Streptococcus thermophilus: Metabolic Properties, Functional Features, and Useful Applications

Streptococcus thermophilus is a Gram-positive, homofermentative lactic acid bacterium classified within the Firmicutes phylum, recognized for its probiotic properties and significant role in promoting human health. This review consolidates existing understanding of its metabolic pathways, functional metabolites, and diverse applications, highlighting evidence-based insights to enhance scientific integrity. S. thermophilus predominantly ferments lactose through the Embden-Meyerhof-Parnas pathway, resulting in L(+)-lactic acid as the primary end-product, along with secondary metabolites including acetic acid, formic acid, and pyruvate derivatives. Exopolysaccharides (EPS) are composed of repeating units of glucose, galactose, rhamnose, and N-acetylgalactosamine. They display strain-specific molecular weights ranging from 10 to 2000 kDa and contribute to the viscosity of fermented products, while also providing antioxidant and immunomodulatory benefits. Aromatic compounds such as acetaldehyde and phenylacetic acid are products of amino acid catabolism and carbohydrate metabolism, playing a significant role in the sensory characteristics observed in dairy fermentations. Bacteriocins, such as thermophilins (e.g., Thermophilin 13, 110), exhibit extensive antimicrobial efficacy against pathogens including Listeria monocytogenes and Bacillus cereus. Their activity is modulated by quorum-sensing mechanisms that involve the blp gene cluster, and they possess significant stability under heat and pH variations, making them suitable for biopreservation applications. In food applications, S. thermophilus functions as a Generally Recognized as Safe (GRAS) starter culture in the production of yogurt and cheese, working in conjunction with Lactobacillus delbrueckii subsp. bulgaricus to enhance acidification and improve texture. Specific strains have been identified to mitigate lactose intolerance, antibiotic-related diarrhea, and inflammatory bowel diseases through the modulation of gut microbiota, the production of short-chain fatty acids, and the inhibition of Helicobacter pylori. The genome, characterized by a G + C content of approximately 37 mol%, facilitates advancements in Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-Cas technology and heterologous protein expression, with applications extending to non-dairy fermentations and the development of postbiotics. This review emphasizes the adaptability of S. thermophilus, showcasing the variability among strains and the necessity for thorough preclinical and clinical validation to fully utilize its potential in health, sustainable agriculture, and innovation. It also addresses challenges such as susceptibility to bacteriophages and limitations in proteolytic activity.