Galactokinase activity in Streptococcus thermophilus

Relationship between lactose utilization of lactic acid bacteria and browning of cheese during storage

To produce processed cheese turning hardly brown during transportation and storage at room temperature, natural cheese showing less discoloration should be used as a raw material. The purpose of this study was to clarify the relationship between the lactose utilization of lactic acid bacteria and the browning of cheese during storage. Three type-cultures (Lactobacillus plantarum and Streptococcus thermophilus) and five isolates from Japanese pickles (Lactobacillus spp.) were used. Cheese curds inoculated with these bacteria were prepared and stored. The L. plantarum-inoculated curds showed smaller ΔE-values after storage, an indicator for the browning, compared to the others. Accumulation of galactose was observed in the curd to which S. thermophilus was inoculated. The sample showed larger ΔE-value after storage. These results showed the lactose utilization of bacteria affected galactose concentration in cheese and its browning during storage. L. plantarum might be a good starter for preparing cheese turning hardly brown.

A Novel Millet-Based Probiotic Fermented Food for the Developing World

Probiotic yogurt, comprised of a Fiti sachet containing Lactobacillus rhamnosus GR-1 and Streptococcus thermophilus C106, has been used in the developing world, notably Africa, to alleviate malnutrition and disease. In sub-Saharan African countries, fermentation of cereals such as millet, is culturally significant. The aim of this study was to investigate the fermentation capability of millet when one gram of the Fiti sachet consortium was added. An increase of 1.8 and 1.4 log CFU/mL was observed for S. thermophilus C106 and L. rhamnosus GR-1 when grown in 8% millet in water. Single cultures of L. rhamnosus GR-1 showed the highest μ_max when grown in the presence of dextrose, galactose and fructose. Single cultures of S. thermophilus C106 showed the highest μ_max when grown in the presence of sucrose and lactose. All tested recipes reached viable counts of the probiotic bacteria, with counts greater than 10⁶ colony-forming units (CFU)/mL. Notably, a number of organic acids were quantified, in particular phytic acid, which was shown to decrease when fermentation time increased, thereby improving the bioavailability of specific micronutrients. Millet fermented in milk proved to be the most favorable, according to a sensory evaluation. In conclusion, this study has shown that sachets being provided to African communities to produce fermented milk, can also be used to produce fermented millet. This provides an option for when milk supplies are short, or if communities wish to utilize the nutrient-rich qualities of locally-grown millet.

Molecular and biochemical analysis of the galactose phenotype of dairy Streptococcus thermophilus strains reveals four different fermentation profiles

Lactose-limited fermentations of 49 dairy Streptococcus thermophilus strains revealed four distinct fermentation profiles with respect to galactose consumption after lactose depletion. All the strains excreted galactose into the medium during growth on lactose, except for strain IMDOST40, which also displayed extremely high galactokinase (GalK) activity. Among this strain collection eight galactose-positive phenotypes sensu stricto were found and their fermentation characteristics and Leloir enzyme activities were measured. As the gal promoter seems to play an important role in the galactose phenotype, the galR-galK intergenic region was sequenced for all strains yielding eight different nucleotide sequences (NS1 to NS8). The gal promoter played an important role in the Gal-positive phenotype but did not determine it exclusively. Although GalT and GalE activities were detected for all Gal-positive strains, GalK activity could only be detected for two out of eight Gal-positive strains. This finding suggests that the other six S. thermophilus strains metabolize galactose via an alternative route. For each type of fermentation profile obtained, a representative strain was chosen and four complete Leloir gene clusters were sequenced. It turned out that Gal-positive strains contained more amino acid differences within their gal genes than Gal-negative strains. Finally, the biodiversity regarding lactose-galactose utilization among the different S. thermophilus strains used in this study was shown by RAPD-PCR. Five Gal-positive strains that contain nucleotide sequence NS2 in their galR-galK intergenic region were closely related.

Characterization of a galactokinase-positive recombinant strain of Streptococcus thermophilus

The lactic acid bacterium Streptococcus thermophilus is widely used by the dairy industry for its ability to transform lactose, the primary sugar found in milk, into lactic acid. Unlike the phylogenetically related species Streptococcus salivarius, S. thermophilus is unable to metabolize and grow on galactose and thus releases substantial amounts of this hexose into the external medium during growth on lactose. This metabolic property may result from the inability of S. thermophilus to synthesize galactokinase, an enzyme of the Leloir pathway that phosphorylates intracellular galactose to generate galactose-1-phosphate. In this work, we report the complementation of Gal(-) strain S. thermophilus SMQ-301 with S. salivarius galK, the gene that codes for galactokinase, and the characterization of recombinant strain SMQ-301K01. The recombinant strain, which was obtained by transformation of strain SMQ-301 with pTRKL2TK, a plasmid bearing S. salivarius galK, grew on galactose with a generation time of 55 min, which was almost double the generation time on lactose. Data confirmed that (i) the ability of SMQ-301K01 to grow on galactose resulted from the expression of S. salivarius galK and (ii) transcription of the plasmid-borne galK gene did not require GalR, a transcriptional regulator of the gal and lac operons, and did not interfere with the transcription of these operons. Unexpectedly, recombinant strain SMQ-301K01 still expelled galactose during growth on lactose, but only when the amount of the disaccharide in the medium exceeded 0.05%. Thus, unlike S. salivarius, the ability to metabolize galactose was not sufficient for S. thermophilus to simultaneously metabolize the glucose and galactose moieties of lactose. Nevertheless, during growth in milk and under time-temperature conditions that simulated those used to produce mozzarella cheese, the recombinant Gal(+) strain grew and produced acid more rapidly than the Gal(-) wild-type strain.

Changes in galactose and lactic acid content of sweet whey during storage

Whey is often stored or transported for a period of time prior to processing. During this time period, galactose and lactic acid concentrations may accumulate, reducing the quality of spray-dried whey powders in regard to stickiness and agglomeration. This study surveyed industry samples of Cheddar and mozzarella cheese whey streams to determine how galactose and lactic acid concentrations changed with storage at appropriate (4 degrees C) and abuse (37.8 degrees C) temperatures. Samples stored at 4 degrees C did not exhibit significant increases in levels of lactic acid or galactose. Mozzarella whey accumulated the greatest amount of galactose and lactic acid with storage at 37.8 degrees C. Whey samples derived from cheese made from single strains of starter culture were also evaluated to determine each culture's contribution to galactose and lactic acid production. Starter cultures evaluated included Streptococcus salivarius ssp. thermophilus. Lactobacillus helveticus, Lactobacillus delbrueckii ssp. bulgaricus, Lactococcus lactis ssp. cremoris, and Lactococcus lactis ssp. lactis. Whey derived from L. helveticus accumulated a significantly greater amount of lactic acid upon storage at 37.8 degrees C as compared with the other cultures. Galactose accumulation was significantly decreased in whey from L. lactis ssp. lactis stored at 37.8 degrees C in comparison with the other cultures. Results from this study indicate that proper storage conditions (4 degrees C) for whey prevent accumulation of galactose and lactic acid while the extent of accumulation during storage at 37.8 degrees C varies depending on the culture(s) used in cheese production.

Biochemistry, genetics, and applications of exopolysaccharide production in Streptococcus thermophilus: a review

Many strains of Streptococcus thermophilus synthesize extracellular polysaccharides. These molecules may be produced as capsules that are tightly associated with the cell, or they may be liberated into the medium as a loose slime (i.e., "ropy" polysaccharide). Although the presence of exopolysaccharide does not confer any obvious advantage to growth or survival of S. thermophilus in milk, in situ production by this species or other dairy lactic acid bacteria typically imparts a desirable "ropy" or viscous texture to fermented milk products. Recent work has also shown that exopolysaccharide-producing S. thermophilus can enhance the functional properties of Mozzarella cheese, but they are not phage-proof. As our understanding of the genetics, physiology, and functionality of bacterial exopolysaccharides continues to improve, novel applications for polysaccharides and polysaccharide-producing cultures are likely to emerge inside and outside the dairy industry. This article provides an overview of biochemistry, genetics, and applications of exopolysaccharide production in S. thermophilus.

Galactose and lactose genes from the galactose-positive bacterium Streptococcus salivarius and the phylogenetically related galactose-negative bacterium Streptococcus thermophilus: organization, sequence, transcription, and activity of the gal gene products

Streptococcus salivarius is a lactose- and galactose-positive bacterium that is phylogenetically closely related to Streptococcus thermophilus, a bacterium that metabolizes lactose but not galactose. In this paper, we report a comparative characterization of the S. salivarius and S. thermophilus gal-lac gene clusters. The clusters have the same organization with the order galR (codes for a transcriptional regulator and is transcribed in the opposite direction), galK (galactokinase), galT (galactose-1-P uridylyltransferase), galE (UDP-glucose 4-epimerase), galM (galactose mutarotase), lacS (lactose transporter), and lacZ (beta-galactosidase). An analysis of the nucleotide sequence as well as Northern blotting and primer extension experiments revealed the presence of four promoters located upstream from galR, the gal operon, galM, and the lac operon of S. salivarius. Putative promoters with virtually identical nucleotide sequences were found at the same positions in the S. thermophilus gal-lac gene cluster. An additional putative internal promoter at the 3' end of galT was found in S. thermophilus but not in S. salivarius. The results clearly indicated that the gal-lac gene cluster was efficiently transcribed in both species. The Shine-Dalgarno sequences of galT and galE were identical in both species, whereas the ribosome binding site of S. thermophilus galK differed from that of S. salivarius by two nucleotides, suggesting that the S. thermophilus galK gene might be poorly translated. This was confirmed by measurements of enzyme activities.

Enhanced exopolysaccharide production by metabolic engineering of Streptococcus thermophilus

It is possible that the low levels of production of exopolysaccharides (EPSs) by lactic acid bacteria could be improved by altering the levels of enzymes in the central metabolism that influence the production of precursor nucleotide sugars. To test this hypothesis, we identified and cloned the galU gene, which codes for UDP glucose pyrophosphorylase (GalU) in Streptococcus thermophilus LY03. Homologous overexpression of the gene led to a 10-fold increase in GalU activity but did not have any effect on the EPS yield when lactose was the carbon source. However, when galU was overexpressed in combination with pgmA, which encodes phosphoglucomutase (PGM), the EPS yield increased from 0.17 to 0.31 g/mol of carbon from lactose. A galactose-fermenting LY03 mutant (Gal(+)) with increased activities of the Leloir enzymes was also found to have a higher EPS yield (0.24 g/mol of carbon) than the parent strain. The EPS yield was further improved to 0.27 g/mol of carbon by overexpressing galU in this strain. However, the highest EPS yield, 0.36 g/mol of carbon, was obtained when pgmA was knocked out in the Gal(+) strain. Measurements of the levels of intracellular metabolites in the cultures revealed that the Gal(+) strains had considerably higher glucose 1-phosphate levels than the other strains, and the strain lacking PGM activity had threefold-higher levels of glucose 1-phosphate than the other Gal(+) strains. These results show that it is possible to increase EPS production by altering the levels of enzymes in the central carbohydrate metabolism.

Biosynthesis, characterisation, and design of bacterial exopolysaccharides from lactic acid bacteria

Lactic acid bacteria (LAB) are characterised by their conversion of a large proportion of their carbon feed, fermentable sugars, to lactic acid. However, in addition to lactic acid production, the LAB are able to divert a small proportion of fermentable sugars towards the biosynthesis of exopolysaccharides (EPSs) that are independent of the cell surface and cell wall material. These microbial EPSs when suspended or dissolved in aqueous solution provide thickening and gelling properties, and, as such, there is great interest in using EPSs from food grade microorganisms (such as the LAB that are traditionally used for food fermentations) for use as thickening agents. The current review includes a brief summary of the recent literature describing features of the biosynthetic pathways leading to EPS production. Many aspects of EPS biosynthesis in LAB are still not fully understood and a number of inferences are made regarding the similarity of the pathway to those involved in the synthesis of other cell polysaccharides, e.g., cell wall components. The main body of the review will cover practical aspects concerned with the isolation and characterisation of EPS structures. In the last couple of years, a substantial number of structures have been published and a summary of the common elements of these structures is included as is a suggestion for a system for representing structures. A brief highlight of the attempts that are being made to design 'tailor'-made polysaccharides using genetic modification and control of metabolic flux is presented.

Requirement for phosphoglucomutase in exopolysaccharide biosynthesis in glucose- and lactose-utilizing Streptococcus thermophilus

To study the influence of phosphoglucomutase (PGM) activity on exopolysaccharide (EPS) synthesis in glucose- and lactose-growing Streptococcus thermophilus, a knockout PGM mutant and a strain with elevated PGM activity were constructed. The pgmA gene, encoding PGM in S. thermophilus LY03, was identified and cloned. The gene was functional in Escherichia coli and was shown to be expressed from its own promoter. The pgmA-deficient mutant was unable to grow on glucose, while the mutation did not affect growth on lactose. Overexpression of pgmA had no significant effect on EPS production in glucose-growing cells. Neither deletion nor overexpression of pgmA changed the growth or EPS production on lactose. Thus, the EPS precursors in lactose-utilizing S. thermophilus are most probably formed from the galactose moiety of lactose via the Leloir pathway, which circumvents the need for a functional PGM.

Activation of silent gal genes in the lac-gal regulon of Streptococcus thermophilus

Streptococcus thermophilus strain CNRZ 302 is unable to ferment galactose, neither that generated intracellularly by lactose hydrolysis nor the free sugar. Nevertheless, sequence analysis and complementation studies with Escherichia coli demonstrated that strain CNRZ 302 contained structurally intact genes for the Leloir pathway enzymes. These were organized into an operon in the order galKTE, which was preceded by a divergently transcribed regulator gene, galR, and followed by a galM gene and the lactose operon lacSZ. Results of Northern blot analysis showed that the structural gal genes were transcribed weakly, and only in medium containing lactose, by strain CNRZ 302. However, in a spontaneous galactose-fermenting mutant, designated NZ302G, the galKTE genes were well expressed in cells grown on lactose or galactose. In both CNRZ 302 and the Gal(+) mutant NZ302G, the transcription of the galR gene was induced by growth on lactose. Disruption of galR indicated that it functioned as a transcriptional activator of both the gal and lac operons while negatively regulating its own expression. Sequence analysis of the gal promoter regions of NZ302G and nine other independently isolated Gal(+) mutants of CNRZ 302 revealed mutations at three positions in the galK promoter region, which included substitutions at positions -9 and -15 as well as a single-base-pair insertion at position -37 with respect to the main transcription initiation point. Galactokinase activity measurements and analysis of gusA reporter gene fusions in strains containing the mutated promoters suggested that they were gal promoter-up mutations. We propose that poor expression of the gal genes in the galactose-negative S. thermophilus CNRZ 302 is caused by naturally occurring mutations in the galK promoter.

Metabolic engineering of sugar catabolism in lactic acid bacteria

Lactic acid bacteria are characterized by a relatively simple sugar fermentation pathway that, by definition, results in the formation of lactic acid. The extensive knowledge of traditional pathways and the accumulating genetic information on these and novel ones, allows for the rerouting of metabolic processes in lactic acid bacteria by physiological approaches, genetic methods, or a combination of these two. This review will discuss past and present examples and future possibilities of metabolic engineering of lactic acid bacteria for the production of important compounds, including lactic and other acids, flavor compounds, and exopolysaccharides.

Identification and characterization of the eps (Exopolysaccharide) gene cluster from Streptococcus thermophilus Sfi6

We report the identification and characterization of the eps gene cluster of Streptococcus thermophilus Sfi6 required for exopolysaccharide (EPS) synthesis. This report is the first genetic work concerning EPS production in a food microorganism. The EPS secreted by this strain consists of the following tetrasaccharide repeating unit:-->3)-beta-D-Galp-(1-->3)-[alpha-D-Galp-(1-->6)]-beta-D- D-Galp-(1-->3)-alpha-D-Galp-D-GalpNAc-(1-->. The genetic locus The genetic locus was identified by Tn916 mutagenesis in combination with a plate assay to identify Eps mutants. Sequence analysis of the gene region, which was obtained from subclones of a genomic library of Sfi6, revealed a 15.25-kb region encoding 15 open reading frames. EPS expression in the non-EPS-producing heterologous host, Lactococcus lactis MG1363, showed that within the 15.25-kb region, a region with a size of 14.52 kb encoding the 13 genes epsA to epsM was capable of directing EPS synthesis and secretion in this host. Homology searches of the predicted proteins in the Swiss-Prot database revealed high homology (40 to 68% identity) for epsA, B, C, D, and E and the genes involved in capsule synthesis in Streptococcus pneumoniae and Streptococcus agalactiae. Moderate to low homology (37 to 18% identity) was detected for epsB, D, F, and H and the genes involved in capsule synthesis in Staphylococcus aureus for epsC, D, and E and the genes involved in exopolysaccharide I (EPSI) synthesis in Rhizobium meliloti for epsC to epsJ and the genes involved in lipopolysaccharide synthesis in members of the Enterobacteriaceae, and finally for eps K and lipB of Neisseria meningitidis. Genes (epsJ, epsL, and epsM) for which the predicted proteins showed little or no homology with proteins in the Swiss-Prot database were shown to be involved in EPS synthesis by single-crossover gene disruption experiments.

Cloning and characterization of the galactokinase gene from Streptococcus thermophilus

The objective of this research was to clone and characterize the galactokinase gene (galK) from Streptococcus thermophilus F410. Partially digested genomic DNA was cloned into pBR322 and transformed into galK Escherichia coli, and a galactose-fermenting transformant was isolated. Restriction analysis revealed that the transformant resulted from a Sau3A-HindIII 4.0-kb fragment. Galactokinase activity in the recombinant was 10 times that of the parent strain. Analysis of the DNA sequence showed the presence of a 1.3-kb open reading frame that had high homology with the galK gene from other organisms. A putative ribosome-binding site, start and stop codons, and -10 and -35 sequences were identified. The predicted protein had a molecular mass of 49 kDa, which corresponded to the estimated size of a band apparent by SDS-PAGE. Amino acid sequence homologies with other galactokinases ranged from 50 to 62% similarity. Northern blots were performed between the galK gene and mRNA from S. thermophilus. No hybridization signals were observed for cells grown in glucose, but cells grown in lactose or galactose gave moderate and strong signals. The results suggest that repression of the galK gene by glucose may be responsible for the galactose-releasing phenotype in these strains.

Genetics of lactose utilization in lactic acid bacteria

Lactose utilization is the primary function of lactic acid bacteria used in industrial dairy fermentations. The mechanism by which lactose is transported determines largely the pathway for the hydrolysis of the internalized disaccharide and the fate of the glucose and galactose moieties. Biochemical and genetic studies have indicated that lactose can be transported via phosphotransferase systems, transport systems dependent on ATP binding cassette proteins, or secondary transport systems including proton symport and lactose-galactose antiport systems. The genetic determinants for the group translocation and secondary transport systems have been identified in lactic acid bacteria and are reviewed here. In many cases the lactose genes are organized into operons or operon-like structures with a modular organization, in which the genes encoding lactose transport are tightly linked to those for lactose hydrolysis. In addition, in some cases the genes involved in the galactose metabolism are linked to or co-transcribed with the lactose genes, suggesting a common evolutionary pathway. The lactose genes show characteristic configurations and very high sequence identity in some phylogenetically distant lactic acid bacteria such as Leuconostoc and Lactobacillus or Lactococcus and Lactobacillus. The significance of these results for the adaptation of lactic acid bacteria to the industrial milk environment in which lactose is the sole energy source is discussed.

Energy transduction in lactic acid bacteria

In the discovery of some general principles of energy transduction, lactic acid bacteria have played an important role. In this review, the energy transducing processes of lactic acid bacteria are discussed with the emphasis on the major developments of the past 5 years. This work not only includes the biochemistry of the enzymes and the bioenergetics of the processes, but also the genetics of the genes encoding the energy transducing proteins. The progress in the area of carbohydrate transport and metabolism is presented first. Sugar translocation involving ATP-driven transport, ion-linked cotransport, heterologous exchange and group translocation are discussed. The coupling of precursor uptake to product product excretion and the linkage of antiport mechanisms to the deiminase pathways of lactic acid bacteria is dealt with in the second section. The third topic relates to metabolic energy conservation by chemiosmotic processes. There is increasing evidence that precursor/product exchange in combination with precursor decarboxylation allows bacteria to generate additional metabolic energy. In the final section transport of nutrients and ions as well as mechanisms to excrete undesirable (toxic) compounds from the cells are discussed.

Lactose Uptake Driven by Galactose Efflux in Streptococcus thermophilus : Evidence for a Galactose-Lactose Antiporter

Galactose-nonfermenting (Gal - ) Streptococcus thermophilus TS2 releases galactose into the extracellular medium when grown in medium containing excess lactose. Starved and de-energized Gal - cells, however, could be loaded with galactose to levels approximately equal to the extracellular concentration (0 to 50 mM). When loaded cells were separated from the medium and resuspended in fresh broth containing 5 mM lactose, galactose efflux occurred. De-energized, galactose-loaded cells, resuspended in buffer or medium, accumulated [ 14 C]lactose at a greater rate and to significantly higher intracellular concentrations than unloaded cells. Uptake of lactose by loaded cells was inhibited more than that by unloaded cells in the presence of extracellular galactose, indicating that a galactose gradient was involved in the exchange system. When de-energized, galactose-loaded cells were resuspended in carbohydrate-free medium at pH 6.7, a proton motive force (Δp) of 86 to 90 mV was formed, whereas de-energized, nonloaded cells maintained a Δp of about 56 mV. However, uptake of lactose by loaded cells occurred when the proton motive force was abolished by the addition of an uncoupler or in the presence of a proton-translocating ATPase inhibitor. These results support the hypothesis that galactose efflux in Gal - S. thermophilus is electrogenic and that the exchange reaction (lactose uptake and galactose efflux) probably occurs via an antiporter system.

Precursor/product antiport in bacteria

Many microorganisms metabolize their substrates (precursors) only partially and excrete the products of the metabolism into the medium. Although uptake of precursor and exit of product can proceed as two independent steps, there is increasing evidence that these processes are often linked and that transport is facilitated by a single antiport mechanism. Features of antiport mechanisms and advantages for the organism of catalysing precursor/product antiport will be illustrated by discussing a number of well-characterized systems. Based on precursor-product conversion stoichiometries, structural relatedness between precursors and products, and energetic and kinetic considerations, new examples of antiport systems will be proposed.

Carbohydrate utilization in Streptococcus thermophilus: characterization of the genes for aldose 1-epimerase (mutarotase) and UDPglucose 4-epimerase

The complete nucleotide sequences of the genes encoding aldose 1-epimerase (mutarotase) (galM) and UDPglucose 4-epimerase (galE) and flanking regions of Streptococcus thermophilus have been determined. Both genes are located immediately upstream of the S. thermophilus lac operon. To facilitate the isolation of galE, a special polymerase chain reaction-based technique was used to amplify the region upstream of galM prior to cloning. The galM protein was homologous to the mutarotase of Acinetobacter calcoaceticus, whereas the galE protein was homologous to UDPglucose 4-epimerase of Escherichia coli and Streptomyces lividans. The amino acid sequences of galM and galE proteins also showed significant similarity with the carboxy-terminal and amino-terminal domains, respectively, of UDPglucose 4-epimerase from Kluyveromyces lactis and Saccharomyces cerevisiae, suggesting that the yeast enzymes contain an additional, yet unidentified (mutarotase) activity. In accordance with the open reading frames of the structural genes, galM and galE were expressed as polypeptides with apparent molecular masses of 39 and 37 kilodaltons, respectively. Significant activities of mutarotase and UDPglucose 4-epimerase were detected in lysates of E. coli cells containing plasmids encoding galM and galE. Expression of galE in E. coli was increased 300-fold when the gene was placed downstream of the tac promoter. The gene order for the gal-lac gene cluster of S. thermophilus is galE-galM-lacS-lacZ. The flanking regions of these genes were searched for consensus promoter sequences and further characterized by primer extension analysis. Analysis of mRNA levels for the gal and lac genes in S. thermophilus showed a strong reduction upon growth in medium containing glucose instead of lactose. The activities of the lac (lactose transport and beta-galactosidase) and gal (UDPglucose 4-epimerase) proteins of lactose- and glucose-grown S. thermophilus cells matched the mRNA levels.

Glycolysis and related reactions during cheese manufacture and ripening

Fermentation of lactose to lactic acid by lactic acid bacteria is an essential primary reaction in the manufacture of all cheese varieties. The reduced pH of cheese curd, which reaches 4.5 to 5.2, depending on the variety, affects at least the following characteristics of curd and cheese: syneresis (and hence cheese composition), retention of calcium (which affects cheese texture), retention and activity of coagulant (which influences the extent and type of proteolysis during ripening), the growth of contaminating bacteria. Most (98%) of the lactose in milk is removed in the whey during cheesemaking, either as lactose or lactic acid. The residual lactose in cheese curd is metabolized during the early stages of ripening. During ripening lactic acid is also altered, mainly through the action of nonstarter bacteria. The principal changes are (1) conversion of L-lactate to D-lactate such that a racemic mixture exists in most cheeses at the end of ripening; (2) in Swiss-type cheeses, L-lactate is metabolized to propionate, acetate, and CO2, which are responsible for eye formation and contribute to typical flavor; (3) in surface mold, and probably in surface bacterially ripened cheese, lactate is metabolized to CO2 and H2O, which contributes to the increase in pH characteristic of such cheeses and that is responsible for textural changes, (4) in Cheddar and Dutch-type cheeses, some lactate may be oxidized to acetate by Pediococci. Cheese contains a low level of citrate, metabolism of which by Streptococcus diacetylactis leads to the production of diacetyl, which contributes to the flavor and is responsible for the limited eye formation characteristic of such cheeses.

Lactose transport system of Streptococcus thermophilus: a hybrid protein with homology to the melibiose carrier and enzyme III of phosphoenolpyruvate-dependent phosphotransferase systems

The gene responsible for the transport of lactose into Streptococcus thermophilus (lacS) was cloned in Escherichia coli as a 4.2-kilobase fragment from an EcoRI library of chromosomal DNA by using the vector pKK223-3. From deletion analysis, the gene for lactose transport mapped to two HindIII fragments with a total size of 2.8 kilobases. The gene was transcribed in E. coli from its own promoter. Functional expression of lactose transport activity was shown by assaying for the uptake and exchange of lactose both in intact cells and in membrane vesicles. The nucleotide sequence of lacS and 200 to 300 bases of 3' and 5' flanking regions were determined. The gene was 1,902 base pairs long, encoding a 69,454-dalton protein with an NH2-terminal hydrophobic region and a COOH-terminal hydrophilic region. The NH2-terminal end was homologous with the melibiose carrier of E. coli (23% similarity overall; greater than 50% similarity for regions with at least 16 amino acids), whereas the COOH-terminal end showed 34 to 41% similarity with the enzyme III (domain) of three different phosphoenolpyruvate-dependent phosphotransferase systems. Among the conserved amino acids were two histidyl residues, of which one has been postulated to be phosphorylated by HPr. Since sugars are not phosphorylated during translocation by the lactose transport system, it is suggested that the enzyme III-like region serves a regulatory function in this protein. The lacS gene also appears similar to the partially sequenced lactose transport gene of Lactobacillus bulgaricus (lacL; greater than 60% similarity). Furthermore, the 3' flanking sequence of the S. thermophilus lactose transport gene showed approximately 50% similarity with the N-terminal portion of the beta-galactosidase gene of L. bulgaricus. In both organisms, the lactose transport gene and the beta-galactosidase appear to be separated by a 3-base-pair intercistronic region.

A comparison of beta-galactosidase specific activities in strains of Streptococcus thermophilus

Six Streptococcus thermophilus strains were examined for growth, acid production, and beta-galactosidase activity per milligram protein (specific activity), so that strain comparisons could be made. A wide range in activity was observed. Activity depended on growth time in M17 broth and, for most strains, continued to increase after cells had reached stationary phase. Maximum activity was at 16 h and ranged from 0 to 58 units/mg protein. Strain ST exhibited no beta-galactosidase activity but had trace phospho-beta-galactosidase activity (.8 units/mg protein after 2 h of growth). Strains 3641 and TS2B exhibited slower growth rates and lower beta-galactosidase activities in milk as compared to M17 broth. Further, strain 3641 exhibited 10 times the activity of strain TS2B (2.86 vs. .24 units) after 4 h of growth in milk.

Cloning and expression of the beta-D-galactosidase gene from Streptococcus thermophilus in Escherichia coli

The beta-D-galactosidase (beta-gal) gene from Streptococcus thermophilus was cloned to isolate and characterize it for potential use as a selection marker in a food-grade cloning vector. Chromosomal DNA from S. thermophilus 19258 was cleaved with the restriction enzyme PstI and ligated to pBR322 for transformation into Escherichia coli JM108. A beta-galactosidase-positive clone was detected by its blue color on a medium supplemented with 5-bromo-4-chloro-3-indolyl-beta-D-galactoside. This transformant possessed a single plasmid, designated pRH116, which contained, in addition to the vector DNA, a 7.0-kilobase (kb) PstI insertion fragment coding for beta-gal activity. An extract from JM108(pRH116) contained a beta-gal protein with the same electrophoretic mobility as the beta-gal from S. thermophilus 19258. Compared with the beta-gal from E. coli HB101, the S. thermophilus beta-gal was of lower molecular weight. A restriction map of pRH116 was constructed from cleavage of both the plasmid and the purified insert. The construction of deletion derivatives of pRH116 with BglII, BstEII, and HindIII revealed the approximate location of the gene on the 7.0-kb fragment. The beta-gal gene was further localized to a 3.85-kb region.

Galactose transport in Streptococcus thermophilus

Although Streptococcus thermophilus accumulated [14C]lactose in the absence of an endogenous energy source, galactose-fermenting (Gal+) cells were unable to accumulate [14C]galactose unless an additional energy source was added to the test system. Both Gal+ and galactose-nonfermenting (Gal-) strains transported galactose when preincubated with sucrose. Accumulation was inhibited 50 or 95% when 10 mM sodium fluoride or 1.0 mM iodoacetic acid, respectively, was added to sucrose-treated cells, indicating that ATP was required for galactose transport activity. Proton-conducting ionophores also inhibited galactose uptake, although N,N'-dicyclohexyl carbodiimide had no effect. The results suggest that galactose transport in S. thermophilus occurs via an ATP-dependent galactose permease and that a proton motive force is involved. The galactose permease in S. thermophilus TS2b (Gal+) had a Km for galactose of 0.25 mM and a Vmax of 195 micromol of galactose accumulated per min per g (dry weight) of cells. Several structurally similar sugars inhibited galactose uptake, indicating that the galactose permease had high affinities for these sugars.