Urease biogenesis in Streptococcus thermophilus

Determination of Technological Properties and CRISPR Profiles of Streptococcus thermophilus Isolates Obtained from Local Yogurt Samples

The aim of this study was to obtain data on Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) profiles of Streptococcus thermophilus (S. thermophilus) isolates resulting from acquired immune memory in addition to their technological starter properties for the selection of potential starter cultures from local yogurt samples. A total of 24 S. thermophilus isolates were collected from six local yogurt samples including Afyon/Dinar, Uşak, Konya/Karapınar, and Tokat provinces of Türkiye. Strain-specific CRISPR I-II-III and IV primers were used to determine the CRISPR profiles of the isolates. The isolates commonly had CRISPR II and IV profiles, while only one isolate had a CRISPR III profile. Polymerase chain reaction (PCR)-based and culture-based analyses were also carried out to obtain data on the technological properties of the isolates. The PCR analyses were performed for the prtS gene for protease activity, the ureC gene for urease enzyme, the gdh gene for glutamate dehydrogenase, the cox gene for competence frequency, the csp gene involved in heat-shock stress resistance of the isolates with specific primers. Culture-based analyses including antimicrobial activity and acid-production ability of the isolates were completed, and proteolytic and lipolytic properties were also screened. Native spacer sequences resulting from acquired immune memory were obtained for CRISPR IV profiles of yogurt samples from the Konya-Karapınar and Tokat provinces and CRISPR III profiles of yogurt samples from the Uşak province. In conclusion, our study results suggest that it is possible to select the isolates with the desired level of technological characteristics, prioritizing the ones with the most diverse CRISPR profiles and with native spacers for potential industrial application as starter cultures. We believe that this study provides data for further biological studies on the impact of centuries of human domestication on evolutionary adaptations and how these microorganisms manage survival and symbiosis.

Substitution of Asp29 with Asn29 in the metallochaperone UreE of Streptococcus thermophilus DSM 20617T increases the urease activity and anticipates urea hydrolysis during milk fermentation

Urease operon is highly conserved within the species Streptococcus thermophilus and urease-negative strains are rare in nature. S. thermophilus MIMO1, isolated from commercial yogurt, was previously characterized as urease-positive Ni-dependent strain. Beside a mutation in ureQ, coding for a nickel ABC transporter permease, the strain MIMO1 showed a mutation in ureE gene which code for a metallochaperone involved in Ni delivery to the urease catalytic site. The single base mutation in ureE determined a substitution of Asp29 with Asn29 in the metallochaperone in a conserved protein region not involved in the catalytic activity. With the aim to investigate the role Asp29vs Asn29 substitution in UreE on the urease activity of S. thermophilus, ureE gene of the reference strain DSM 20617T (ureEDSM20617) was replaced by ureE gene of strain MIMO1 (ureEMIMO1) to obtain the recombinant ES3. In-gel detection of urease activity revealed that the substitution of Asp29 with Asn29 in UreE resulted in a higher stability of the enzyme complexes. Moreover, the recombinant ES3 showed higher level of urease activity compared to the wildtype without any detectable increase in the expression level of ureC gene, thus highlighting the role of UreE not only in Ni assembly but also on the level of urease activity. During the growth in milk, the recombinant ES3 showed an anticipated urease activity compared to the wildtype, and analogous milk fermentation performance. The overall data obtained by comparing urease-positive and urease-negative strains/mutants confirmed that urease activity strongly impacts on the milk fermentation process and specifically on the yield of the homolactic fermentation.

Unveiling the Probiotic Potential of Streptococcus thermophilus MCC0200: Insights from In Vitro Studies Corroborated with Genome Analysis

Streptococcus thermophilus is widely used as a starter culture in the dairy industry and has garnered attention as a beneficial bacterium owing to its health-promoting functionalities in humans. In this study, the probiotic potential of S. thermophilus MCC0200 isolated from a dairy product was investigated through a combinatorial approach of in vitro and in silico studies. MCC0200 demonstrated the ability to survive harsh gastrointestinal (GI) transit, adhere to intestinal mucosa and exert health-promoting traits in in vitro studies. These findings were corroborated with in silico evidence, wherein, MCC0200 genome harboured genes associated with tolerance to GI conditions, intestinal adhesion and colonization. Genome mapping also highlighted the ability of MCC0200 to produce compounds advantageous for the host (folate, bacteriocins), to release antioxidant enzymes that can quench the free radicals (superoxide dismutase, NADH peroxidase), and to metabolize food components that can be harmful to sensitive people (lactose). MCC0200 also demonstrated a positive effect on reducing cholesterol levels, proving to be a potential candidate for food and pharmaceutical applications. The absence of transmissible antibiotic resistance genes and virulence genes underscored the generally regarded as safe (GRAS) nature of MCC0200. This study explored the potential of Streptococcus thermophilus for its probable applications as a probiotic beyond the dairy industry.

Genome-scale reconstruction of the metabolic network in Streptococcus thermophilus S-3 and assess urea metabolism

BACKGROUND

Streptococcus thermophilus is an important strain widely used in dairy fermentation, with distinct urea metabolism characteristics compared to other lactic acid bacteria. The conversion of urea by S. thermophilus has been shown to affect the flavor and acidification characteristics of milk. Additionally, urea metabolism has been found to significantly increase the number of cells and reduce cell damage under acidic pH conditions, resulting in higher activity. However, the physiological role of urea metabolism in S. thermophilus has not been fully evaluated. A deep understanding of this metabolic feature is of great significance for its production and application. Genome-scale metabolic network models (GEMs) are effective tools for investigating the metabolic network of organisms using computational biology methods. Constructing an organism-specific GEM can assist us in comprehending its characteristic metabolism at a systemic level.

RESULTS

In the present study, we reconstructed a high-quality GEM of S. thermophilus S-3 (iCH492), which contains 492 genes, 608 metabolites and 642 reactions. Growth phenotyping experiments were employed to validate the model both qualitatively and quantitatively, yielding satisfactory predictive accuracy (95.83%), sensitivity (93.33%) and specificity (100%). Subsequently, a systematic evaluation of urea metabolism in S. thermophilus was performed using iCH492. The results showed that urea metabolism reduces intracellular hydrogen ions and creates membrane potential by producing and transporting ammonium ions. This activation of glycolytic fluxes and ATP synthase produces more ATP for biomass synthesis. The regulation of fluxes of reactions involving NAD(P)H by urea metabolism improves redox balance.

CONCLUSION

Model iCH492 represents the most comprehensive knowledge-base of S. thermophilus to date, serving as a potent tool. The evaluation of urea metabolism led to novel insights regarding the role of urease. © 2023 Society of Chemical Industry.

The intricate symbiotic relationship between lactic acid bacterial starters in the milk fermentation ecosystem

Fermentation is one of the most effective methods of food preservation. Since ancient times, food has been fermented using lactic acid bacteria (LAB). Fermented milk is a very intricate fermentation ecosystem, and the microbial metabolism of fermented milk largely determines its metabolic properties. The two most frequently used dairy starter strains are Streptococcus thermophilus (S. thermophilus) and Lactobacillus delbrueckii subsp. bulgaricus (L. bulgaricus). To enhance both the culture growth rate and the flavor and quality of the fermented milk, it has long been customary to combine S. thermophilus and L. bulgaricus in milk fermentation due to their mutually beneficial and symbiotic relationship. On the one hand, the symbiotic relationship is reflected by the nutrient co-dependence of the two microbes at the metabolic level. On the other hand, more complex interaction mechanisms, such as quorum sensing between cells, are involved. This review summarizes the application of LAB in fermented dairy products and discusses the symbiotic mechanisms and interactions of milk LAB starter strains from the perspective of nutrient supply and intra- and interspecific quorum sensing. This review provides updated information and knowledge on microbial interactions in a fermented milk ecosystem.

In Streptococcus thermophilus, Ammonia from Urea Hydrolysis Paradoxically Boosts Acidification and Reveals a New Regulatory Mechanism of Glycolysis

Streptococcus thermophilus is widely used in the dairy industry for the manufacturing of fermented milk and cheeses and probiotic formulations. S. thermophilus evolved from closely phylogenetically related pathogenic streptococci through loss-of-function events counterbalanced by the acquisition of relevant traits, such as lactose and urea utilization for the adaptation to the milk environment. In the context of regressive evolution, the urease gene cluster accounts for 0.9% of the total coding sequence belonging to known functional categories. The fate of ammonia and carbon dioxide derived by urea hydrolysis in several biosynthetic pathways have been depicted, and the positive effect of urease activity on S. thermophilus growth fitness and lactic acid fermentation in milk has been already addressed by several authors. However, the mechanistic effect of urea hydrolysis on the energetic metabolisms of S. thermophilus is still unclear. This study aimed to assess the effect of urease activity on the growth and energy metabolism of Streptococcus thermophilus in milk. In milk, ¹³C-urea was completely hydrolyzed in the first 150 min of S. thermophilus growth, and urea hydrolysis was accompanied by an increase in cell density and a reduction in the generation time. By using energetically discharged cells with gene transcription and translation blocked, we showed that in the presence of fermentable carbon sources, urease activity, specifically the production of ammonia, could dramatically boost glycolysis and, in cascade, homolactic fermentation. Furthermore, we showed that ammonia, specifically ammonium ions, were potent effectors of phosphofructokinase, a key glycolytic enzyme.

IMPORTANCE Finding that ammonia-generating enzymes, such as urease, and exogenous ammonia act on phosphofructokinase activity shed new light on the regulatory mechanisms that govern glycolysis. Phosphofructokinase is the key enzyme known to exert a regulatory role on glycolytic flux and, therefore, ammonia as an effector of phosphofructokinase acts, in cascade, modulating the glycolytic pathway. Apart from S. thermophilus, due to the high conservation of glycolytic enzymes in all branches of the tree of life and being aware of the role of ammonia as an effector of phosphofructokinase, we propose to reevaluate the physiological role of the ammonia production pathways in all organisms whose energy metabolism is supported by glycolysis.

Genetic Engineering Production of Ethyl Carbamate Hydrolase and Its Application in Degrading Ethyl Carbamate in Chinese Liquor

Ethyl carbamate (EC), classified as a Group 2A carcinogen, is most abundant in the fermented foods, such as Cachaca, Shaoxing wine, and Chinese liquor (baijiu). Although biodegradation can reduce its concentration, a high ethanol concentration and acidic environment often limit its degradation. In the present study, a novel ethyl carbamate hydrolase (ECH) with high specificity to EC was isolated from Acinetobacter calcoaceticus, and its enzymatic properties and EC degradability were investigated. ECH was immobilized to resist extreme environmental conditions, and the flavor substance changes were explored by gas chromatography-mass spectrometry (GC/MS). The specific enzymatic activity of ECH was 68.31 U/mg. Notably, ECH exhibited excellent thermal stability and tolerance to sodium chloride and high ethanol concentration (remaining at 40% activity in 60% (v/v) ethanol, 1 h). The treatment of immobilized ECH for 12 h decreased the EC concentration in liquor by 71.6 μg/L. Furthermore, the immobilized ECH exerted less effect on its activity and on the flavor substances, which could be easily filtrated during industrial production.

Comparative Genomics of Streptococcus thermophilus Support Important Traits Concerning the Evolution, Biology and Technological Properties of the Species

Streptococcus thermophilus is a major starter for the dairy industry with great economic importance. In this study we analyzed 23 fully sequenced genomes of S. thermophilus to highlight novel aspects of the evolution, biology and technological properties of this species. Pan/core genome analysis revealed that the species has an important number of conserved genes and that the pan genome is probably going to be closed soon. According to whole genome phylogeny and average nucleotide identity (ANI) analysis, most S. thermophilus strains were grouped in two major clusters (i.e., clusters A and B). More specifically, cluster A includes strains with chromosomes above 1.83 Mbp, while cluster B includes chromosomes below this threshold. This observation suggests that strains belonging to the two clusters may be differentiated by gene gain or gene loss events. Furthermore, certain strains of cluster A could be further subdivided in subgroups, i.e., subgroup I (ASCC 1275, DGCC 7710, KLDS SM, MN-BM-A02, and ND07), II (MN-BM-A01 and MN-ZLW-002), III (LMD-9 and SMQ-301), and IV (APC151 and ND03). In cluster B certain strains formed one distinct subgroup, i.e., subgroup I (CNRZ1066, CS8, EPS, and S9). Clusters and subgroups observed for S. thermophilus indicate the existence of lineages within the species, an observation which was further supported to a variable degree by the distribution and/or the architecture of several genomic traits. These would include exopolysaccharide (EPS) gene clusters, Clustered Regularly Interspaced Short Palindromic Repeats (CRISPRs)-CRISPR associated (Cas) systems, as well as restriction-modification (R-M) systems and genomic islands (GIs). Of note, the histidine biosynthetic cluster was found present in all cluster A strains (plus strain NCTC12958^T) but was absent from all strains in cluster B. Other loci related to lactose/galactose catabolism and urea metabolism, aminopeptidases, the majority of amino acid and peptide transporters, as well as amino acid biosynthetic pathways were found to be conserved in all strains suggesting their central role for the species. Our study highlights the necessity of sequencing and analyzing more S. thermophilus complete genomes to further elucidate important aspects of strain diversity within this starter culture that may be related to its application in the dairy industry.

Short communication: Lactose utilization of Streptococcus thermophilus and correlations with β-galactosidase and urease

The ability to use lactose is critical for the application of Streptococcus thermophilus in fermented dairy products. Most studies have evaluated the use of lactose of S. thermophilus by measuring lactose utilization, but its correlation with β-galactosidase and urease has rarely been investigated. In this study, 10 strains of S. thermophilus isolated from fermented yak milk exhibited a diversity of β-galactosidase and urease activities, growth, and acid production in de Man, Rogosa, and Sharpe-lactose. Among the strains, 15G5 possessed the highest β-galactosidase activity and showed the highest cell growth, lactic acid production, and titratable acidity during fermentation. In contrast, 7G10, with the weakest β-galactosidase activity, produced the lowest lactic acid content and change in titratable acidity. Further investigation indicated that β-galactosidase activity of S. thermophilus showed significant positive correlations with the growth of cell densities, the production of lactic acid, and titratable acidity, and urease activity of S. thermophilus showed a significant correlation with the use of lactose and the production of lactic acid and acetaldehyde. These findings suggest that the differences of β-galactosidase and urease activities are essential for the performance in the lactose metabolism, growth, and acid production of S. thermophilus, providing new insights into strain selection and application.

Development of omics-based protocols for the microbiological characterization of multi-strain formulations marketed as probiotics: the case of VSL#3

The growing commercial interest in multi-strain formulations marketed as probiotics has not been accompanied by an equal increase in the evaluation of quality levels of these biotechnological products. The multi-strain product VSL#3 was used as a model to setup a microbiological characterization that could be extended to other formulations with high complexity. Shotgun metagenomics by deep Illumina sequencing was applied to DNA isolated from the commercial VSL#3 product to confirm strains identity safety and composition. Single-cell analysis was used to evaluate the cell viability, and β-galactosidase and urease activity have been used as marker to monitor the reproducibility of the production process. Similarly, these lots were characterized in detail by a metaproteomics approach for which a robust protein extraction protocol was combined with advanced mass spectrometry. The results identified over 1600 protein groups belonging to all strains present in the VSL#3 formulation. Of interest, only 3.2 % proteins showed significant differences mainly related to small variations in strain abundance. The protocols developed in this study addressed several quality criteria that are relevant for marketed multi-strain products and these represent the first efforts to define the quality of complex probiotic formulations such as VSL#3.

Role of Temperate Bacteriophage ϕ20617 on Streptococcus thermophilus DSM 20617T Autolysis and Biology

Streptococcus thermophilus DSM 20167^T showed autolytic behavior when cultured in lactose- and sucrose-limited conditions. The amount of cell lysis induced was inversely related to the energetic status of the cells, as demonstrated by exposing cells to membrane-uncoupling and glycolysis inhibitors. Genome sequence analysis of strain DSM 20617^T revealed the presence of a pac-type temperate bacteriophage, designated Φ20617, whose genomic organization and structure resemble those of temperate streptococcal bacteriophages. The prophage integrated at the 3'-end of the gene encoding the glycolytic enzyme enolase (eno), between eno and the lipoteichoic acid synthase-encoding gene ltaS, affecting their transcription. Comparative experiments conducted on the wild-type strain and a phage-cured derivative strain revealed that the cell-wall integrity of the lysogenic strain was compromised even in the absence of detectable cell lysis. More importantly, adhesion to solid surfaces and heat resistance were significantly higher in the lysogenic strain than in the phage-cured derivative. The characterization of the phenotype of a lysogenic S. thermophilus and its phage-cured derivative is relevant to understanding the ecological constraints that drive the stable association between a temperate phage and its bacterial host.

Extracellular urease from Arthrobacter creatinolyticus MTCC 5604: scale up, purification and its cytotoxic effect thereof

Urease is a potent metalloenzyme with diverse applications. This paper describes the scale up and purification of an extracellular urease from Arthrobacter creatinolyticus MTCC 5604. The urease production was scaled-up in 3.7 L and 20 L fermentor. A maximum activity of 27 and 27.8 U/mL and a productivity of 0.90 and 0.99 U/mL/h were obtained at 30 h and 28 h in 3.7 and 20 L fermentor, respectively. Urease was purified to homogeneity with 49.85-fold purification by gel filtration and anion exchange chromatography with a yield of 36% and a specific activity of 1044.37 U/mg protein. The enzyme showed three protein bands with molecular mass of 72.6, 11.2 and 6.1 kDa on SDS-PAGE and ~ 270 kDa on native PAGE. The cytotoxic effect of urease was assessed in vitro using cancer cell lines (A549 and MG-63) and normal cell line (HEK 293). Urease showed its inhibitory effects on cancer cell lines through the generation of toxic ammonia, which in turn increased the pH of the surrounding medium. This increase in extracellular pH, enhanced the cytotoxic effect of weak base chemotherapeutic drugs, doxorubicin (50 µM) and vinblastine (100 µM) in the presence of urease (5 U/mL) and urea (0-4 mM) significantly.

Stress Physiology of Lactic Acid Bacteria

Lactic acid bacteria (LAB) are important starter, commensal, or pathogenic microorganisms. The stress physiology of LAB has been studied in depth for over 2 decades, fueled mostly by the technological implications of LAB robustness in the food industry. Survival of probiotic LAB in the host and the potential relatedness of LAB virulence to their stress resilience have intensified interest in the field. Thus, a wealth of information concerning stress responses exists today for strains as diverse as starter (e.g., Lactococcus lactis), probiotic (e.g., several Lactobacillus spp.), and pathogenic (e.g., Enterococcus and Streptococcus spp.) LAB. Here we present the state of the art for LAB stress behavior. We describe the multitude of stresses that LAB are confronted with, and we present the experimental context used to study the stress responses of LAB, focusing on adaptation, habituation, and cross-protection as well as on self-induced multistress resistance in stationary phase, biofilms, and dormancy. We also consider stress responses at the population and single-cell levels. Subsequently, we concentrate on the stress defense mechanisms that have been reported to date, grouping them according to their direct participation in preserving cell energy, defending macromolecules, and protecting the cell envelope. Stress-induced responses of probiotic LAB and commensal/pathogenic LAB are highlighted separately due to the complexity of the peculiar multistress conditions to which these bacteria are subjected in their hosts. Induction of prophages under environmental stresses is then discussed. Finally, we present systems-based strategies to characterize the "stressome" of LAB and to engineer new food-related and probiotic LAB with improved stress tolerance.

Streptococcus thermophilus urease activity boosts Lactobacillus delbrueckii subsp. bulgaricus homolactic fermentation

The proto-cooperation between Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus in the yogurt consortium enhances the growth rate and size of each population. In contrast, the independent growth of the two species in milk leads to a slower growth rate and a smaller population size. In this study, we report the first evidence that the urease activity of S. thermophilus increases the intracellular pH of L. delbrueckii in the absence of carbon source. However, in milk, in the presence of lactose the alkalizing effect of urea-derived ammonia was not detectable. Nevertheless, based on glucose consumption and lactic acid production at different pH_in, L. delbrueckii showed an optimum of glycolysis and homolactic fermentation at alkaline pH values. In milk, we observed that ammonia provided by urea hydrolysis boosted lactic acid production in S. thermophilus and in L. delbrueckii when the species were grown alone or in combination. Therefore, we propose that urease activity acts as an altruistic cooperative trait, which is costly for urease-positive individuals but provides a local benefit because other individuals can take advantage of urease-dependent ammonia release.

Streptococcus thermophilus, an emerging and promising tool for heterologous expression: Advantages and future trends

Streptococcus thermophilus is the second most used bacterium in dairy industry. It is daily consumed by millions of people through the worldwide consumption of yogurts, cheeses and fermented milks. S. thermophilus presents many features that make it a good candidate for the production of heterologous proteins. First, its ability to be naturally transformable allows obtaining swiftly and easily recombinant strains using various genetic tools available. Second, its Generally Recognised As Safe status and its ability to produce beneficial molecules or to liberate bioactive peptides from milk proteins open up the way for the development of new functional foods to maintain health and well-being of consumers. Finally, its ability to survive the intestinal passage and to be metabolically active in gastrointestinal tract allows considering S. thermophilus as a potential tool for delivering various biological molecules to the gastrointestinal tract. The aim of this review is therefore to take stock of various genetic tools which can be employed in S. thermophilus to produce heterologous proteins and to highlight the advantages and future trends of use of this bacterium as a heterologous expression host.

High-level expression and characterization of recombinant acid urease for enzymatic degradation of urea in rice wine

Ethylcarbamate, a carcinogenic compound, is formed from urea and ethanol in rice wine, and enzymatic elimination of urea is always attractive. In the present work, we amplified the acid urease gene cluster ureABCEFGD from Lactobacillus reuteri CICC6124 and constructed robust Lactococcus lactis cell factories for the production of acid urease. The titer of the recombinant acid urease was increased from 1,550 to 11,560 U/L by optimization of the cultivation process. Meanwhile, the enzyme showed satisfied properties toward urea elimination in the rice wine model system. By incubating the enzyme (50 U/L) at 20 °C for 60 h, about 95.8% of urea in rice wine was removed. Interestingly, this acid urease also exhibited activity toward ethylcarbamate. The results demonstrated that this recombinant acid urease has great potential in the elimination of urea in rice wine.

Food environments select microorganisms based on selfish energetic behavior

Nutrient richness, and specifically the abundance of mono- and disaccharides that characterize several food matrixes, such as milk and grape juice, has allowed the speciation of lactic acid bacteria and yeasts with a high fermentation capacity instead of energetically favorable respiratory metabolism. In these environmental contexts, rapid sugar consumption and lactic acid or ethanol production, accumulation, and tolerance, together with the ability to propagate in the absence of oxygen, are several of the "winning" traits that have apparently evolved and become specialized to perfection in these fermenting microorganisms. Here, we summarize and discuss the evolutionary context that has driven energetic metabolism in food-associated microorganisms, using the dairy species Lactococcus lactis and Streptococcus thermophilus among prokaryotes and the bakers' yeast Saccharomyces cerevisiae among eukaryotes as model organisms.

Alkalizing reactions streamline cellular metabolism in acidogenic microorganisms

An understanding of the integrated relationships among the principal cellular functions that govern the bioenergetic reactions of an organism is necessary to determine how cells remain viable and optimise their fitness in the environment. Urease is a complex enzyme that catalyzes the hydrolysis of urea to ammonia and carbonic acid. While the induction of urease activity by several microorganisms has been predominantly considered a stress-response that is initiated to generate a nitrogen source in response to a low environmental pH, here we demonstrate a new role of urease in the optimisation of cellular bioenergetics. We show that urea hydrolysis increases the catabolic efficiency of Streptococcus thermophilus, a lactic acid bacterium that is widely used in the industrial manufacture of dairy products. By modulating the intracellular pH and thereby increasing the activity of β-galactosidase, glycolytic enzymes and lactate dehydrogenase, urease increases the overall change in enthalpy generated by the bioenergetic reactions. A cooperative altruistic behaviour of urease-positive microorganisms on the urease-negative microorganisms within the same environment was also observed. The physiological role of a single enzymatic activity demonstrates a novel and unexpected view of the non-transcriptional regulatory mechanisms that govern the bioenergetics of a bacterial cell, highlighting a new role for cytosol-alkalizing biochemical pathways in acidogenic microorganisms.

The relevance of carbon dioxide metabolism in Streptococcus thermophilus

Streptococcus thermophilus is a major component of dairy starter cultures used for the manufacture of yoghurt and cheese. In this study, the CO(2) metabolism of S. thermophilus DSM 20617(T), grown in either a N(2) atmosphere or an enriched CO(2) atmosphere, was analysed using both genetic and proteomic approaches. Growth experiments performed in a chemically defined medium revealed that CO(2) depletion resulted in bacterial arginine, aspartate and uracil auxotrophy. Moreover, CO(2) depletion governed a significant change in cell morphology, and a high reduction in biomass production. A comparative proteomic analysis revealed that cells of S. thermophilus showed a different degree of energy status depending on the CO(2) availability. In agreement with proteomic data, cells grown under N(2) showed a significantly higher milk acidification rate compared with those grown in an enriched CO(2) atmosphere. Experiments carried out on S. thermophilus wild-type and its derivative mutant, which was inactivated in the phosphoenolpyruvate carboxylase and carbamoyl-phosphate synthase activities responsible for fixing CO(2) to organic molecules, suggested that the anaplerotic reactions governed by these enzymes have a central role in bacterial metabolism. Our results reveal the capnophilic nature of this micro-organism, underlining the essential role of CO(2) in S. thermophilus physiology, and suggesting potential applications in dairy fermentation processes.

Aspartate biosynthesis is essential for the growth of Streptococcus thermophilus in milk, and aspartate availability modulates the level of urease activity

We investigated the carbon dioxide metabolism of Streptococcus thermophilus, evaluating the phenotype of a phosphoenolpyruvate carboxylase-negative mutant obtained by replacement of a functional ppc gene with a deleted and inactive version, Deltappc. The growth of the mutant was compared to that of the parent strain in a chemically defined medium and in milk, supplemented or not with L-aspartic acid, the final product of the metabolic pathway governed by phosphoenolpyruvate carboxylase. It was concluded that aspartate present in milk is not sufficient for the growth of S. thermophilus. As a consequence, phosphoenolpyruvate carboxylase activity was considered fundamental for the biosynthesis of L-aspartic acid in S. thermophilus metabolism. This enzymatic activity is therefore essential for growth of S. thermophilus in milk even if S. thermophilus was cultured in association with proteinase-positive Lactobacillus delbrueckii subsp. bulgaricus. It was furthermore observed that the supplementation of milk with aspartate significantly affected the level of urease activity. Further experiments, carried out with a p(ureI)-gusA recombinant strain, revealed that expression of the urease operon was sensitive to the aspartate concentration in milk and to the cell availability of glutamate, glutamine, and ammonium ions.

Urease production by Streptococcus thermophilus

In order to identify potential alternative sources of urease for the removal of urea from alcoholic beverages, 205 strains of lactic acid bacteria belonging to 27 different species were screened for urease production. Only Streptococcus thermophilus produced urease. Cell permeabilization with toluene allowed to increase activity significantly. Optimal pH for urease activity in whole and permeabilized cells and of cell free extracts differed slightly, but was in the range 6.0-7.0. Significant activity was retained at pH 3.0 and 8.0, and, for cell free extracts, at pH 4.0 in the presence of ethanol. Urease production was evaluated in fermentations with pH control (5.25-6.5) and without pH control. Very little urease was produced in absence of urea, which at 5g/l slowed growth significantly in fermentations without pH control, but prevented a decrease in pH below 5.1 and resulted in higher final biomass. Optimal pH for growth was between 6.0 and 6.5 but specific urease activity was higher for fermentations at low pH at the beginning of the exponential phase. However, a higher total urease activity was obtained at pH 6.0 and 6.5 because of higher biomass. Potential technological applications of urease production by S. thermophilus are discussed.