Safety aspects, genetic diversity and technological characterisation of wild-type Streptococcus thermophilus strains isolated from north Italian traditional cheeses

Genetic and technological diversity of Streptococcus thermophilus isolated from the Saint-Nectaire PDO cheese-producing area

Streptococcus thermophilus is of major importance for cheese manufacturing to ensure rapid acidification; however, studies indicate that intensive use of commercial strains leads to the loss of typical characteristics of the products. To strengthen the link between the product and its geographical area and improve the sensory qualities of cheeses, cheese-producing protected designations of origin (PDO) are increasingly interested in the development of specific autochthonous starter cultures. The present study is therefore investigating the genetic and functional diversity of S. thermophilus strains isolated from a local cheese-producing PDO area. Putative S. thermophilus isolates were isolated and identified from milk collected in the Saint-Nectaire cheese-producing PDO area and from commercial starters. Whole genomes of isolates were sequenced, and a comparative analysis based on their pan-genome was carried out. Important functional properties were studied, including acidifying and proteolytic activities. Twenty-two isolates representative of the diversity of the geographical area and four commercial strains were selected for comparison. The resulting phylogenetic trees do not correspond to the geographical distribution of isolates. The clustering based on the pan-genome analysis indicates that isolates are divided into five distinct groups. A Kyoto Encyclopedia of Genes and Genomes (KEGG) functional annotation of the accessory genes indicates that the accessory gene contents of isolates are involved in different functional categories. High variability in acidifying activities and less diversity in proteolytic activities were also observed. These results indicate that high genetic and functional variabilities of the species S. thermophilus may arise from a small (1,800 km2) geographical area and may be exploited to meet demand for use as autochthonous starters.

A rapid screening method to evaluate acidifying activity by lactic acid bacteria

The determination of pH in fermented milk is an important parameter for monitoring the production of acid by lactic acid bacteria (LAB). In this work, a colorimetric method is proposed that allows a fast determination of LAB acidification ability by evaluation of pH drop in whey fermentation. The proposed method uses spectrophotometry to measure the pH change by bacteria and uses bromocresol purple as a pH indicator dye. The absorbance at 430 nm of a buffer solution with bromocresol purple was found to be correlated with pH values. This colorimetric assay was linear within the pH range of 4.6-7.0. Upon regression analysis, linear equation y = -0.1267× + 0.9196 was obtained having r² value of 0.9927. The assay was validated by the use of LAB fermentation in sweet whey and comparison to the values obtained by glass electrode/pH meter. Estimation of acidification activity of LAB in whey was found to be similar in both methods (r = 0.801, p > 0.05). The proposed procedure presents a viable alternative to the measurement of pH by the standard method and allows the simultaneous and fast screening of LAB acidifying activity.

Structural characterisation of EPS of Streptococcus thermophilus S-3 and its application in milk fermentation

The application of Streptococcus thermophilus S-3 into yogurt production was studied and the structural properties of the generated exopolysaccharides (EPS-S3) were characterized. The proposed structure of EPS-S3 was obtained. EPS-S3 contained a high ratio of N-Acetyl-galactosamine with the Mw of 574 kDa, which was higher than that of AR333 (314 kD) leading to higher apparent viscosity. Streptococcus thermophilus strain S-3 was co-cultured with Lactobacillus delbrueckii for yogut production which highly increased the acidifying rate and post-acidification rate. The quality of the co-cultured yogurts in terms of apparent viscosity, syneresis capacity, water holding capacity and rheological properties were much better than that by using Lactobacillus bulgaricus only. The production mechanism of EPS-S3 from gene regulated level was also discussed which is helpful to facilitate the application of Streptococcus thermophilus strain into milk production.

A critical review of antibiotic resistance in probiotic bacteria

Probiotics are defined as live microorganisms that, when administered in adequate amounts, confer a health benefit upon the host. At present, probiotics are gaining popularity worldwide and are widely used in food and medicine. Consumption of probiotics is increasing with further in-depth research on the relationship between intestinal flora and host health. Most people pay more attention to the function of probiotics but ignore their potential risks, such as infection and antibiotic resistance transfer to pathogenic microbes. Physiological functions, effects and mechanisms of action of probiotics were covered in this review, as well as the antibiotic resistance phenotypes, mechanisms and genes found in probiotics. Typical cases of antibiotic resistance of probiotics were also highlighted, as well as the potential risks (including pathogenicity, infectivity and excessive immune response) and corresponding strategies (dosage, formulation, and administration route). This timely study provides an avenue for further research, development and application of probiotics.

Diversity of non-starter lactic acid bacteria in autochthonous dairy products from Western Balkan Countries - Technological and probiotic properties

The aim of this review was to summarize the data regarding diversity of non-starter lactic acid bacteria (NSLAB) isolated from various artisanal dairy products manufactured in Western Balkan Countries. The dairy products examined were manufactured from raw cow's, sheep's or goat's milk or mixed milk, in the traditional way without the addition of commercial starter cultures. Dairy products such as white brined cheese, fresh cheese, hard cheese, yogurt, sour cream and kajmak were sampled in the households of Serbia, Croatia, Slovenia, Bosnia and Herzegovina, Montenegro, and North Macedonia. It has been established that the diversity of lactic acid bacteria (LAB) from raw milk artisanal dairy products is extensive. In the reviewed literature, 28 LAB species and a large number of strains belonging to the Lactobacillus, Lactococcus, Enterococcus, Streptococcus, Pediococcus, Leuconostoc and Weissella genera were isolated from various dairy products. Over 3000 LAB strains were obtained and characterized for their technological and probiotic properties including: acidification and coagulation of milk, production of aromatic compounds, proteolytic activity, bacteriocins production and competitive exclusion of pathogens, production of exopolysaccharides, aggregation ability and immunomodulatory effect. Results show that many of the isolated NSLAB strains had one, two or more of the properties mentioned. The data presented emphasize the importance of artisanal products as a valuable source of NSLAB with unique technological and probiotic features important both as a base for scientific research as well as for designing novel starter cultures for functional dairy food.

Selection of raw cow’s milk thermophilic lactic acid bacteria obtained from southwest Parana, Brazil, with potential use as autochthonous starter

Abstract This work aimed to isolate and identify Streptococcus and Lactobacillus species from raw cow’s milk obtained from Southwest Paraná - Brazil. We used randomly amplified polymorphic DNA (RAPD)-PCR to identify and type 58Streptococcus and 48 Lactobacillus isolates, of which 04 Streptococcus thermophilus and 02 Streptpcoccus macedonicus were confirmed by species-specific PCR and by sequencing of the 16S ribosomal RNA of 02Streptococcus lutetiensis/infantarius, 10 Lactobacillus fermentum, 03 Lactobacillus delbrueickii subspecies bulgaricus, 01 Lactobacillus rhamnosus/casei and 02 Lactobacillus helveticus. The results indicated predominance of Streptococcus thermophillus and Lactobacillus fermentum. Streptococcus thermophilus and Lactobacillus delbrueickii subspecies bulgaricus strains were tested on the basis of their acidification kinetics. Considerable variation between the Streptococcus thermophilus was observed for the maximum rate of acidification (Vm), with a maximum of -4.5 and minimum of -4.2 pH milliunits min-1. The Lactobacillus delbrueickii subspecies bulgaricus showed values between -8.4 and -7.1 pH milliunits min-1. These results suggest that strains characterized as having a high acidifying capacity, could be used as starters in cheesemaking. The ferments presented an excellent performance in the acidification process, generating adequate curves, characteristics of a starter culture.

Isolation and characterization of an exopolysaccharide-producing Leuconostoc citreum strain from artisanal cheese

High molar mass exopolysaccharides (EPS) produced from sucrose by lactic acid bacteria (LAB) are of great interest as natural additives to use in foods, medical and pharmaceutical industry. This study aimed to identify the EPS produced by Leuconostoc citreum L3C1E7 isolated from Pico cheese and characterize the strain for technological and probiotic potential. Purified EPS was isolated from the culture of L. citreum L3C1E7 by ethanol precipitation, with a yield of 520 mg ml^-1 . The EPS-producing strain had a mucoid phenotype and average molecular weight of 5·88 × 10⁶  Da. The structural characterization of the purified EPS was determined by ¹ H, ¹³ C and two-dimensional NMR spectroscopy. EPS was composed of alternating α-(1→6)-linked and α-(1→3)-linked D-glucopyranyl units, suggesting the existence of an alternan. The strain was slow acidifying, produced diacetyl and displayed high esterase/lipase and aminopeptidase activities, which promote the desirable flavours in dairy products. Moreover, L. citreum showed moderate resistance to the adverse conditions of the gastrointestinal (GI) tract and high adhesion to GI cells. This work provides a better understanding of EPS produced by L. citreum and the potential application of EPS-producing strain in food and/or as a probiotic culture. SIGNIFICANCE AND IMPACT OF THE STUDY: Some LAB strains are known to use extracellular glycoside-hydrolase enzymes for synthesizing a diversity of exopolysaccharides (EPS) with potential application as natural additives to foods. Previous studies have identified an EPS-producing Leuconostoc citreum strain with immunomodulatory properties. This work provides a better understanding of EPS produced by this strain and the potential application of the strain in food fermentation and/or as a probiotic culture.

Technological properties assessment and two component systems distribution of Streptococcus thermophilus strains isolated from fermented milk

Streptococcus thermophilus is one of the economically most representatives of lactic acid bacteria, which is widely used as a starter to produce fermented milk products. In this study, 22 S. thermophilus strains were isolated from 26 fermented milk samples. Most isolates showed the ability to ferment a broad range of carbohydrates. Interestingly, eight strains are galactose positive, which is a desirable property in various industrial dairy fermentations. Four different nucleotide sequences were found in the galR-galK intergenic regions. The 16S-23S intergenic spacer region sequences of most isolates were determined as ITS-St-II type, which are related with protease positive and fast acidification. CS18 presented excellent technological performances, and showed potential as a promising starter candidate. To gain a comprehensive view of stress response mechanisms of strains, the distribution of all the two-component systems (TCSs) in strains were investigated. TCS analysis indicated that the nucleotide sequence of TCSs have obvious differences in different strains. And the strains with the special nucleotide sequences of TCS have distinctive traits. Therefore, it was speculated that there is a certain connection between the traits' difference and the TCS difference of strains.

New advances in exopolysaccharides production of Streptococcus thermophilus

Streptococcus thermophilus is the most important thermophilic dairy starter, and is widely used in the dairy industry. Streptococcus thermophilus exopolysaccharides received wide attention over recent decades, because they can improve the properties of the dairy product and confer beneficial health effects. The understanding of the regulatory and biosynthetic mechanisms of EPS will improve the EPS biosynthesis, increase the productivity of EPSs, and develop EPSs with desirable properties. The structure of EPSs is the focus of this study. Revealing the structure-function relationship can lead to increase the knowledge base and from there to increased research of EPS. The EPS yield is a key limiting factor in the research and utilization of EPS. In the present review, biosynthetic pathways and genetics of S. thermophilus EPSs were described and reviewed. At the same time, functional properties and applications of EPS, and strategies for enhancement of EPS production are discussed.

Proteolytic Activity and Production of γ-Aminobutyric Acid by Streptococcus thermophilus Cultivated in Microfiltered Pasteurized Milk

A set of 191 strains of Streptococcus thermophilus were preliminarily screened for the presence of the genes codifying for cell envelope-associated proteinase (prtS) and for glutamate decarboxylase (gadB) responsible for γ-aminobutyric acid (GABA) production. The growth and proteolytic activity of the gadB-positive strains (9 presenting the prtS gene and 11 lacking it) were studied in microfiltered pasteurized milk. Degradation of both caseins (capillary electrophoresis) and soluble nitrogen fractions (HPLC) and changes in the profile of free amino acids (FAAs; ion-exchange chromatography) were evaluated at inoculation and after 6 and 24 h of incubation at 41 °C. None of the strains was capable of hydrolyzing caseins and β-lactoglobulin, and only two hydrolyzed part of α-lactalbumin, these proteins being present in their native states in pasteurized milk. Contrarily, most strains were able to hydrolyze peptones and peptides. For initial growth, most strains relied on the FAAs present in milk, whereas, after 6 h, prtS⁺ strains released variable amounts of FAA. One prtS⁺ strain expressed a PrtS^- phenotype, and two prtS^- strains showed a rather intense proteolytic activity. Only five strains (all prtS⁺) produced GABA, in variable quantities (up to 100 mg/L) and at different rates, depending on the acidification strength. Addition of glutamate did not induce production of GABA in nonproducing strains that, however, unexpectedly were shown to adopt the degradation of arginine into citrulline and ornithine as an alternative acid resistance system and likely as a source of ATP.

New Insights into Various Production Characteristics of Streptococcus thermophilus Strains

Streptococcus thermophilus is one of the most valuable homo-fermentative lactic acid bacteria, which, for a long time, has been widely used as a starter for the production of fermented dairy products. The key production characteristics of S. thermophilus, for example the production of extracellular polysaccharide, proteolytic enzymes and flavor substances as well as acidifying capacity etc., have an important effect on the quality of dairy products. The acidification capacity of the strains determines the manufacturing time and quality of dairy products. It depends on the sugar utilization ability of strains. The production of extracellular polysaccharide is beneficial for improving the texture of dairy products. Flavor substances increase the acceptability of dairy products. The proteolytic activity of the strain influences not only the absorption of the nitrogen source, but also the formation of flavor substances. Different strains have obvious differences in production characteristics via long-time evolution and adaptation to environment. Gaining new strains with novel and desirable characteristics is an important long-term goal for researchers and the fermenting industry. The understanding of the potential molecular mechanisms behind important characteristics of different strains will promote the screening and breeding of excellent strains. In this paper, key technological and functional properties of different S. thermophilus strains are discussed, including sugar metabolism, proteolytic system and amino acid metabolism, and polysaccharide and flavor substance biosynthesis. At the same time, diversity of genomes and plasmids of S. thermophilus are presented. Advances in research on key production characteristics and molecular levels of S. thermophilus will increase understanding of molecular mechanisms of different strains with different important characteristics, and improve the industrialization control level for fermented foods.

Technological characterisation, antibiotic susceptibility and antimicrobial activity of wild-type Leuconostoc strains isolated from North Italian traditional cheeses

Genotypic and technological properties, antibiotic susceptibility and antimicrobial activity of 35 Leuconostoc strains, isolated from different Italian raw milk cheeses, were investigated. RAPD-PCR was used to study genetic variability and to distinguish closely related strains. The results showed a high degree of heterogeneity among isolates. All the strains had weak acidifying activity and showed low proteolytic and lipolytic activities. Reduction activity, was generally low. All the Leuconostoc were susceptible to ampicillin, mupirocin, erythromycin, quinupristin/dalfopristin and tetracycline. Many strains were classified as resistant to oxacillin, ciprofloxacin and nitrofurantonin, while all isolates were found resistant to vancomycin. PCR-based detection did not identify any of the common genetic determinants for vancomycin (vanA, vanB, vanC1, vanC2, vanC3, vanD, vanE, vanG) or erythromycin (ermB and ermC). Tetracycline resistance genes were detected in 25 tetracycline susceptible strains, the most frequent one being tetM. One strain, belonging to Ln. pseudomesenteroides species, was positive for the presence of the int gene of the Tn916/Tn1545 trasposon family. This is the first time the conjugative transposon Tn916 has been detected inside the Leuconostoc species. All strains showed antimicrobial activity against Enterococcus faecalis and Ent. faecium. The presence of genes encoding amino-acid decarboxylases (hdc and tdc) was not detected. Some strains are interesting in view of their use in cheese production as starter and non starter cultures.