Hidden Genomic Islands Reveal How Yogurt Bacterium Evolved Probiotic Powers

Streptococcus thermophilus is one of the most familiar bacteria on Earth, even if almost nobody has heard its name. Every spoonful of yogurt and most blocks of cheese owe their existence to this lactic acid bacterium, which is classified as GRAS (Generally Recognized As Safe) in the United States and holds Qualified Presumption of Safety status in Europe. Yet for all its industrial importance, the genomic machinery behind its adaptation to dairy environments and its potential as a probiotic has remained only partly explained. A new study published in BMC Genomics by Juan Carlos Ariute, Flávia Figueira Aburjaile and colleagues at the Federal University of Minas Gerais in Brazil and INRAE in France now points to a compelling answer: stretches of foreign DNA called genomic islands, acquired through horizontal gene transfer, appear to carry much of the equipment that makes individual strains of S. thermophilus distinctive, metabolically versatile and potentially probiotic.

Genomic islands are discrete regions of a bacterial chromosome that often originate from other microbes. They typically differ in nucleotide composition from the rest of the genome, are flanked by repeats or insertion sequences, and can encode anything from secretion systems to metabolic enzymes. Because they move between lineages, they act as hotspots of genomic plasticity, allowing bacteria to acquire new functions in single evolutionary leaps rather than through slow accumulation of point mutations. For S. thermophilus, whose transition from a commensal ancestor living in mammalian guts to an industrial workhorse in fermenters involved extensive genomic remodeling, these islands are a natural place to look for the traits that set high-performing strains apart from the rest.

The research team assembled a dataset of 109 S. thermophilus genomes and subjected them to a battery of comparative and predictive bioinformatics analyses. Phylogenomic trees built from single-copy orthologs and pangenome reconstructions revealed that the species divides into two major lineages, which the authors labeled STH1 and STH2. This clustering mattered, because the two branches turned out to differ in their evolutionary dynamics. Strains belonging to STH2 showed markedly greater genomic plasticity, with twenty equivalent genomic islands shared among members of this lineage, suggesting that this branch has been more receptive to horizontal gene acquisition or has exchanged DNA more frequently among its members.

To capture the full functional landscape of the species, the researchers analyzed the pangenome, the sum of all genes found across all strains. They found that genes associated with probiotic-related functions, including cell wall and membrane biosynthesis, amino acid and carbohydrate metabolism, and secondary metabolite production, were concentrated predominantly in the accessory and unique fractions of the genome rather than the core. In other words, the traits most relevant to probiotic potential are not universal properties of the species but strain-specific endowments, and genomic islands appear to be a major vehicle delivering them. Among island categories, metabolic islands were the most abundant and accounted for the majority of probiotic-associated functions, followed by virulence islands, whose limited cargo is itself informative about the species’ biosafety.

That biosafety picture is one of the study’s most reassuring findings. Although S. thermophilus is a close relative of streptococci that cause disease, the team found that both the panvirulome, the complete set of virulence-related genes across all 109 strains, and the panresistome, the corresponding repertoire of antimicrobial resistance genes, were small. Features such as capsule-associated genes, rather than classical virulence factors, may contribute to probiotic performance by helping the bacterium persist and interact with host tissues. The limited virulence and resistance gene repertoire, the authors conclude, further supports the biosafety of the species and reinforces its suitability for industrial and probiotic applications, a conclusion consistent with its long GRAS and QPS regulatory standing.

The researchers went beyond gene prediction to test actual metabolic behavior. Using Biolog phenotyping, which measures a bacterium’s ability to metabolize a panel of substrates under different oxygen conditions, they examined three candidate strains: CIRM-BIA 16, CIRM-BIA 336 and CIRM-BIA 961. Overall, metabolic activity was limited to a few substrates under both aerobic and anaerobic conditions, reflecting the well-known nutritional specialization of S. thermophilus to the lactose-rich environment of milk. However, one strain stood out. The metabolic island identified in CIRM-BIA 336 carries genes that may support enhanced galactose utilization under anaerobic conditions through the tagatose-6-phosphate pathway, a route for fermenting this milk sugar that many dairy strains lack.

This detail has practical resonance for the dairy industry. Galactose accumulates when S. thermophilus ferments lactose in milk, and its incomplete metabolism can cause problems in products such as yogurt and reduced-lactose dairy foods, where residual galactose affects texture, sweetness and shelf life. A strain equipped with a genomic island enabling tagatose-6-phosphate pathway-dependent galactose consumption could therefore offer both a technological advantage in fermentation and a route to gentler products for galactose-sensitive consumers. The finding illustrates how a single mobile genetic element can translate directly into a phenotype of commercial interest, and it supports the authors’ broader conclusion that genomic islands are associated with strain-specific probiotic potential and metabolic versatility.

The mobilome analysis extended beyond islands proper. The team also mapped integrative conjugative elements and integrative mobilizable elements, two classes of mobile DNA that can carry entire functional modules between cells, sometimes across species boundaries. A striking pattern emerged: most genomic islands, as well as these integrative elements, occupied conserved positions in the chromosome across strains. This suggests that integration is not random but prefers specific hotspots, likely reflecting insertion-site preferences of the phage and plasmid machinery that mediates transfer, as well as selection against disrupting essential genes. Conserved landing pads for mobile DNA mean that parallel acquisitions can occur in different lineages, generating equivalent islands with related cargo that shape the species’ evolution in reproducible ways.

The study also probed the potential for bacteriocin production and secondary metabolite biosynthesis, traits prized in probiotics because bacteriocins can inhibit competing pathogens. Using antiSMASH to detect biosynthetic gene clusters and the DRAMP database to identify putative bacteriocins, the researchers documented these features across the dataset and linked them to the accessory genome and its mobile elements. Combined with the capsule-associated genes and the enriched cargo of metabolic islands, the picture is of a species whose probiotic-relevant toolkit is distributed unevenly among strains and is built, to a substantial degree, from horizontally acquired parts. This aligns with the emerging discipline of probiogenomics, which the authors list among the study’s keywords alongside horizontal gene transfer, lactic acid bacteria, comparative genomics and bioinformatics.

For strain designers and fermentation technologists, the practical message is that selecting S. thermophilus strains for next-generation fermented foods or probiotic formulations should involve screening their genomic islands, not just their core genomes. Two strains with nearly identical housekeeping genes may differ sharply in carbohydrate metabolism, defense molecules and surface features because of what their islands carry. The work also offers a caution: genomic islands act as major hotspots of genomic plasticity, particularly within the STH2 lineage, so genetic stability over industrial propagation and storage deserves attention when a desirable trait is island-encoded. With this study, the humble yogurt bacterium reveals itself as a genomic patchwork, stitched together over millennia of horizontal gene transfer, and the seams of that patchwork turn out to hold precisely the traits that make it valuable in our food and potentially in our guts.

Subject of Research: Genomic islands underlying host adaptation and probiotic potential in Streptococcus thermophilus

Article Title: Genomic islands associated with host adaptation and probiotic-related functions in Streptococcus thermophilus

Article References: Ariute, J. C., Roland, N., Azevedo, V., Le Loir, Y., Aburjaile, F. F., & Guédon, E. (2026). Genomic islands associated with host adaptation and probiotic-related functions in Streptococcus thermophilus. BMC Genomics. https://doi.org/10.1186/s12864-026-13419-x

Image Credits: AI Generated

DOI: 10.1186/s12864-026-13419-x

Keywords: Streptococcus thermophilus, genomic islands, probiotics, horizontal gene transfer, pangenomics, lactic acid bacteria, panvirulome, panresistome, galactose metabolism, dairy fermentation, BMC Genomics, biosafety