes Mn-catalase and Mn-superoxide dismutase were successfully constructed using the pSIP expression system. The Mn-catalase and Mn-SOD-expressing strains produced activities of 873 U/ml and 1213 U/ml and could minimize the ROS formation in the cell, resulting in fourfold and sevenfold higher biomass formation, respectively.

CONCLUSIONS

Expression of Mn-catalase and Mn-SOD in L. reuteri KUB-AC5 successfully reduced oxidative stress and enhanced growth. This finding could be applied for other lactic acid bacteria that are subject to oxidative stress and will be beneficial for applications of lactic acid bacteria for cell factory applications.

Secretion of Bacillus amyloliquefaciens Transglutaminase from Lactococcus lactis and Its Enhancement of Food Gel Properties

(1) Background: Microbial transglutaminases (MTGase) catalyze protein crosslink. This is useful in the food industry to improve gelation, water holding capacity, and emulsifying capacity during foodstuff manufacturing. The production of MTGase in wild-type strains renders low yield and high costs of downstream purification, limiting its industrial applications. (2) Methods: In this work, MTGase from Bacillus amyloliquefaciens BH072 (BaMTGase) has been heterologously expressed in Lactococcus lactis, using the signal peptide Usp45 to direct the secretion of recombinant BaMTGase out of the cell for easier purification. (3) Results: In these conditions, MTGase was purified with a high yield (48.7 ± 0.2 mg/L) and high enzyme activity (28.6 ± 0.5 U/mg). Next, BaMTGase was tested for industrial applications. Recombinant BaMTGase and commercial MTGase were used for SPI solution crosslinking. BaMTGase formed a harder gel with higher water-holding capacity and a dense and smooth gel microstructure. (4) Conclusions: This work provides an attractive food-grade cell factory for the food industry and offers a suitable chassis for MTGase production.

Functional Expression and Characterization of the Highly Promiscuous Lanthipeptide Synthetase SyncM, Enabling the Production of Lanthipeptides with a Broad Range of Ring Topologies

Lanthipeptides are ribosomally synthesized and post-translationally modified peptides characterized by the presence of lanthionine rings that provide stability and functionality. Genome mining techniques have shown their huge diversity and potential for the discovery of novel active molecules. However, in many cases, they are not easily produced under laboratory conditions. The heterologous expression of these molecules using well-characterized lanthipeptide biosynthetic enzymes is rising as an alternative system for the design and production of new lanthipeptides with biotechnological or clinical properties. Nevertheless, the substrate-enzyme specificity limits the complete modification of the desired peptides and hence, their full stability and/or biological activity. New low substrate-selective biosynthetic enzymes are therefore necessary for the heterologous production of new-to-nature peptides. Here, we have identified, cloned, and heterologously expressed in Lactococcus lactis the most promiscuous lanthipeptide synthetase described to date, i.e., SyncM from the marine cyanobacteria Synechococcus MITS9509. We have characterized the functionality of SyncM by the successful expression of 15 out of 18 different SyncA substrates, subsequently determining the dehydration and cyclization processes in six representatives of them. This characterization highlights the very relaxed substrate specificity of SyncM toward its precursors and the ability to catalyze the formation of exceptionally large rings in a variety of topologies. Our results suggest that SyncM could be an attractive enzyme to design and produce a wide variety of new-to-nature lanthipeptides with a broad range of ring topologies.

Nisin- and Ripcin-Derived Hybrid Lanthipeptides Display Selective Antimicrobial Activity against Staphylococcus aureus

Lanthipeptides are (methyl)lanthionine ring-containing ribosomally synthesized and post-translationally modified peptides (RiPPs). Many lanthipeptides show strong antimicrobial activity against bacterial pathogens, including antibiotic-resistant bacterial pathogens. The group of disulfide-bond-containing antimicrobial peptides (AMPs) is well-known in nature and forms a rich source of templates for the production of novel peptides with corresponding (methyl)lanthionine analogues instead of disulfides. Here, we show that novel macrocyclic lanthipeptides (termed thanacin and ripcin) can be synthesized using the known antimicrobials thanatin and rip-thanatin as templates. Notably, the synthesized nisin(1-20)-ripcin hybrid lanthipeptides (ripcin B-G) showed selective antimicrobial activity against S. aureus, including an antibiotic-resistant MRSA strain. Interestingly, ripcin B-G, which are hybrid peptides of nisin(1-20) and ripcin that are each inactive against Gram-negative pathogens, showed substantial antimicrobial activity against the tested Gram-negative pathogens. Moreover, ripcin B-G was highly resistant against the nisin resistance protein (NSR; a peptidase that removes the C-terminal 6 amino acids of nisin and strongly reduces its antimicrobial activity), opposed to nisin itself. This study provides an example of converting disulfide-bond-based AMPs into (methyl)lanthionine-based macrocyclic hybrid lanthipeptides and can yield antimicrobial peptides with selective antimicrobial activity against S. aureus.

Importance of the leader peptide sequence on the lanthipeptide secretion level

Lanthipeptides are ribosomally synthesized and posttranslationally modified peptides. Their precursor peptide comprises of an N-terminal leader peptide and a C-terminal core peptide. Here, the leader peptide is crucial for enzyme recognition especially for the modification enzymes and acts furthermore as a secretion signal for the lanthipeptide exporter. The core peptide is the target site for the posttranslational modifications and contains dehydrated amino acids and lanthionine rings. Nisin produced by the Gram-positive bacterium Lactococcus lactis is one of the best-studied lanthipeptides and used as a model system to study their modification and secretion processes. Nisin is secreted as a precursor peptide. Here, we present an in vivo secretion analysis of NisT in the absence of the modification machinery allowing the secretion of leader peptide mutants and their impact solely on the secretion activity of NisT. Additionally, we created leader peptide hybrids to provide new insights, how the secretion is effected by unnatural leader peptides. The focus on the secretion activity of the transporter alone enabled us to determine the recognition site of NisT within the leader peptide of nisin.

Insights in the Antimicrobial Potential of the Natural Nisin Variant Nisin H

Lantibiotics are a growing class of antimicrobial peptides, which possess antimicrobial activity against mainly Gram-positive bacteria including the highly resistant strains such as methicillin-resistant Staphylococcus aureus or vancomycin-resistant enterococci. In the last decades numerous lantibiotics were discovered in natural habitats or designed with bioengineering tools. In this study, we present an insight in the antimicrobial potential of the natural occurring lantibiotic nisin H from Streptococcus hyointestinalis as well as the variant nisin H F₁I. We determined the yield of the heterologously expressed peptide and quantified the cleavage efficiency employing the nisin protease NisP. Furthermore, we analyzed the effect on the modification via mass spectrometry analysis. With standardized growth inhibition assays we benchmarked the activity of pure nisin H and the variant nisin H F₁I, and their influence on the activity of the nisin immunity proteins NisI and NisFEG from Lactococcus lactis and the nisin resistance proteins SaNSR and SaNsrFP from Streptococcus agalactiae COH1. We further checked the antibacterial activity against clinical isolates of Staphylococcus aureus, Enterococcus faecium and Enterococcus faecalis via microdilution method. In summary, nisin H and the nisin H F₁I variant possessed better antimicrobial potency than the natural nisin A.

Construction of Genetically Modified Lactococcus lactis Producing Anti-human-CTLA-4 Single-Chain Fragment Variable

Lactic acid bacteria are human commensal organisms that have immunomodulatory and metabolism-promoting effects. In addition, due to the increasing demand for biopharmaceuticals, genetically modified lactic acid bacteria (gmLAB) that produce recombinant proteins are expected to be used as microbial therapeutics and next-generation probiotics. In this study, we constructed a gmLAB strain that produces anti-human cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) single-chain fragment variable (CTLA4scFv) for possible use in a cancer treatment strategy using gmLAB. CTLA-4, an immune checkpoint molecule, suppresses the anti-cancer immune response; thus, inhibition of CTLA-4 signaling is important in cancer therapy. In this study, we designed a CTLA4scFv composed of a heavy and light chain of the variable region from anti-human CTLA-4 antibody connected by a flexible peptide linker. CTLA4scFv was expressed using nisin controlled gene expression (NICE) system, a lactococcal inducible gene expression system, and the DNA sequence encoding CTLA4scFv was inserted downstream of the P_nisA promoter of the gene expression vector pNZ8148#2. Furthermore, expression of recombinant CTLA4scFv was confirmed by Western blotting, and the immunoreactivity of recombinant CTLA4scFv against human CTLA-4 protein was examined using ELISA. We speculate that gmLAB producing bioactive CTLA4scFv will become an attractive approach for cancer treatment.

Mimicry of a Non-ribosomally Produced Antimicrobial, Brevicidine, by Ribosomal Synthesis and Post-translational Modification

The group of bacterial non-ribosomally produced peptides (NRPs) forms a rich source of antibiotics, such as daptomycin, vancomycin, and teixobactin. The difficulty of functionally expressing and engineering the corresponding large biosynthetic complexes is a bottleneck in developing variants of such peptides. Here, we apply a strategy to synthesize mimics of the recently discovered antimicrobial NRP brevicidine. We mimicked the molecular structure of brevicidine by ribosomally synthesized, post-translationally modified peptide (RiPP) synthesis, introducing several relevant modifications, such as dehydration and thioether ring formation. Following this strategy, in two rounds peptides were engineered in vivo, which showed antibacterial activity against Gram-negative pathogenic bacteria susceptible to wild-type brevicidine. This study demonstrates the feasibility of a strategy to structurally and functionally mimic NRPs by employing the synthesis and post-translational modifications typical for RiPPs. This enables the future generation of large genetically encoded peptide libraries of NRP-mimicking structures to screen for antimicrobial activity against relevant pathogens.

Feasability of Introducing a Thioether Ring in Vasopressin by nisBTC Co-expression in Lactococcus lactis

Introducing one or more intramolecular thioether bridges in a peptide provides a promising approach to create more stable molecules with improved pharmacodynamic properties and especially to protect peptides against proteolytic degradation. Lanthipeptides are compounds that naturally possess thioether bonds in their structure. The model lanthipeptide, nisin, is produced by Lactococcus lactis as a core peptide fused to a leader peptide. The modification machinery responsible for nisin production, including the Ser/Thr-dehydratase NisB and the cyclase NisC, can be applied for introducing a thioether bridge into peptides fused to the nisin leader peptide, e.g., to replace a disulfide bond. Vasopressin plays a key role in water homeostasis in the human body and helps to constrict blood vessels. There are two cysteine residues in the structure of wild type vasopressin, which form a disulfide bridge in the mature peptide. Here, we show it is possible to direct the biosynthesis of vasopressin variants in such a way that the disulfide bridge is replaced by a thioether bridge using the nisin modification machinery NisBTC, albeit at low efficiency. Vasopressin mutants were fused either to the nisin leader peptide directly (Type A), after the first three rings of nisin (Type B/C), or after full nisin (Type D). The type B strategy was optimal for expression. LC-MS/MS data verified the formation of a thioether bridge, which provides proof of principle for this modification in vasopressin. This is a first step prior to the necessary increase of the production yield and further purification of these peptides to finally test their biological activity in tissue and animal models.

Distinct and Specific Role of NlpC/P60 Endopeptidases LytA and LytB in Cell Elongation and Division of Lactobacillus plantarum

Peptidoglycan (PG) is an essential lattice of the bacterial cell wall that needs to be continuously remodeled to allow growth. This task is ensured by the concerted action of PG synthases that insert new material in the pre-existing structure and PG hydrolases (PGHs) that cleave the PG meshwork at critical sites for its processing. Contrasting with Bacillus subtilis that contains more than 35 PGHs, Lactobacillus plantarum is a non-sporulating rod-shaped bacterium that is predicted to possess a minimal set of 12 PGHs. Their role in morphogenesis and cell cycle remains mostly unexplored, except for the involvement of the glucosaminidase Acm2 in cell separation and the NlpC/P60 D, L-endopeptidase LytA in cell shape maintenance. Besides LytA, L. plantarum encodes three additional NlpC/P60 endopeptidases (i.e., LytB, LytC and LytD). The in silico analysis of these four endopeptidases suggests that they could have redundant functions based on their modular organization, forming two pairs of paralogous enzymes. In this work, we investigate the role of each Lyt endopeptidase in cell morphogenesis in order to evaluate their distinct or redundant functions, and eventually their synthetic lethality. We show that the paralogous LytC and LytD enzymes are not required for cell shape maintenance, which may indicate an accessory role such as in PG recycling. In contrast, LytA and LytB appear to be key players of the cell cycle. We show here that LytA is required for cell elongation while LytB is involved in the spatio-temporal regulation of cell division. In addition, both PGHs are involved in the proper positioning of the division site. The absence of LytA activity is responsible for the asymmetrical positioning of septa in round cells while the lack of LytB results in a lateral misplacement of division planes in rod-shaped cells. Finally, we show that the co-inactivation of LytA and LytB is synthetically affecting cell growth, which confirms the key roles played by both enzymes in PG remodeling during the cell cycle of L. plantarum. Based on the large distribution of NlpC/P60 endopeptidases in low-GC Gram-positive bacteria, these enzymes are attractive targets for the discovery of novel antimicrobial compounds.

β-Galactosidase from Lactobacillus helveticus DSM 20075: Biochemical Characterization and Recombinant Expression for Applications in Dairy Industry

β-Galactosidase encoding genes lacLM from Lactobacillus helveticus DSM 20075 were cloned and successfully overexpressed in Escherichia coli and Lactobacillus plantarum using different expression systems. The highest recombinant β-galactosidase activity of ∼26 kU per L of medium was obtained when using an expression system based on the T7 RNA polymerase promoter in E. coli, which is more than 1000-fold or 28-fold higher than the production of native β-galactosidase from L. helveticus DSM 20075 when grown on glucose or lactose, respectively. The overexpression in L. plantarum using lactobacillal food-grade gene expression system resulted in ∼2.3 kU per L of medium, which is approximately 10-fold lower compared to the expression in E. coli. The recombinant β-galactosidase from L. helveticus overexpressed in E. coli was purified to apparent homogeneity and subsequently characterized. The K_m and v_max values for lactose and o-nitrophenyl-β-d-galactopyranoside (oNPG) were 15.7 ± 1.3 mM, 11.1 ± 0.2 µmol D-glucose released per min per mg protein, and 1.4 ± 0.3 mM, 476 ± 66 µmol o-nitrophenol released per min per mg protein, respectively. The enzyme was inhibited by high concentrations of oNPG with K_i,s = 3.6 ± 0.8 mM. The optimum pH for hydrolysis of both substrates, lactose and oNPG, is pH 6.5 and optimum temperatures for these reactions are 60 and 55 °C, respectively. The formation of galacto-oligosaccharides (GOS) in discontinuous mode using both crude recombinant enzyme from L. plantarum and purified recombinant enzyme from E. coli revealed high transgalactosylation activity of β-galactosidases from L. helveticus; hence, this enzyme is an interesting candidate for applications in lactose conversion and GOS formation processes.

Recurrent group A Streptococcus tonsillitis is an immunosusceptibility disease involving antibody deficiency and aberrant TFH cells

"Strep throat" is highly prevalent among children, yet it is unknown why only some children develop recurrent tonsillitis (RT), a common indication for tonsillectomy. To gain insights into this classic childhood disease, we performed phenotypic, genotypic, and functional studies on pediatric group A Streptococcus (GAS) RT and non-RT tonsils from two independent cohorts. GAS RT tonsils had smaller germinal centers, with an underrepresentation of GAS-specific CD4+ germinal center T follicular helper (GC-TFH) cells. RT children exhibited reduced antibody responses to an important GAS virulence factor, streptococcal pyrogenic exotoxin A (SpeA). Risk and protective human leukocyte antigen (HLA) class II alleles for RT were identified. Lastly, SpeA induced granzyme B production in GC-TFH cells from RT tonsils with the capacity to kill B cells and the potential to hobble the germinal center response. These observations suggest that RT is a multifactorial disease and that contributors to RT susceptibility include HLA class II differences, aberrant SpeA-activated GC-TFH cells, and lower SpeA antibody titers.

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.

Increasing the Antimicrobial Activity of Nisin-Based Lantibiotics against Gram-Negative Pathogens

Lantibiotics are ribosomally synthesized and posttranslationally modified antimicrobial compounds containing lanthionine and methyl-lanthionine residues. Nisin, one of the most extensively studied and used lantibiotics, has been shown to display very potent activity against Gram-positive bacteria, and stable resistance is rarely observed. By binding to lipid II and forming pores in the membrane, nisin can cause the efflux of cellular constituents and inhibit cell wall biosynthesis. However, the activity of nisin against Gram-negative bacteria is much lower than that against Gram-positive bacteria, mainly because lipid II is located at the inner membrane, and the rather impermeable outer membrane in Gram-negative bacteria prevents nisin from reaching lipid II. Thus, if the outer membrane-traversing efficiency of nisin could be increased, the activity against Gram-negative bacteria could, in principle, be enhanced. In this work, several relatively short peptides with activity against Gram-negative bacteria were selected from literature data to be fused as tails to the C terminus of either full or truncated nisin species. Among these, we found that one of three tails (tail 2 [T2; DKYLPRPRPV], T6 [NGVQPKY], and T8 [KIAKVALKAL]) attached to a part of nisin displayed improved activity against Gram-negative microorganisms. Next, we rationally designed and reengineered the most promising fusion peptides. Several mutants whose activity significantly outperformed that of nisin against Gram-negative pathogens were obtained. The activity of the tail 16 mutant 2 (T16m2) construct against several important Gram-negative pathogens (i.e., Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, Enterobacter aerogenes) was increased 4- to 12-fold compared to that of nisin. This study indicates that the rational design of nisin can selectively and significantly improve its outer membrane-permeating capacity as well as its activity against Gram-negative pathogens.IMPORTANCE Lantibiotics are antimicrobial peptides that are highly active against Gram-positive bacteria but that have relatively poor activity against most Gram-negative bacteria. Here, we modified the model lantibiotic nisin by fusing parts of it to antimicrobial peptides with known activity against Gram-negative bacteria. The appropriate selection of peptidic moieties that could be attached to (parts of) nisin could lead to a significant increase in its inhibitory activity against Gram-negative bacteria. Using this strategy, hybrids that outperformed nisin by displaying 4- to 12-fold higher levels of activity against relevant Gram-negative bacterial species were produced. This study shows the power of modified peptide engineering to alter target specificity in a desired direction.

PBP2b plays a key role in both peripheral growth and septum positioning in Lactococcus lactis

Lactococcus lactis is an ovoid bacterium that forms filaments during planktonic and biofilm lifestyles by uncoupling cell division from cell elongation. In this work, we investigate the role of the leading peptidoglycan synthase PBP2b that is dedicated to cell elongation in ovococci. We show that the localization of a fluorescent derivative of PBP2b remains associated to the septal region and superimposed with structural changes of FtsZ during both vegetative growth and filamentation indicating that PBP2b remains intimately associated to the division machinery during the whole cell cycle. In addition, we show that PBP2b-negative cells of L. lactis are not only defective in peripheral growth; they are also affected in septum positioning. This septation defect does not simply result from the absence of the protein in the cell growth machinery since it is also observed when PBP2b-deficient cells are complemented by a catalytically inactive variant of PBP2b. Finally, we show that round cells resulting from β-lactam treatment are not altered in septation, suggesting that shape elongation as such is not a major determinant for selection of the division site. Altogether, we propose that the specific PBP2b transpeptidase activity at the septum plays an important role for tagging future division sites during L. lactis cell cycle.

Specificity and Application of the Lantibiotic Protease NisP

Lantibiotics are ribosomally produced and posttranslationally modified peptides containing several lanthionine residues. They exhibit substantial antimicrobial activity against Gram-positive bacteria, including relevant pathogens. The production of the model lantibiotic nisin minimally requires the expression of the modification and export machinery. The last step during nisin maturation is the cleavage of the leader peptide. This liberates the active compound and is catalyzed by the cell wall-anchored protease NisP. Here, we report the production and purification of a soluble variant of NisP. This has enabled us to study its specificity and test its suitability for biotechnological applications. The ability of soluble NisP to cleave leaders from various substrates was tested with two sets of nisin variants. The first set was designed to investigate the influence of amino acid variations in the leader peptide or variations around the cleavage site. The second set was designed to study the influence of the lanthionine ring topology on the proteolytic efficiency. We show that the substrate promiscuity is higher than has previously been suggested. Our results demonstrate the importance of the arginine residue at the end of the leader peptide and the importance of lanthionine rings in the substrate for specific cleavage. Collectively, these data indicate that NisP is a suitable protease for the activation of diverse heterologously expressed lantibiotics, which is required to release active antimicrobial compounds.

Semi-microbiological synthesis of an active lysinoalanine-bridged analog of glucagon-like-peptide-1

Some modified glucagon-like-peptide-1 (GLP-1) analogs are highly important for treating type 2 diabetes. Here we investigated whether GLP-1 analogs expressed in Lactococcus lactis could be substrates for modification and export by the nisin dehydratase and transporter enzyme. Subsequently we introduced a lysinoalanine by coupling a formed dehydroalanine with a lysine and investigated the structure and activity of the formed lysinoalanine-bridged GLP-1 analog. Our data show: (i) GLP-1 fused to the nisin leader peptide is very well exported via the nisin transporter NisT, (ii) production of leader-GLP-1 via NisT is higher than via the SEC system, (iii) leader-GLP-1 exported via NisT was more efficiently dehydrated by the nisin dehydratase NisB than when exported via the SEC system, (iv) individual serines and threonines in GLP-1 are dehydrated by NisB to a significantly different extent, (v) an introduced Ser30 is well dehydrated and can be coupled to Lys34 to form a lysinoalanine-bridged GLP-1 analog, (vi) a lysinoalanine(30-34) variant's conformation shifts in the presence of 25% trifluoroethanol towards a higher alpha helix content than observed for wild type GLP-1 under identical condition, (vii) a lysinoalanine(30-34) GLP-1 variant has retained significant activity. Taken together the data extend knowledge on the substrate specificities of NisT and NisB and their combined activity relative to export via the Sec system, and demonstrate that introducing a lysinoalanine bridge is an option for modifying therapeutic peptides.

Plasmid Complement of Lactococcus lactis NCDO712 Reveals a Novel Pilus Gene Cluster

Lactococcus lactis MG1363 is an important gram-positive model organism. It is a plasmid-free and phage-cured derivative of strain NCDO712. Plasmid-cured strains facilitate studies on molecular biological aspects, but many properties which make L. lactis an important organism in the dairy industry are plasmid encoded. We sequenced the total DNA of strain NCDO712 and, contrary to earlier reports, revealed that the strain carries 6 rather than 5 plasmids. A new 50-kb plasmid, designated pNZ712, encodes functional nisin immunity (nisCIP) and copper resistance (lcoRSABC). The copper resistance could be used as a marker for the conjugation of pNZ712 to L. lactis MG1614. A genome comparison with the plasmid cured daughter strain MG1363 showed that the number of single nucleotide polymorphisms that accumulated in the laboratory since the strains diverted more than 30 years ago is limited to 11 of which only 5 lead to amino acid changes. The 16-kb plasmid pSH74 was found to contain a novel 8-kb pilus gene cluster spaCB-spaA-srtC1-srtC2, which is predicted to encode a pilin tip protein SpaC, a pilus basal subunit SpaB, and a pilus backbone protein SpaA. The sortases SrtC1/SrtC2 are most likely involved in pilus polymerization while the chromosomally encoded SrtA could act to anchor the pilus to peptidoglycan in the cell wall. Overexpression of the pilus gene cluster from a multi-copy plasmid in L. lactis MG1363 resulted in cell chaining, aggregation, rapid sedimentation and increased conjugation efficiency of the cells. Electron microscopy showed that the over-expression of the pilus gene cluster leads to appendices on the cell surfaces. A deletion of the gene encoding the putative basal protein spaB, by truncating spaCB, led to more pilus-like structures on the cell surface, but cell aggregation and cell chaining were no longer observed. This is consistent with the prediction that spaB is involved in the anchoring of the pili to the cell.

From by-product to valuable components: Efficient enzymatic conversion of lactose in whey using β-galactosidase from Streptococcus thermophilus

β-Galactosidase from Streptococcus thermophilus was overexpressed in a food-grade organism, Lactobacillus plantarum WCFS1. Laboratory cultivations yielded 11,000 U of β-galactosidase activity per liter of culture corresponding to approximately 170 mg of enzyme. Crude cell-free enzyme extracts obtained by cell disruption and subsequent removal of cell debris showed high stability and were used for conversion of lactose in whey permeate. The enzyme showed high transgalactosylation activity. When using an initial concentration of whey permeate corresponding to 205 g L^-1 lactose, the maximum yield of galacto-oligosaccharides (GOS) obtained at 50°C reached approximately 50% of total sugar at 90% lactose conversion, meaning that efficient valorization of the whey lactose was obtained. GOS are of great interest for both human and animal nutrition; thus, efficient conversion of lactose in whey into GOS using an enzymatic approach will not only decrease the environmental impact of whey disposal, but also create additional value.

Mutagenesis of nisin's leader peptide proline strongly modulates export of precursor nisin

The lantibiotic nisin is produced by Lactococcus lactis as a precursor peptide comprising a 23 amino acid leader peptide and a 34 amino acid post-translationally modifiable core peptide. We previously demonstrated that the conserved FNLD part of the leader is essential for intracellular enzyme-catalyzed introduction of lanthionines in the core peptide and also for transporter-mediated export, whereas other positions are subject to large mutational freedom. We here demonstrate that, in the absence of the extracellular leader peptidase, NisP, export of precursor nisin via the modification and transporter enzymes, NisBTC, is strongly affected by multiple substitutions of the leader residue at position -2, but not by substitution of positions in the vicinity of this site. Export levels of precursor nisin increased by more than 70% for position -2 mutants Asp, Thr, Ser, Trp, Lys, Val and decreased more than 70% for Cys, His, Met. In a strain with leader peptidase, the Pro-2Lys and Pro-2Asp precursor nisins were less efficiently cleaved by NisP than wild type precursor nisin. Taken together, the wild type precursor nisin with a proline at position -2 allows balanced export and cleavage efficiencies by precursor nisin's transporter and leader peptidase.

Discovery, Production and Modification of Five Novel Lantibiotics Using the Promiscuous Nisin Modification Machinery

To find the right conditions to isolate natively expressed antimicrobial peptides from a wide range of different microorganisms can be a challenge. Here, we exploited a heterologous expression system to produce and characterize several novel lantibiotics. We identified 54 novel putative class I and class II lantibiotics after inspecting all publicly available prokaryotic genomes using the in-house developed mining tool BAGEL3. The genes encoding these new lantibiotics fused to the nisin leader peptide gene sequence were synthesized, and the constructs were plugged into the nisin expression and modification system. Using this approach 30 peptides could be expressed, 27 of which were dehydrated by NisBC on at least 1 predicted position. Good antimicrobial activity against several pathogenic bacteria could be demonstrated for 5 novel heterologously modified lantibiotics. Lantibiotics from Corynebacterium lipophiloflavum DSM 44291 and Streptococcus agalactiae ATCC 13813, named flavucin and agalacticin, respectively, were fully modified and displayed high antimicrobial activity. The efficiency of functional expression was significantly enhanced when we made use of the native nisin leader cleavage site, instead of an artificial factor Xa site. Thus, we describe an efficient way for heterologous production of active lantibiotics, facilitating a rapid identification of promising molecules.

Display of a β-mannanase and a chitosanase on the cell surface of Lactobacillus plantarum towards the development of whole-cell biocatalysts

BACKGROUND

Lactobacillus plantarum is considered as a potential cell factory because of its GRAS (generally recognized as safe) status and long history of use in food applications. Its possible applications include in situ delivery of proteins to a host, based on its ability to persist at mucosal surfaces of the human intestine, and the production of food-related enzymes. By displaying different enzymes on the surface of L. plantarum cells these could be used as whole-cell biocatalysts for the production of oligosaccharides. In this present study, we aimed to express and display a mannanase and a chitosanase on the cell surface of L. plantarum.

RESULTS

ManB, a mannanase from Bacillus licheniformis DSM13, and CsnA, a chitosanase from Bacillus subtilis ATCC 23857 were fused to different anchoring motifs of L. plantarum for covalent attachment to the cell surface, either via an N-terminal lipoprotein anchor (Lp_1261) or a C-terminal cell wall anchor (Lp_2578), and the resulting fusion proteins were expressed in L. plantarum WCFS1. The localization of the recombinant proteins on the bacterial cell surface was confirmed by flow cytometry and immunofluorescence microscopy. The highest mannanase and chitosanase activities obtained for displaying L. plantarum cells were 890 U and 1360 U g dry cell weight, respectively. In reactions with chitosan and galactomannans, L. plantarum CsnA- and ManB-displaying cells produced chito- and manno-oligosaccharides, respectively, as analyzed by high performance anion exchange chromatography (HPAEC) and mass spectrometry (MS). Surface-displayed ManB is able to break down galactomannan (LBG) into smaller manno-oligosaccharides, which can support growth of L. plantarum.

CONCLUSION

This study shows that mannanolytic and chitinolytic enzymes can be anchored to the cell surface of L. plantarum in active forms. L. plantarum chitosanase- and mannanase-displaying cells should be of interest for the production of potentially 'prebiotic' oligosaccharides. This approach, with the enzyme of interest being displayed on the cell surface of a food-grade organism, may also be applied in production processes relevant for food industry.

Development of Chemically Defined Media to Express Trp-Analog-Labeled Proteins in a Lactococcus lactis Trp Auxotroph

Chemically defined media for growth of Lactococcus lactis strains contain about 50 components, making them laborious and expensive growth media. However, they are crucial for metabolism studies as well as for expression of heterologous proteins labeled with unnatural amino acids. In particular, the L. lactis Trp auxotroph PA1002, overexpressing the tryptophanyl tRNA synthetase enzyme of L. lactis, is very suitable for the biosynthetic incorporation of Trp analogs in proteins because of its most relaxed substrate specificity reported towards Trp analogs. Here we present two much simpler defined media for L. lactis, which consist of only 24 or 31 components, respectively, and with which the L. lactis Trp auxotroph shows similar growth characteristics as with a 50-component chemically defined medium. Importantly, the expression levels of two recombinant proteins used for evaluation were up to 2-3 times higher in these new media than in the 50-component medium, without affecting the Trp analog incorporation efficiency. Taken together, the simplest chemically defined media reported so far for L. lactis are presented. Since L. lactis also shows auxotrophy for Arg, His, Ile, Leu Val, and Met, our simplified media may also be useful for the biosynthetic incorporation of analogs of these five amino acids.

Incorporation of tryptophan analogues into the lantibiotic nisin

Lantibiotics are posttranslationally modified peptides with efficient inhibitory activity against various Gram-positive bacteria. In addition to the original modifications, incorporation of non-canonical amino acids can render new properties and functions to lantibiotics. Nisin is the most studied lantibiotic and contains no tryptophan residues. In this study, a system was constructed to incorporate tryptophan analogues into nisin, which included the modification machinery (NisBTC) and the overexpression of tryptophanyl-tRNA synthetase (TrpRS). Tryptophan and three different tryptophan analogues (5-fluoroTrp (5FW), 5-hydroxyTrp (5HW) and 5-methylTrp (5MeW)) were successfully incorporated at four different positions of nisin (I1W, I4W, M17W and V32W). The incorporation efficiency of tryptophan analogues into mutants I1W, M17W and V32W was over 97 %, while the mutant I4W showed relatively low incorporation efficiency (69–93 %). The variants with 5FW showed relatively higher production yield, while 5MeW-containing variants showed the lowest yield. The dehydration efficiency of serines or threonines was affected by the tryptophan mutants of I4W and V32W. The affinity of the peptides for the cation-ion exchange and reverse phase chromatography columns was significantly reduced when 5HW was incorporated. The antimicrobial activity of IIW and its 5FW analogue both decreased two times compared to that of nisin, while that of its 5HW analogue decreased four times. The 5FW analogue of I4W also showed two times decreased activity than nisin. However, the mutant M17W and its 5HW analogue both showed 32 times reduced activity relative to that of nisin.

Potentiating the Activity of Nisin against Escherichia coli

Lantibiotics are antimicrobial (methyl)lanthionine-containing peptides produced by various Gram-positive bacteria. The model lantibiotic, nisin, binds lipid II in the cell membrane. Additionally, after binding it can insert into the membrane creating a pore. Nisin can efficiently inhibit the growth of Gram-positive bacteria and resistance is rarely observed. However, the activity of lantibiotics is at least 100-fold lower against certain Gram-negative bacteria. This is caused by the fact that Gram-negative bacteria have an outer membrane that hinders the peptides to reach lipid II, which is located in the inner membrane. Improving the activity of lantibiotics against Gram-negative bacteria could be achieved if the outer membrane traversing efficiency is increased. Here, several anti-Gram-negative peptides (e.g., apidaecin 1b, oncocin), or parts thereof, were fused to the C-terminus of either a truncated version of nisin containing the first three/five rings or full length nisin. The activities of these fusion peptides were tested against Gram-negative pathogens. Our results showed that when an eight amino acids (PRPPHPRL) tail from apidaecin 1b was attached to nisin, the activity of nisin against Escherichia coli CECT101 was increased more than two times. This research presents a new and promising method to increase the anti-Gram-negative activity of lantibiotics.

Implementation of the agmatine-controlled expression system for inducible gene expression in Lactococcus lactis

BACKGROUND

Lactococcus lactis has been safely consumed in fermented foods for millennia. This Gram-positive bacterium has now become of industrial importance as an expression host for the overproduction of lipopolysaccharide-free recombinant proteins used as food ingredients, therapeutic proteins and biotechnological enzymes.

RESULTS

This paper reports an agmatine-controlled expression (ACE) system for L. lactis, comprising the lactococcal agmatine-sensor/transcriptional activator AguR and its target promoter P(aguB). The usefulness and efficiency of this system was checked via the reporter gene gfp and by producing PEP (Myxococcus xanthus prolyl-endopeptidase), an enzyme of biomedical interest able to degrade the immunotoxic peptides produced during the gastrointestinal breakdown of gluten.

CONCLUSION

The ACE system developed in this work was suitable for the efficient expression of the functional recombinant proteins GFP and PEP. The expression system was tightly regulated by the agmatine concentration and allowed high protein production without leakiness.

Versatile vector suite for the extracytoplasmic production and purification of heterologous His-tagged proteins in Lactococcus lactis

Recent studies have shown that the Gram-positive bacterium Lactococcus lactis can be exploited for the expression of heterologous proteins; however, a versatile set of vectors suitable for inducible extracellular protein production and subsequent purification of the expressed proteins by immobilized metal affinity chromatography was so far lacking. Here we describe three novel vectors that, respectively, facilitate the nisin-inducible production of N- or C-terminally hexa-histidine (His6)-tagged proteins in L. lactis. One of these vectors also encodes a tobacco etch virus (TEV) protease cleavage site allowing removal of the N-terminal His6-tag from expressed proteins. Successful application of the developed vectors for protein expression, purification and/or functional studies is exemplified with six different cell wall-bound or secreted proteins from Staphylococcus aureus. The results show that secretory production of S. aureus proteins is affected by the position, N- or C-terminal, of the His6-tag. This seems to be due to an influence of the His6-tag on protein stability. Intriguingly, the S. aureus IsdB protein, which is phosphorylated in S. aureus, was also found to be phosphorylated when heterologously produced in L. lactis, albeit not on the same Tyr residue. This implies that this particular post-translational protein modification is to some extent conserved in S. aureus and L. lactis. Altogether, we are confident that the present vector set combined with the L. lactis expression host has the potential to become a very useful tool in optimization of the expression, purification and functional analysis of extracytoplasmic bacterial proteins.

AguR, a Transmembrane Transcription Activator of the Putrescine Biosynthesis Operon in Lactococcus lactis, Acts in Response to the Agmatine Concentration

Dairy industry fermentative processes mostly use Lactococcus lactis as a starter. However, some dairy L. lactis strains produce putrescine, a biogenic amine that raises food safety and spoilage concerns, via the agmatine deiminase (AGDI) pathway. The enzymatic activities responsible for putrescine biosynthesis in this bacterium are encoded by the AGDI gene cluster. The role of the catabolic genes aguB, aguD, aguA, and aguC has been studied, but knowledge regarding the role of aguR (the first gene in the cluster) remains limited. In the present work, aguR was found to be a very low level constitutively expressed gene that is essential for putrescine biosynthesis and is transcribed independently of the polycistronic mRNA encoding the catabolic genes (aguBDAC). In response to agmatine, AguR acts as a transcriptional activator of the aguB promoter (PaguB), which drives the transcription of the aguBDAC operon. Inverted sequences required for PaguB activity were identified by deletion analysis. Further work indicated that AguR is a transmembrane protein which might function as a one-component signal transduction system that senses the agmatine concentration of the medium and, accordingly, regulates the transcription of the aguBDAC operon through a C-terminal cytoplasmic DNA-binding domain typically found in LuxR-like proteins.

Physiological and Transcriptional Responses of Different Industrial Microbes at Near-Zero Specific Growth Rates

The current knowledge of the physiology and gene expression of industrially relevant microorganisms is largely based on laboratory studies under conditions of rapid growth and high metabolic activity. However, in natural ecosystems and industrial processes, microbes frequently encounter severe calorie restriction. As a consequence, microbial growth rates in such settings can be extremely slow and even approach zero. Furthermore, uncoupling microbial growth from product formation, while cellular integrity and activity are maintained, offers perspectives that are economically highly interesting. Retentostat cultures have been employed to investigate microbial physiology at (near-)zero growth rates. This minireview compares information from recent physiological and gene expression studies on retentostat cultures of the industrially relevant microorganisms Lactobacillus plantarum, Lactococcus lactis, Bacillus subtilis, Saccharomyces cerevisiae, and Aspergillus niger. Shared responses of these organisms to (near-)zero growth rates include increased stress tolerance and a downregulation of genes involved in protein synthesis. Other adaptations, such as changes in morphology and (secondary) metabolite production, were species specific. This comparison underlines the industrial and scientific significance of further research on microbial (near-)zero growth physiology.

Lantibiotic Reductase LtnJ Substrate Selectivity Assessed with a Collection of Nisin Derivatives as Substrates

Lantibiotics are potent antimicrobial peptides characterized by the presence of dehydrated amino acids, dehydroalanine and dehydrobutyrine, and (methyl)lanthionine rings. In addition to these posttranslational modifications, some lantibiotics exhibit additional modifications that usually confer increased biological activity or stability on the peptide. LtnJ is a reductase responsible for the introduction of D-alanine in the lantibiotic lacticin 3147. The conversion of L-serine into D-alanine requires dehydroalanine as the substrate, which is produced in vivo by the dehydration of serine by a lantibiotic dehydratase, i.e., LanB or LanM. In this work, we probe the substrate specificity of LtnJ using a system that combines the nisin modification machinery (dehydratase, cyclase, and transporter) and the stereospecific reductase LtnJ in Lactococcus lactis. We also describe an improvement in the production yield of this system by inserting a putative attenuator from the nisin biosynthesis gene cluster in front of the ltnJ gene. In order to clarify the sequence selectivity of LtnJ, peptides composed of truncated nisin and different mutated C-terminal tails were designed and coexpressed with LtnJ and the nisin biosynthetic machinery. In these tails, serine was flanked by diverse amino acids to determine the influence of the surrounding residues in the reaction. LtnJ successfully hydrogenated peptides when hydrophobic residues (Leu, Ile, Phe, and Ala) were flanking the intermediate dehydroalanine, while those in which dehydroalanine was flanked by one or two polar residues (Ser, Thr, Glu, Lys, and Asn) or Gly were either less prone to be modified by LtnJ or not modified at all. Moreover, our results showed that dehydrobutyrine cannot serve as a substrate for LtnJ.

Heterologous expression of a recombinant lactobacillal β-galactosidase in Lactobacillus plantarum: effect of different parameters on the sakacin P-based expression system

Background

Two overlapping genes lacL and lacM (lacLM) encoding for heterodimeric β-galactosidase from Lactobacillus reuteri were previously cloned and over-expressed in the food-grade host strain Lactobacillus plantarum WCFS1, using the inducible lactobacillal pSIP expression system. In this study, we analyzed different factors that affect the production of recombinant L. reuteri β-galactosidase.

Results

Various factors related to the cultivation, i.e. culture pH, growth temperature, glucose concentration, as well as the induction conditions, including cell concentration at induction point and inducer concentration, were tested. Under optimal fermentation conditions, the maximum β-galactosidase levels obtained were 130 U/mg protein and 35–40 U/ml of fermentation broth corresponding to the formation of approximately 200 mg of recombinant protein per litre of fermentation medium. As calculated from the specific activity of the purified enzyme (190 U/mg), β-galactosidase yield amounted to roughly 70% of the total soluble intracellular protein of the host organism. It was observed that pH and substrate (glucose) concentration are the most prominent factors affecting the production of recombinant β-galactosidase.

Conclusions

The over-expression of recombinant L. reuteri β-galactosidase in a food-grade host strain was optimized, which is of interest for applications of this enzyme in the food industry. The results provide more detailed insight into these lactobacillal expression systems and confirm the potential of the pSIP system for efficient, tightly controlled expression of enzymes and proteins in lactobacilli.

Constructing a recombinant hyaluronic acid biosynthesis operon and producing food-grade hyaluronic acid in Lactococcus lactis

Hyaluronic acid (HA), a natural high molecular weight polysaccharide, is produced by Streptococcus zooepidemicus. However, Streptococcus has several drawbacks including its potential to produce exotoxins, so there is demand for an alternative HA source. Here, a recombinant HA biosynthesis operon, as well as the HA biosynthesis operon of S. zooepidemicus were introduced into L. lactis using the nisin-controlled expression system, respectively. HA was successfully synthesized by recombinant L. lactis. Furthermore, overexpression of the endogenous enzymes directing the synthesis of precursor sugars was effective at increasing HA production, and increasing the supply of UDP-activated monosaccharide donors aided synthesis of monodisperse HA polysaccharides. Besides GRAS host strain (L. lactis) and NICE system, the selecting marker (lacF gene) of the recombinant strain is also food grade. Therefore, HA produced by recombinant L. lactis overcomes the problems associated with Streptococcus and provides a source of food-grading HA appropriate for widespread biotechnological applications.

The length of a lantibiotic hinge region has profound influence on antimicrobial activity and host specificity

Lantibiotics are ribosomally synthesized (methyl)lanthionine containing peptides which can efficiently inhibit the growth of Gram-positive bacteria. As lantibiotics kill bacteria efficiently and resistance to them is difficult to be obtained, they have the potential to be used in many applications, e.g., in pharmaceutical industry or food industry. Nisin can inhibit the growth of Gram-positive bacteria by binding to lipid II and by making pores in their membrane. The C-terminal part of nisin is known to play an important role during translocation over the membrane and forming pore complexes. However, as the thickness of bacterial membranes varies between different species and environmental conditions, this property could have an influence on the pore forming activity of nisin. To investigate this, the so-called “hinge region” of nisin (residues NMK) was engineered to vary from one to six amino acid residues and specific activity against different indicators was compared. Antimicrobial activity in liquid culture assays showed that wild type nisin is most active, while truncation of the hinge region dramatically reduced the activity of the peptide. However, one or two amino acids extensions showed only slightly reduced activity against most indicator strains. Notably, some variants (+2, +1, −1, −2) exhibited higher antimicrobial activity than nisin in agar well diffusion assays against Lactococcus lactis MG1363, Listeria monocytogenes, Enterococcus faecalis VE14089, Bacillus sporothermodurans IC4 and Bacillus cereus 4153 at certain temperatures.

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.

Secretion of biologically active human interleukin 22 (IL-22) by Lactococcus lactis

Interleukin-22 (IL-22) participates in the modulation of innate immunity and inflammation. This cytokine has important therapeutic potential, such as with ulcerative colitis, liver and lung injury, and infection, in different animal models. We generated a Lactococcus lactis strain that secretes human IL-22 under the regulation of the nisin-inducible promoter. Identification and secretion of this cytokine was demonstrated using western blots of culture supernatants from IL-22-expressing bacteria. The recombinant IL-22 protein produced by L. lactis was biologically active as determined by its ability to induce IL-10 secretion when co-cultured with a colon epithelial cell line in vitro. We consider this novel strain a promising live vaccine for various therapeutic applications.

Tuning constitutive recombinant gene expression in Lactobacillus plantarum

Background

Lactobacillus plantarum constitutes a well-recognized food-grade system for the expression of recombinant proteins in the field of industrial and medical biotechnology. For applications in vivo or in biotechnological processes, the level of expression of e.g. antigens or enzymes is often critical, as expression levels should be of a certain effectiveness, yet, without putting too much strain to the overall system. The key factors that control gene expression are promoter strength, gene copy number and translation efficiency. In order to estimate the impact of these adjusting screws in L. plantarum CD033, we have tested several constitutive promoters in combination with high and low copy number plasmid backbones and varying space between the Shine-Dalgarno sequence and the start-codon.

Results

By combining strong promoters, such as transcription elongation factor promoters, isolated from L. plantarum CD033 and L. buchneri CD034, a synthetic promoter, originally derived from L. plantarum WCSF1 and a heterologous promoter derived from L. buchneri CD034 with a high and a low copy number origin of replication we demonstrated various expression levels of the model protein mCherry. All promoters were feasible for protein expression and in all cases, the high copy number origin of replication increased expression twofold. We found that the optimal spacer between the Shine-Dalgarno sequence and the start codon in L. plantarum consists of 8 nucleotides and elongation as well as shortening this sequence gradually down-regulates gene expression.

Conclusions

We have evaluated the effects of a set of gene regulatory tools to fine tune recombinant gene expression in L. plantarum CD033. We have thus, provided potential expression vectors useful for constitutive protein expression in lactic acid bacteria ranging from moderate to strong production levels.

SdrC induces staphylococcal biofilm formation through a homophilic interaction

The molecular pathogenesis of many Staphylococcus aureus infections involves growth of bacteria as biofilm. In addition to polysaccharide intercellular adhesin (PIA) and extracellular DNA, surface proteins appear to mediate the transition of bacteria from planktonic growth to sessile lifestyle as well as biofilm growth, and can enable these processes even in the absence of PIA expression. However, the molecular mechanisms by which surface proteins contribute to biofilm formation are incompletely understood. Here we demonstrate that self-association of the serine-aspartate repeat protein SdrC promotes both bacterial adherence to surfaces and biofilm formation. However, this homophilic interaction is not required for the attachment of bacteria to abiotic surfaces. We identified the subdomain that mediates SdrC dimerization and subsequent cell-cell interactions. In addition, we determined that two adjacently located amino acid sequences within this subdomain are required for the SdrC homophilic interaction. Comparative amino acid sequence analysis indicated that these binding sites are conserved. In summary, our study identifies SdrC as a novel molecular determinant in staphylococcal biofilm formation and describes the mechanism responsible for intercellular interactions. Furthermore, these findings contribute to a growing body of evidence suggesting that homophilic interactions between surface proteins present on neighbouring bacteria induce biofilm growth.

Development of Streptococcus pneumoniae Vaccines Using Live Vectors

Streptococcus pneumoniae still causes severe morbidity and mortality worldwide, especially in young children and the elderly. Much effort has been dedicated to developing protein-based universal vaccines to conquer the current shortcomings of capsular vaccines and capsular conjugate vaccines, such as serotype replacement, limited coverage and high costs. A recombinant live vector vaccine delivering protective antigens is a promising way to achieve this goal. In this review, we discuss the researches using live recombinant vaccines, mainly live attenuated Salmonella and lactic acid bacteria, to deliver pneumococcal antigens. We also discuss both the limitations and the future of these vaccines.

Metabolic engineering of Lactococcus lactis influence of the overproduction of lipase enzyme

The dairy industry uses lipase extensively for hydrolysis of milk fat. Lipase is used in the modification of the fatty acid chain length, to enhance the flavours of various chesses. Therefore finding the unlimited source of lipase is a concern of dairy industry. Due to the importance of lipase, this study was an attempt to express the lipase from Burkholderia cepacia in Lactococcus lactis. To achieve this, a gene associated with lipase transport was amplified and subcloned in inducible pNZ8148 vector, and subsequently transformed into Lc. lactis NZ9000. The enzyme assay as well as SDS-PAGE and western blotting were carried out to analysis the recombinant lipase expression. Nucleotide sequencing of the DNA insert from the clone revealed that the lipase activity corresponded to an open reading frame consisting of 1092 bp coding for a 37·5-kDa size protein. Blue colour colonies on nile blue sulphate agar and sharp band on 37·5-kD size on SDS-PAGE and western blotting results confirm the successful expression of lipase by Lc. lactis. The protein assay also showed high expression, approximately 152·2 μg/ml.h, of lipase by recombinant Lc. lactis. The results indicate that Lc. lactis has high potential to overproduce the recombinant lipase which can be used commercially for industrially purposes.

Designing and producing modified, new-to-nature peptides with antimicrobial activity by use of a combination of various lantibiotic modification enzymes

Lanthipeptides are peptides that contain several post-translationally modified amino acid residues and commonly show considerable antimicrobial activity. After translation, the amino acid residues of these peptides are modified by a distinct set of modification enzymes. This process results in peptides containing one or more lanthionine rings and dehydrated Ser and Thr residues. Previously, an in vivo lanthipeptide production system based on the modification machinery of the model lantibiotic nisin was reported. Here, we present the addition of the modification enzymes LtnJ and GdmD to this production system. With these enzymes we can now produce lanthipeptides that contain d-alanines or a C-terminal aminovinyl-cysteine. We show experimentally that the decarboxylase GdmD is responsible for the C-terminal decarboxylation. Our results demonstrate that different lanthipeptide modification enzymes can work together in an in vivo production system. This yields a plug-and-play system that can be used to select different sets of modification enzymes to work on diverse, specifically designed substrates.

O-glycosylation as a novel control mechanism of peptidoglycan hydrolase activity

Acm2, the major autolysin of Lactobacillus plantarum, is a tripartite protein. Its catalytic domain is surrounded by an O-glycosylated N-terminal region rich in Ala, Ser, and Thr (AST domain), which is of low complexity and unknown function, and a C-terminal region composed of five SH3b peptidoglycan (PG) binding domains. Here, we investigate the contribution of these two accessory domains and of O-glycosylation to Acm2 functionality. We demonstrate that Acm2 is an N-acetylglucosaminidase and identify the pattern of O-glycosylation (21 mono-N-acetylglucosamines) of its AST domain. The O-glycosylation process is species-specific as Acm2 purified from Lactococcus lactis is not glycosylated. We therefore explored the functional role of O-glycosylation by purifying different truncated versions of Acm2 that were either glycosylated or non-glycosylated. We show that SH3b domains are able to bind PG and are responsible for Acm2 targeting to the septum of dividing cells, whereas the AST domain and its O-glycosylation are not involved in this process. Notably, our data reveal that the lack of O-glycosylation of the AST domain significantly increases Acm2 enzymatic activity, whereas removal of SH3b PG binding domains dramatically reduces this activity. Based on this antagonistic role, we propose a model in which access of the Acm2 catalytic domain to its substrate may be hindered by the AST domain where O-glycosylation changes its conformation and/or mediates interdomain interactions. To the best of our knowledge, this is the first time that O-glycosylation is shown to control the activity of a bacterial enzyme.

Zirex: a novel zinc-regulated expression system for Lactococcus lactis

Here, we report a new zinc-inducible expression system for Lactococcus lactis, called Zirex, consisting of the pneumococcal repressor SczA and PczcD. PczcD tightly regulates the expression of green fluorescent protein in L. lactis. We show the applicability of Zirex together with the nisin-controlled expression system, enabling simultaneous but independent regulation of different genes.

Identification of key peptidoglycan hydrolases for morphogenesis, autolysis, and peptidoglycan composition of Lactobacillus plantarum WCFS1

BACKGROUND

Lactobacillus plantarum is commonly used in industrial fermentation processes. Selected strains are also marketed as probiotics for their health beneficial effects. Although the functional role of peptidoglycan-degrading enzymes is increasingly documented to be important for a range of bacterial processes and host-microbe interactions, little is known about their functional roles in lactobacilli. This knowledge holds important potential for developing more robust strains resistant to autolysis under stress conditions as well as peptidoglycan engineering for a better understanding of the contribution of released muramyl-peptides as probiotic immunomodulators.

RESULTS

Here, we explored the functional role of the predicted peptidoglycan hydrolase (PGH) complement encoded in the genome of L. plantarum by systematic gene deletion. From twelve predicted PGH-encoding genes, nine could be individually inactivated and their corresponding mutant strains were characterized regarding their cell morphology, growth, and autolysis under various conditions. From this analysis, we identified two PGHs, the predicted N-acetylglucosaminidase Acm2 and NplC/P60 D,L-endopeptidase LytA, as key determinants in the morphology of L. plantarum. Acm2 was demonstrated to be required for the ultimate step of cell separation of daughter cells, whereas LytA appeared to be required for cell shape maintenance and cell-wall integrity. We also showed by autolysis experiments that both PGHs are involved in the global autolytic process with a dominant role for Acm2 in all tested conditions, identifying Acm2 as the major autolysin of L. plantarum WCFS1. In addition, Acm2 and the putative N-acetylmuramidase Lys2 were shown to play redundant roles in both cell separation and autolysis under stress conditions. Finally, the analysis of the peptidoglycan composition of Acm2- and LytA-deficient derivatives revealed their potential hydrolytic activities by the disappearance of specific cleavage products.

CONCLUSION

In this study, we showed that two PGHs of L. plantarum have a predominant physiological role in a range of growth conditions. We demonstrate that the N-acetylglucosaminidase Acm2 is the major autolysin whereas the D,L-endopeptidase LytA is a key morphogenic determinant. In addition, both PGHs have a direct impact on PG structure by generating a higher diversity of cleavage products that could be of importance for interaction with the innate immune system.